Ratproxy is a passive web application security assessment tool that we’ve been using internally at Google. It is designed to transparently analyze legitimate, browser-driven interactions with a tested web property and automatically pinpoint, annotate, and prioritize potential flaws or areas of concern. The tool analyzes problems such as cross-site script inclusion threats, caching issues, and caching issues. We believe responsible security research brings a net overall benefit to the safety of the Web as a whole, and have released this tool explicitly to support that kind of research.”]
Source: https://security.googleblog.com/2008/07/meet-ratproxy-our-passive-web-security.html