CVE-2026-63179: Path Traversal
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.customcss field (backend.managebranding) — compiled inline into every backend page's <style> block. - Editor Settings EditorSetting.htmlcustomstyles field (backend.manageeditor) — compiled inline into every backend page's <style> block. - Mail Brand Settings MailBrandSetting colour-picker fields (system.managemailtemplates) — values are concatenated into LESS source via LessParser::ModifyVars() with no escaping, so any value the form validator does not reject can carry an @import directive. - Theme .less/.sass/.scss assets (cms.manageassets) — compiled through System\Classes\CombineAssets when served, with the same LessParser 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 APPKEY 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: LessParser was instantiated without a safe import resolver, and its LessFileManager::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 LessParser::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 themespath(), pluginspath(), and basepath('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 https://github.com/wintercms/storm/commit/af770331c683e628533a6ec2991285d6e10a4d6c and https://github.com/wintercms/winter/commit/130f0ea43e9228bf0d129b481da1cdfbcc4b4456 manually. As an interim mitigation, remove cms.manageassets from any non-trusted role and clear any non-empty value from the Brand Settings customcss and Editor Settings htmlcustomstyles fields.
References
See https://github.com/octobercms/october/security/advisories/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
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/winter coreto a version that resolves this vulnerability.Fixed in v1.2.13 - Upgrade
Upgrade
wintercms/winter stormto a version that resolves this vulnerability.Fixed in v1.2.13 - Configuration
As interim mitigation, clear any non-empty value in Brand Settings → `BrandSetting.custom_css` for all instances/tenants, because it is compiled inline into the backend `<style>` block and can be used to inject `@import (inline) "<path>"`.
Winter CMS Brand Settings BrandSetting.custom_css (backend.manage_branding) = (clear/empty) - Configuration
As interim mitigation, clear any non-empty value in Editor Settings → `EditorSetting.html_custom_styles` for all instances/tenants, because it is compiled inline into the backend `<style>` block and can be used to inject `@import (inline) "<path>"`.
Winter CMS Editor Settings EditorSetting.html_custom_styles (backend.manage_editor) = (clear/empty) - Configuration
As interim mitigation, remove the `cms.manage_assets` permission from any non-trusted role (defaults include the built-in Developer role), since theme `.less`/`.sass`/`.scss` assets are compiled using the same vulnerable Less import behavior.
Winter CMS Roles/Permissions cms.manage_assets = remove from non-trusted role(s) - Compensating control
If you cannot upgrade, apply the referenced manual commits: wintercms/storm commit `af770331c683e628533a6ec2991285d6e10a4d6c` and wintercms/winter commit `130f0ea43e9228bf0d129b481da1cdfbcc4b4456` (these add the `LessImportResolver`-based protection for the four sinks described in the advisory).
Event History
Frequently Asked Questions
Which backend roles can exploit the issue?
An authenticated backend user needs at least one of these permissions: backend.manage_branding, backend.manage_editor, system.manage_mail_templates, or cms.manage_assets. Each permission corresponds to a separate input path that can reach the vulnerable LESS compilation process.
What information could be exposed?
The issue can disclose arbitrary files that are readable by the PHP/web-process account. Both absolute filesystem paths and parent-directory traversal from theme asset locations were exploitable.