Impact

Affected versions of Winter CMS allow authenticated backend users with the following permissions to disclose arbitrary files readable by the PHP process by injecting @import (inline) "<path>" directives into LESS source that the backend compiles. Four entry points share the same root cause:

• Brand Settings BrandSetting.custom_css field (backend.manage_branding) — compiled inline into every backend page's <style> block.
• Editor Settings EditorSetting.html_custom_styles field (backend.manage_editor) — compiled inline into every backend page's <style> block.
• Mail Brand Settings MailBrandSetting colour-picker fields (system.manage_mail_templates) — values are concatenated into LESS source via Less_Parser::ModifyVars() with no escaping, so any value the form validator does not reject can carry an @import directive.
• Theme .less/.sass/.scss assets (cms.manage_assets) — compiled through System\Classes\CombineAssets when served, with the same Less_Parser configuration. Both absolute paths and .. traversal escape from the asset's own tree were exploitable.

The leaked content includes any file the web process can read, most importantly the application .env file (disclosing APP_KEY and database credentials).

To actively exploit this issue, an attacker would need an authenticated backend account with one of the permissions listed above. By default these are assigned to the built-in Developer role.

Patches

The root cause is in the wikimedia/less.php integration in Winter Storm: Less_Parser was instantiated without a safe import resolver, and its Less_FileManager::getFilePath() falls back to the raw attacker-supplied path when no candidate root matches. Storm now ships a LessImportResolver that uses the callable form of Less_Parser::SetImportDirs() to refuse any @import whose resolved path lies outside the calling context's allowed roots, defeating both absolute paths and .. traversal at the parser level.

The four sinks have been updated to use the resolver. The three settings models pass no allowed roots (deny-all) because the bundled stylesheets ship no @import directives and the user fields have no legitimate use for them. System\Classes\CombineAssets configures the theme-asset compiler with themes_path(), plugins_path(), and base_path('modules') as allowed roots, preserving real cross-tree imports observed in shipped themes and plugins.

This security issue has been fixed in v1.2.13 (Winter core) and v1.2.13 (Winter Storm).

Workarounds

If you cannot upgrade, apply wintercms/storm@af77033 and 130f0ea manually. As an interim mitigation, remove cms.manage_assets from any non-trusted role and clear any non-empty value from the Brand Settings custom_css and Editor Settings html_custom_styles fields.

References

See GHSA-3888-q23f-x7qh for the related (but distinct in scope) October CMS advisory addressing the theme-asset compiler path. The Brand/Editor/Mail Brand Settings sinks reported in this advisory are not covered by the October patch.

Credit to Nguyen Van Hiep (@hypnguyen1209) from Lo Security for reporting the issue.

For more information

If you have any questions or comments about this advisory:

• Email us at hello@wintercms.com