When a web application is vulnerable to this type of attack, it will pass unvalidated input sent through requests to the client. The common modus operandi of the attack includes a design step, in which the attacker creates and tests an offending URI, a social engineering step, in which she convinces her victims to load this URI on their browsers, and the eventual execution of the offending code - using the victim's credentials.

Commonly the attacker's code is written in the Javascript language, but other scripting languages are also used, e.g., ActionScript and VBScript.

Attackers typically leverage these vulnerabilities to install key loggers, steal victim cookies, perform clipboard theft, and change the content of the page (e.g., download links).

One of the important matters about exploiting XSS vulnerabilities is character encoding. In some cases, the web server or the web application may not be filtering some encodings of characters, so, for example, the web application might filter out "<script>", but might not filter "%3Cscript%3E" which simply includes another encoding of tags. A nice tool for testing character encodings is OWASP's CAL9000.