Six security issues affect WordPress versions 5.4 and earlier; version 5.4.1 fixes them, so you’ll want to upgrade. If you haven’t yet updated to 5.4, there are also updated versions of 5.3 and earlier that fix the security issues.
Props to Muaz Bin Abdus Sattar and Jannes who both independently reported an issue where password reset tokens were not properly invalidated
Props to ka1n4t for finding an issue where certain private posts can be viewed unauthenticated
Props to Evan Ricafort for discovering an XSS issue in the Customizer
Props to Ben Bidner from the WordPress Security Team who discovered an XSS issue in the search block
Props to Nick Daugherty from WPVIP.com / WordPress Security Team who discovered an XSS issue in wp-object-cache
Props to Ronnie Goodrich (Kahoots) and Jason Medeiros who independently reported an XSS issue in file uploads.
Additionally, an authenticated XSS issue in the block editor was discovered by Nguyen the Duc in WordPress 5.4 RC1 and RC2. It was fixed in 5.4 RC5. We wanted to be sure to give credit and thank them for all of their work in making WordPress more secure.