In DOMPurify through 3.3.3, function predicates supplied via ADD_ATTR or ADD_TAGS to DOMPurify.sanitize() persist in internal state (EXTRA_ELEMENT_HANDLING) across subsequent sanitize() calls on the same instance. If a later call on the same instance provides ADD_ATTR or ADD_TAGS as an array rather than a function, the previously set function handler is neither cleared nor overwritten, so it continues to approve attacker-controlled attributes or tags. This can allow dangerous event-handler attributes or forbidden tags (bypassing FORBID_TAGS) to survive sanitization, resulting in cross-site scripting. The vendor (Cure53) considers this an edge case outside DOMPurify's threat model; the referenced advisory lists 3.4.0 as the patched version.