Index: wp-signup.php
===================================================================
--- wp-signup.php	(revision 52041)
+++ wp-signup.php	(working copy)
@@ -296,7 +296,7 @@
  *               See wpmu_validate_user_signup() for details.
  */
 function validate_user_form() {
-	return wpmu_validate_user_signup( $_POST['user_name'], $_POST['user_email'] );
+	return wpmu_validate_user_signup( sanitize_text_field($_POST['user_name']), sanitize_email($_POST['user_email']) );
 }
 
 /**