Over the last years, the complexity of web applications has grown significantly, challenging desktop programs in terms of functionality and design. Along with the rising popularity of web applications, the number of exploitable bugs has also increased significantly. Web application flaws, such as cross-site scripting or SQL injection bugs, now account for more than two thirds of the reported security vulnerabilities. Black-box testing techniques are a common approach to improve software quality and detect bugs before deployment. There exist a number of vulnerability scanners, or fuzzers, that expose web applications to a barrage of malformed inputs in the hope to identify input validation errors. Unfortunately, these scanners often fail to test a substantial fraction of a web application's logic, especially when this logic is invoked from pages that can only be reached after filling out complex forms that aggressively check the correctness of the provided values. In this paper, we present an automated testing tool that can find reflected and stored cross-site scripting (XSS) vulnerabilities in web applications. The core of our system is a black-box vulnerability scanner. This scanner is enhanced by techniques that allow one to generate more comprehensive test cases and explore a larger fraction of the application. Our experiments demonstrate that our approach is able to test more thoroughly these programs and identify more bugs than a number of open-source and commercial web vulnerability scanners.
