GHSA-vxr8-fq34-vvx9: DOMPurify: Trusted Types policy survives `clearConfig()` and can poison later `RETURN_TRUSTED_TYPE` output

Impact A 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. If 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. dompurify-trusted-types-policy-survives-clearconfig-poc.js Affected version Tested against DOMPurify 3.4.8, repository commit 825e617753ac1169306a542d3174a77f717a0cf6. Root cause _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. Relevant code: - src/purify.ts:786-812 accepts and stores cfg.TRUSTED_TYPES_POLICY. - src/purify.ts:813-832 does not reset an existing policy when config has no policy or has TRUSTED_TYPES_POLICY: null. - src/purify.ts:2123-2125 signs the final serialized HTML with the retained policy when RETURN_TRUSTED_TYPE is true. - src/purify.ts:2133-2136 clearConfig() only clears CONFIG and SET_CONFIG; it does not reset trustedTypesPolicy or emptyHTML. Local PoC Run from the DOMPurify checkout, or set DOMPURIFY_REPO: node /home/dompurify-trusted-types-policy-survives-clearconfig-poc.js Observed output: { "result": { "baseline": "<b>baseline</b>", "duringPolicy": "<img src=x onerror=alert(\"TT_POLICY_SURVIVED_CLEARCONFIG\")>", "afterClearString": "<img src=\"x\">", "afterClearTrustedType": "[object TrustedHTML]", "afterClearTrusted": "<img src=x onerror=alert(\"TT_POLICY_SURVIVED_CLEARCONFIG\")>", "afterNullTrusted": "<img src=x onerror=alert(\"TT_POLICY_SURVIVED_CLEARCONFIG\"