diff --git src/wp-includes/class-walker-nav-menu.php src/wp-includes/class-walker-nav-menu.php
index f46f14c177..25f0042ca6 100644
--- src/wp-includes/class-walker-nav-menu.php
+++ 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 noreferrer';
+		} else {
+			$atts['rel'] = $item->xfn;
+		}
 		$atts['href']         = ! empty( $item->url ) ? $item->url : '';
 		$atts['aria-current'] = $item->current ? 'page' : '';
 
diff --git tests/phpunit/tests/menu/walker-nav-menu.php tests/phpunit/tests/menu/walker-nav-menu.php
new file mode 100644
index 0000000000..e60a9550d4
--- /dev/null
+++ tests/phpunit/tests/menu/walker-nav-menu.php
@@ -0,0 +1,69 @@
+<?php
+/**
+ * @group navmenus
+ * @group walker
+ */
+class Tests_Walker_Nav_Menu extends WP_UnitTestCase {
+
+	/**
+	 * @var \Walker_Nav_Menu The instance of the walker.
+	 */
+	public $walker;
+
+	/**
+	 * Setup.
+	 */
+	public function setUp() {
+		global $_wp_nav_menu_max_depth;
+
+		parent::setUp();
+
+		/** Walker_Nav_Menu_Edit class */
+		require_once ABSPATH . 'wp-includes/class-walker-nav-menu.php';
+		$this->walker = new Walker_Nav_Menu();
+
+		$this->_wp_nav_menu_max_depth = $_wp_nav_menu_max_depth;
+		parent::setUp();
+	}
+
+	/**
+	 * Tear down
+	 */
+	public function tearDown() {
+		global $_wp_nav_menu_max_depth;
+
+		$_wp_nav_menu_max_depth = $this->_wp_nav_menu_max_depth;
+		parent::tearDown();
+	}
+
+	/**
+	 * Tests when an items target it _blank, that rel="'noopener noreferrer" is added.
+	 *
+	 * @ticket #43290
+	 */
+	public function test_noopener_no_referrer_for_target_blank() {
+		$expected   = '';
+		$post_id    = $this->factory->post->create();
+		$post_title = get_the_title( $post_id );
+
+		$item = array(
+			'ID'        => $post_id,
+			'object_id' => $post_id,
+			'title'     => $post_title,
+			'target'    => '_blank',
+			'xfn'       => '',
+			'current'   => false,
+		);
+
+		$args = array(
+			'before'      => '',
+			'after'       => '',
+			'link_before' => '',
+			'link_after'  => '',
+		);
+
+		$this->walker->start_el( $expected, (object) $item, 0, (object) $args );
+
+		$this->assertSame( "<li id=\"menu-item-{$post_id}\" class=\"menu-item-{$post_id}\"><a target=\"_blank\" rel=\"noopener noreferrer\">{$post_title}</a>", $expected );
+	}
+}