Rundll32 Spawned Via Explorer.EXE

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

Detects execution of "rundll32.exe" with a parent process of Explorer.exe. This has been observed by variants of Raspberry Robin, as first reported by Red Canary.

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: Rundll32 Spawned Via Explorer.EXE
id: 1723e720-616d-4ddc-ab02-f7e3685a4713
status: test
description: Detects execution of "rundll32.exe" with a parent process of Explorer.exe. This has been observed by variants of Raspberry Robin, as first reported by Red Canary.
references:
    - https://redcanary.com/blog/raspberry-robin/
    - https://thedfirreport.com/2022/09/26/bumblebee-round-two/
author: CD_ROM_
date: 2022-05-21
modified: 2023-08-31
tags:
    - attack.stealth
logsource:
    category: process_creation
    product: windows
detection:
    selection_parent:
        ParentImage|endswith: '\explorer.exe'
    selection_img:
        - Image|endswith: '\rundll32.exe'
        - OriginalFileName: 'RUNDLL32.EXE'
    filter_main_generic:
        - CommandLine|contains: ' C:\Windows\System32\' # The space at the start is required
        - CommandLine|endswith: ' -localserver 22d8c27b-47a1-48d1-ad08-7da7abd79617' # Windows 10 volume control
    condition: all of selection_* and not 1 of filter_main_*
falsepositives:
    - Unknown
level: medium

Stages and Predicates

Stage 0: `condition`

all of selection_* and not 1 of filter_main_*

Stage 1: `selection_parent`

selection_parent:
    ParentImage|endswith: '\explorer.exe'

Stage 2: `selection_img`

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

Stage 3: `not filter_main_generic`

filter_main_generic:
    - CommandLine|contains: ' C:\Windows\System32\'
    - CommandLine|endswith: ' -localserver 22d8c27b-47a1-48d1-ad08-7da7abd79617'

Exclusions

The rule actively suppresses these predicates.

Field	Kind	Excluded values	Search
`CommandLine`	ends_with	`-localserver 22d8c27b-47a1-48d1-ad08-7da7abd79617`	`excludes:CommandLine` `field:"CommandLine" value:" -localserver 22d8c27b-47a1-48d1-ad08-7da7abd79617"`
`CommandLine`	match	`C:\Windows\System32\`	`excludes:CommandLine` `field:"CommandLine" value:" C:\Windows\System32\"`

Indicators

These rows show field, operator, and value matches.

Field	Kind	Values	Search
`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	`\explorer.exe` corpus 13 (sigma 13)	`field:"ParentImage" kind:ends_with value:"\explorer.exe"`