GHSA-rpr9-rxv7-x643 Apostrophe has default XSS via `xmp` raw-text passthrough in `sanitize-html`

• npm sanitize-html: >= 2.17.3, < 2.17.4 → fixed in 2.17.4

Summary Under the default configuration, `sanitize-html` can turn attacker-controlled content inside a disallowed `xmp` element into live HTML or JavaScript. This is a sanitizer bypass in the default `disallowedTagsMode: 'discard'` path and can lead to stored XSS in applications that render sanitized output back to users. Details In `[email protected]`, the default `nonTextTags` list includes only `script`, `style`, `textarea`, and `option` in `index.js` lines 138-142. That means disallowed `xmp` tags are not treated as "drop the entire contents" tags. Later, in the `ontext` handler at `index.js` lines 569-577, the code special-cases `textarea` and `xmp` and appends their text content directly to the output without escaping: ```js } else if ((options.disallowedTagsMode === 'discard' || options.disallowedTagsMode === 'completelyDiscard') && (tag === 'textarea' || tag === 'xmp')) { result += text; } ``` Because `htmlparser2` treats `xmp` as a raw-text element, markup inside `xmp` is parsed as text on input but becomes live markup again once it is appended unescaped to the sanitized output. This creates a default sanitizer bypass. For example, a disallowed `` wrapper can be used to smuggle `` or event-handler payloads through sanitization. The README also appears to contradict the implementation. In the "Discarding the entire contents of a disallowed tag" section, the documented exception list names only `style`, `script`, `textarea`, and `option`, and does not mention `xmp`. PoC Tested locally against `[email protected]` on Node.js `v25.2.1`. 1. Install the package: ```bash npm install sanitize-html ``` 2. Run the following script: ```js const sanitizeHtml = require('sanitize-html'); console.log(sanitizeHtml('alert(1)')); console.log(sanitizeHtml('')); console.log(sanitizeHtml('alert(1)')); ``` 3. Observed output: ```html alert(1) alert(1) ``` 4. Render any of the returned strings in a browser context that trusts `sanitize-html` output, for example: ```js const dirty = 'alert(1)'; const clean = sanitizeHtml(dirty); ``` If `clean` is inserted into the DOM or stored and later rendered as trusted HTML, the attacker-controlled script executes. Impact This is a cross-site scripting vulnerability in the default sanitizer behavior. Any application that uses `sanitize-html` defaults and then renders the returned HTML as trusted output is impacted. A remote attacker who can submit HTML content can trigger execution of arbitrary JavaScript in another user's browser when that content is viewed.