Mod_security is an Apache module whose purpose is to protect vulnerable applications and reject human or automated attacks. It is an open source intrusion detection and prevention system for Apache. In addition to request filtering, it also creates Web application audit logs. Requests are filtered using regular expressions. Some of the things possible are:

• Apply filters against any part of the request (URI, headers, either GET or POST)
• Apply filters against individual parameters
• Reject SQL injection attacks
• Reject Cross site scripting attacks

Changes in version 1.7 include:

• Output filtering has been added to Apache 2.x.
• The ability to filter cookies directly has been added.
• Apache can now pretend to be some other Web server through the SecServerSignature directive.
• Three new actions: "allow" to finish filter processing and let the request through, "chain" to chain several filter together (logical AND), and "skipnext" to skip over filters.
• A new anti-evasion technique to fight null-byte attacks.
• Finally, the module now runs on Netware.

Software download page on Help Net Security: mod_security 1.7.