{"id":"CVE-2026-65899","aliases":["GHSA-vxr8-fq34-vvx9"],"url":"https://o3.security/vulnerability/CVE-2026-65899","summary":"DOMPurify before 3.4.9 Trusted Types Policy State Contamination","details":"## Impact\n\nA DOMPurify instance that is reused across trust boundaries can stay bound to a previously supplied `TRUSTED_TYPES_POLICY` even after `clearConfig()` is called. A later caller that requests `RETURN_TRUSTED_TYPE` receives a `TrustedHTML` object created by the old policy, not by a clean default configuration.\n\nIf the old policy is unsafe or controlled by a less-trusted integration, this turns a later \"default\" sanitize call into script execution at a Trusted Types sink. `TRUSTED_TYPES_POLICY: null` on the later call also does not clear the retained policy.\n[dompurify-trusted-types-policy-survives-clearconfig-poc.js](https://github.com/user-attachments/files/28604913/dompurify-trusted-types-policy-survives-clearconfig-poc.js)\n\n\n## Affected version\n\nTested against DOMPurify `3.4.8`, repository commit `825e617753ac1169306a542d3174a77f717a0cf6`.\n\n## Root cause\n\n`_parseConfig()` overwrites `trustedTypesPolicy` when `cfg.TRUSTED_TYPES_POLICY` is truthy, but the default/null path only initializes the internal policy when `trustedTypesPolicy === undefined`. Once a custom policy has been set, later default config parsing leaves it in place.\n\nRelevant code:\n\n- `src/purify.ts:786-812` accepts and stores `cfg.TRUSTED_TYPES_POLICY`.\n- `src/purify.ts:813-832` does not reset an existing policy when config has no policy or has `TRUSTED_TYPES_POLICY: null`.\n- `src/purify.ts:2123-2125` signs the final serialized HTML with the retained policy when `RETURN_TRUSTED_TYPE` is true.\n- `src/purify.ts:2133-2136` `clearConfig()` only clears `CONFIG` and `SET_CONFIG`; it does not reset `trustedTypesPolicy` or `emptyHTML`.\n\n## Local PoC\n\nRun from the DOMPurify checkout, or set `DOMPURIFY_REPO`:\n\n```bash\nnode /home/dompurify-trusted-types-policy-survives-clearconfig-poc.js\n```\n\nObserved output:\n\n```json\n{\n  \"result\": {\n    \"baseline\": \"<b>baseline</b>\",\n    \"duringPolicy\": \"<img src=x onerror=alert(\\\"TT_POLICY_SURVIVED_CLEARCONFIG\\\")>\",\n    \"afterClearString\": \"<img src=\\\"x\\\">\",\n    \"afterClearTrustedType\": \"[object TrustedHTML]\",\n    \"afterClearTrusted\": \"<img src=x onerror=alert(\\\"TT_POLICY_SURVIVED_CLEARCONFIG\\\")>\",\n    \"afterNullTrusted\": \"<img src=x onerror=alert(\\\"TT_POLICY_SURVIVED_CLEARCONFIG\\\")>\",\n    \"mountedHTML\": \"<img src=\\\"x\\\" onerror=\\\"alert(&quot;TT_POLICY_SURVIVED_CLEARCONFIG&quot;)\\\">\"\n  },\n  \"dialogs\": [\n    \"TT_POLICY_SURVIVED_CLEARCONFIG\"\n  ]\n}\n```\n\nThe important part is the split behavior after cleanup:\n\n- `purify.clearConfig(); purify.sanitize(...);` returns a normal sanitized string (`<img src=\"x\">`), because the later call is not asking for a Trusted Type.\n- `purify.clearConfig(); purify.sanitize(..., { RETURN_TRUSTED_TYPE: true });` still uses the old policy and returns attacker-controlled `TrustedHTML`.\n- Passing `{ TRUSTED_TYPES_POLICY: null, RETURN_TRUSTED_TYPE: true }` also still returns attacker-controlled `TrustedHTML`.\n\n## Preconditions\n\nThis is a shared-instance state contamination issue. It matters when one DOMPurify instance is reused by multiple integrations, plugins, request handlers, or components with different trust levels, and a cleanup step relies on `clearConfig()` to restore safe defaults.\n\nThis is not a default string-input bypass. An attacker must be able to influence a prior `TRUSTED_TYPES_POLICY` on the reused instance, or a less-trusted integration must have installed an unsafe policy.\n\n## Severity\n\n impact is XSS at a Trusted Types sink in applications that reuse a DOMPurify instance across trust boundaries. Attack complexity is high because exploitation depends on prior policy injection or a less-trusted integration and a later `RETURN_TRUSTED_TYPE` sink.\n\n## Suggested fix\n\nMake `clearConfig()` reset Trusted Types state as part of restoring defaults, or have `_parseConfig()` explicitly clear `trustedTypesPolicy` and `emptyHTML` when `TRUSTED_TYPES_POLICY: null` is supplied.","published":"2026-07-23T13:16:18.389Z","modified":"2026-10-21T02:30:18.810512937Z","cvss":null,"epss":{"score":0.00266,"percentile":0.18464,"asOf":"2026-09-01"},"cisaKev":null,"exploitsKnown":0,"affectedPackages":[{"ecosystem":"npm","name":"dompurify","fixedVersion":"3.4.9"}],"fix":{"url":"https://github.com/cure53/DOMPurify/commit/825e617753ac1169306a542d3174a77f717a0cf6","label":"cure53/DOMPurify@825e617"},"references":[{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/65xxx/CVE-2026-65899.json"},{"type":"ADVISORY","url":"https://github.com/cure53/DOMPurify/security/advisories/GHSA-vxr8-fq34-vvx9"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-65899"},{"type":"ADVISORY","url":"https://www.vulncheck.com/advisories/dompurify-before-trusted-types-policy-state-contamination"},{"type":"FIX","url":"https://github.com/cure53/DOMPurify/commit/825e617753ac1169306a542d3174a77f717a0cf6"},{"type":"PACKAGE","url":"https://github.com/cure53/DOMPurify"}],"provenance":{"sources":["OSV.dev","FIRST.org (EPSS)"],"lastVerified":"2026-10-21T02:30:18.810512937Z"}}