The bug, discovered by developer Lemi Ergin, lets anyone log into an admin account using the username “root” with no password. This works when attempting to access an administrator’s account on an unlocked Mac, and it also provides access at the login screen of a locked Mac.
To replicate, follow these steps from any kind of Mac account, admin or guest:
1. Open System Preferences
2. Choose Users & Groups
3. Click the lock to make changes
4. Type “root” in the username field
5. Move the mouse to the Password field and click there, but leave it blank
6. Click unlock, and it should allow you full access to add a new administrator account.
At the login screen, you can also use the root trick to gain access to a Mac after the feature has been enabled in System Preferences. At the login screen, click “Other,” and then enter “root” again with no password.
This allows for admin-level access directly from the locked login screen, with the account able to see everything on the computer.
It appears that this bug is present in the current version of macOS High Sierra, 10.13.1, and the macOS 10.13.2 beta that is in testing at the moment. It’s not clear how such a significant bug got past Apple, but it’s likely this is something that the company will immediately address.
Update: An Apple spokesperson told MacRumors that a fix is in the works:
“We are working on a software update to address this issue. In the meantime, setting a root password prevents unauthorized access to your Mac. To enable the Root User and set a password, please follow the instructions here: https://support.apple.com/en-us/HT204012. If a Root User is already enabled, to ensure a blank password is not set, please follow the instructions from the ‘Change the root password’ section.”
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