Update 2:52PM Thursday November 29, 2017:
Apple has released an update that fixes this vulnerability. Please update your Mac ASAP.
Here is the information regarding the update.
A security hole was recently discovered in the latest version of Mac OS X High Sierra which allows for anyone to access your computer without a password. We have provided a link to a USA Today article below, but there are many articles on the web reporting this issue. While Apple is working on a fix via update, you should take immediate action to fix the issue by following the instructions here. Basically, all you need to do is set a Root User password so please navigate to those instructions in the aforementioned link. If you need further assistance, please contact email@example.com.
USA Today Article