Windows Suspicious Execution of Regasm/Regsvcs With Uncommon Command-Line Extension

Flags Regasm.exe/Regsvcs.exe runs that include unusual extensions in the command line, which may indicate stealthy misuse.

FreeUnreviewedSigmamediumv1

windows-suspicious-execution-of-regasm-regsvcs-with-uncommon-command-line-extens-e9f8f8cc

title: Windows Suspicious Execution of Regasm/Regsvcs With Uncommon Command-Line Extension
id: a92be7ac-019f-47e4-aac7-3ccf3e04d470
related:
  - id: cc368ed0-2411-45dc-a222-510ace303cb2
    type: derived
  - id: e9f8f8cc-07cc-4e81-b724-f387db9175e4
    type: derived
status: test
description: This rule flags process creation events where Regasm.exe or Regsvcs.exe is executed and the command line contains an uncommon file extension (e.g., .dat, .gif, .jpeg, .jpg, .png, .txt). Attackers may use these Windows utilities to register or manipulate components while disguising activity via atypical extension arguments. It relies on Windows process creation telemetry including the executed image path and the full command line.
references:
  - https://www.fortiguard.com/threat-signal-report/4718?s=09
  - https://lolbas-project.github.io/lolbas/Binaries/Regasm/
  - https://lolbas-project.github.io/lolbas/Binaries/Regsvcs/
  - https://github.com/SigmaHQ/sigma/blob/master/rules/windows/process_creation/proc_creation_win_regasm_regsvcs_uncommon_extension_execution.yml
author: Nasreddine Bencherchali (Nextron Systems), Huntrule Team
date: 2023-02-13
tags:
  - attack.stealth
  - attack.t1218.009
logsource:
  category: process_creation
  product: windows
detection:
  selection_img:
    - Image|endswith:
        - \Regsvcs.exe
        - \Regasm.exe
    - OriginalFileName:
        - RegSvcs.exe
        - RegAsm.exe
  selection_extension:
    CommandLine|contains:
      - .dat
      - .gif
      - .jpeg
      - .jpg
      - .png
      - .txt
  condition: all of selection_*
falsepositives:
  - Unknown
level: medium
license: DRL-1.1

What it detects

This rule flags process creation events where Regasm.exe or Regsvcs.exe is executed and the command line contains an uncommon file extension (e.g., .dat, .gif, .jpeg, .jpg, .png, .txt). Attackers may use these Windows utilities to register or manipulate components while disguising activity via atypical extension arguments. It relies on Windows process creation telemetry including the executed image path and the full command line.

Known false positives

Unknown

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