**** B E W A R E ****
There is a security-bug in the first versions of macOS 10.13 High Sierra.
In version 10.13 (10.13.0) and 10.13.1 of macOS High Sierra, it is possible to take control of your Mac by just logging in as ‘root’ and leaving the password blank…
Upgrade to macOS High_Sierra 10.13.2 or newer !
Apple has fixed this by releasing an updated version of High_Sierra, so for your own safety, you should upgrade your macOS !
Note : this security-bug was introduced in High_Sierra, so it is no problem in earlier versions of macOS and OSX, including macOS 10.12 Sierra