Cross-site scripting

Cross-site scripting (XSS) is a type of computer security vulnerability typically found in Web applications. XSS enables attackers to inject client-side script into Web pages viewed by other users. A cross-site scripting vulnerability may be used by attackers to bypass access controls such as the same origin policy. Cross-site scripting carried out on websites accounted for roughly 84% of all security vulnerabilities documented by Symantec as of 2007.^[1] Their effect 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 mitigation implemented by the site's owner.

In computing, the same-origin policy is an important concept in the web application security model. The policy permits scripts running on pages originating from the same site – a combination of scheme, hostname, and port number^[1] – to access each other's DOM with no specific restrictions, but prevents access to DOM on different sites.^[1] The same-origin policy also applies to XMLHttpRequests unless the server provides an Access-Control-Allow-Origin (CORS) header. Notably, WebSockets are not subject to the same-origin policy.

This mechanism bears a particular significance for modern web applications that extensively depend on HTTP cookies to maintain authenticated user sessions, as servers act based on the HTTP cookie information to reveal sensitive information or take state-changing actions. A strict separation between content provided by unrelated sites must be maintained on the client side to prevent the loss of data confidentiality or integrity.