A system for detecting XSS vulnerabilities includes determining the context in which a probe supplied as an input to a webpage or an application exists in a script associated with the webpage or application. A payload is generated based on, at least in part, the context such that during execution of the script, an executable code fragment in the payload can escape out of the context in which the probe exists and into a the global context of the script. The payload may include additional characters that prevent the payload from causing errors in the execution of the script.