A few days ago, a bug was discovered that enabled root user access without a password when anyone attempted to login as root on a system without the root user enabled. The next day Apple released a security update for High Sierra and apologized to all Mac users for the mistake.
If you're running a beta version of High Sierra, we recommend you update as soon as possible.
Developers can download the macOS Developer Beta Access Utility from the link below...
Download [via MacRumors]