WordPress 4.9.2 is now available. This is a maintenance and security release for all WordPress versions since 3.7, and you need to update your websites immediately.
An XSS vulnerability was discovered in the Flash fallback files in MediaElement, a library that is included with WordPress. Because the Flash files are no longer needed for most use cases, they have been removed from WordPress.
MediaElement has released a new version that contains a fix for the bug, and a WordPress plugin containing the fixed files is available in the plugin repository.
While WordPress.org already started automatic background updates that do not require you to do anything it may take some time for them to get to your website. If it does not update automatically today, do it yourself (if you are confident enough) or contact Dusk Owl for help.