genieacs-mcp is an MCP server for GenieACS written in Go. Prior to 0.3.2, the Streamable HTTP transport in cmd/server/main.go creates an unauthenticated /mcp listener on the default MCP_LISTEN_ADDR value 127.0.0.1:8080 when MCP_AUTH_TOKEN is unset and the httpSrv.Start(addr) branch does not validate the Host or Origin headers. A malicious website can use DNS rebinding to send browser requests with attacker-controlled Host and Origin values to the loopback listener, initialize an MCP session, list tools, and invoke operations against the GenieACS NBI configured by ACS_URL. Successful exploitation can expose or modify CPE management state, including device reboots, firmware tasks, TR-069 parameter changes, presets, provisions, tags, connection requests, and task operations. The npm wrapper is not affected because it forces TRANSPORT=stdio and does not expose an HTTP listener. This issue is fixed in version 0.3.2.

Affected: Go: 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 defau…