Two pretty serious vulnerabilities have been found in PHP, diclosed between yesterday and today.
The issue has already been fixed, and the problem is how fast your web host will update their PHP installation. All the version 8 releases of PHP are patched to fix the vulnerabilities.
Sitely use PHP for some features, but even if it didn’t, PHP is generally added to websites automatically by web hosts, and can be sufficient to be vulnerable.
This is a big enough problem that your web host will certainly update very quickly, so I wouldn’t fret over it. You don’t need to do anything in Sitely, once the web hosts updates the server, you’re fine.
What I would suggest doing, if your web host control panel allows for it, is setting your website’s PHP version to 8.x. There will be no fix for PHP 7.x, and Sitely works fine with both PHP 7 and PHP 8, so setting your website to use PHP 8.x ensures that as soon as your web host updates their PHP installation, your website gets it.