CVE-2026-54256: Winter: Authenticated IDOR in backend FileUpload widget allows cross-user access to attachment metadata

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.