GHSA-v9jr-rg53-9pgp DOMPurify: Prototype Pollution to XSS Bypass via CUSTOM_ELEMENT_HANDLING Fallback

• npm dompurify: >= 3.0.1, < 3.4.0 → fixed in 3.4.0

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 blocks that would set `tagNameCheck` and `attributeNameCheck` from the config are never entered. As a result, `CUSTOM_ELEMENT_HANDLING.tagNameCheck` and `CUSTOM_ELEMENT_HANDLING.attributeNameCheck` resolve via the prototype chain. If an attacker has polluted `Object.prototype.tagNameCheck` and `Object.prototype.attributeNameCheck` with permissive values (e.g., `/.*/`), these polluted values flow into DOMPurify's custom element validation at lines 973-977 and attribute validation, causing all custom elements and all attributes to be allowed. Impact • Attack type: XSS bypass via prototype pollution chain • Prerequisites: Attacker must have a prototype pollution primitive in the same execution context (e.g., vulnerable version of lodash, jQuery.extend, query-string parser, deep merge utility, or any other PP gadget) • Config required: Default. No special DOMPurify configuration needed. The standard `DOMPurify.sanitize(userInput)` call is affected. • Payload: Any HTML custom element (name containing a hyphen) with event handler attributes survives sanitization Proof of Concept ```javascript // Step 1: Attacker exploits a prototype pollution gadget elsewhere in the application Object.prototype.tagNameCheck = /.*/; Object.prototype.attributeNameCheck = /.*/; // Step 2: Application sanitizes user input with DEFAULT config const clean = DOMPurify.sanitize(''); // Step 3: "Sanitized" output still contains the event handler console.log(clean); // Output: // Step 4: When injected into DOM, XSS executes document.body.innerHTML = clean; // alert() fires ``` Tested configurations that are vulnerable: | Call Pattern | Vulnerable? | |---|---| | `DOMPurify.sanitize(input)` | YES | | `DOMPurify.sanitize(input, {})` | YES | | `DOMPurify.sanitize(input, { CUSTOM_ELEMENT_HANDLING: null })` | YES | | `DOMPurify.sanitize(input, { CUSTOM_ELEMENT_HANDLING: {} })` | NO (explicit object triggers L591 path) | Suggested Fix Change line 590 from: ```javascript CUSTOM_ELEMENT_HANDLING = cfg.CUSTOM_ELEMENT_HANDLING || {}; ``` To: ```javascript CUSTOM_ELEMENT_HANDLING = cfg.CUSTOM_ELEMENT_HANDLING || create(null); ``` The `create(null)` function (already used elsewhere in DOMPurify, e.g., in `clone()`) creates an object with no prototype, preventing prototype chain inheritance. Alternative application-level mitigation: Applications can protect themselves by always providing an explicit `CUSTOM_ELEMENT_HANDLING` in their config: ```javascript DOMPurify.sanitize(input, { CUSTOM_ELEMENT_HANDLING: { tagNameCheck: null, attributeNameCheck: null } }); ``` Timeline • 2026-04-04: Vulnerability discovered during automated DOMPurify fuzzing research (Fermat project) • 2026-04-04: Confirmed in Chrome browser with DOMPurify 3.3.3 • 2026-04-04: Verified distinct from GHSA-cj63-jhhr-wcxv and CVE-2024-45801 • 2026-04-04: Advisory drafted, responsible disclosure initiated Credit https://github.com/trace37labs