DOMPurify is a DOM-only cross-site scripting sanitizer for HTML, MathML, and SVG. Versions prior to 3.4.0 have an inconsistency between FORBIDTAGS and FORBIDATTR handling when function-based ADDTAGS is used. Commit c361baa added an early exit for FORBIDATTR at line 1214. 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. This allows forbidden elements to survive sanitization with their attributes intact. Version 3.4.0 patches the issue.