Cross-site scripting (XSS)

Cross-site scripting (XSS) attacks exploit vulnerabilities in Web page validation by injecting client-side script code. Common vulnerabilities that make your Web applications susceptible to cross-site scripting attacks include failing to properly validate input, failing to encode output, and trusting the data retrieved from a shared database. To protect your application against cross-site scripting attacks, assume that all input is malicious. Constrain and validate all input. Encode all output that could, potentially, include HTML characters. This includes data read from files and databases.

The injected script code embeds itself in response data, which is sent back to an unsuspecting user. The user's browser then runs the script code. Because the browser downloads the script code from a trusted site, the browser has no way of recognizing that the code is not legitimate, and Microsoft Internet Explorer security zones provide no defense. Cross-site scripting attacks also work over HTTP and HTTPS (SSL) connections.

One of the most serious examples of a cross-site scripting attack occurs when an attacker writes script to retrieve the authentication cookie that provides access to a trusted site and then posts the cookie to a Web address known to the attacker. This enables the attacker to spoof the legitimate user's identity and gain illicit access to the Web site.

source : msdn