fix: restrict supervisor drain to loopback

server/supervisor/supervisor.go

@@ -7,7 +7,9 @@ import (
 	"context"
 	"encoding/json"
 	"fmt"
+	"net"
 	"net/http"
+	"net/url"
 	"os"
 	"os/exec"
 	"strconv"
@@ -63,9 +65,32 @@ func New(config Config) (*Supervisor, error) {
 	if config.HTTPClient == nil {
 		config.HTTPClient = http.DefaultClient
 	}
+	if (config.DrainURL == "") != (config.DrainToken == "") {
+		return nil, fmt.Errorf("drain URL and token must be configured together")
+	}
+	if config.DrainURL != "" {
+		if err := validateLocalDrainURL(config.DrainURL); err != nil {
+			return nil, err
+		}
+	}
 	return &Supervisor{config: config, client: config.HTTPClient}, nil
 }
 
+func validateLocalDrainURL(raw string) error {
+	parsed, err := url.Parse(raw)
+	if err != nil || (parsed.Scheme != "http" && parsed.Scheme != "https") || parsed.Host == "" || parsed.Path == "" || parsed.User != nil || parsed.RawQuery != "" || parsed.Fragment != "" {
+		return fmt.Errorf("drain URL must be a loopback HTTP endpoint")
+	}
+	host := parsed.Hostname()
+	if host != "localhost" {
+		ip := net.ParseIP(host)
+		if ip == nil || !ip.IsLoopback() {
+			return fmt.Errorf("drain URL must be a loopback HTTP endpoint")
+		}
+	}
+	return nil
+}
+
 // Start launches the process and marks Agones Ready only after the explicit
 // readiness probe succeeds. No stdout/log scraping is used. With no SDK URL,
 // this is direct/Compose mode and the command is simply started.