Winter: Stored XSS through Backend List widget image columns

Impact

Backend\Widgets\Lists::evalImageTypeValue() interpolated the resolved image URL into a single-quoted src attribute without escaping it. Where a list column of type image rendered an attacker-influenced value, that value could break out of the attribute and inject arbitrary attributes — including event handlers — into the backend list, executing in the session of whichever backend user viewed it.

Winter core ships no image list column, so a default installation is unaffected. Exploitation requires a plugin that renders attacker-influenced values through one.

The supported image sources are plugins, themes, the Media Library, the uploads directory, and Attach\File models; an arbitrary external URL is not among them. ImageResizer::filterGetUrl() returns the value it was given when it cannot resolve an image, however, so an unsupported value reached the attribute verbatim rather than being rejected.

As with previous issues of this kind the severity is limited: an attacker must already be able to store the value, and a backend user must then open the affected list. Note that validating a stored URL was not necessarily sufficient — a payload can survive filter_var($url, FILTER_VALIDATE_URL), since / is valid both in a URL path and after a quoted HTML attribute value.

Patches

evalImageTypeValue() now escapes the image URL, along with the width and height taken from the column configuration, before interpolating them into the tag.

This security issue has been fixed as of v1.2.14 (commit 0941c9816181095fe35d58e78e6d0c4a49238967).

Workarounds

Apply the commit above to modules/backend/widgets/Lists.php manually if you are unable to upgrade to v1.2.14.

References

• https://github.com/wintercms/winter/security/advisories/GHSA-43w4-4j3c-jx29
• https://github.com/wintercms/winter/commit/0941c9816181095fe35d58e78e6d0c4a49238967

Credit to Seungyeon Park (@sy460129) for reporting the issue.

For more information

If you have any questions or comments about this advisory:

• Email us at hello@wintercms.com