Index: src/wp-signup.php
===================================================================
--- src/wp-signup.php	(revision 34747)
+++ src/wp-signup.php	(working copy)
@@ -37,6 +37,23 @@
 	die();
 }
 
+$network_site_url = network_site_url( 'wp-signup.php' );
+/**
+ * Filter the signup location for this WordPress installation.
+ *
+ * @since 4.3.0
+ *
+ * @param string $network_site_url URL of the signup page for WordPress.
+ */
+$signup_location = apply_filters( 'wp_signup_location', $network_site_url );
+$signup_location_parts = explode( '?', $signup_location );
+
+// Redirect to the true signup location if the URL differs from default.
+if ( $signup_location_parts[0] !== $network_site_url ) {
+	wp_redirect( $signup_location );
+	exit;
+}
+
 // Fix for page title
 $wp_query->is_404 = false;