XSS-GUARD: Precise dynamic prevention of cross-site scripting attacks

This paper focuses on defense mechanisms for cross-site scripting attacks, the top threat on web applications today. It is believed that input validation,(or filtering) can effectively prevent XSS attacks on the server side. In this paper, we discuss several recent real-world XSS attacks and analyze the reasons for the failure of filtering mechanisms in defending these attacks. We conclude that while filtering is useful as a first level of defense against XSS attacks, it is ineffective in preventing several instances of attack, especially when user input includes content-rich HTML. We then propose XSS-GUARD, a new framework that is designed to be a prevention mechanism against XSS attacks on the server side. XSS-GUARD works by dynamically learning the set of scripts that a web application intends to create for any HTML request. Our approach also includes a robust mechanism for identifying scripts at the server side and removes any script in the output that is not intended by the web application. We discuss extensive experimental results that demonstrate the resilience of XSS-GUARD in preventing a number of real-world XSS exploits.