This is not always due to a hack, mostly, it is a site administrators failure.
I have had a spate of new Users appearing in my User Manager.
I am the only authorised user on my sites (Super User) - so how do these idiot spammers get in; and how to block them in future?
I've received email messages from my website, telling me that a new user has registered.
1. There is no user registration form on the website
2. These appear to be hacks
The symptom checklist is as follows: