High Sierra Security Update

Apple has released Security Update 2017-001 to fix a serious vulnerability that enables access to the root superuser account with a blank password on any Mac running macOS High Sierra version 10.13.1.
 
The critical bug lets anyone gain administrator privileges by simply entering the username "root" and a blank password in System Preferences > Users & Groups.

The security update is on the Mac App Store now, and it should be installed by all users running macOS High Sierra as soon as possible. Regardless Apple said the security update will be automatically installed on all Macs running macOS High Sierra 10.13.1.

Apple has since apologized for the vulnerability:
“Security is a top priority for every Apple product, and regrettably we stumbled with this release of macOS.

When our security engineers became aware of the issue we immediately began working on an update that closes the security hole. The update is available for download, and starting November 30,  automatically installed on all systems running the latest version (10.13.1) of macOS High Sierra.

We greatly regret this error and we apologize to all Mac users, both for releasing with this vulnerability and for the concern it has caused. Our customers deserve better. We are auditing our development processes to help prevent this from happening again.
The vulnerability does not affect macOS Sierra or any other previous version of the operating system.”

To ensure you have updated your Mac, go to the App store, click on the “Updates” icon and verify you have installed this update.