Web Application Firewall

Description
A web application firewall (WAF) is an appliance, server plugin, or filter that applies a set of rules to an HTTP conversation. Generally, these rules cover common attacks such as XSS and SQL Injection. By customizing the rules to your application, many attacks can be identified and blocked. The effort to perform this customization can be significant and needs to be maintained as the application is modified.

A far more detailed description is available at Wikipedia

Important Selection Criteria

 * Very Few False Positives (i.e., should NEVER dissallow an authorized request)
 * Strength of Default (Out of the Box) Defenses
 * Power and Ease of Learn Mode
 * Types of Vulnerabilities it can prevent
 * Ability to keep individual users constrained to exactly what they have seen in the current session
 * Ability to be configured to prevent ANY specific problem (i.e., Emergency Patches)
 * Form Factor: Software vs. Hardware (Hardware generally preferred)

You may also find the Web Application Firewall Evaluation Criteria useful for evaluating the performance and other characteristics of a WAF.

The London Chapter WAF event has some comparative info amongst the WAF Vendors that participated in the event.

OWASP Tools Of This Type
The OWASP Stinger Project is not a full blown WAF, but it is a strong Java/J2EE input validation filter that can be put in front of your application.

Well Known Open Source Tools Of This Type
ModSecurity

Commercial Tools from OWASP Members Of This Type
These vendors have decided to support OWASP by becoming members. OWASP appreciates the support from these organizations, but cannnot endorse any commercial products or services.


 * F5 - TrafficShield
 * Breach - WebDefend

Other Well Known Commercial Tools Of This Type

 * Citrix - Application Firewall
 * NetContinuum - Application Firewall
 * Imperva - SecureSphere™ Web Application Firewall