Cross-site scripting

Cross-site scripting (XSS) is a type of security vulnerability that can be found in some web applications. XSS attacks enable attackers to inject client-side scripts 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 up until 2007. XSS effects vary in range from petty nuisance to 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 network. OWASP considers the term cross-site scripting to be a misnomer. It initially was an attack that was used for breaching data across sites, but gradually started to include other forms of data injection attacks. Security on the web depends on a variety of mechanisms, including an underlying concept of trust known as the same-origin policy. This essentially states that if content from one site (such as https://mybank.example1.com) is granted permission to access resources (like cookies etc.) on a web browser, then content from any URL with the same (1) URI scheme (e.g. ftp, http, or https), (2) host name, and (3) port number will share these permissions. Content from URLs where any of these three attributes are different will have to be granted permissions separately. Cross-site scripting attacks use known vulnerabilities in web-based applications, their servers, or the plug-in systems on which they rely. Exploiting one of these, attackers fold malicious content into the content being delivered from the compromised site. When the resulting combined content arrives at the client-side web browser, it has all been delivered from the trusted source, and thus operates under the permissions granted to that system. By finding ways of injecting malicious scripts into web pages, an attacker can gain elevated access-privileges to sensitive page content, to session cookies, and to a variety of other information maintained by the browser on behalf of the user.