Description:

ModSecurity is an Apache 1.x/2.x module whose purpose is to tighten the Web
application security. Effectively, it is an intrusion detection and prevention
system for the web server.

At the moment its main features are:
* Audit log; store full request details in a separate file, including POST
payloads.
* Request filtering; incoming requests can be analysed and offensive requests
can be rejected (or simply logged, if that is what you want). This feature
can be used to prevent many types of attacks (e.g. XSS attacks, SQL
injection, ...) and even allow you to run insecure applications on your
servers (if you have no other choice, of course).