Adversaries may use hidden users to hide the presence of user accounts they create or modify. Administrators may want to hide users when there are many user accounts on a given system or if they want to hide their administrative or other management accounts from other users.
In macOS, adversaries can create or modify a user to be hidden through manipulating plist files, folder attributes, and user attributes. To prevent a user from being shown on the login screen and in System Preferences, adversaries can set the userID to be under 500 and set the key value Hide500Users to TRUE in the /Library/Preferences/com.apple.loginwindow plist file.[1] Every user has a userID associated with it. When the Hide500Users key value is set to TRUE, users with a userID under 500 do not appear on the login screen and in System Preferences. Using the command line, adversaries can use the dscl utility to create hidden user accounts by setting the IsHidden attribute to 1. Adversaries can also hide a user’s home folder by changing the chflags to hidden.[2]
Adversaries may similarly hide user accounts in Windows. Adversaries can set the HKLM\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Winlogon\SpecialAccounts\UserList Registry key value to 0 for a specific user to prevent that user from being listed on the logon screen.[3][4]
On Linux systems, adversaries may hide user accounts from the login screen, also referred to as the greeter. The method an adversary may use depends on which Display Manager the distribution is currently using. For example, on an Ubuntu system using the GNOME Display Manger (GDM), accounts may be hidden from the greeter using the gsettings command (ex: sudo -u gdm gsettings set org.gnome.login-screen disable-user-list true).[5] Display Managers are not anchored to specific distributions and may be changed by a user or adversary.
| ID | Name | Description |
|---|---|---|
| G0035 | Dragonfly |
Dragonfly has modified the Registry to hide created user accounts.[4] |
| G0094 | Kimsuky |
Kimsuky has run |
| S0649 | SMOKEDHAM |
SMOKEDHAM has modified the Registry to hide created user accounts from the Windows logon screen. [3] |
| ID | Mitigation | Description |
|---|---|---|
| M1028 | Operating System Configuration |
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 |
Monitor for users that may be hidden from the login screen but still present in additional artifacts of usage such as directories and authentication logs.
Monitor processes and command-line events for actions that could be taken to add a new user and subsequently hide it from login screens. Monitor Registry events for modifications to the HKLM\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Winlogon\SpecialAccounts\UserList key.
In macOS, monitor for commands, processes, and file activity in combination with a user that has a userID under 500.[1] Monitor for modifications to set the Hide500Users key value to TRUE in the /Library/Preferences/com.apple.loginwindow plist file. Monitor the command line for usage of the dscl . create command with the IsHidden attribute set to 1.[2]