Web Application Firewall

Description
A web application firewall 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 will need to maintained as the application is modified.

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