The WordPress core team has released an emergency release of WordPress 5.5.3, just one day after the release of version 5.5.2. This emergency release was done to remedy an issue introduced in WordPress 5.5.2 making it impossible to install WordPress on a brand new website without a database connection configured. In preparing for this emergency release, a second issue caused a number of sites to be erroneously updated to version 5.5.3-alpha.
According to the release notes, between approximately 15:30 and 16:00 UTC on October 30, the WordPress auto-update system updated some sites from version 5.5.2 to 5.5.3-alpha. This occurred because the WordPress Core team disabled the download of the 5.5.2 release in an attempt to prevent new users from using this version. By disabling the download for 5.5.2, the wordpress.org API returned the alpha version 5.5.3-alpha-49449 as the version to which WordPress should update.
An analysis of the 5.5.3-alpha-49449 release found little difference between the WordPress 5.5.2 release and WordPress 5.5.3-alpha-49449 as much of the core functionality is the same. No reported site functionality was lost due to the error. However, with that autoupdate, a number of additional Twenty- themes were installed along with the Akismet plugin.
To fix both issues, the Core team initially re-enabled download 5.5.2 to prevent sites from updating to the alpha version followed by the emergency release of WordPress 5.5.3 to address the issue which prevented new installations.
What Should I Do?
If your site was updated to WordPress 5.5.3-alpha, you may have additional themes installed on your site. You might also have Akismet installed. These themes and plugin were not activated if installed as a part of the pre-release package. Check your themes and plugin installations. No other plugins would have been installed or removed.
Update your sites normally to WordPress 5.5.3, just as you would for any other WordPress update. If you are allowing your site to autoupdate, the 5.5.3 version may already be installed.
If you had not yet updated to WordPress 5.5.2, updating to 5.5.3 is essentially the same update with a minor fix. Updating your site is safe to do.