{"id":"GHSA-v9jr-rg53-9pgp","summary":"DOMPurify: Prototype Pollution to XSS Bypass via CUSTOM_ELEMENT_HANDLING Fallback","details":"## Summary\n\nDOMPurify 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.\n\n## Affected Versions\n\n- **3.0.1 through 3.3.3** (current latest) — all affected\n- **3.0.0 and all 2.x versions** — NOT affected (used `Object.create(null)` for initialization, no `|| {}` reassignment)\n- The vulnerable `|| {}` reassignment was introduced in the 3.0.0→3.0.1 refactor\n- This is **distinct** from GHSA-cj63-jhhr-wcxv (USE_PROFILES Array.prototype pollution, fixed in 3.3.2)\n- This is **distinct** from CVE-2024-45801 / GHSA-mmhx-hmjr-r674 (__depth prototype pollution, fixed in 3.1.3)\n\n## Root Cause\n\nIn `purify.js` at line 590, during config parsing:\n\n```javascript\nCUSTOM_ELEMENT_HANDLING = cfg.CUSTOM_ELEMENT_HANDLING || {};\n```\n\nWhen 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`.\n\nLines 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.\n\nAs 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.\n\n## Impact\n\n- **Attack type:** XSS bypass via prototype pollution chain\n- **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)\n- **Config required:** Default. No special DOMPurify configuration needed. The standard `DOMPurify.sanitize(userInput)` call is affected.\n- **Payload:** Any HTML custom element (name containing a hyphen) with event handler attributes survives sanitization\n\n## Proof of Concept\n\n```javascript\n// Step 1: Attacker exploits a prototype pollution gadget elsewhere in the application\nObject.prototype.tagNameCheck = /.*/;\nObject.prototype.attributeNameCheck = /.*/;\n\n// Step 2: Application sanitizes user input with DEFAULT config\nconst clean = DOMPurify.sanitize('\u003cx-x onfocus=alert(document.cookie) tabindex=0 autofocus\u003e');\n\n// Step 3: \"Sanitized\" output still contains the event handler\nconsole.log(clean);\n// Output: \u003cx-x onfocus=\"alert(document.cookie)\" tabindex=\"0\" autofocus=\"\"\u003e\n\n// Step 4: When injected into DOM, XSS executes\ndocument.body.innerHTML = clean; // alert() fires\n```\n\n### Tested configurations that are vulnerable:\n\n| Call Pattern | Vulnerable? |\n|---|---|\n| `DOMPurify.sanitize(input)` | YES |\n| `DOMPurify.sanitize(input, {})` | YES |\n| `DOMPurify.sanitize(input, { CUSTOM_ELEMENT_HANDLING: null })` | YES |\n| `DOMPurify.sanitize(input, { CUSTOM_ELEMENT_HANDLING: {} })` | NO (explicit object triggers L591 path) |\n\n## Suggested Fix\n\nChange line 590 from:\n```javascript\nCUSTOM_ELEMENT_HANDLING = cfg.CUSTOM_ELEMENT_HANDLING || {};\n```\n\nTo:\n```javascript\nCUSTOM_ELEMENT_HANDLING = cfg.CUSTOM_ELEMENT_HANDLING || create(null);\n```\n\nThe `create(null)` function (already used elsewhere in DOMPurify, e.g., in `clone()`) creates an object with no prototype, preventing prototype chain inheritance.\n\n### Alternative application-level mitigation:\n\nApplications can protect themselves by always providing an explicit `CUSTOM_ELEMENT_HANDLING` in their config:\n\n```javascript\nDOMPurify.sanitize(input, {\n  CUSTOM_ELEMENT_HANDLING: {\n    tagNameCheck: null,\n    attributeNameCheck: null\n  }\n});\n```\n\n## Timeline\n\n- **2026-04-04:** Vulnerability discovered during automated DOMPurify fuzzing research (Fermat project)\n- **2026-04-04:** Confirmed in Chrome browser with DOMPurify 3.3.3\n- **2026-04-04:** Verified distinct from GHSA-cj63-jhhr-wcxv and CVE-2024-45801\n- **2026-04-04:** Advisory drafted, responsible disclosure initiated\n\n## Credit\n\nhttps://github.com/trace37labs","aliases":["CVE-2026-41238"],"modified":"2026-07-17T21:05:05.513995953Z","published":"2026-04-22T17:31:32Z","database_specific":{"severity":"MODERATE","github_reviewed":true,"github_reviewed_at":"2026-04-22T17:31:32Z","nvd_published_at":"2026-04-23T16:16:26Z","cwe_ids":["CWE-1321","CWE-79"]},"references":[{"type":"WEB","url":"https://github.com/cure53/DOMPurify/security/advisories/GHSA-v9jr-rg53-9pgp"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-41238"},{"type":"PACKAGE","url":"https://github.com/cure53/DOMPurify"},{"type":"WEB","url":"https://github.com/cure53/DOMPurify/releases/tag/3.4.0"}],"affected":[{"package":{"name":"dompurify","ecosystem":"npm","purl":"pkg:npm/dompurify"},"ranges":[{"type":"SEMVER","events":[{"introduced":"3.0.1"},{"fixed":"3.4.0"}]}],"database_specific":{"source":"https://github.com/github/advisory-database/blob/main/advisories/github-reviewed/2026/04/GHSA-v9jr-rg53-9pgp/GHSA-v9jr-rg53-9pgp.json"}}],"schema_version":"1.9.0","severity":[{"type":"CVSS_V3","score":"CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:C/C:H/I:L/A:N"}]}