Fix reachable security issues found in the audit

internal/agents/preview.go

@@ -112,6 +112,26 @@ func (m *Manager) servePreview(port int) error {
 		ln.Close()
 		return err
 	}
+	// HTTP previews also arrive through the app's loopback forwards. Reject
+	// attacker-controlled hostnames before proxying: DNS rebinding makes those
+	// requests same-origin, so cross-origin protection alone cannot stop them.
+	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+	defer cancel()
+	status, err := lc.StatusWithoutPeers(ctx)
+	if err != nil {
+		ln.Close()
+		return fmt.Errorf("get preview hostnames: %w", err)
+	}
+	hosts := map[string]bool{}
+	if status.Self != nil {
+		name := strings.ToLower(strings.TrimSuffix(status.Self.DNSName, "."))
+		hosts[name] = true
+		hosts[strings.SplitN(name, ".", 2)[0]] = true
+	}
+	for _, ip := range status.TailscaleIPs {
+		hosts[ip.String()] = true
+	}
+
 	tracked := &previewListener{Listener: ln, conns: map[net.Conn]bool{}}
 	m.previewMu.Lock()
 	m.previewListeners[port] = tracked
@@ -126,6 +146,16 @@ func (m *Manager) servePreview(port int) error {
 		FlushInterval: -1,
 	}
 	handler := http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		host := r.Host
+		if h, _, err := net.SplitHostPort(host); err == nil {
+			host = h
+		}
+		host = strings.ToLower(strings.TrimSuffix(host, "."))
+		ip := net.ParseIP(host)
+		if host == "" || !(hosts[host] || host == "localhost" || strings.HasSuffix(host, ".localhost") || (ip != nil && ip.IsLoopback())) {
+			http.Error(w, "forbidden preview host", http.StatusForbidden)
+			return
+		}
 		who, err := lc.WhoIs(r.Context(), r.RemoteAddr)
 		if err != nil {
 			http.Error(w, err.Error(), http.StatusForbidden)