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 upload metadata can add files to knowledge bases without write permission

Summary

Open WebUI's file upload background processing trusts the client-supplied metadata.knowledgeid value and inserts a knowledgefile 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={"knowledgeid":"<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 knowledge base but not write access.

Source Evidence

The normal single-file knowledge add endpoint checks write permission before processing or inserting the relationship:

- backend/openwebui/routers/knowledge.py - addfiletoknowledgebyid - Lines 714-728 reject callers who are not owner, admin, or granted write access. - Lines 750-766 then process and insert the file only after that authorization gate.

The upload auto-link path does not perform the same check:

- backend/openwebui/routers/files.py - processuploadedfile - Lines 178-186 read knowledgeid from upload metadata and immediately call Knowledges.addfiletoknowledgebyid(...). - Lines 187-192 call processfile(... collectionname=knowledgeid ...) after the insert.

The model method inserts the relationship without validating the caller's write access to the knowledge base:

- backend/openwebui/models/knowledge.py - addfiletoknowledgebyid - Lines 646-677 create and commit a KnowledgeFile row for the supplied knowledgeid, fileid, and userid.

The later vector write check exists, but it runs too late:

- backend/openwebui/routers/retrieval.py - processfile - Lines 1587-1592 call validatecollectionaccess(..., accesstype='write') when a collection is supplied.

Because the unauthorized KnowledgeFile row is already committed before that check runs, the failed vector processing does not undo the knowledge-base file association. The upload code catches the exception at backend/openwebui/routers/files.py lines 194-195 and logs a warning while leaving the row in place.

The unauthorized relationship affects file access decisions:

- backend/openwebui/utils/accesscontrol/files.py - hasaccesstofile - Lines 41-53 grant file access when a file is associated with a knowledge base the user can access.

So once the attacker's file is inserted into the target KnowledgeFile table, target knowledge-base readers/owners can see and fetch that file through normal knowledge/file routes.

Reproduction Steps

Use a local Open WebUI instance with two verified users:

1. As user owner, create a knowledge base. 2. Grant user reader read access to the knowledge base, but do not grant write access. 3. As reader, confirm the normal add-file endpoint is blocked:

http POST /api/v1/knowledge/<knowledgeid>/file/add Authorization: Bearer <reader token> Content-Type: application/json

{"fileid":"<reader-owned-file-id>"}

Expected and observed behavior for the normal route: it rejects the request because reader lacks knowledge-base write access.

4. As reader, upload a new file with the same target knowledge ID embedded in upload metadata:

http POST /api/v1/files/?process=true&processinbackground=false Authorization: Bearer <reader token> Content-Type: multipart/form-data

file=@attacker-note.txt metadata={"knowledgeid":"<knowledgeid>"}

5. Observe that the upload request succeeds and returns the uploaded file record. 6. As owner, request the knowledge-base files:

http GET /api/v1/knowledge/<knowledgeid>/files Authorization: Bearer <owner token>

7. Observe that attacker-note.txt appears in the target knowledge base even though reader did not have write access. 8. As owner, request the file content:

http GET /api/v1/files/<attackerfileid>/content Authorization: Bearer <owner token>

9. Observe that the file is retrievable because hasaccesstofile derives access from the unauthorized knowledge-base membership.

Expected Behavior

The upload auto-link path should enforce the same authorization contract as /api/v1/knowledge/{id}/file/add:

- The target knowledge base must exist. - The caller must be the knowledge owner, an admin, or have write access. - The supplied directoryid, if present, must belong to the target knowledge base. - The KnowledgeFile association should only be inserted after authorization and processing succeed.

Actual Behavior

metadata.knowledgeid causes Knowledges.addfiletoknowledgebyid(...) to insert a KnowledgeFile row before write authorization is checked. The later collection write validation can fail, but the unauthorized membership row remains committed.

Suggested Fix

Move knowledge-base authorization before the insert in the upload auto-link path. The upload path should share the same write-access and directory validation logic used by the dedicated knowledge endpoints.

One safe pattern:

1. Load the target knowledge base. 2. Require owner/admin/write access before calling Knowledges.addfiletoknowledgebyid. 3. Validate that directoryid, if supplied, belongs to the same knowledge base. 4. Run vector processing before inserting the membership row, or wrap processing plus insertion in a transaction/compensating cleanup so a denied or failed process cannot leave a stale unauthorized row.