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 ` clone from the selected 2. DOMPurify walks and sanitizes that generated clone. 3. DOMPurify reaches the original and removes selected=javascript:1 4. The browser refreshes the 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 `.

PoC

const dirty =
  '' +
  '' +
  'x' +
  '';

const clean = DOMPurify.sanitize(dirty);
console.log(clean);

document.body.innerHTML = clean;

Observed "sanitized" output in Chromium 148/WebKit 625: ``html xx ``

After reinsertion, the browser updates the live DOM and strips the handler from the displayed clone, but the onerror has already fired: ``html xx ``

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.