WordPress has released a new maintenance/security update today to WordPress core: version 4.9.1. Version 4.9.1 comes with minor fixes which the WordPress core team deemed impactful enough to warrant a release. The release includes both security hardening fixes and a handful of smaller bugs.
As with all security-related updates, WordPress has released patches for older functional versions as well. As a WP Engine customer, no action is required. Our team is already working to push out automatic updates to your site(s) to ensure they are in the next secured version. You will receive an email notification when the update has been completed. Our team is always attentive to our promise to provide a secure WordPress experience for all WP Engine customers.