DOMPurify before 3.4.0 contains a logic error in the ADD_TAGS function where short-circuit evaluation allows forbidden tags to bypass FORBID_TAGS restrictions. Attackers can craft input containing tags listed in FORBID_TAGS that are also added via ADD_TAGS function, causing them to be retained in sanitized output.

Affected: npm: dompurify

## 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)

Affected: Ubuntu:22.04:LTS: node-dompurify, Ubuntu:24.04:LTS: node-dompurify, Ubuntu:26.04:LTS: node-dompurify

DOMPurify before 3.4.0 contains a logic error in the ADD_TAGS function where short-circuit evaluation allows forbidden tags to bypass FORBID_TAGS restrictions. Attackers can craft input containing tags listed in FORBID_TAGS that are also added via ADD_TAGS function, causing them to be retained in sanitized output.