GHSA-87xg-pxx2-7hvx

Описание

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

Observed "sanitized" output in Chromium 148/WebKit 625:

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

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.