Winter CMS stored XSS through privileged upload of SVG file

Impact

Users with the backend.manage_branding permission can upload SVGs as the application logo. Previously, SVG uploads were not sanitized, 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 developer or super user level permissions in Winter CMS. 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 (1.2.3) to ensure their systems remain secure.

Patches

This issue has been patched in v1.2.3 through the inclusion of full support for SVG uploads and automatic sanitization of uploaded SVG files.

Workarounds

Apply https://github.com/wintercms/storm/commit/186d85d8fea2cae43afc807d39f68553c24e56be & https://github.com/wintercms/winter/commit/fa50b4c7489b67ea80072f8ac9fe5294fce1df1c manually if unable to upgrade to v1.2.3.