# Winter Backend Module stored XSS in cached styles

Source: https://junglewise.ai/threats/winter-backend-module-stored-xss-in-cached-styles-3dde5f4d  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-08-20. Updated: 2026-09-25  
License: CC BY 4.0 with a link back

Severity: medium · CVSS 4.5 · Published 2026-08-20

Winter CMS's backend styling system allows administrators with branding or editor permissions to inject custom CSS that is cached and rendered on every backend page, including unauthenticated login pages. A previous patch applied sanitization only to the first cache retrieval, leaving subsequent cached copies vulnerable to cross-site scripting attacks. An attacker with administrative styling permissions could execute arbitrary JavaScript in the context of backend users and unauthenticated sign-in pages.

Technologies: [winter/wn-backend-module (Packagist)](https://junglewise.ai/threats/technologies/winter-wn-backend-module). Vendors: [Packagist](https://junglewise.ai/threats/vendors/packagist).

## Quick answers

**Is This vulnerability exploited in the wild?** No. Junglewise Threat Intelligence has no report of This vulnerability being exploited in the wild.

**How severe is This vulnerability?** Medium severity, with a CVSS score of 4.5, published 20 August 2026.

**Which products and versions are affected?** winter/wn-backend-module (Packagist) <= 1.2.13.

**Is there a fix for This vulnerability?** A patch was published on 2026-08-20: Fix released in v1.2.14 Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is This vulnerability attacked?** It is attacked over the network, in a software library.

**Am I affected?** Check whether you run winter/wn-backend-module (Packagist) at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## Technical details

This vulnerability is a stored cross-site scripting (XSS) flaw in Winter CMS's BrandSetting::renderCss() and EditorSetting::renderCss() methods. The root cause is incomplete sanitization of LESS CSS compiler output: the raw compiled CSS is cached via Cache::forever() without sanitization, and strip_tags() is applied only when the cached value is first retrieved on a cache miss. Every subsequent cache hit returns the unsanitized HTML/CSS directly into the backend &lt;style&gt; element, allowing injected payloads to execute from the second page render onwards. The vulnerability requires high privileges (backend.manage_branding or backend.manage_editor permissions) and user interaction (viewing the affected page), but the impact is severe because the custom styles are rendered on all backend pages and also on unauthenticated sign-in, password restore, and password reset pages. The fix, released in v1.2.14, applies strip_tags() to the cached value when it is read back, neutralizing both new and previously poisoned cache entries.

## Affected products

- Winter wn-backend-module <= 1.2.13

## Timeline

- 2026-08-20: disclosed: Advisory published by Winter CMS
- 2026-08-20: patched: Fix released in v1.2.14

## References

- [https://api.github.com/users/NRAwwad](https://api.github.com/users/NRAwwad)
- [https://github.com/NRAwwad](https://github.com/NRAwwad)
- [https://api.github.com/users/NRAwwad/gists%7B/gist_id%7D](https://api.github.com/users/NRAwwad/gists%7B/gist_id%7D)
- [https://api.github.com/users/NRAwwad/repos](https://api.github.com/users/NRAwwad/repos)
- [https://avatars.githubusercontent.com/u/39842950?v=4](https://avatars.githubusercontent.com/u/39842950?v=4)
- [https://api.github.com/users/NRAwwad/events%7B/privacy%7D](https://api.github.com/users/NRAwwad/events%7B/privacy%7D)

## Related threats

- [Winter CMS local file inclusion via LESS @import directives](https://junglewise.ai/threats/cve-2026-63179-winter-cms-local-file-inclusion-via-less-import-directives)
- [Winter CMS FileUpload widget IDOR in attachment metadata](https://junglewise.ai/threats/cve-2026-54256-winter-cms-fileupload-widget-idor-in-attachment-metadata)
- [Winter CMS SQL injection in Backend Filter Widget numberrange scope](https://junglewise.ai/threats/cve-2026-32593-winter-cms-sql-injection-in-backend-filter-widget-numberrange)
- [Winter CMS authorization bypass in Users controller](https://junglewise.ai/threats/cve-2026-35445-winter-cms-authorization-bypass-in-users-controller)
- [Winter CMS stored XSS in Editor Settings custom styles](https://junglewise.ai/threats/cve-2026-32258-winter-cms-stored-xss-in-editor-settings-custom-styles)