Affected: npm: dompurify

## Summary

DOMPurify versions 3.0.1 through 3.3.3 (latest) are vulnerable to a prototype pollution-based XSS bypass. When an application uses `DOMPurify.sanitize()` with the default configuration (no `CUSTOM_ELEMENT_HANDLING` option), a prior prototype pollution gadget can inject permissive `tagNameCheck` and `attributeNameCheck` regex values into `Object.prototype`, causing DOMPurify to allow arbitrary custom elements with arbitrary attributes — including event handlers — through sanitization.

## Affected Versions

- **3.0.1 through 3.3.3** (current latest) — all affected - **3.0.0 and all 2.x versions** — NOT affected (used `Object.create(null)` for initialization, no `|| {}` reassignment) - The vulnerable `|| {}` reassignment was introduced in the 3.0.0→3.0.1 refactor - This is **distinct** from GHSA-cj63-jhhr-wcxv (USE_PROFILES Array.prototype pollution, fixed in 3.3.2) - This is **distinct** from CVE-2024-45801 / GHSA-mmhx-hmjr-r674 (__depth prototype pollution, fixed in 3.1.3)

## Root Cause

In `purify.js` at line 590, during config parsing:

```javascript CUSTOM_ELEMENT_HANDLING = cfg.CUSTOM_ELEMENT_HANDLING || {}; ```

When no `CUSTOM_ELEMENT_HANDLING` is specified in the config (the default usage pattern), `cfg.CUSTOM_ELEMENT_HANDLING` is `undefined`, and the fallback `{}` is used. This plain object inherits from `Object.prototype`.

Lines 591-598 then check `cfg.CUSTOM_ELEMENT_HANDLING` (the original config property) — which is `undefined` — so the conditional b…