Apple MacOS High Sierra (10.13) contains a flaw in how it authenticates disabled accounts. When a privileged action prompts the user for administrative credentials, the user can simply enter the user of "root" with an empty password. The first attempt appears to fail, but in actuality, this action causes MacOS High Sierra to enable the ability to log in as root using the credentials specified. A second attempt to authenticate using the same credentials successfully takes the action with root administrative privileges. Once this vulnerability has been triggered by an authenticated user (either locally, or via remote access such as SSH), the root account will be available to use as a viable authentication mechanism to the system.

It is important to note that simply confirming the vulnerability on a system will have the side effect of enabling the root account for use in authenticating to the system.

A local or remote user of a MacOS High Sierra system can obtain root privileges without requiring credentials. Any system that has the root account enabled (e.g. via testing for this vulnerability) may also expose the root account for use with remote administrative capabilities, such as the built-in "Screen Sharing" or "Remote Management" capabilities

Apply an update

This issue is addressed in Security Update 2017-001. Please also consider the following workaround if you are unable to install the update:

Set the root password

1. As a user with administrative privileges, launch Terminal
2. Type sudo passwd -u root
3. Enter a strong password