DOMPurify's ADD_TAGS function form bypasses FORBID_TAGS due to short-circuit evaluation

Summary

In src/purify.ts:1117-1123, ADD_TAGS as a function (via EXTRA_ELEMENT_HANDLING.tagCheck) bypasses FORBID_TAGS due to short-circuit evaluation.

The condition:

!(tagCheck(tagName)) && (!ALLOWED_TAGS[tagName] || FORBID_TAGS[tagName])

When tagCheck(tagName) returns true, the entire condition is false and the element is kept — FORBID_TAGS[tagName] is never evaluated.

Inconsistency

This contradicts the attribute-side pattern at line 1214 where FORBID_ATTR explicitly wins first:

if (FORBID_ATTR[lcName]) { continue; }

For tags, FORBID should also take precedence over ADD.

Impact

Applications using both ADD_TAGS as a function and FORBID_TAGS simultaneously get unexpected behavior — forbidden tags are allowed through. Config-dependent but a genuine logic inconsistency.

Suggested Fix

Check FORBID_TAGS before tagCheck:

if (FORBID_TAGS[tagName]) { /* remove */ }
else if (tagCheck(tagName) || ALLOWED_TAGS[tagName]) { /* keep */ }

Affected Version

v3.3.3 (commit 883ac15)