GHSA-5cwr-5jxg-pcf6: Winter: Stored XSS through cached Brand Settings and Editor Settings custom styles

Users with the backend.manage_branding (“Customize the back-end”) or backend.manage_editor (“Manage global code editor preferences”) permission can provide custom CSS through Settings → Customize Backend → Styles or Settings → Editor Settings → Markup Styles that is compiled through the LESS CSS parser and rendered on every backend page.

v1.2.13 addressed CVE-2026-32257 and CVE-2026-32258 by applying strip_tags() to the compiled output of BrandSetting::renderCss() and EditorSetting::renderCss(). That fix was incomplete. Both methods cache the raw compiler output with Cache::forever() and applied strip_tags() only to the value returned on a cache miss, so every subsequent cache hit returned the unsanitized value directly into the backend <style> element. The first page render after saving therefore appears sanitized while priming an unsafe cache entry, and the stored XSS remains exploitable from the second render onwards.

Because the custom styles partial is included by both the standard backend layout head and the backend authentication layout, an injected payload also renders on the unauthenticated backend sign-in, password restore, and password reset pages.

Although this is a valid security issue, it’s important to note that its severity is relatively low. To exploit the vulnerability, an attacker would already need to have trusted access to the Winter CMS backend with a specific administrative permission. The Winter CMS maintainers recommend that the backend.manage_branding and backend.manage_editor permissions only be granted to trusted administrators and developers.

All users are advised to update to the latest version to ensure their systems remain secure.