GHSA-6g2f-w7g3-77vf 9router has an Incomplete Fix: Local-Only Access Gate Bypass in 9router via Host Header SpoofING
SummaryThe 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`: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 ScenarioScenario 1: Cloudflare Tunnel / Tailscale Funnel9router 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 identifiersScenario 2: DNS RebindingAttacker controls `evil.com` DNS, initially resolving to attacker IPVictim'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 opensExploitation (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 stdinThe 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 hostSteps to ReproduceDeploy 9router behind a reverse proxy or tunnelFrom a remote host, send:Observe: SSE connection established, `endpoint` event received with message URLPOST arbitrary JSON-RPC to the message endpointImpactAn 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
CVSS v3.1 score: 7.5