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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 VirtualHost,
Location, LocationMatch,
Directory, 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. The first version to use a given
directive is given in the Version sections below.
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 -
modsecurity_crs_15_customrules.conf 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 evaluated 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.
It is highly encouraged that you do not edit the Core rules files
themselves but rather place all changes (such as
SecRuleRemoveByID, 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.
Description: 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 SecRule.
Syntax: SecAction
action1,action2,action3
Example Usage: SecAction
nolog,phase:1,initcol:RESOURCE=%{REQUEST_FILENAME}
Processing Phase: Any
Scope: Any
Version: 2.0.0
Dependencies/Notes: None
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
initcol to initialize collections.
Description: Specifies which character to use
as separator for
application/x-www-form-urlencoded content. Defaults to
&. Applications are sometimes
(very rarely) written to use a semicolon (;).
Syntax: SecArgumentSeparator character
Example Usage: SecArgumentSeparator ;
Processing Phase: Any
Scope: Main
Version: 2.0.0
Dependencies/Notes: None
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.
Description: Configures the audit logging engine.
Syntax: SecAuditEngine On|Off|RelevantOnly
Example Usage: SecAuditEngine On
Processing Phase: N/A
Scope: Any
Version: 2.0.0
Dependencies/Notes: Can be set/changed with
the "ctl" action for the current transaction.
Example: The following example shows the various audit directives used together.
SecAuditEngine RelevantOnly SecAuditLog logs/audit/audit.log SecAuditLogParts ABCFHZ SecAuditLogType concurrent SecAuditLogStorageDir logs/audit SecAuditLogRelevantStatus ^(?:5|4\d[^4])
Possible values are:
On - log all transactions
by default.
Off - do not log
transactions by default.
RelevantOnly - 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 SecAuditLogRelevantStatus).
Description: Defines the path to the main audit log file.
Syntax: SecAuditLog
/path/to/auditlog
Example Usage: SecAuditLog
/usr/local/apache/logs/audit.log
Processing Phase: N/A
Scope: Any
Version: 2.0.0
Dependencies/Notes: This file is open on startup when the server typically still runs as root. You should not allow non-root users to have write privileges for this file or for the directory it is stored in..
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:
SecAuditLog "|/path/to/mlogc /path/to/mlogc.conf"
Description: Defines the path to the
secondary audit log index file when concurrent logging is enabled. See
SecAuditLog2 for more details.
Syntax: SecAuditLog2
/path/to/auditlog2
Example Usage: SecAuditLog2
/usr/local/apache/logs/audit2.log
Processing Phase: N/A
Scope: Any
Version: 2.1.2
Dependencies/Notes: A main audit log must be
defined via SecAuditLog 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 not be used for non-concurrent
audit logging.
Description: Configures the mode
(permissions) of any directories created for concurrent audit logs using
an octal mode (as used in chmod). See SecAuditLogFileMode for controlling the mode
of audit log files.
Syntax: SecAuditLogDirMode octal_mode|"default"
Example Usage: SecAuditLogDirMode 02750
Processing Phase: N/A
Scope: Any
Version: 2.5.10
Dependencies/Notes: This feature is not available on operating systems not supporting octal file modes. The default mode (0600) only grants read/write access to the account writing the file. If access from another account is needed (using mpm-itk is a good example), then this directive may be required. However, 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.
The process umask may still limit the mode if it is being more restrictive than the mode set using this directive.
Description: Configures the mode
(permissions) of any files created for concurrent audit logs using an
octal mode (as used in chmod). See SecAuditLogDirMode for controlling the mode of
created audit log directories.
Syntax: SecAuditLogFileMode
octal_mode|"default"
Example Usage: SecAuditLogFileMode 00640
Processing Phase: N/A
Scope: Any
Version: 2.5.10
Dependencies/Notes: This feature is not available on operating systems not supporting octal file modes. The default mode (0600) only grants read/write access to the account writing the file. If access from another account is needed (using mpm-itk is a good example), then this directive may be required. However, 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.
The process umask may still limit the mode if it is being more restrictive than the mode set using this directive.
Description: Defines which part of each transaction are going to be recorded in audit log. Each part is assigned a single letter. If a letter appears in the list then the equivalent part of each transactions will be recorded. See below for the list of all parts.
Syntax: SecAuditLogParts PARTS
Example Usage: SecAuditLogParts ABCFHZ
Processing Phase: N/A
Scope: Any
Version: 2.0.0
Dependencies/Notes: At this time ModSecurity
does not log response bodies of stock Apache responses (e.g. 404), or the Server and Date response headers.
Default: ABCFHZ.
Please refer to the ModSecurity Data Formats document for a detailed description of every available part.
Available audit log parts:
A - audit log header
(mandatory)
B - request headers
C - request body (present
only if the request body exists and ModSecurity is configured to
intercept it)
D - RESERVED for
intermediary response headers, not implemented yet.
E - 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).
F - final response headers
(excluding the Date and Server headers, which are always added by
Apache in the late stage of content delivery).
G - RESERVED for the actual
response body, not implemented yet.
H - audit log
trailer
I - This part is a
replacement for part C. It will log the same data as C in all cases
except when multipart/form-data
encoding in used. In this case it will log a fake application/x-www-form-urlencoded 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.
J - RESERVED. This part,
when implemented, will contain information about the files uploaded
using multipart/form-data encoding.
K - 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
Z - final boundary,
signifies the end of the entry (mandatory)
Description: Configures which response status code is to be considered relevant for the purpose of audit logging.
Syntax: SecAuditLogRelevantStatus REGEX
Example Usage: SecAuditLogRelevantStatus
^(?:5|4\d[^4])
Processing Phase: N/A
Scope: Any
Version: 2.0.0
Dependencies/Notes: Must have the
SecAuditEngine set to
RelevantOnly. The parameter is a regular
expression.
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.
Description: Configures the storage directory where concurrent audit log entries are to be stored.
Syntax: SecAuditLogStorageDir
/path/to/storage/dir
Example Usage: SecAuditLogStorageDir
/usr/local/apache/logs/audit
Processing Phase: N/A
Scope: Any
Version: 2.0.0
Dependencies/Notes: 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.
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.
Description: Configures the type of audit logging mechanism to be used.
Syntax: SecAuditLogType Serial|Concurrent
Example Usage: SecAuditLogType Serial
Processing Phase: N/A
Scope: Any
Version: 2.0.0
Dependencies/Notes: Must specify
SecAuditLogStorageDir if you use concurrent
logging.
Possible values are:
Serial - 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.
Concurrent - 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.
Description: Controls caching of transformations. Caching is off by default starting with 2.5.6, when it was deprecated and downgraded back to experimental.
Syntax: SecCacheTransformations On|Off
[options]
Example Usage: SecCacheTransformations On
"minlen:64,maxlen:0"
Processing Phase: N/A
Scope: Any
Version: 2.5.0
Dependencies/Notes: N/A
First parameter:
On - cache transformations
(per transaction, per phase) allowing identical transformations to
be performed only once. (default)
Off - do not cache any
transformations, forcing all transformations to be performed for
each rule executed.
The following options are allowed (comma separated):
incremental:on|off -
enabling this option will cache every transformation instead of just
the final transformation. (default: off)
maxitems:N - do not allow
more than N transformations to be cached. The cache will then be
disabled. A zero value is interpreted as "unlimited". This option
may be useful to limit caching for a form with a large number of
ARGS. (default: 512)
minlen:N - do not cache the
transformation if the value's length is less than N bytes. (default:
32)
maxlen:N - do not cache the
transformation if the value's length is more than N bytes. A zero
value is interpreted as "unlimited". (default: 1024)
Description: Configures the directory path that will be used to jail the web server process.
Syntax: SecChrootDir
/path/to/chroot/dir
Example Usage: SecChrootDir /chroot
Processing Phase: N/A
Scope: Main
Version: 2.0.0
Dependencies/Notes: This feature is not available on Windows builds. 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.builds. Some problems you might encounter with more complex setups:
DNS lookups do not work (this is because this feature requires a shared library that is loaded on demand, after chroot takes place).
You cannot send email from PHP because it uses sendmail and sendmail is outside the jail.
In some cases Apache graceful (reload) no longer works.
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.
mod_fastcgi, mod_fcgid,
mod_cgid), 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.
Description: Appends component signature to the ModSecurity signature.
Syntax: SecComponentSignature
"COMPONENT_NAME/X.Y.Z (COMMENT)"
Example usage: SecComponentSignature
"Core Rules/1.2.3"
Processing Phase: N/A
Scope: Main
Version: 2.5.0
Dependencies/Notes: 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.
Description: Enables content injection using
actions append and prepend.
Syntax: SecContentInjection
(On|Off)
Example Usage: SecContentInjection
On
Processing Phase: N/A
Scope: Any
Version: 2.5.0
Dependencies/Notes: N/A
Description: Selects the cookie format that will be used in the current configuration context.
Syntax: SecCookieFormat 0|1
Example Usage: SecCookieFormat 0
Processing Phase: N/A
Scope: Any
Version: 2.0.0
Dependencies/Notes: None
Possible values are:
0 - use version 0
(Netscape) cookies. This is what most applications use. It is the
default value.
1 - use version 1
cookies.
Description: Path where persistent data (e.g. IP address data, session data, etc) is to be stored.
Syntax: SecDataDir
/path/to/dir
Example Usage: SecDataDir /usr/local/apache/logs/data
Processing Phase: N/A
Scope: Main
Dependencies/Notes: This directive is needed when initcol, setsid an setuid are used. Must be writable by the web server user.
Description: Path to the ModSecurity debug log file.
Syntax: SecDebugLog
/path/to/modsec-debug.log
Example Usage: SecDebugLog
/usr/local/apache/logs/modsec-debug.log
Processing Phase: N/A
Scope: Any
Version: 2.0.0
Dependencies/Notes: None
Description: Configures the verboseness of the debug log data.
Syntax: SecDebugLogLevel 0|1|2|3|4|5|6|7|8|9
Example Usage: SecDebugLogLevel 4
Processing Phase: N/A
Scope: Any
Version: 2.0.0
Dependencies/Notes: Levels 1 - 3 are always sent to the Apache error log.
Therefore you can always use level 0
as the default logging level in production. Level 5 is useful when debugging. It is not
advisable to use higher logging levels in production as excessive
logging can slow down server significantly.
Possible values are:
0 - no logging.
1 - errors (intercepted
requests) only.
2 - warnings.
3 - notices.
4 - details of how
transactions are handled.
5 - as above, but including
information about each piece of information handled.
9 - log everything,
including very detailed debugging information.
Description: Defines the default action to take on a rule match.
Syntax: SecDefaultAction
action1,action2,action3
Example Usage: SecDefaultAction
log,auditlog,deny,status:403,phase:2
Processing Phase: Any
Scope: Any
Version: 2.0.0
Dependencies/Notes: 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.
The default value is minimal (differing from previous versions):
SecDefaultAction phase:2,log,auditlog,pass
SecDefaultAction must specify a disruptive
action and a processing phase and cannot contain metadata
actions.
SecDefaultAction is not
inherited across configuration contexts. (For an example of why this
may be a problem for you, read the following ModSecurity Blog entry
http://blog.modsecurity.org/2008/07/modsecurity-tri.html).
Description: Defines the path to the geographical database file.
Syntax: SecGeoLookupDb /path/to/db
Example Usage: SecGeoLookupDb
/usr/local/geo/data/GeoLiteCity.dat
Processing Phase: N/A
Scope: Any
Version: 2.5.0
Dependencies/Notes: Check out
maxmind.com for free database files.
Description: Configuration directive to use the httpd-guardian script to monitor for Denial of Service (DoS) attacks.
Syntax: SecGuardianLog |/path/to/httpd-guardian
Example Usage: SecGuardianLog
|/usr/local/apache/bin/httpd-guardian
Processing Phase: N/A
Scope: Main
Version: 2.0.0
Dependencies/Notes: By default httpd-guardian will defend against clients that send more than 120 requests in a minute, or more than 360 requests in five minutes.
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.
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 Apache httpd tools project. 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:
SecGuardianLog |/path/to/httpd-guardian
Description: Adds a fixed rule marker in the
ruleset to be used as a target in a skipAfter action.
A SecMarker directive essentially creates a rule that
does nothing and whose only purpose it to carry the given ID.
Syntax: SecMarker
ID
Example Usage: SecMarker 9999
Processing Phase: Any
Scope: Any
Version: 2.5.0
Dependencies/Notes: None
SecRule REQUEST_URI "^/$" \
"chain,t:none,t:urlDecode,t:lowercase,t:normalisePath,skipAfter:99"
SecRule REMOTE_ADDR "^127\.0\.0\.1$" "chain"
SecRule REQUEST_HEADERS:User-Agent \
"^Apache \(internal dummy connection\)$" "t:none"
SecRule &REQUEST_HEADERS:Host "@eq 0" \
"deny,log,status:400,id:08,severity:4,msg:'Missing a Host Header'"
SecRule &REQUEST_HEADERS:Accept "@eq 0" \
"log,deny,log,status:400,id:15,msg:'Request Missing an Accept Header'"
SecMarker 99Description: 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 .pdf extension in the request URI.
Syntax: SecPdfProtect On|Off
Example Usage: SecPdfProtect On
Processing Phase: N/A
Scope: Any
Version: 2.5.0
Dependencies/Notes: None
Description: Configure desired protection
method to be used when requests for PDF files are detected. Possible
values are TokenRedirection and
ForcedDownload. 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!").
Syntax: SecPdfProtectMethod method
Example Usage: SecPdfProtectMethod TokenRedirection
Processing Phase: N/A
Scope: Any
Version: 2.5.0
Dependencies/Notes: None
Default:
TokenRedirection
Description: 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.
Syntax: SecPdfProtectSecret secret
Example Usage: SecPdfProtectSecret
MyRandomSecretString
Processing Phase: N/A
Scope: Any
Version: 2.5.0
Dependencies/Notes: None
Description: 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.
Syntax: SecPdfProtectTimeout timeout
Example Usage: SecPdfProtectTimeout 10
Processing Phase: N/A
Scope: Any
Version: 2.5.0
Dependencies/Notes: None
Default: 10
Description: 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.
Syntax: SecPdfProtectTokenName name
Example Usage: SecPdfProtectTokenName PDFTOKEN
Processing Phase: N/A
Scope: Any
Version: 2.5.0
Dependencies/Notes: None
Default: PDFTOKEN
Description: Configures whether request bodies will be buffered and processed by ModSecurity by default.
Syntax: SecRequestBodyAccess On|Off
Example Usage: SecRequestBodyAccess On
Processing Phase: N/A
Scope: Any
Version: 2.0.0
Dependencies/Notes: This directive is
required if you plan to inspect POST_PAYLOAD. 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.
Possible values are:
On - access request
bodies.
Off - do not attempt to
access request bodies.
Description: Configures the maximum request body size ModSecurity will accept for buffering.
Syntax: SecRequestBodyLimit NUMBER_IN_BYTES
Example Usage: SecRequestBodyLimit 134217728
Scope: Any
Version: 2.0.0
Dependencies/Notes: 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.
Description: 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 SecRequestBodyLimit 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.
Syntax: SecRequestBodyNoFilesLimit
NUMBER_IN_BYTES
Example Usage: SecRequestBodyLimit 131072
Scope: Any
Version: 2.5.0
Dependencies/Notes: 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 413
Request Entity Too Large. There is a hard limit of 1
GB.
Description: Configures the maximum request body size ModSecurity will store in memory.
Syntax: SecRequestBodyInMemoryLimit
NUMBER_IN_BYTES
Example Usage: SecRequestBodyInMemoryLimit 131072
Processing Phase: N/A
Scope: Any
Version: 2.0.0
Dependencies/Notes: None
By default the limit is 128 KB:
# Store up to 128 KB in memory SecRequestBodyInMemoryLimit 131072
Description: Configures the maximum response body size that will be accepted for buffering.
Syntax: SecResponseBodyLimit NUMBER_IN_BYTES
Example Usage: SecResponseBodyLimit 524228
Processing Phase: N/A
Scope: Any
Version: 2.0.0
Dependencies/Notes: 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.
By default this limit is configured to 512 KB:
# Buffer response bodies of up to 512 KB in length SecResponseBodyLimit 524288
Description: Controls what happens once a
response body limit, configured with
SecResponseBodyLimit, 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.
Syntax: SecResponseBodyLimitAction
Reject|ProcessPartial
Example Usage:
SecResponseBodyLimitAction ProcessPartial
Processing Phase: N/A
Scope: Any
Version: 2.5.0
Dependencies/Notes: None
Description: Configures which MIME types are to be considered for response
body buffering.
Syntax: SecResponseBodyMimeType mime/type
Example Usage: SecResponseBodyMimeType text/plain
text/html
Processing Phase: N/A
Scope: Any
Version: 2.0.0
Dependencies/Notes: Multiple SecResponseBodyMimeType directives can be
used to add MIME types.
The default value is text/plaintext/html:
SecResponseBodyMimeType text/plain text/html
Description: Clears the list of MIME types considered for response body
buffering, allowing you to start populating the list from
scratch.
Syntax: SecResponseBodyMimeTypesClear
Example Usage: SecResponseBodyMimeTypesClear
Processing Phase: N/A
Scope: Any
Version: 2.0.0
Dependencies/Notes: None
Description: Configures whether response bodies are to be buffer and analysed or not.
Syntax: SecResponseBodyAccess On|Off
Example Usage: SecResponseBodyAccess On
Processing Phase: N/A
Scope: Any
Version: 2.0.0
Dependencies/Notes: 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.
Possible values are:
On - access response bodies
(but only if the MIME type matches, see above).
Off - do not attempt to
access response bodies.
Description: SecRule is the main ModSecurity directive. It
is used to analyse data and perform actions based on the results.
Syntax: SecRule
VARIABLES OPERATOR [ACTIONS]
Example Usage: SecRule REQUEST_URI "attack" \
"phase:1,t:none,t:urlDecode,t:lowercase,t:normalisePath"
Processing Phase: Any
Scope: Any
Version: 2.0.0
Dependencies/Notes: None
In general, the format of this rule is as follows:
SecRule VARIABLES OPERATOR [ACTIONS]
The second part, OPERATOR,
specifies how they are going to be checked. The third (optional) part,
ACTIONS, specifies what to do
whenever the operator used performs a successful match against a
variable.
The first part, VARIABLES,
specifies which variables are to be checked. For example, the
following rule will reject a transaction that has the word
dirty in the URI:
SecRule ARGS dirty
Each rule can specify one or more variables:
SecRule ARGS|REQUEST_HEADERS:User-Agent dirty
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.
SecRule XML:/xPath/Expression dirty
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.
A variable can contain one or many pieces of data, depending on the nature of the variable and the way it is used. We've seen examples of both approaches in the previous section. When a variable can contain more than one value we refer to it as a collection.
Collections are always expanded before a rule is run. For example, the following rule:
SecRule ARGS dirty
will be expanded to:
SecRule ARGS:p dirty SecRule ARGS:q dirty
in a requests that has only two parameters, named
p and q.
Collections come in several flavours:
Created at runtime using transaction data. For example:
ARGS (contains a list of all request
parameter values) and REQUEST_HEADERS
(contains a list of all request header values).
The TX collection is created (empty)
for every transaction. Rules can read from it and write to it
(using the setvar action, for example), but
the information stored in this collection will not survive the
end of transaction.
There are several collections that can be written to, but
which are persisted to the storage backend. These collections
are used to track clients across transactions. Examples of
collections that fall into this type are IP,
SESSION and USER.
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
rx (regular expression) operator.
You can also explicitly specify the operator you want to use by using
@, followed by the name of an
operator, at the beginning of the second SecRule
parameter:
SecRule ARGS "@rx dirty"
Note how we had to use double quotes to delimit the second rule parameter. This is because the second parameter now has 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.
The @ can be the second character if you are using negation to negate the result returned by the operator:
SecRule &ARGS "!@rx ^0$"
Operator results can be negated by using an exclamation mark at
the beginning of the second parameter. The following rule matches if
the word dirty does not appear
in the User-Agent request header:
SecRule REQUEST_HEADERS:User-Agent !dirty
You can use the exclamation mark in combination with any parameter. If you do, the exclamation mark needs to go first, followed by the explicit operator reference. The following rule has the same effect as the previous example:
SecRule REQUEST_HEADERS:User-Agent "!@rx dirty"
If you need to use negation in a rule that is going to be applied to several variables then it may not be immediately clear what will happen. Consider the following example:
SecRule ARGS:p|ARGS:q !dirty
The above rule is identical to:
SecRule ARGS:p !dirty SecRule ARGS:q !dirty
Negation is applied to operations against individual operations, not agains the entire variable list.
The third parameter, ACTIONS,
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.
Description: 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).
Syntax: SecRuleInheritance On|Off
Example Usage: SecRuleInheritance Off
Processing Phase: Any
Scope: Any
Version: 2.0.0
Dependencies/Notes: Resource-specific
contexts (e.g. Location, Directory, etc) cannot override
phase1 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.
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.
SecRuleEngine On SecDefaultAction log,pass,phase:2 ... <VirtualHost *:80> ServerName app1.com ServerAlias www.app1.com SecRuleInheritance Off SecDefaultAction log,deny,phase:1,redirect:http://www.site2.com ... </VirtualHost> <VirtualHost *:80> ServerName app2.com ServerAlias www.app2.com SecRuleInheritance On SecRule ARGS "attack" ... </VirtualHost>
Possible values are:
On - inherit rules from the
parent context.
Off - do not inherit rules
from the parent context.
Configuration contexts are an Apache concept. Directives
<Directory>,
<Files>,
<Location> and
<VirtualHost> are all used to create
configuration contexts. For more information please go to the
Apache documentation section Configuration
Sections.
Description: Configures the rules engine.
Syntax: SecRuleEngine On|Off|DetectionOnly
Example Usage: SecRuleEngine On
Processing Phase: Any
Scope: Any
Version: 2.0.0
Dependencies/Notes: This directive can also be controlled by the ctl action (ctl:ruleEngine=off) for per rule processing.
Possible values are:
On - process rules.
Off - do not process
rules.
DetectionOnly - process
rules but never intercept transactions, even when rules are
configured to do so.
Description: Removes matching rules from the parent contexts.
Syntax: SecRuleUpdateActionById RULEID
ACTIONLIST
Example Usage: SecRuleRemoveByID 1 2 "9000-9010"
Processing Phase: Any
Scope: Any
Version: 2.0.0
Dependencies/Notes: 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.
SecRuleRemoveById 1 2 5 10-20 "400-556" 673
Description: Removes matching rules from the parent contexts.
Syntax: SecRuleRemoveByMsg REGEX
Example Usage: SecRuleRemoveByMsg "FAIL"
Processing Phase: Any
Scope: Any
Version: 2.0.0
Dependencies/Notes: This directive supports
multiple parameters. Each parameter is a regular expression that will be
applied to the message (specified using the msg action).
Description: This directive creates a special
rule that executes a Lua script to decide whether to match or not. The
main difference from SecRule 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 SecRule.
Syntax: SecRuleScript
/path/to/script.lua [ACTIONS]
Example Usage: SecRuleScript "/path/to/file.lua"
"block"
Processing Phase: Any
Scope: Any
Version: 2.5.0
Dependencies/Notes: None
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.
Example script:
-- Your script must define the main 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("ARGS", "lowercase"); -- 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 must contain the name -- of the variable where the problem is located. -- return "Variable ARGS:p looks suspicious!" -- Otherwise, simply return nil. return nil; end
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.
Example showing use of m.getvars() to retrieve
many variables at once:
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, "<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;
endGo to http://www.lua.org/ to find more about the Lua programming language. The reference manual too is available online, at http://www.lua.org/manual/5.1/.
Lua support is marked as experimental as the way the progamming interface may continue to evolve while we are working for the best implementation style. Any user input into the programming interface is appreciated.
Description: Updates the action list of the specified rule.
Syntax: SecRuleRemoveById RULEID ACTIONLIST
Example Usage: SecRuleUpdateActionById 12345
deny,status:403
Processing Phase: Any
Scope: Any
Version: 2.5.0
Dependencies/Notes: 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.
SecAction \ "t:lowercase,phase:2,id:12345,pass,msg:'The Message',log,auditlog" SecRuleUpdateActionById 12345 "t:compressWhitespace,deny,status:403,msg:'A new message'
The example above will cause the rule to be executed as if it was specified as follows:
SecAction \ "t:lowercase,phase:2,id:12345,log,auditlog,t:compressWhitespace,deny,status:403,msg:'A new message'"
Description: Instructs ModSecurity to change the data presented in the "Server:" response header token.
Syntax: SecServerSignature "WEB SERVER
SOFTWARE"
Example Usage: SecServerSignature
"Netscape-Enterprise/6.0"
Processing Phase: N/A
Scope: Main
Version: 2.0.0
Dependencies/Notes: 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.
Description: Configures the directory where temporary files will be created.
Syntax: SecTmpDir
/path/to/dir
Example Usage: SecTmpDir /tmp
Processing Phase: N/A
Scope: Any
Version: 2.0.0
Dependencies/Notes: 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.
Description: Configures the directory where intercepted files will be stored.
Syntax: SecUploadDir
/path/to/dir
Example Usage: SecUploadDir /tmp
Processing Phase: N/A
Scope: Any
Version: 2.0.0
Dependencies/Notes: This directory must be on
the same filesystem as the temporary directory defined with SecTmpDir. This directive is used with
SecUploadKeepFiles.
Description: Configures the mode (permissions) of any uploaded files using an octal mode (as used in chmod).
Syntax: SecUploadFileMode octal_mode|"default"
Example Usage: SecUploadFileMode 0640
Processing Phase: N/A
Scope: Any
Version: 2.1.6
Dependencies/Notes: This feature is not available on operating systems not supporting octal file modes. The default mode (0600) only grants read/write access to the account writing the file. If access from another account is needed (using clamd is a good example), then this directive may be required. However, 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.
The process umask may still limit the mode if it is being more restrictive than the mode set using this directive.
Description: Configures whether or not the intercepted files will be kept after transaction is processed.
Syntax: SecUploadKeepFiles On|Off|RelevantOnly
Example Usage: SecUploadKeepFiles On
Processing Phase: N/A
Scope: Any
Version: 2.0.0
Dependencies/Notes: This directive requires
the storage directory to be defined (using SecUploadDir).
Possible values are:
On - Keep uploaded
files.
Off - Do not keep uploaded
files.
RelevantOnly - This will
keep only those files that belong to requests that are deemed
relevant.
Description: Creates a partition on the server that belongs to one web application.
Syntax: SecWebAppId
"NAME"
Example Usage: SecWebAppId "WebApp1"
Processing Phase: N/A
Scope: Any
Version: 2.0.0
Dependencies/Notes: 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 default.
Example:
<VirtualHost *:80> ServerName app1.com ServerAlias www.app1.com SecWebAppId "App1" SecRule REQUEST_COOKIES:PHPSESSID !^$ chain,nolog,pass SecAction setsid:%{REQUEST_COOKIES.PHPSESSID} ... </VirtualHost> <VirtualHost *:80> ServerName app2.com ServerAlias www.app2.com SecWebAppId "App2" SecRule REQUEST_COOKIES:PHPSESSID !^$ chain,nolog,pass SecAction setsid:%{REQUEST_COOKIES.PHPSESSID} ... </VirtualHost>
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".
SecWebAppId is relevant in two cases:
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.
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.