Security harden your website post Joomla! 3.9.21
Did you see it?
Post Installation Message
After updating an old version of Joomla! CMS to v3.9.21 or later you should be prompted to see the following Post Installation Message.
If you missed it then you can re-read the message when you select Components > Post Installation Messages from the menu at the top of your website's control panel.
What action should you take?
You should edit your website's HTACCESS file as directed in the message.
Need help, just ask.
Web Application Firewall
If you are really serious about hardening website security, get yourself a Web Application Firewall.
We like Akeeba Admin Tools PRO.
Need help configuring Admin Tools, just ask.