Hidden Users

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Hidden Users
Technique
ID T1147
Tactic Defense Evasion
Platform MacOS, OS X
Permissions Required Administrator, root
Data Sources Authentication logs, File monitoring

Every user account in macOS has a userID associated with it. When creating a user, you can specify the userID for that account. There is a property value in /Library/Preferences/com.apple.loginwindow called Hide500Users that prevents users with userIDs 500 and lower from appearing at the login screen. By using the Create Account technique with a userID under 500 and enabling this property (setting it to Yes), an adversary can hide their user accounts much more easily: sudo dscl . -create /Users/username UniqueID 4011.

Mitigation

If the computer is domain joined, then group policy can help restrict the ability to create or hide users. Similarly, preventing the modification of the /Library/Preferences/com.apple.loginwindow Hide500Users value will force all users to be visible.

Detection

This technique prevents the new user from showing up at the log in screen, but all of the other signs of a new user still exist. The user still gets a home directory and will appear in the authentication logs.