diff --git src/wp-includes/script-loader.php src/wp-includes/script-loader.php
index f9813ed87c..7c960a2d91 100644
--- src/wp-includes/script-loader.php
+++ src/wp-includes/script-loader.php
@@ -144,7 +144,7 @@ function wp_default_packages_vendor( $scripts ) {
 		'moment',
 		sprintf(
 			"moment.updateLocale( '%s', %s );",
-			get_user_locale(),
+			esc_js( get_user_locale() ),
 			wp_json_encode(
 				array(
 					'months'         => array_values( $wp_locale->month ),