diff --git a/src/tether/cli.py b/src/tether/cli.py
index dca60bd..e2d7a22 100644
--- a/src/tether/cli.py
+++ b/src/tether/cli.py
@@ -1717,11 +1717,39 @@ def guard(
         raise typer.Exit(1)
 
 
+_LOOPBACK_HOSTS = {"127.0.0.1", "localhost", "::1"}
+
+
+def _warn_insecure_bind(host: str, api_key: str) -> None:
+    """Loudly warn when binding a non-loopback interface with no api_key.
+
+    The default host is 127.0.0.1 (localhost only). If an operator explicitly
+    binds 0.0.0.0 (or any non-loopback address) WITHOUT --api-key, the robot is
+    drivable by anyone on the network — surface that, don't let it pass silent.
+    """
+    if host in _LOOPBACK_HOSTS:
+        return
+    if api_key:
+        return
+    err_console.print(
+        f"[bold yellow]⚠ SECURITY:[/bold yellow] serving on [bold]{host}[/bold] "
+        f"(reachable from the network) with [bold]no --api-key[/bold]. Anyone who "
+        f"can reach this host can send /act commands to the robot.\n"
+        f"  Set [cyan]--api-key <key>[/cyan], or bind localhost only with "
+        f"[cyan]--host 127.0.0.1[/cyan] (the default).",
+        style=None, markup=True,
+    )
+
+
 @app.command()
 def serve(
     export_dir: str = typer.Argument(help="Path to exported model directory"),
     port: int = typer.Option(8000, help="Server port"),
-    host: str = typer.Option("0.0.0.0", help="Server host"),
+    host: str = typer.Option(
+        "127.0.0.1",
+        help="Server host. Defaults to localhost-only; pass 0.0.0.0 to expose "
+             "on the network (use --api-key when you do).",
+    ),
     transport: str = typer.Option(
         "http",
         "--transport",
@@ -2650,6 +2678,8 @@ def _run_mcp_http():
             f"(streamable-http)[/bold green]"
         )
 
+    _warn_insecure_bind(host, api_key)
+
     if transport == "zmq":
         console.print("[bold green]Starting ZMQ server...[/bold green]")
         from tether.runtime.transports.zmq.factory import create_zmq_server
@@ -4122,7 +4152,11 @@ def go(
         help="Where to cache weights. Default: ~/.cache/tether/models/<id>/",
     ),
     port: int = typer.Option(8000, "--port", help="HTTP port for /act + /health"),
-    host: str = typer.Option("0.0.0.0", "--host"),
+    host: str = typer.Option(
+        "127.0.0.1", "--host",
+        help="Server host. Defaults to localhost-only; pass 0.0.0.0 to expose "
+             "on the network (use --api-key when you do).",
+    ),
     api_key: str = typer.Option("", "--api-key", help="If set, /act requires X-Tether-Key header"),
     dry_run: bool = typer.Option(
         False,
@@ -4372,6 +4406,7 @@ def go(
         embodiment_config=embodiment_cfg,
         api_key=api_key or None,
     )
+    _warn_insecure_bind(host, api_key)
     import uvicorn
     uvicorn.run(app_instance, host=host, port=port, log_level="info")
 
diff --git a/tests/test_serve_bind_security.py b/tests/test_serve_bind_security.py
new file mode 100644
index 0000000..9d4e161
--- /dev/null
+++ b/tests/test_serve_bind_security.py
@@ -0,0 +1,47 @@
+"""serve/go bind to localhost by default + warn on insecure public binds."""
+from __future__ import annotations
+
+import inspect
+from unittest.mock import MagicMock
+
+from tether import cli
+
+
+def _default(func, param: str):
+    p = inspect.signature(func).parameters[param].default
+    # typer.Option(...) returns an OptionInfo whose .default holds the value.
+    return getattr(p, "default", p)
+
+
+def test_serve_and_go_default_host_is_localhost():
+    assert _default(cli.serve, "host") == "127.0.0.1"
+    assert _default(cli.go, "host") == "127.0.0.1"
+
+
+def test_warns_on_public_bind_without_api_key(monkeypatch):
+    mock = MagicMock()
+    monkeypatch.setattr(cli, "err_console", mock)
+    cli._warn_insecure_bind("0.0.0.0", "")
+    assert mock.print.called
+
+
+def test_no_warn_on_loopback(monkeypatch):
+    for host in ("127.0.0.1", "localhost", "::1"):
+        mock = MagicMock()
+        monkeypatch.setattr(cli, "err_console", mock)
+        cli._warn_insecure_bind(host, "")
+        assert not mock.print.called, host
+
+
+def test_no_warn_when_api_key_set(monkeypatch):
+    mock = MagicMock()
+    monkeypatch.setattr(cli, "err_console", mock)
+    cli._warn_insecure_bind("0.0.0.0", "a-secret-key")
+    assert not mock.print.called
+
+
+def test_warns_on_arbitrary_public_ip(monkeypatch):
+    mock = MagicMock()
+    monkeypatch.setattr(cli, "err_console", mock)
+    cli._warn_insecure_bind("192.168.1.10", "")
+    assert mock.print.called