Scripting/CommandLine Process Spawned Regsvr32

Status: test
Severity: medium
Log source: category process_creation, product windows
Author: Florian Roth (Nextron Systems), Nasreddine Bencherchali (Nextron Systems)
Source: github.com/SigmaHQ/sigma

Detects various command line and scripting engines/processes such as "PowerShell", "Wscript", "Cmd", etc. spawning a "regsvr32" instance.

Known false positives

Legitimate ".bat", ".hta", ".ps1" or ".vbs" scripts leverage legitimately often. Apply additional filter and exclusions as necessary
Some legitimate Windows services

MITRE ATT&CK coverage

Tactic	Techniques
Stealth	`T1218.010` System Binary Proxy Execution: Regsvr32

Telemetry coverage

Provider	Record / event type
Sysmon	Event ID 1: Process creation
Security-Auditing	Event ID 4688: A new process has been created.

Rule body

title: Scripting/CommandLine Process Spawned Regsvr32
id: ab37a6ec-6068-432b-a64e-2c7bf95b1d22
related:
    - id: 8e2b24c9-4add-46a0-b4bb-0057b4e6187d
      type: obsolete
status: test
description: Detects various command line and scripting engines/processes such as "PowerShell", "Wscript", "Cmd", etc. spawning a "regsvr32" instance.
references:
    - https://web.archive.org/web/20171001085340/https://subt0x10.blogspot.com/2017/04/bypass-application-whitelisting-script.html
    - https://app.any.run/tasks/34221348-072d-4b70-93f3-aa71f6ebecad/
author: Florian Roth (Nextron Systems), Nasreddine Bencherchali (Nextron Systems)
date: 2023-05-26
tags:
    - attack.stealth
    - attack.t1218.010
logsource:
    category: process_creation
    product: windows
detection:
    selection:
        ParentImage|endswith:
            - '\cmd.exe'
            - '\cscript.exe'
            - '\mshta.exe'
            - '\powershell_ise.exe'
            - '\powershell.exe'
            - '\pwsh.exe'
            - '\wscript.exe'
        Image|endswith: '\regsvr32.exe'
    filter_main_rpcproxy:
        ParentImage: C:\Windows\System32\cmd.exe
        CommandLine|endswith: ' /s C:\Windows\System32\RpcProxy\RpcProxy.dll'
    condition: selection and not 1 of filter_main_*
falsepositives:
    - Legitimate ".bat", ".hta", ".ps1" or ".vbs" scripts leverage legitimately often. Apply additional filter and exclusions as necessary
    - Some legitimate Windows services
level: medium # Can be reduced to low if you experience a ton of FP

Stages and Predicates

Stage 0: `condition`

selection and not 1 of filter_main_*

Stage 1: `selection`

selection:
    ParentImage|endswith:
        - '\cmd.exe'
        - '\cscript.exe'
        - '\mshta.exe'
        - '\powershell_ise.exe'
        - '\powershell.exe'
        - '\pwsh.exe'
        - '\wscript.exe'
    Image|endswith: '\regsvr32.exe'

Stage 2: `not filter_main_rpcproxy`

filter_main_rpcproxy:
    ParentImage: C:\Windows\System32\cmd.exe
    CommandLine|endswith: ' /s C:\Windows\System32\RpcProxy\RpcProxy.dll'

Exclusions

The rule actively suppresses these predicates.

Field	Kind	Excluded values	Search
`CommandLine`	ends_with	`/s C:\Windows\System32\RpcProxy\RpcProxy.dll`	`excludes:CommandLine` `field:"CommandLine" value:" /s C:\Windows\System32\RpcProxy\RpcProxy.dll"`
`ParentImage`	eq	`C:\Windows\System32\cmd.exe`	`excludes:ParentImage` `field:"ParentImage" value:"C:\Windows\System32\cmd.exe"`

Indicators

These rows show field, operator, and value matches.

Field	Kind	Values	Search
`Image`	ends_with	`\regsvr32.exe` corpus 64 (sigma 64)	`field:"Image" kind:ends_with value:"\regsvr32.exe"`
`ParentImage`	ends_with	`\cmd.exe` corpus 20 (sigma 20) `\cscript.exe` corpus 17 (sigma 17) `\mshta.exe` corpus 13 (sigma 13) `\powershell.exe` corpus 24 (sigma 24) `\powershell_ise.exe` corpus 3 (sigma 3) `\pwsh.exe` corpus 21 (sigma 21) `\wscript.exe` corpus 19 (sigma 19)	`field:"ParentImage" kind:ends_with`