CVE-2026-59217: Open WebUI: Upload `metadata.knowledge_id` bypasses the knowledge-base write-access check (read-only users can add files to KB)

Open WebUI’s file upload background processing trusts the client-supplied metadata.knowledge_id value and inserts a knowledge_file association before validating that the uploading user has write access to the target knowledge base.

A verified user with only read access to a knowledge base can upload an arbitrary file and set metadata={"knowledge_id":"<target knowledge id>"}. The normal /api/v1/knowledge/{id}/file/add endpoint correctly requires knowledge-base write access, but the upload auto-link path bypasses that authorization check.

The immediate result is unauthorized modification of the target knowledge base’s file membership. The attached attacker-controlled file becomes visible through /api/v1/knowledge/{id}/files, and readers/owners of that knowledge base can retrieve the file through the normal file endpoints because file access is derived from KnowledgeFile membership.