Summary

A vulnerability in ASGI web servers and starlette's trust on those web servers enables an authentication bypass of the OpenAI API AuthenticationMiddleware, which was discovered during @x41sec's source code audit. It allows to use the API without providing the configured VLLMAPIKEY or --api-key.

Details

In https://github.com/vllm-project/vllm/blob/v0.14.0/vllm/entrypoints/openai/apiserver.py#L689-L692 the urlpath is taken from the URL, which is reconstructed by starlette based on the request scope.

py from starlette.datastructures import URL, Headers, MutableHeaders, State

...

urlpath = URL(scope=scope).path.removeprefix(rootpath) headers = Headers(scope=scope) if urlpath.startswith("/v1") and not self.verifytoken(headers): response = JSONResponse(content={"error": "Unauthorized"}, statuscode=401) return response(scope, receive, send) return self.app(scope, receive, send)

The request scope includes the request's Host: header and reconstructs the URL as shown below:

py f"{scheme}://{hostheader}{path}"

Neither starlette nor any of the ASGI servers (including uvicorn, which vllm uses) properly filter the Host: header for invalid characters. This allows an attacker to include special URL characters such as / or ? in the Host: header and thereby control the reconstructed URL and it's .path attribute.

FastAPI/starlette's routing uses the HTTP path and does not depend on the parsed url.path attribute, allowing attackers to reach an endpoint via a certain path while providing a different value in the .path.

Impact - Instances of vllm that use an API Key for the OpenAI API and expose the API to attackers. - Instances behind an RFC-conforming web server (such as nginx) are not affected.