CVE-2026-32257: XSS
Impact
Users with the backend.managebranding ("Customize the back-end") permission can provide custom CSS through Settings → Customize Backend → Styles that is compiled through the LESS CSS parser and rendered on every backend page. Previously, the compiled output was not sanitized, which could have allowed a stored XSS attack.
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.managebranding permission only be granted to trusted administrators and developers.
A related issue exists in the custom styles field at Settings → Editor Settings → Markup Styles (backend.manageeditor permission), which uses the same rendering mechanism and is addressed separately.
All users are advised to update to the latest version to ensure their systems remain secure.
Patches
This issue has been patched in v1.2.13.
Workarounds
Apply https://github.com/wintercms/winter/commit/d28f0b9474af79cfaa80eeb9d691f7a7c4469720 manually if unable to upgrade to v1.2.13.
Resources
- CVE-2025-61676 (identical vulnerability in October CMS, patched in v3.7.13 and v4.0.12)
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Upgrade
Upgrade
composer/winter/wn-backend-moduleto a version that resolves this vulnerability.Fixed in 1.2.13 - Upgrade
Upgrade
Winter CMSto a version that resolves this vulnerability.Fixed in 3.7.13 - Upgrade
Upgrade
Winter CMSto a version that resolves this vulnerability.Fixed in 4.0.12 - Upgrade
Upgrade
Winter CMSto a version that resolves this vulnerability.Fixed in 1.2.13 - Upgrade
Upgrade to a fixed release to a version that resolves this vulnerability.
Patch d28f0b9474af79cfaa80eeb9d691f7a7c4469720 - Compensating control
Ensure the `backend.manage_branding` (“Customize the back-end”) permission is granted only to trusted administrators and developers, since users with this permission can provide custom CSS that is compiled and rendered on every backend page.
Event History
Frequently Asked Questions
What is the severity of CVE-2026-32257?
CVE-2026-32257 has a high severity score of 8.1.
What impact does CVE-2026-32257 have?
CVE-2026-32257 allows users with the 'backend.manage_branding' permission to inject custom CSS that can lead to cross-site scripting vulnerabilities.
How do I fix CVE-2026-32257?
To fix CVE-2026-32257, ensure that custom CSS inputs are properly sanitized to prevent the execution of malicious scripts.
Which software is affected by CVE-2026-32257?
CVE-2026-32257 affects the `composer/winter/wn-backend-module` software.
What type of vulnerability is CVE-2026-32257 classified as?
CVE-2026-32257 is classified as a cross-site scripting (XSS) vulnerability.