CVE-2026-54256

Winter: Authenticated IDOR in backend FileUpload widget allows cross-user access to attachment metadata

Workarounds

There is no supported workaround other than upgrading. If you cannot upgrade immediately, you may apply the fix manually in modules/backend/formwidgets/FileUpload.php:

1. In getFileRecord(), replace $this->getRelationModel()->find(post('file_id')) with $this->getRelationObject()->withDeferred($this->sessionKey)->find(post('file_id')).
2. In onSortAttachments(), filter the posted sortOrder ids to those returned by $this->getRelationObject()->withDeferred($this->sessionKey)->pluck($keyName) before calling setSortableOrder().

The backend FileUpload form widget trusted an attacker-controlled file_id POST parameter when resolving the attachment it operates on. The lookup (FileUpload::getFileRecord()) resolved the posted id against the global system_files table without verifying that the file belonged to the widget's own relation, parent record, or deferred-binding session.

Any authenticated backend user who can reach a form containing a fileupload field, including the built-in My Account avatar field, which requires no specific backend permission, could therefore target a System\Models\File record belonging to another user or record and:

• modify its title and description via onSaveAttachmentConfig, and
• change its sort_order via onSortAttachments (which passed posted ids
  straight to setSortableOrder(), an unscoped UPDATE ... WHERE id = ?).

The same unscoped lookup is reached by onLoadAttachmentConfig, onSaveAttachmentConfig, and onRemoveAttachment. Because all attachments share the single System\Models\File model and system_files table, an attacker was not limited to other users' avatars, any attachment on any model could be referenced by id. Attachment ids are sequential integers and are easily enumerated.

The confirmed impact is unauthorized integrity modification of arbitrary attachment metadata and ordering.

CSRF tokens are still verified on all POST requests, so an attacker must be authenticated to the backend with a valid session. To exploit this issue an attacker needs a backend account with any level of access.

CVE-2026-54256 has a CVSS score of 5.4 (Medium). The vector is network-reachable, low privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (1.2.13); upgrading removes the vulnerable code path.

Kodem intelligence

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The FileUpload widget now scopes every file_id lookup to the widget's own relation, including any files bound through the current deferred-binding session, so a posted id can no longer reference an unrelated System\Models\File record:

• getFileRecord() resolves the id through getRelationObject()->withDeferred($this->sessionKey)->find(...) rather than the global file model. This covers onLoadAttachmentConfig, onSaveAttachmentConfig, and onRemoveAttachment.
• onSortAttachments() intersects the posted ids with the ids that actually belong to the relation before calling setSortableOrder().

This security issue has been fixed as of v1.2.13 (commit 9cb0ae5f9d837db141ab111c6a7de8eed9603d25).

1. What is CVE-2026-54256? CVE-2026-54256 is a medium-severity security vulnerability in winter/wn-backend-module (composer), affecting versions <= 1.2.12. It is fixed in 1.2.13.
2. How severe is CVE-2026-54256? CVE-2026-54256 has a CVSS score of 5.4 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of winter/wn-backend-module are affected by CVE-2026-54256? winter/wn-backend-module (composer) versions <= 1.2.12 is affected.
4. Is there a fix for CVE-2026-54256? Yes. CVE-2026-54256 is fixed in 1.2.13. Upgrade to this version or later.
5. Is CVE-2026-54256 exploitable, and should I be worried? Whether CVE-2026-54256 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-54256 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-54256? Upgrade winter/wn-backend-module to 1.2.13 or later.