Exploiting Browser Parser Quirks

Browser HTML parsers are designed to be forgiving, attempting to render even severely malformed HTML. This fault tolerance, while improving user experience, creates numerous opportunities for XSS bypasses. Different browsers have different parsing quirks, and attackers exploit these differences to find payloads that work in specific browsers. Understanding common parser quirks helps developers anticipate potential bypasses.

Nested tag confusion represents a common parser quirk that enables bypasses. When browsers encounter improperly nested tags, they attempt to fix the structure, potentially creating unexpected results. For example, <script><div></script>alert(1)</div> might be "fixed" in ways that execute the alert despite appearing to be safely contained. Similarly, browsers handle null bytes, control characters, and malformed attributes differently, creating browser-specific bypass opportunities.

The mutation XSS (mXSS) class of attacks exploits how browsers modify HTML during parsing. Certain HTML constructs are transformed by the browser in ways that can introduce XSS vulnerabilities. For example, innerHTML operations might transform seemingly safe HTML into dangerous constructs. A payload that appears safe when validated server-side might mutate into an executable form when processed by the browser. These attacks are particularly challenging because the dangerous transformation happens after all server-side validation.