Resolved
cPanel has issued an official fix which apparently resolves the issue.
We have applied the patches suggested and restored control panel access.
We have also reset any logged-in sessions to err on the side of caution.
Monitoring
We have taken steps to limit access - as far as we can see, no compromise has taken place.
We will await cPanel's official update and fix to restore access to affected areas such as control panels and WHM.
Identified
A cPanel Zero day authentication exploit has been found - out of an abundance of caution while cPanel work on a fix, we have taken action to protect systems from unauthenticated access to the affected area of the control panel.
This is an issue with cPanel itself, and not an issue with hosting or your websites.
This is tracked by cPanel themselves here: https://support.cpanel.net/hc/en-us/articles/40073787579671-Critical-Vulnerability-with-cPanel-WHM-Login-Authentication
