Most Web sites today add dynamic content to a Web page making the experience for the user more enjoyable. Dynamic content is content generated by some server process, which when delivered can behave and display differently to the user depending upon their settings and needs. Dynamic Web sites have a threat that static Web sites don't, called "cross-site scripting," also known as "XSS."

Cross-site scripting (XSS) is a type of computer security vulnerability typically found in web applications that enables malicious attackers to inject client-side script into web pages viewed by other users. An exploited cross-site scripting vulnerability can be used by attackers to bypass access controls such as the same origin policy. Cross-site scripting carried out on websites were roughly 80% of all security vulnerabilities documented by Symantec as of 2007. Their impact may range from a petty nuisance to a significant security risk, depending on the sensitivity of the data handled by the vulnerable site, and the nature of any security mitigations implemented by the site's owner.