Winter is a free, open-source content management system (CMS) based on the Laravel PHP framework. From 1.2.10 through 1.2.12, authenticated backend users with the backend.manage_editor permission can store custom Markup Styles that are compiled by the LESS parser and rendered without sanitization on every backend page, allowing stored cross-site scripting. This issue is fixed in version 1.2.13.

Affected: Packagist: winter/wn-backend-module

### Impact Authenticated Backend Users with the `backend.manage_editor` ("Manage editor settings") permission can provide custom styles through **Settings → Editor Settings → Markup Styles** that are 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.manage_editor` permission only be granted to trusted administrators and developers.

A related issue exists in the custom CSS field at **Settings → Customize Backend → Styles** (`backend.manage_branding` 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. The `renderCss()` method now applies `strip_tags()` to compiled CSS output before returning it.

### Workarounds

Apply [https://github.com/wintercms/w…