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.
Hidden Users
source_name: mitre-attack  url: https://attack.mitre.org/techniques/T1564/002  external_id: T1564.002     source_name: Cybereason OSX Pirrit  description: Amit Serper. (2016). Cybereason Lab Analysis OSX.Pirrit. Retrieved December 10, 2021.  url: https://cdn2.hubspot.net/hubfs/3354902/Content%20PDFs/Cybereason-Lab-Analysis-OSX-Pirrit-4-6-16.pdf     source_name: Apple Support Hide a User Account  description: Apple. (2020, November 30). Hide a user account in macOS. Retrieved December 10, 2021.  url: https://support.apple.com/en-us/HT203998     source_name: FireEye SMOKEDHAM June 2021  description: FireEye. (2021, June 16). Smoking Out a DARKSIDE Affiliate’s Supply Chain Software Compromise. Retrieved September 22, 2021.  url: https://www.fireeye.com/blog/threat-research/2021/06/darkside-affiliate-supply-chain-software-compromise.html     source_name: Hide GDM User Accounts  description: Ji Mingkui. (2021, June 17). How to Hide All The User Accounts in Ubuntu 20.04, 21.04 Login Screen. Retrieved March 15, 2022.  url: https://ubuntuhandbook.org/index.php/2021/06/hide-user-accounts-ubuntu-20-04-login-screen/     source_name: US-CERT TA18-074A  description: US-CERT. (2018, March 16). Alert (TA18-074A): Russian Government Cyber Activity Targeting Energy and Other Critical Infrastructure Sectors. Retrieved June 6, 2018.  url: https://www.us-cert.gov/ncas/alerts/TA18-074A     