Upstream announcement:

WordPress 4.9.2 is now available. This is a security and maintenance release for all versions since WordPress 3.7. We strongly encourage you to update your sites 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.

Thank you to the reporters of this issue for practicing responsible security disclosure: Enguerran Gillier and Widiz.

21 other bugs were fixed in WordPress 4.9.2. Particularly of note were:

• JavaScript errors that prevented saving posts in Firefox have been fixed.
• The previous taxonomy-agnostic behavior of get_category_link() and category_description() was restored.
• Switching themes will now attempt to restore previous widget assignments, even when there are no sidebars to map.

The Codex has more information about all of the issues fixed in 4.9.2, if you'd like to learn more.