Ticket #17630: 17630.1.diff

src/wp-signup.php

@@ -32 +32 @@
         die();
 }
 
+$signup_location = apply_filters( 'wp_signup_location', network_site_url( 'wp-signup.php' ) );
+$signup_location_parts = explode( '?', $signup_location );
+if ( $signup_location_parts[0] !== network_site_url( 'wp-signup.php' ) ) {
+        wp_redirect( $signup_location );
+        die();
+}
+
 // Fix for page title
 $wp_query->is_404 = false;