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\")>",
    "mountedHTML": "<img src=\"x\" onerror=\"alert(&quot;TT_POLICY_SURVIVED_CLEARCONFIG&quot;)\">"
  },
  "dialogs": [
    "TT_POLICY_SURVIVED_CLEARCONFIG"
  ]
}

The important part is the split behavior after cleanup:

• purify.clearConfig(); purify.sanitize(...); returns a normal sanitized string (<img src="x">), because the later call is not asking for a Trusted Type.
• purify.clearConfig(); purify.sanitize(..., { RETURN_TRUSTED_TYPE: true }); still uses the old policy and returns attacker-controlled TrustedHTML.
• Passing { TRUSTED_TYPES_POLICY: null, RETURN_TRUSTED_TYPE: true } also still returns attacker-controlled TrustedHTML.

Preconditions

This 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.

This 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.

Severity

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.

Suggested fix

Make 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.