Open WebUI upload metadata can add files to knowledge bases without write permission

Summary

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.

Affected Version

• Repository: open-webui/open-webui
• Tested source commit: 02dc3e689ceac915a870b373318b99c029ddf603
• Package version observed in package.json: 0.9.6
• Package name: open-webui

Impact

A read-only knowledge-base collaborator can perform a write operation against that knowledge base by attaching arbitrary uploaded files.

Security impact:

• Unauthorized knowledge-base membership modification.
• Integrity impact on shared knowledge-base file listings.
• Attacker-controlled files become readable to other users who can read the target knowledge base.
• If an owner/admin later reprocesses or globally reindexes the knowledge base, the unauthorized file can be indexed into the knowledge collection, turning the membership bypass into RAG/content poisoning.

This is not an unauthenticated issue. It requires a verified Open WebUI account and a valid target knowledge-base ID. The clearest exploit path is a user who legitimately has read access to a...