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Windows: Detect reg.exe Changing Screen Saver Registry Settings for .scr Payloads

Flags reg.exe command lines that modify HKCU desktop screensaver settings and configure a .scr screen saver payload.

FreeUnreviewedSigmamediumv1
title: "Windows: Detect reg.exe Changing Screen Saver Registry Settings for .scr Payloads"
id: 7c8a256e-2028-4152-8cf0-bafeb4f4468a
status: test
description: This rule identifies executions of reg.exe whose command line targets the current user desktop screensaver-related registry keys and sets ScreenSaveActive, ScreenSaveTimeout, and ScreenSaverIsSecure. It further matches registry value changes that reference an executables path ending in .scr via the SCRNSAVE.EXE value. Such behavior matters because it can allow an attacker to configure a user inactivity screen saver to run malicious content. The detection relies on Windows process creation telemetry including the process image path and full command-line arguments.
references:
  - https://github.com/redcanaryco/atomic-red-team/blob/f339e7da7d05f6057fdfcdd3742bfcf365fee2a9/atomics/T1546.002/T1546.002.md
  - https://www.welivesecurity.com/wp-content/uploads/2017/08/eset-gazer.pdf
  - https://github.com/SigmaHQ/sigma/blob/master/rules/windows/process_creation/proc_creation_win_reg_screensaver.yml
author: frack113, Huntrule Team
date: 2021-08-19
modified: 2022-06-02
tags:
  - attack.persistence
  - attack.privilege-escalation
  - attack.t1546.002
logsource:
  category: process_creation
  product: windows
detection:
  selection_reg:
    Image|endswith: \reg.exe
    CommandLine|contains:
      - HKEY_CURRENT_USER\Control Panel\Desktop
      - HKCU\Control Panel\Desktop
  selection_option_1:
    CommandLine|contains|all:
      - /v ScreenSaveActive
      - /t REG_SZ
      - /d 1
      - /f
  selection_option_2:
    CommandLine|contains|all:
      - /v ScreenSaveTimeout
      - /t REG_SZ
      - "/d "
      - /f
  selection_option_3:
    CommandLine|contains|all:
      - /v ScreenSaverIsSecure
      - /t REG_SZ
      - /d 0
      - /f
  selection_option_4:
    CommandLine|contains|all:
      - /v SCRNSAVE.EXE
      - /t REG_SZ
      - "/d "
      - .scr
      - /f
  condition: selection_reg and 1 of selection_option_*
falsepositives:
  - GPO
level: medium
license: DRL-1.1
related:
  - id: 0fc35fc3-efe6-4898-8a37-0b233339524f
    type: derived

What it detects

This rule identifies executions of reg.exe whose command line targets the current user desktop screensaver-related registry keys and sets ScreenSaveActive, ScreenSaveTimeout, and ScreenSaverIsSecure. It further matches registry value changes that reference an executables path ending in .scr via the SCRNSAVE.EXE value. Such behavior matters because it can allow an attacker to configure a user inactivity screen saver to run malicious content. The detection relies on Windows process creation telemetry including the process image path and full command-line arguments.

Known false positives

  • GPO

Detection content is published as a reviewed draft. Tune thresholds and exclusions against your own telemetry before enabling this rule for alerting.