Description

9router has an Incomplete Fix: Local-Only Access Gate Bypass in 9router via Host Header SpoofING

Summary

The fix for CVE-2026-46339 (unauthenticated RCE via unprotected MCP plugin routes) introduced a local-only access gate in src/dashboardGuard.js that restricts spawn-capable routes (/api/mcp/*, /api/tunnel/*, /api/cli-tools/*) to loopback requests. The gate determines "local" by inspecting the Host and Origin HTTP headers rather than the TCP source address. When 9router is deployed behind a reverse proxy, tunnel (Cloudflare Tunnel, Tailscale — both natively supported), or is subject to DNS rebinding, these headers are attacker-controlled, allowing the local-only gate to be bypassed.

A second factor (CLI token or JWT cookie) is required by canAccessLocalOnlyRoute(), but the CLI token is a deterministic HMAC of the machine ID (getConsistentMachineId), which is stable and predictable on cloud VMs. If the attacker can obtain or guess the machine ID (e.g., via another information disclosure, or on shared-tenant infrastructure), the full chain to MCP child process stdin injection is reachable.

This is a variant / incomplete fix of CVE-2026-46339 — the same attack surface (remote → MCP child process stdin) remains reachable under specific but realistic deployment configurations.

Root Cause

isLocalRequest() at src/dashboardGuard.js:93-101:

function isLocalRequest(request) {
  if (!isLoopbackHostname(request.headers.get("host"))) return false;
  const origin = request.headers.get("origin");
  if (origin) {
    try {
      if (!isLoopbackHostname(new URL(origin).hostname)) return false;
    } catch { return false; }
  }...
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This function trusts Host and Origin headers as proof of local origin. Both are attacker-controlled in any proxied deployment. The LOOPBACK_HOSTS set (localhost, 127.0.0.1, ::1) is checked against these headers, not against the actual connection source IP.

Attack Scenario

Scenario 1: Cloudflare Tunnel / Tailscale Funnel

9router natively supports Cloudflare Tunnel and Tailscale (see LOCAL_ONLY_PATHS entries for /api/tunnel/*). When exposed via tunnel:

Attacker sends request to https://<tunnel-domain>/api/mcp/<plugin>/sse

Sets Host: localhost:3000 and Origin: http://localhost:3000

isLocalRequest() returns true

canAccessLocalOnlyRoute() then requires CLI token or (local + JWT)

CLI token is getConsistentMachineId("9r-cli-auth") — a deterministic HMAC of the machine's hardware/OS identifiers

Scenario 2: DNS Rebinding

Attacker controls evil.com DNS, initially resolving to attacker IP

Victim's browser navigates to evil.com (or via iframe/redirect)

DNS rebinding switches evil.com → 127.0.0.1

Subsequent fetch to evil.com:3000/api/mcp/<plugin>/message reaches 9router

Host header is evil.com:3000 — this is blocked by the current check (not in LOOPBACK_HOSTS)

However, if the attacker uses localhost:3000 as the request host via CORS or service worker tricks, and the browser sends Host: localhost:3000, the gate opens

Exploitation (when CLI token is obtained)

Once past the gate, the attacker can:

GET /api/mcp/<plugin>/sse — establish SSE session, get sessionId

POST /api/mcp/<plugin>/message — send arbitrary JSON-RPC to the child process stdin

The child process is one of: npx, node, python, python3, uvx, bunx, bun

Depending on the MCP plugin implementation, this can achieve arbitrary code execution on the host

Steps to Reproduce

Deploy 9router behind a reverse proxy or tunnel

From a remote host, send:

GET /api/mcp/browser/sse HTTP/1.1
Host: localhost:3000
Origin: http://localhost:3000
x-9r-cli-token: <machine-id-derived-token>

Observe: SSE connection established, endpoint event received with message URL

POST arbitrary JSON-RPC to the message endpoint

Impact

An attacker who can reach a proxied/tunneled 9router instance and obtain the deterministic CLI token can bypass the local-only restriction and interact with MCP child processes (node, python, npx, etc.) via stdin. This achieves the same impact as CVE-2026-46339: remote code execution on the host.

The severity is reduced from CVE-2026-46339's CVSS 10.0 because:

Requires proxied/tunneled deployment (not default localhost-only)

Requires obtaining the CLI token (deterministic but not trivially guessable without another primitive)

Remediation

Check actual source IP, not headers. Use request.ip, request.socket.remoteAddress, or a trusted X-Forwarded-For header with known proxy configuration instead of Host/Origin for the local-only gate.

Make CLI token non-deterministic. Generate a random token on first run and persist it, rather than deriving from machine ID. Machine IDs are often predictable or discoverable on cloud infrastructure.

Bind MCP routes to loopback at the network layer. If MCP is local-only by design, the server should bind those routes to 127.0.0.1 only, not rely on middleware header checks.

Credit: @snailsploit

Mitigation

Aliases

References