Cross-site scripting (XSS) attacks target web sites with Cookie-based session management, resulting in the leakage of privacy information. Although several server-side countermeasures for XSS attacks do exist, such techniques have not been applied in a universal manner, because of their deployment overhead and the poor understanding of XSS problems. This paper proposes a client-side system that automatically detects XSS vulnerability by manipulating either request or server response. The system also shares the indication of vulnerability via a central repository. The purpose of the proposed system is twofold: to protect users from XSS attacks, and to warn the web servers with XSS vulnerabilities.