Cross-site Scripting (XSS) is a client-side code injection attack. The aggressor aims to execute malicious scripts in a web browser of the victims, including malicious code in a legitimate web page or web application. The actual attack occurs when the victim visits the web page or web application that executes the malicious code. The web page or web application becomes a vehicle to deliver the malicious script to the user’s browser. Vulnerable vehicles that are commonly used for Cross-site Scripting attacks are forums, message boards, and web pages that allow comments on.

A web page or web application is vulnerable to XSS if it uses unsanitized user input, on the output that it generates. This user input must then be parsed by the victim’s browser. XSS attacks are possible in VBScript, ActiveX, Flash, and even CSS. However, they are most common in JavaScript, primarily because JavaScript is fundamental to most browsing experiences.