{"id":"GHSA-vxr8-fq34-vvx9","summary":"DOMPurify: Trusted Types policy survives `clearConfig()` and can poison later `RETURN_TRUSTED_TYPE` output","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\": \"\u003cb\u003ebaseline\u003c/b\u003e\",\n    \"duringPolicy\": \"\u003cimg src=x onerror=alert(\\\"TT_POLICY_SURVIVED_CLEARCONFIG\\\")\u003e\",\n    \"afterClearString\": \"\u003cimg src=\\\"x\\\"\u003e\",\n    \"afterClearTrustedType\": \"[object TrustedHTML]\",\n    \"afterClearTrusted\": \"\u003cimg src=x onerror=alert(\\\"TT_POLICY_SURVIVED_CLEARCONFIG\\\")\u003e\",\n    \"afterNullTrusted\": \"\u003cimg src=x onerror=alert(\\\"TT_POLICY_SURVIVED_CLEARCONFIG\\\")\u003e\",\n    \"mountedHTML\": \"\u003cimg src=\\\"x\\\" onerror=\\\"alert("TT_POLICY_SURVIVED_CLEARCONFIG")\\\"\u003e\"\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 (`\u003cimg src=\"x\"\u003e`), 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.","aliases":["CVE-2026-65899"],"modified":"2026-07-23T14:34:40.188380Z","published":"2026-06-15T20:12:53Z","database_specific":{"severity":"LOW","github_reviewed":true,"github_reviewed_at":"2026-06-15T20:12:53Z","nvd_published_at":null,"cwe_ids":["CWE-693"]},"references":[{"type":"WEB","url":"https://github.com/cure53/DOMPurify/security/advisories/GHSA-vxr8-fq34-vvx9"},{"type":"PACKAGE","url":"https://github.com/cure53/DOMPurify"}],"affected":[{"package":{"name":"dompurify","ecosystem":"npm","purl":"pkg:npm/dompurify"},"ranges":[{"type":"SEMVER","events":[{"introduced":"0"},{"fixed":"3.4.9"}]}],"database_specific":{"source":"https://github.com/github/advisory-database/blob/main/advisories/github-reviewed/2026/06/GHSA-vxr8-fq34-vvx9/GHSA-vxr8-fq34-vvx9.json"}}],"schema_version":"1.9.0","severity":[{"type":"CVSS_V4","score":"CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:A/VC:N/VI:N/VA:N/SC:L/SI:L/SA:N"}]}