fix(mcp): listen on localhost by default

MCP.md

@@ -121,7 +121,7 @@ nais alpha mcp serve [flags]
 | Flag | Default | Description |
 |------|---------|-------------|
 | `--transport`, `-t` | `stdio` | Transport: `stdio`, `http`, or `sse` |
-| `--listen`, `-l` | `:8080` | Listen address (for http/sse) |
+| `--listen`, `-l` | `127.0.0.1:8080` | Listen address (for http/sse) |
 | `--rate-limit`, `-r` | `10` | Max requests per minute (0 = unlimited) |
 | `--log-file` | - | Write logs to file instead of stderr |
 

internal/mcp/command/flag/flag.go

@@ -23,7 +23,7 @@ func (t *Transport) AutoComplete(context.Context, *naistrix.Arguments, string, a
 type Serve struct {
 	*MCP
 	Transport  Transport `name:"transport" usage:"Transport type (stdio, http, sse)."`
-	ListenAddr string    `name:"listen" short:"l" usage:"Address to listen on (for http/sse transports)."`
+	ListenAddr string    `name:"listen" short:"l" usage:"Address to listen on (for http/sse transports). The server has no authentication; only bind to non-loopback addresses on trusted networks."`
 	RateLimit  int       `name:"rate-limit" short:"r" usage:"Maximum requests per minute (0 = unlimited)."`
 	LogFile    string    `name:"log-file" usage:"Write logs to file instead of stderr."`
 }

internal/mcp/command/mcp.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"io"
 	"log/slog"
+	"net"
 	"os"
 
 	alpha "github.com/nais/cli/internal/alpha/command/flag"
@@ -29,7 +30,7 @@ func serveCommand(parentFlags *flag.MCP) *naistrix.Command {
 	flags := &flag.Serve{
 		MCP:        parentFlags,
 		Transport:  "stdio",
-		ListenAddr: ":8080",
+		ListenAddr: "127.0.0.1:8080",
 		RateLimit:  25,
 	}
 
@@ -43,10 +44,10 @@ Examples:
   nais alpha mcp serve
 
   # Start with HTTP transport on a specific port
-  nais alpha mcp serve --transport http --listen :8080
+  nais alpha mcp serve --transport http --listen 127.0.0.1:8080
 
   # Start with SSE transport
-  nais alpha mcp serve --transport sse --listen :8080
+  nais alpha mcp serve --transport sse --listen 127.0.0.1:8080
 
   # Restrict to specific teams
   nais alpha mcp serve --team my-team --team other-team
@@ -106,8 +107,24 @@ func runServe(ctx context.Context, flags *flag.Serve, out *naistrix.OutputWriter
 		out.Println("Starting MCP server with transport:", string(flags.Transport))
 		if flags.Transport == "http" || flags.Transport == "sse" {
 			out.Println("Listening on:", flags.ListenAddr)
+			if !isLoopbackAddr(flags.ListenAddr) {
+				out.Println("WARNING: the MCP server has no authentication and uses your Nais credentials. Anyone who can reach this address can query the Nais API as you.")
+			}
 		}
 	}
 
 	return server.Serve(ctx)
 }
+
+// isLoopbackAddr reports whether addr (host:port) only binds to a loopback interface.
+func isLoopbackAddr(addr string) bool {
+	host, _, err := net.SplitHostPort(addr)
+	if err != nil {
+		return false
+	}
+	if host == "localhost" {
+		return true
+	}
+	ip := net.ParseIP(host)
+	return ip != nil && ip.IsLoopback()
+}

internal/mcp/command/mcp_test.go

@@ -0,0 +1,20 @@
+package command
+
+import "testing"
+
+func TestIsLoopbackAddr(t *testing.T) {
+	for addr, want := range map[string]bool{
+		"127.0.0.1:8080": true,
+		"localhost:8080": true,
+		"[::1]:8080":     true,
+		":8080":          false,
+		"0.0.0.0:8080":   false,
+		"10.0.0.1:8080":  false,
+		"example.com:80": false,
+		"invalid":        false,
+	} {
+		if got := isLoopbackAddr(addr); got != want {
+			t.Errorf("isLoopbackAddr(%q) = %v, want %v", addr, got, want)
+		}
+	}
+}

internal/mcp/options.go

@@ -48,7 +48,7 @@ type Option func(*Options)
 func DefaultOptions() *Options {
 	return &Options{
 		Transport:  TransportStdio,
-		ListenAddr: ":8080",
+		ListenAddr: "127.0.0.1:8080",
 		RateLimit:  10,
 		Logger:     slog.Default(),
 	}