fix(mcp): send the control token only to a loopback address (#3814)

crates/mcp/src/backend.rs

@@ -53,6 +53,15 @@ impl Remote {
         self.conn = None;
         let mut last = std::io::Error::new(std::io::ErrorKind::NotFound, format!("cannot resolve {}", self.addr));
         for sa in self.addr.to_socket_addrs()? {
+            // The token goes over plain TCP, so only to this machine: a network peer could read it,
+            // and the app's control port listens on loopback only.
+            if !sa.ip().is_loopback() {
+                last = std::io::Error::new(
+                    std::io::ErrorKind::PermissionDenied,
+                    format!("{sa} is not a loopback address: the control token is sent only to 127.0.0.1 or ::1"),
+                );
+                continue;
+            }
             match TcpStream::connect_timeout(&sa, Duration::from_secs(2)) {
                 Ok(s) => {
                     s.set_nodelay(true).ok();

crates/mcp/src/tests.rs

@@ -482,6 +482,9 @@ fn remote_backend_over_tcp() {
     drop(s);
     handle.join().unwrap();
     assert!(Remote::connect_with_token("127.0.0.1:1", "x".into()).is_err());
+    // A network address is refused before anything, the token included, is sent.
+    let err = Remote::connect_with_token("192.0.2.1:7979", "x".into()).err().unwrap();
+    assert_eq!(err.kind(), std::io::ErrorKind::PermissionDenied, "{err}");
 }
 
 #[test]