Understanding XSS Types and Their Detection

XSS vulnerabilities manifest in three primary forms, each requiring different detection approaches. Reflected XSS occurs when user input immediately reflects back in the response without proper encoding. Stored XSS persists malicious payloads in databases or files, executing when other users view the infected content. DOM-based XSS executes entirely in the browser when client-side JavaScript unsafely processes user input. ZAP employs distinct techniques to identify each type, understanding these differences improves detection effectiveness.

Reflected XSS detection forms the core of ZAP's automated scanning capabilities. The scanner injects various JavaScript payloads into parameters and analyzes responses for unencoded reflections. Simple payloads like <script>alert(1)</script> test basic filtering, while complex payloads bypass common filters. ZAP's detection algorithms look for exact payload reflections and partial reflections that might still execute in specific contexts.

Stored XSS presents unique detection challenges as payloads persist across requests. ZAP identifies potential stored XSS by injecting unique markers during scanning, then checking if these markers appear in subsequent responses. This detection requires careful session management and comprehensive site coverage. Manual testing often proves more effective for stored XSS, especially in complex workflows where automated tools struggle to understand data flow.

DOM-based XSS detection requires different approaches since vulnerabilities exist in client-side code rather than server responses. ZAP's Ajax Spider helps identify DOM XSS by executing JavaScript and observing DOM modifications. However, manual analysis of JavaScript code often reveals vulnerabilities that automated tools miss. Understanding JavaScript sources and sinks helps identify where user input might reach dangerous functions.