Improperly Controlled Modification of Object Prototype Attributes ('Prototype Pollution') in dompurify

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. Affected versions are prior to 3.3.2.