DOMPurify USE_PROFILES prototype pollution allows event handlers

Summary

When USE_PROFILES is enabled, DOMPurify rebuilds ALLOWED_ATTR as a plain array before populating it with the requested allowlists. Because the sanitizer still looks up attributes via ALLOWED_ATTR[lcName], any Array.prototype property that is polluted also counts as an allowlisted attribute. An attacker who can set Array.prototype.onclick = true (or a runtime already subject to prototype pollution) can thus force DOMPurify to keep event handlers such as onclick even when they are normally forbidden. The provided PoC sanitizes <img onclick=...> with USE_PROFILES and adds the sanitized output to the DOM; the polluted prototype allows the event handler to survive and execute, turning what should be a blocklist into a silent XSS vector.

Impact

Prototype pollution makes DOMPurify accept dangerous event handler attributes, which bypasses the sanitizer and results in DOM-based XSS once the sanitized markup is rendered.

Credits

Identified by Cantina’s Apex (https://www.cantina.security).