WordPress.org developers have just released the WordPress 4.9.2 update (Billy Tipton) to fix some security gaps.
The change log starts by saying that all older versions of WordPress are affected by a vulnerability XSS in the Flash fallback files of MediaElement 4.x, a library included in the WordPress 4.9.
In addition to the above security issue, the new WorPress 4.9.2 contains other 21 bug fixes that you can see in official announcement.
Of course, as with any other supplier regardless of supplier, immediate upgrading is recommended. You can make a new installation with the package, or upgrade automatically through the admin panel of the web application.