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Hide Artifacts: Hidden Users

Description from ATT&CK

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 <code>Hide500Users</code> to <code>TRUE</code> in the <code>/Library/Preferences/com.apple.loginwindow</code> plist file.(Citation: Cybereason OSX Pirrit) Every user has a userID associated with it. When the <code>Hide500Users</code> key value is set to <code>TRUE</code>, 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 <code>dscl</code> utility to create hidden user accounts by setting the <code>IsHidden</code> attribute to <code>1</code>. Adversaries can also hide a user’s home folder by changing the <code>chflags</code> to hidden.(Citation: Apple Support Hide a User Account)

Adversaries may similarly hide user accounts in Windows. Adversaries can set the <code>HKLM\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Winlogon\SpecialAccounts\UserList</code> Registry key value to <code>0</code> for a specific user to prevent that user from being listed on the logon screen.(Citation: FireEye SMOKEDHAM June 2021)(Citation: US-CERT TA18-074A)

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 <code>gsettings</code> command (ex: <code>sudo -u gdm gsettings set org.gnome.login-screen disable-user-list true</code>).(Citation: Hide GDM User Accounts) Display Managers are not anchored to specific distributions and may be changed by a user or adversary. https://www.aleksandrhovhannisyan.com/blog/how-to-add-a-copy-to-clipboard-button-to-your-jekyll-blog/

Atomic Tests

Atomic Test #1 - Create Hidden User using UniqueID < 500

Add a hidden user on macOS using Unique ID < 500 (users with that ID are hidden by default)

Supported Platforms: macos

auto_generated_guid: 4238a7f0-a980-4fff-98a2-dfc0a363d507

Inputs:

Name Description Type Default Value
user_name username to add string APT

Attack Commands: Run with sh! Elevation Required (e.g. root or admin)

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sudo dscl . -create /Users/#{user_name} UniqueID 333

Cleanup Commands:

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sudo dscl . -delete /Users/#{user_name}

Atomic Test #2 - Create Hidden User using IsHidden option

Add a hidden user on macOS using IsHidden optoin

Supported Platforms: macos

auto_generated_guid: de87ed7b-52c3-43fd-9554-730f695e7f31

Inputs:

Name Description Type Default Value
user_name username to add string APT

Attack Commands: Run with sh! Elevation Required (e.g. root or admin)

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sudo dscl . -create /Users/#{user_name} IsHidden 1

Cleanup Commands:

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sudo dscl . -delete /Users/#{user_name}

source