Suspicious Workstation Locking via Rundll32

Status: test
Severity: medium
Log source: category process_creation, product windows
Author: frack113
Source: github.com/SigmaHQ/sigma

Detects a suspicious call to the user32.dll function that locks the user workstation

Known false positives

Scripts or links on the user desktop used to lock the workstation instead of Windows+L or the menu option

MITRE ATT&CK coverage

Tactic	Techniques
Stealth	No specific technique

Telemetry coverage

Provider	Record / event type
Sysmon	Event ID 1: Process creation

Rule body

title: Suspicious Workstation Locking via Rundll32
id: 3b5b0213-0460-4e3f-8937-3abf98ff7dcc
status: test
description: Detects a suspicious call to the user32.dll function that locks the user workstation
references:
    - https://app.any.run/tasks/2aef9c63-f944-4763-b3ef-81eee209d128/
author: frack113
date: 2022-06-04
modified: 2023-02-09
tags:
    - attack.stealth
logsource:
    category: process_creation
    product: windows
detection:
    selection_call_img:
        - Image|endswith: '\rundll32.exe'
        - OriginalFileName: 'RUNDLL32.EXE'
    selection_call_parent:
        ParentImage|endswith: '\cmd.exe'
    selection_call_cli:
        CommandLine|contains: 'user32.dll,'
    selection_function:
        CommandLine|contains: 'LockWorkStation'
    condition: all of selection_*
falsepositives:
    - Scripts or links on the user desktop used to lock the workstation instead of Windows+L or the menu option
level: medium

Stages and Predicates

Stage 0: `condition`

all of selection_*

Stage 1: `selection_call_img`

selection_call_img:
    - Image|endswith: '\rundll32.exe'
    - OriginalFileName: 'RUNDLL32.EXE'

Stage 2: `selection_call_parent`

selection_call_parent:
    ParentImage|endswith: '\cmd.exe'

Stage 3: `selection_call_cli`

selection_call_cli:
    CommandLine|contains: 'user32.dll,'

Stage 4: `selection_function`

selection_function:
    CommandLine|contains: 'LockWorkStation'

Indicators

These rows show field, operator, and value matches.

Field	Kind	Values	Search
`CommandLine`	match	`LockWorkStation` `user32.dll,`	`field:"CommandLine" kind:match`
`Image`	ends_with	`\rundll32.exe` corpus 94 (sigma 94)	`field:"Image" kind:ends_with value:"\rundll32.exe"`
`OriginalFileName`	eq	`RUNDLL32.EXE` corpus 78 (sigma 35, elastic 22, splunk 21)	`field:"OriginalFileName" kind:eq value:"RUNDLL32.EXE"`
`ParentImage`	ends_with	`\cmd.exe` corpus 20 (sigma 20)	`field:"ParentImage" kind:ends_with value:"\cmd.exe"`

Suspicious Workstation Locking via Rundll32

Known false positives

MITRE ATT&CK coverage

Telemetry coverage

Rule body

Stages and Predicates

Stage 0: condition

Stage 1: selection_call_img

Stage 2: selection_call_parent

Stage 3: selection_call_cli

Stage 4: selection_function

Indicators

Stage 0: `condition`

Stage 1: `selection_call_img`

Stage 2: `selection_call_parent`

Stage 3: `selection_call_cli`

Stage 4: `selection_function`