Cross Site Scripting (XSS) is a vulnerability of a Web Application that is essentially caused by the failure of the application to check up on user input before returning it to the client's web browser. Without an adequate validation, user input may include malicious code that may be sent to other clients and unexpectedly executed by their browsers, thus causing a security attack.Techniques to prevent this type of attacks require that all application input must be checked up and filtered, encoded, or validated before sending them to any user. In order to discover the XSS vulnerabilities in a Web application, traditional source code analysis techniques can be exploited. In this paper, in order to assess the XSS vulnerability of a Web application, an approach that combines static and dynamic analysis of the Web application is presented. Static analysis based criteria have been defined to detect potential vulnerabilities in the server pages of a Web application, while a process of dynamic analysis has been proposed in order to detect actual vulnerabilities. Some case studies have been carried out, giving encouraging results.