Index: wp-includes/class-wp-customize-manager.php
===================================================================
--- wp-includes/class-wp-customize-manager.php	(revision 21018)
+++ wp-includes/class-wp-customize-manager.php	(working copy)
@@ -78,6 +78,9 @@
 	 * @since 3.4.0
 	 */
 	public function setup_theme() {
+		if ( is_admin() && ! defined( 'DOING_AJAX' ) )
+   			auth_redirect();
+
 		send_origin_headers();
 
 		$this->original_stylesheet = get_stylesheet();
@@ -964,4 +967,4 @@
 		return '#' . $unhashed;
 
 	return $color;
-}
\ No newline at end of file
+}