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Detection rules › Sigma

ScreenSaver Registry Key Set

Status
test
Severity
medium
Log source
category registry_set, product windows
Author
Jose Luis Sanchez Martinez (@Joseliyo_Jstnk)
Source
github.com/SigmaHQ/sigma

Detects registry key established after masqueraded .scr file execution using Rundll32 through desk.cpl

Known false positives

  • Legitimate use of screen saver

MITRE ATT&CK coverage

TacticTechniques
Stealth

Telemetry coverage

ProviderRecord / event type
SysmonEvent ID 13: RegistryEvent (Value Set)

Rule body

title: ScreenSaver Registry Key Set
id: 40b6e656-4e11-4c0c-8772-c1cc6dae34ce
status: test
description: Detects registry key established after masqueraded .scr file execution using Rundll32 through desk.cpl
references:
    - https://twitter.com/VakninHai/status/1517027824984547329
    - https://twitter.com/pabraeken/status/998627081360695297
    - https://jstnk9.github.io/jstnk9/research/InstallScreenSaver-SCR-files
author: Jose Luis Sanchez Martinez (@Joseliyo_Jstnk)
date: 2022-05-04
modified: 2023-08-17
tags:
    - attack.stealth
    - attack.t1218.011
logsource:
    product: windows
    category: registry_set
detection:
    selection:
        Image|endswith: '\rundll32.exe'
    registry:
        TargetObject|contains: '\Control Panel\Desktop\SCRNSAVE.EXE'
        Details|endswith: '.scr'
    filter:
        Details|contains:
            - 'C:\Windows\System32\'
            - 'C:\Windows\SysWOW64\'
    condition: selection and registry and not filter
falsepositives:
    - Legitimate use of screen saver
level: medium

Stages and Predicates

Stage 0: condition

selection and registry and not filter

Stage 1: selection

selection:
    Image|endswith: '\rundll32.exe'

Stage 2: registry

registry:
    TargetObject|contains: '\Control Panel\Desktop\SCRNSAVE.EXE'
    Details|endswith: '.scr'

Stage 3: not filter

filter:
    Details|contains:
        - 'C:\Windows\System32\'
        - 'C:\Windows\SysWOW64\'

Exclusions

The rule actively suppresses these predicates.

FieldKindExcluded valuesSearch
DetailsmatchC:\Windows\SysWOW64\excludes:Details field:"Details" value:"C:\Windows\SysWOW64\"
DetailsmatchC:\Windows\System32\excludes:Details field:"Details" value:"C:\Windows\System32\"

Indicators

These rows show field, operator, and value matches.

FieldKindValuesSearch
Detailsends_with
  • .scr
field:"Details" kind:ends_with value:".scr"
Imageends_with
  • \rundll32.exe corpus 94 (sigma 94)
field:"Image" kind:ends_with value:"\rundll32.exe"
TargetObjectmatch
  • \Control Panel\Desktop\SCRNSAVE.EXE corpus 2 (sigma 2)
field:"TargetObject" kind:match value:"\Control Panel\Desktop\SCRNSAVE.EXE"