Hidden Users

Executive Summary

Hidden Users (T1564.002) is a MITRE ATT&CK technique associated with Stealth. Adversaries may use hidden users to hide the presence of user accounts they create or modify.

Why Attackers Use It

Attackers use Hidden Users because it provides a reliable way to advance their objective within the Stealth tactic, often with a favorable balance of impact versus detectability on Linux, macOS, Windows environments. Defenders should assess this behavior in the context of the affected platform and adjacent activity rather than treating it as a standalone indicator.

MITRE Description

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.

Attack Flow

1. Attacker gains the prerequisite access or context described below.
2. Attacker executes Hidden Users to achieve its tactical objective (Stealth).
3. Resulting access/data/effect is leveraged to advance the broader attack chain (see Related Techniques).

Prerequisites

• Platform(s): Linux, macOS, Windows
• ATT&CK does not define one universal permission requirement for this technique. Establish the required access from the observed implementation and affected platform.

Common Tools

• Tool attribution is implementation-specific. Use ATT&CK procedure examples and local telemetry to identify the binaries, services, scripts, accounts, or cloud resources involved.

Commands

No universal command represents Hidden Users. Capture the exact command line, arguments, parent process, account, host, and execution time from the investigated environment; do not operationalize unverified examples.

Network Traffic

• Network observability is implementation-dependent. Review DNS, proxy, firewall, flow, authentication, and packet telemetry around the activity window, then correlate remote endpoints and protocol behavior with host evidence.

Windows Events

Sysmon Events

Detection Opportunities

No MITRE detection guidance published for this technique.

Relevant ATT&CK Data Sources: N/A

Sigma Rules

A universal Sigma rule would create unreliable results because this technique has no single guaranteed observable. Build detection logic from a documented behavior and supported data source, scope it to the affected platform, and validate it against benign administrative activity before deployment.

Splunk Queries

Start with the data sources named in the detection section. Scope searches by asset, identity, and time window; correlate the primary behavior with preceding access and subsequent actions. A portable query is intentionally not provided where the technique lacks a universal schema or observable.

Investigation Workflow

1. Confirm that the observed behavior is consistent with Hidden Users and rule out expected administrative or application activity.
2. Establish the first-seen time, initiating identity, source system, target system, and affected resources.
3. Collect relevant host, identity, network, cloud, and application telemetry for the surrounding time window.
4. Correlate parent and child activity, remote connections, file or configuration changes, and related ATT&CK techniques.
5. Determine scope by searching for the same observable across peer assets and identities.
6. Preserve volatile evidence and record confidence, assumptions, and telemetry gaps before containment.

Containment

1. Isolate affected host(s)/account(s) identified during investigation.
2. Revoke or rotate any credentials/tokens potentially exposed.
3. Apply the mitigations listed below where not already enforced.
4. Validate no related techniques (see Related Techniques) were chained against the same asset.

Mitigation

• M1028 -- Operating System Configuration: Operating System Configuration involves adjusting system settings and hardening the default configurations of an operating system (OS) to mitigate adversary exploitation and prevent abuse of system functionality.

Related Techniques

• T1564