diff --git a/src/wp-includes/class-walker-nav-menu.php b/src/wp-includes/class-walker-nav-menu.php
index 5e2335a..1a5e475 100644
--- a/src/wp-includes/class-walker-nav-menu.php
+++ b/src/wp-includes/class-walker-nav-menu.php
@@ -172,7 +172,11 @@ class Walker_Nav_Menu extends Walker {
 		$atts           = array();
 		$atts['title']  = ! empty( $item->attr_title ) ? $item->attr_title : '';
 		$atts['target'] = ! empty( $item->target ) ? $item->target : '';
-		$atts['rel']    = ! empty( $item->xfn ) ? $item->xfn : '';
+		if ( '_blank' === $item->target && empty( $item->xfn ) ) {
+			$atts['rel'] = 'noopener';
+		} else {
+			$atts['rel'] = ! empty( $item->xfn ) ? $item->xfn : '';
+		}
 		$atts['href']   = ! empty( $item->url ) ? $item->url : '';
 
 		/**