Getting Started with mod_security

or

/usr/sbin/apxs2 -cia mod_security.c

See the ModSecurity User Guide, or the mod_security source code's INSTALL file, for more information on installing mod_security from source.

The mod_security module, like all other Apache modules, is controlled from httpd.conf in Apache 1.3, or apache2.conf in Apache 2.x. On the one hand, mod_security's configuration parameters are straightforward to use and well documented. But on the other hand, as of this writing, there is no default configuration; the assumption is that you know enough about your environment and about Web security to create your own configuration from scratch.

And indeed, only you (and your Web developers) know what sorts of input are legitimate for the Web applications on your particular server. However, a minimum default configuration would be nice to start out with, wouldn't it? Luckily, one is provided, in the ModSecurity User Guide.

The rest of this article consists of a dissection of this minimum configuration, which should give you a taste of mod_security's power. For more a complete reference on mod_security configuration parameters and more-advanced examples, see the on-line Resources for this article.

Rather than presenting the entire configuration in one imposing list, let's break it up into manageable chunks. Listing 1 contains some basic settings.

<IfModule mod_security.c>
	SecFilterEngine On
	SecFilterDefaultAction "deny,log,status:403"
	SecFilterScanPOST On
	SecFilterCheckURLEncoding On
	SecFilterCheckUnicodeEncoding Off
	SecFilterForceByteRange 1 255

The first line in Listing 1 simply checks to see whether mod_security even has been enabled; if it isn't, the subsequent parameters are ignored. The parameter SecFilterEngine controls whether mod_security's filtering engine is enabled. The default value is Off, so you need to set this explicitly either to On, which causes mod_security to inspect all data, or DynamicOnly, which turns filtering on but tells mod_security to ignore requests for static content (specifically, it ignores requests with null handlers). Note that the DynamicOnly setting may not behave precisely how you expect; although it can save CPU cycles, some testing is in order if you use DynamicOnly.

SecFilterDefaultAction is very important. It defines the default action to take on filter matches. In Listing 1, this is set both to log the matching request and deny it with a status code 403 message. Obviously, you can specify multiple actions, separated by commas.

SecFilterScanPOST, if set to On, tells mod_security to inspect not only GET requests, but POST payloads as well.

Setting SecFilterCheckURLEncoding to On causes hexadecimal-encoded values within URLs to be checked for valid values (0-9, A-F).

SecFilterCheckUnicodeEncoding can be set to On if your Web server understands Unicode and uses UTF-8 encoding.

Finally, SecFilterForceByteRange specifies the range of allowable ASCII values in GET requests and in form data within POST requests.

On to our next set of parameters—Listing 2 shows some settings related to logging.

SecUploadDir /tmp
SecUploadKeepFiles Off
SecAuditEngine RelevantOnly
SecAuditLog logs/audit_log
SecFilterDebugLog logs/modsec_debug_log
SecFilterDebugLevel 0

SecUploadDir specifies a place for mod_security to store files uploaded via POST requests for processing, but it won't actually use this unless SecUploadKeepFiles is set to On. You probably don't want to enable this feature unless you've got a script, specified by a SecUploadApproveScript directive, that's ready to scan such files, for example, a script that invokes ClamAV and can return the results to mod_security. See the ModSecurity User Guide for more information on the SecUploadApproveScript parameter.

Setting SecAuditEngine to On, RelevantOnly or DynamicOrRelevant enables mod_security's powerful logging facility, which captures much more information than Apache's default logs. On causes all requests to be logged by mod_security, RelevantOnly logs only those requests that trigger mod_security filters and DynamicOrRelevant logs both relevant requests and requests with non-null handlers. SecAuditLog specifies the file to which mod_security should write its logs.

SecFilterDebugLog, obviously enough, specifies the file to which mod_security should log internal debugging information. Setting SecFilterDebugLevel to 0 turns off debug-logging; if you're actually having problems with mod_security, or are fine-tuning its configuration, you can set this to 1 for significant events (which will also be written to the audit log), 2 for info messages or 3 for still-more-detailed info messages.

Now, at last, we arrive at the real power of mod_security: customized filters. Listing 3 shows three such filters.