WordPress 4.9.1 Security and Maintenance Release

WordPress 4.9.1 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.

WordPress versions 4.9 and earlier are affected by four security issues which could potentially be exploited as part of a multi-vector attack. As part of the core team's ongoing commitment to security hardening, the following fixes have been implemented in 4.9.1:

Use a properly generated hash for the newbloguser key instead of a determinate substring.
Add escaping to the language attributes used on html elements.
Ensure the attributes of enclosures are correctly escaped in RSS and Atom feeds.
Remove the ability to upload JavaScript files for users who do not have the unfiltered_html capability.

Thank you to the reporters of these issues for practicing responsible security disclosure: Rahul Pratap Singh and John Blackbourn.

Eleven other bugs were fixed in WordPress 4.9.1. Particularly of note were:

Issues relating to the caching of theme template files.
A MediaElement JavaScript error preventing users of certain languages from being able to upload media files.
The inability to edit theme and plugin files on Windows based servers.

This post has more information about all of the issues fixed in 4.9.1 if you'd like to learn more.

Download WordPress 4.9.1 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 4.9.1:

Alain Schlesser, Andrea Fercia, Angelika Reisiger, Blobfolio, bobbingwide, Chetan Prajapati, Dion Hulse, Dominik Schilling (ocean90), edo888, Erich Munz, Felix Arntz, Florian TIAR, Gary Pendergast, Igor Benic, Jeff Farthing, Jeffrey Paul, jeremyescott, Joe McGill, John Blackbourn, johnpgreen, Kelly Dwan, lenasterg, Marius L. J., Mel Choyce, Mário Valney, natacado, odyssey, precies, Saša, Sergey Biryukov, and Weston Ruter.