Impact

The Trix editor, in versions prior to 2.1.18, is vulnerable to XSS when crafted HTML is pasted into the editor. The HTMLParser processed a mock attachment, a <span> carrying an empty data-trix-attachment="{}". The empty attachment object caused the element to bypass attachment handling, so its data-trix-attributes were applied to a plain string piece. The pre-2.1.18 StringPiece.fromJSON accepted the href without validation, so an attacker-supplied javascript: URI was carried into the document model and emitted verbatim into the serialized HTML, executing when the content was rendered and clicked.

This is a stored XSS in any application that accepts untrusted rich text through Trix and renders the serialized output to other users. Applications that apply server-side HTML sanitization, such as the Rails built-in sanitizer, are additionally protected because the payload is neutralized on save.

This vulnerability shares its fix with GHSA-53p3-c7vp-4mcc. Both are resolved by the StringPiece.fromJSON sanitization added in 2.1.18. This advisory covers the paste and HTMLParser entry vector, while GHSA-53p3-c7vp-4mcc covers the drag-and-drop path through the fallback Level0InputController.

References

The vulnerability was responsibly reported by HackerOne researcher newbiefromcoma.