DOMPurify: Trusted Types policy survives `clearConfig()` and can poison later `RETURN_TRUSTED_TYPE` output
Date: 2026-06
Affected: npm/dompurify
Attack vector: xss
Category: vulnerability-disclosure
Severity: Low
CWE: CWE-693

## 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](https://github.com/user-attachments/files/28604913/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/


Mitigations:

Architectural layers (MAESTRO):