Index: src/wp-includes/bookmark-template.php
===================================================================
--- src/wp-includes/bookmark-template.php	(revision 51519)
+++ src/wp-includes/bookmark-template.php	(working copy)
@@ -101,7 +101,20 @@
 		if ( '' !== $title ) {
 			$title = ' title="' . $title . '"';
 		}
+
 		$rel = $bookmark->link_rel;
+		if ( '_blank' === $bookmark->link_target ) {
+			if ( ! empty( $rel ) ) {
+				$rels   = explode( ' ', $rel );
+				if ( false === in_array( 'noopener', $rels, true ) ) {
+					$rels[] = 'noopener';
+				}
+				$rel    = implode( ' ', $rels );
+			} else {
+				$rel = 'noopener';
+			}
+		}
+
 		if ( '' !== $rel ) {
 			$rel = ' rel="' . esc_attr( $rel ) . '"';
 		}