Winter CMS Stored XSS through privileged upload of Media Manager file followed by renaming

Impact

Users with the media.manage_media permission can upload files to the Media Manager and rename them after uploading. Previously, media manager files were only sanitized on upload, not on renaming, which could have allowed a stored XSS attack.

Although this was a security issue, it’s important to note that its severity is low. To exploit the vulnerability, an attacker would already need to have trusted permissions in the Winter CMS backend. This means they would already have extensive access and control within the system. Additionally, to execute the XSS, the attacker would need to convince the victim to directly visit the URL of the maliciously uploaded SVG, and the application would have to be using local storage where uploaded files are served under the same domain as the application itself instead of a CDN. This is because all SVGs in Winter CMS are rendered through an img tag, which prevents any payloads from being executed directly.

These two factors significantly limit the potential harm of this vulnerability. That being said, all users are advised to update to the latest version (1.2.4) to ensure their systems remain secure.

Patches

This issue has been patched in v1.2.4.

Workarounds

Apply https://github.com/wintercms/winter/commit/2969daeea8dee64d292dbaa3778ea251e2a7e491 and https://github.com/wintercms/winter/commit/2969daeea8dee64d292dbaa3778ea251e2a7e491 manually if unable to upgrade to v1.2.4.

References

Related: https://github.com/wintercms/winter/security/advisories/GHSA-wjw2-4j7j-6gc3