T1546.002 Screensaver

> Sub-technique of: T1546

High-Level Description

Adversaries may establish persistence by executing malicious content triggered by user inactivity. Screensavers are programs that execute after a configurable time of user inactivity and consist of Portable Executable (PE) files with a .scr file extension. The Windows screensaver application scrnsave.scr is located in <code>C:\Windows\System32\</code>, and <code>C:\Windows\sysWOW64\</code> on 64-bit Windows systems, along with screensavers included with base Windows installations.

The following screensaver settings are stored in the Registry (<code>HKCU\Control Panel\Desktop\</code>) and could be manipulated to achieve persistence:

• <code>SCRNSAVE.exe</code> - set to malicious PE path
• <code>ScreenSaveActive</code> - set to '1' to enable the screensaver
• <code>ScreenSaverIsSecure</code> - set to '0' to not require a password to unlock
• <code>ScreenSaveTimeout</code> - sets user inactivity timeout before screensaver is executed

Adversaries can use screensaver settings to maintain persistence by setting the screensaver to run malware after a certain timeframe of user inactivity.

Kill Chain Phase

• Privilege Escalation (TA0004)
• Persistence (TA0003)

Platforms: Windows

What to Check

• ] Identify if Screensaver technique is applicable to target environment
• ] Check Windows systems for indicators of Screensaver
• ] Verify mitigations are bypassed or absent (2 known mitigations)
• ] Assess detection coverage (1 detection strategies)

How to Test

Atomic Red Team Tests

The following tests are from Atomic Red Team and provide actionable ways to test this technique:

Atomic Test 1: Set Arbitrary Binary as Screensaver

This test copies a binary into the Windows System32 folder and sets it as the screensaver so it will execute for persistence. Requires a reboot and logon.

Supported Platforms: windows Elevation Required: Yes

cmd
reg export "HKEY_CURRENT_USER\Control Panel\Desktop" %userprofile%\backup.reg
copy #{input_binary} "%SystemRoot%\System32\evilscreensaver.scr"
reg.exe add "HKEY_CURRENT_USER\Control Panel\Desktop" /v ScreenSaveActive /t REG_SZ /d 1 /f
reg.exe add "HKEY_CURRENT_USER\Control Panel\Desktop" /v ScreenSaveTimeout /t REG_SZ /d 60 /f
reg.exe add "HKEY_CURRENT_USER\Control Panel\Desktop" /v ScreenSaverIsSecure /t REG_SZ /d 0 /f
reg.exe add "HKEY_CURRENT_USER\Control Panel\Desktop" /v SCRNSAVE.EXE /t REG_SZ /d "%SystemRoot%\System32\evilscreensaver.scr" /f
if #{reboot} NEQ 0 shutdown /r /t 0

Manual Testing

If Atomic Red Team tests are not applicable, manually verify the technique by:

1. Identify Attack Surface: Determine if the target environment is susceptible to Screensaver by examining the target platforms (Windows).

1. Assess Existing Defenses: Review whether mitigations for T1546.002 are in place. If defenses are absent or misconfigured, this technique may be exploitable.

1. Execute Test: Use tools and methods described in the MITRE ATT&CK page and external references below.

Remediation Guide

M1038 Execution Prevention

Block .scr files from being executed from non-standard locations.

M1042 Disable or Remove Feature or Program

Use Group Policy to disable screensavers if they are unnecessary.

Detection

Detect Screensaver-Based Persistence via Registry and Execution Chains

Risk Assessment

| Finding | Severity | Impact | | -------------------------------- | -------- | -------------------- | | Screensaver technique applicable | Low | Privilege Escalation |

CWE Categories

| CWE ID | Title | | ------- | ----------------------------- | | CWE-269 | Improper Privilege Management |

References

• ESET Gazer Aug 2017
• Wikipedia Screensaver
• Atomic Red Team - T1546.002
• MITRE ATT&CK - T1546.002