CVE-2026-49353: 9router has an Incomplete Fix Local-Only Access Gate Bypass Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-6g2f-w7g3-77vf",
  "modified" : "2026-07-02T21:13:19Z",
  "published" : "2026-07-02T21:13:19Z",
  "aliases" : [ "CVE-2026-49353" ],
  "summary" : "9router has an Incomplete Fix: Local-Only Access Gate Bypass in 9router via Host Header SpoofING",
  "details" : "## Summary\n\nThe 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.\n\nA 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.\n\nThis 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.\n\n## Root Cause\n\n`isLocalRequest()` at `src/dashboardGuard.js:93-101`:\n\n```javascript\nfunction isLocalRequest(request) {\n  if (!isLoopbackHostname(request.headers.get(\"host\"))) return false;\n  const origin = request.headers.get(\"origin\");\n  if (origin) {\n    try {\n      if (!isLoopbackHostname(new URL(origin).hostname)) return false;\n    } catch { return false; }\n  }\n  return true;\n}\n```\n\nThis 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.\n\n## Attack Scenario\n\n### Scenario 1: Cloudflare Tunnel / Tailscale Funnel\n\n9router natively supports Cloudflare Tunnel and Tailscale (see `LOCAL_ONLY_PATHS` entries for `/api/tunnel/*`). When exposed via tunnel:\n\n1. Attacker sends request to `https://<tunnel-domain>/api/mcp/<plugin>/sse`\n2. Sets `Host: localhost:3000` and `Origin: http://localhost:3000`\n3. `isLocalRequest()` returns `true`\n4. `canAccessLocalOnlyRoute()` then requires CLI token or (local + JWT)\n5. CLI token is `getConsistentMachineId(\"9r-cli-auth\")` — a deterministic HMAC of the machine's hardware/OS identifiers\n\n### Scenario 2: DNS Rebinding\n\n1. Attacker controls `evil.com` DNS, initially resolving to attacker IP\n2. Victim's browser navigates to `evil.com` (or via iframe/redirect)\n3. DNS rebinding switches `evil.com` → `127.0.0.1`\n4. Subsequent fetch to `evil.com:3000/api/mcp/<plugin>/message` reaches 9router\n5. `Host` header is `evil.com:3000` — this is **blocked** by the current check (not in LOOPBACK_HOSTS)\n6. 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\n\n### Exploitation (when CLI token is obtained)\n\nOnce past the gate, the attacker can:\n\n1. `GET /api/mcp/<plugin>/sse` — establish SSE session, get `sessionId`\n2. `POST /api/mcp/<plugin>/message` — send arbitrary JSON-RPC to the child process stdin\n3. The child process is one of: `npx`, `node`, `python`, `python3`, `uvx`, `bunx`, `bun`\n4. Depending on the MCP plugin implementation, this can achieve arbitrary code execution on the host\n\n## Steps to Reproduce\n\n1. Deploy 9router behind a reverse proxy or tunnel\n2. From a remote host, send:\n\n```http\nGET /api/mcp/browser/sse HTTP/1.1\nHost: localhost:3000\nOrigin: http://localhost:3000\nx-9r-cli-token: <machine-id-derived-token>\n```\n\n3. Observe: SSE connection established, `endpoint` event received with message URL\n4. POST arbitrary JSON-RPC to the message endpoint\n\n## Impact\n\nAn 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.\n\nThe severity is reduced from CVE-2026-46339's CVSS 10.0 because:\n- Requires proxied/tunneled deployment (not default localhost-only)\n- Requires obtaining the CLI token (deterministic but not trivially guessable without another primitive)\n\n## Remediation\n\n1. **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.\n\n2. **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.\n\n3. **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.\n\n\nCredit: @snailsploit",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:L/I:H/A:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "npm",
      "name" : "9router"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "last_affected" : "0.4.55"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/decolua/9router/security/advisories/GHSA-6g2f-w7g3-77vf"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/decolua/9router/commit/5e1c1261368e06dced1cbc650684561b2c8844db"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/decolua/9router/commit/bb86808582067e4fc6f004508a919efb9970d1d5"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/decolua/9router"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-290" ],
    "severity" : "HIGH",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-07-02T21:13:19Z",
    "nvd_published_at" : null
  }
}