GPUStack: Authentication bypass via trusted loopback source (reverse-proxy TCP peer or X-GPUStack-Real-IP header) grants unauthenticated admin

Impact

In GPUStack before v2.2.0, get_current_user (in gpustack/api/auth.py) promotes any request whose source is considered loopback to the built-in administrator account when force_auth_localhost is False (the default). The "source" is derived by client_ip_getter, which can be made to report 127.0.0.1 without any credentials in two ways:

1. Same-host reverse proxy (TCP peer). In the canonical self-hosted shape — GPUStack behind a same-host reverse proxy with proxy_pass http://127.0.0.1:<port> — every remote request arrives at GPUStack with TCP peer 127.0.0.1, so the loopback auto-admin branch fires. No header or credential is required. Verified end-to-end against HEAD 6bee5883.
2. X-GPUStack-Real-IP header. A forged X-GPUStack-Real-IP: 127.0.0.1 header can cause the request source to be treated as loopback.

Either path yields full, unauthenticated administrative access over the network: enumerate and modify users, models, workers, and configuration; mint persistent admin API keys (scope ["*"], valid from any source IP thereafter); read worker credentials and cloud/provider secrets. Combined with admin-only functionality such as POST /v2/inference-backends/from yaml, this leads to code execution on worker hosts.

This vulnerability has been observed exploited in the wild against a v2.1.1 deployment (attacker-created custom backends running reverse shells).

Root cause

Trusting the network source of a request (socket peer, or a client-controllable header) as an authentication signal. When GPUStack is fronted by a same-host reverse proxy, the socket peer is always loopback, so the "localhost is trusted" assumption no longer holds.

Patches

Fixed in gpustack 2.2.0: the loopback auto-admin path was removed and client-IP / forwarding-header handling was hardened. Upgrade to 2.2.0 or later.

Workarounds

• Set force_auth_localhost=true to require authentication even for loopback requests.
• Ensure the reverse proxy / gateway cannot cause GPUStack to treat external traffic as loopback, and strip any client-supplied X-GPUStack-Real-IP header at the edge.
• Keep the API off untrusted networks (VPN / edge authentication).

Remediation (in the fix)

Do not derive authentication from the request's network source. Remove implicit loopback-based admin, and only accept a real client-IP header from an authenticated/trusted gateway (never from arbitrary clients).

Credit

Independently reported by Nikko Enggaliano; Wei Yunzhou; and Rui Yang & Haoyu Wang (Johns Hopkins University). Active in the-wild exploitation was reported by Javier Sixto (ulex.fr).