During a web application assessment of an online forum, a penetration tester suspects a stored cross-site scripting (XSS) vulnerability in the comment submission feature. The tester's initial attempts to inject a simple <script>alert(1)</script> payload are blocked by a basic input filter. Which of the following techniques is the most effective next step to confirm the vulnerability by attempting to bypass the filter?
Encode the entire <script> payload using URL encoding and resubmit it.
Inject a payload using a different HTML tag and an event handler, such as <img src=x onerror=alert('XSS')>.
Use an automated SQL injection tool's XSS module to find a valid payload.
Send the payload in an HTTP header like User-Agent to see if it is reflected elsewhere.
The most effective next step is to attempt a bypass using alternative syntax that still results in script execution. Basic filters often blacklist common tags like <script>, but may overlook event handlers like onerror combined with other tags such as <img>. This method directly tests the filter's resilience. URL encoding the payload would be decoded by the server before reaching the application logic, and thus the filter would still likely block it. Testing for a different vulnerability (reflected XSS in a header) does not confirm the suspected stored XSS vulnerability in the comment field. While some automated tools have XSS modules, a manual, targeted bypass attempt is a more precise and common next step for a penetration tester.
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What is stored XSS and how is it different from reflected XSS?
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How does the 'onerror' attribute work in tags like `<img>` for an XSS attack?
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What is URL encoding and why isn't it effective against input filters in this scenario?