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:
The Codex has more information about all of the issues fixed in 4.9.2, if you'd like to learn more.
Download WordPress 4.9.2 or venture over to Dashboard → Updates and click "Update Now." Sites that support automatic background updates are already beginning to update automatically.
Thank you to everyone who contributed to WordPress 4.9.2:
0x6f0, Aaron Jorbin, Andrea Fercia, Andrew Duthie, Andrew Ozz, Blobfolio, Boone Gorges, Caleb Burks, Carolina Nymark, chasewg, Chetan Prajapati, Dion Hulse, Hardik Amipara, ionvv, Jason Caldwell, Jeffrey Paul, Jeremy Felt, Joe McGill, johnschulz, Juhi Patel, Konstantin Obenland, Mark Jaquith, Nilambar Sharma, Peter Wilson, Rachel Baker, Rinku Y, Sergey Biryukov, and Weston Ruter.