--- id: CVE-2026-49353 title: >- 9router has an Incomplete Fix: Local-Only Access Gate Bypass in 9router via Host Header SpoofING summary: >- 9router has an Incomplete Fix: Local-Only Access Gate Bypass in 9router via Host Header SpoofING severity: high cvss: 7.5 cwe: - CWE-290 vendor: 9router product: 9router ecosystem: npm affected: - 9router <= 0.4.55 published: '2026-07-02' updated: '2026-07-02' source: GHSA sourceUrl: 'https://github.com/advisories/GHSA-6g2f-w7g3-77vf' references: - url: 'https://github.com/decolua/9router/security/advisories/GHSA-6g2f-w7g3-77vf' - url: >- https://github.com/decolua/9router/commit/5e1c1261368e06dced1cbc650684561b2c8844db - url: >- https://github.com/decolua/9router/commit/bb86808582067e4fc6f004508a919efb9970d1d5 - url: 'https://github.com/advisories/GHSA-6g2f-w7g3-77vf' tags: - ghsa - npm ingestedAt: '2026-07-02T21:44:45.079Z' epss: 0.00357 epssPercentile: 0.26656 --- ## Overview ## 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`: ```javascript 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; } } return true; } ``` 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: 1. Attacker sends request to `https:///api/mcp//sse` 2. Sets `Host: localhost:3000` and `Origin: http://localhost:3000` 3. `isLocalRequest()` returns `true` 4. `canAccessLocalOnlyRoute()` then requires CLI token or (local + JWT) 5. CLI token is `getConsistentMachineId("9r-cli-auth")` — a deterministic HMAC of the machine's hardware/OS identifiers ### Scenario 2: DNS Rebinding 1. Attacker controls `evil.com` DNS, initially resolving to attacker IP 2. Victim's browser navigates to `evil.com` (or via iframe/redirect) 3. DNS rebinding switches `evil.com` → `127.0.0.1` 4. Subsequent fetch to `evil.com:3000/api/mcp//message` reaches 9router 5. `Host` header is `evil.com:3000` — this is **blocked** by the current check (not in LOOPBACK_HOSTS) 6. 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: 1. `GET /api/mcp//sse` — establish SSE session, get `sessionId` 2. `POST /api/mcp//message` — send arbitrary JSON-RPC to the child process stdin 3. The child process is one of: `npx`, `node`, `python`, `python3`, `uvx`, `bunx`, `bun` 4. Depending on the MCP plugin implementation, this can achieve arbitrary code execution on the host ## Steps to Reproduce 1. Deploy 9router behind a reverse proxy or tunnel 2. From a remote host, send: ```http GET /api/mcp/browser/sse HTTP/1.1 Host: localhost:3000 Origin: http://localhost:3000 x-9r-cli-token: ``` 3. Observe: SSE connection established, `endpoint` event received with message URL 4. 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 1. **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. 2. **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. 3. **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 ## Affected packages - `9router <= 0.4.55` ## Remediation Refer to the advisory for the patched release.