Executive brief
Winter CMS is a popular open-source content management system. A stored cross-site scripting (XSS) vulnerability in the backend List widget's image column feature allows an attacker to inject malicious code that executes when a backend administrator or staff member views an affected list. While default Winter installations are not vulnerable, plugins that use the image list column type to display user-controlled image URLs could be exploited to compromise the session of backend users.
Technical details
The vulnerability is a stored XSS (CWE-79) in the Backend\Widgets\Lists::evalImageTypeValue() method. The function interpolated a resolved image URL directly into a single-quoted HTML src attribute without HTML escaping, allowing an attacker to inject additional attributes and event handlers. The vulnerable component accepts image URLs from multiple sources (plugins, themes, Media Library, uploads directory, Attach\File models), and ImageResizer::filterGetUrl() returns unsupported URL values verbatim rather than rejecting them, enabling the XSS payload to reach the vulnerable interpolation point. Attack preconditions are stringent: an attacker must first be able to store a malicious URL value (requiring access to a plugin that uses the image column type), and a backend user with appropriate permissions must then view the affected list. The payload can evade basic URL validation (e.g., filter_var with FILTER_VALIDATE_URL) because forward slashes are valid both in URL paths and after quoted HTML attributes. The fix escapes the image URL and column configuration values (width, height) before interpolating them into the tag, which was patched in v1.2.14.
Affected products
- Winter CMS wn-backend-module >= 1.1.0, < 1.2.14
Timeline
- 2026-08-19: disclosed: Advisory published
- 2026-08-16: patched: Fix committed to repository
- 2026-08-20: advisory: GitHub Advisory Database entry created