DOMPurify before 3.4.12 fails to execute afterSanitizeElements hook for custom elements allowed via CUSTOM_ELEMENT_HANDLING.tagNameCheck, allowing attributes to bypass application security policies. Attackers can preserve sensitive attributes on custom elements that later re-inject them into innerHTML sinks, creating second-order XSS gadgets.

Affected: npm: dompurify

## Summary

There is a possible hook-policy inconsistency in DOMPurify 3.4.11 involving `CUSTOM_ELEMENT_HANDLING`.

When a custom element is allowed via `CUSTOM_ELEMENT_HANDLING.tagNameCheck`, it appears that the element does not go through `afterSanitizeElements` in the same way as a normal element. As a result, an application that relies on `afterSanitizeElements` as a security policy layer to strip sensitive attributes from all elements may see those attributes removed from normal elements but preserved on allowed custom elements.

This does not appear to be a direct DOMPurify XSS or a case where DOMPurify directly allows executable payloads. The preserved value is still inert at sanitize time. The issue becomes relevant when the allowed custom element later re-injects that attribute value into an HTML sink such as `innerHTML`, creating a second-order XSS gadget.

## Details

The issue appears to originate from the control flow in `src/purify.ts`: line 1672~1691

```tsx const _sanitizeDisallowedNode = function ( currentNode: any, tagName: string ): boolean { /* Check if we have a custom element to handle */ if (!FORBID_TAGS[tagName] && _isBasicCustomElement(tagName)) { if ( CUSTOM_ELEMENT_HANDLING.tagNameCheck instanceof RegExp && regExpTest(CUSTOM_ELEMENT_HANDLING.tagNameCheck, tagName) ) { return false; }

if ( CUSTOM_ELEMENT_HANDLING.tagNameCheck instanceof Function && CUSTOM_ELEMENT_HAND…

Affected: Ubuntu:22.04:LTS: node-dompurify, Ubuntu:24.04:LTS: node-dompurify, Ubuntu:26.04:LTS: node-dompurify

DOMPurify before 3.4.12 fails to execute afterSanitizeElements hook for custom elements allowed via CUSTOM_ELEMENT_HANDLING.tagNameCheck, allowing attributes to bypass application security policies. Attackers can preserve sensitive attributes on custom elements that later re-inject them into innerHTML sinks, creating second-order XSS gadgets.

Ein Angreifer kann mehrere Schwachstellen in IBM DataPower Gateway ausnutzen, um Sicherheitsvorkehrungen zu umgehen oder einen Cross Site Scripting Angriff durchzuführen.