9Router has an Authentication Bypass in Public LLM API via Spoofable X-9r-Real-Ip Header

Summary

9router determines whether an incoming request originates from localhost by trusting the X-9r-Real-Ip HTTP request header. This header is intended to be produced and sanitized exclusively by the bundled custom-server.js layer from the TCP socket address. In deployment modes where requests reach Next.js directly (the header is never stripped/regenerated), a remote, unauthenticated attacker can simply send X-9r-Real-Ip: 127.0.0.1 and be treated as a local client. This bypasses the API-key requirement on the public LLM API (/api/v1/*), granting unauthenticated access to the instance owner's configured provider resources.

Affected Component

• src/dashboardGuard.js
• isLocalRequest() — trusts the client-supplied X-9r-Real-Ip header to decide loopback origin
• canAccessPublicLlmApi() — grants access to /api/v1/* when isLocalRequest() returns true, skipping API-key validation
• Verified affected route: GET /api/v1/models
• Product version tested: 9router-app 0.5.4 (Next.js 16.2.9)

Root Cause

The authorization layer makes a security decision based on a client-controllable HTTP header. isLocalRequest() reads X-9r-Real-Ip and, if its value is a loopback address (127.0.0.1), classifies the request as local. The design assumes this header can only be set by the trusted custom-server.js wrapper (which derives it from the unspoofable socket address and strips any inbound copy). When the application is served without that wrapper, Next.js passes the attacker-supplied header through unchanged, so the trust assumption is violated:

Untrusted Client Input
        ↓
X-9r-Real-Ip: 127.0.0.1
        ↓
isLocalRequest()  → true
        ↓
canAccessPublicLlmApi()  → allowed (API key not required)
        ↓
200 OK

Attack Scenario

1. The instance is deployed in a mode that does not use custom-server.js, and the LLM API is reachable by the attacker (the default bind is 0.0.0.0).
2. The attacker sends a normal request to /api/v1/models and receives 401 Unauthorized (API key required for remote access).
3. The attacker re-sends the identical request with the single added header X-9r-Real-Ip: 127.0.0.1.
4. The request is classified as local, the API-key check is skipped, and the attacker receives 200 OK with the owner's model catalog and ongoing access to the LLM API.

Proof of Concept

Baseline Request

Exploit Request

The only difference between the two requests is the addition of X-9r-Real-Ip: 127.0.0.1.

Impact

An unauthenticated remote attacker who can reach the service can bypass API-key enforcement on the public LLM API and act as a trusted local client. Consequences include:

• Unauthorized use of the owner's configured LLM provider connections
• Consumption of paid API credits / financial loss to the instance owner
• Abuse of upstream provider accounts via the proxy
• Enumeration of configured providers and available models

Remediation

• Do not trust X-9r-Real-Ip (or any X-9r-* header) when received directly from clients.
• Derive the client address for authorization from a trusted transport-level source, e.g. req.socket.remoteAddress, rather than a request header.
• If custom-server.js is required for the security model, fail closed when its trusted marker is absent, and explicitly strip/reject any inbound client-supplied X-9r-* headers at the edge.
• Document supported, secure startup modes so the application is not run in a configuration where the header is attacker-controllable.