Winter: Stored XSS through Backend List widget image columns

Summary

### 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`](https://wintercms.com/docs/v1.2/docs/backend/lists#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](https://wintercms.com/docs/v1.2/docs/services/image-resizing#available-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`](https://github.com/wintercms/winter/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](https://github.com/sy460129)) for reporting the issue. ### For more information If you have any questions or comments about this advisory: - Email us at [[email protected]](mailto:[email protected])