Cross-User File Access via Unchecked file_id in Folder Knowledge and Knowledge-Base Attach Endpoints

Summary

Multiple endpoints accept a user-supplied file_id and attach the referenced file to a resource the caller controls (folder knowledge, knowledge-base contents) without verifying that the caller owns or has been granted access to the file. The file's content then becomes reachable through the downstream RAG / file-content paths, allowing any authenticated user to exfiltrate any other user's private file — and on the knowledge-base path, also to overwrite it — given knowledge of the file's UUID.

Affected code paths

Path 1 — Folder knowledge ingestion via folders.update

backend/open_webui/routers/folders.py:156 — POST /api/v1/folders/{id}/update accepts a FolderUpdateForm whose data: Optional[dict] field is written verbatim into the folder. The folder consumer at backend/open_webui/utils/middleware.py:2409 spreads folder.data['files'] directly into form_data['files'] for the next chat completion, which becomes RAG context. There is no per-file ownership check at the writer (the update handler) and no per-file ownership check at the reader (the middleware folder consumer) — only the folder list endpoint (folders.py:78-94) cleans up by stripping inaccessible files, and that runs lazily at folder-list time rather than at chat time. An attacker with a victim's file UUID can write data: {"files": [{"id": "<victim>", "type": "file"}]} into their own folder, immediately chat in that folder, and have the LLM return the victim's document content via RAG. The cleanup pass strips the file from persistence later, but the exfiltration has already happened.

Path 2 — Knowledge-base attach via knowledge.{id}/file/add and knowledge.{id}/files/batch/add