Today WordPress team released WordPress version 5.4.2. This consist 23 fixes and enhancements. Plus, it adds a number of security fixes.
Same security related bugfixes have been made in older WordPress series also what have been affected with this issuses (5.3, 5.2, 5.1, 4.9).
It is highly recommended to update WordPress to latest version!
It is possible to download new version here, or you can update your page manually by visiting in admin panel Dashboard → Updates and click Update Now.
If your site support automatic background updates, they’ve already started the update process.