Windows Regsvr32.exe Executed with Suspicious File Extension Masquerading as DLL

Alerts when REGSVR32.exe runs with a command-line argument ending in a suspicious masquerade file extension.

FreeUnreviewedSigmahighv1

windows-regsvr32-exe-executed-with-suspicious-file-extension-masquerading-as-dll-089fc3d2

title: Windows Regsvr32.exe Executed with Suspicious File Extension Masquerading as DLL
id: 467147ba-50c5-49e6-9e0e-8ad2120ee55d
related:
  - id: 8e2b24c9-4add-46a0-b4bb-0057b4e6187d
    type: obsolete
  - id: 089fc3d2-71e8-4763-a8a5-c97fbb0a403e
    type: derived
status: test
description: This rule flags process creation events where REGSVR32.exe is executed and the command line ends with one of several file extensions commonly used for masquerading. Attackers may abuse regsvr32 to load a DLL via a crafted argument while disguising the payload filename with a benign-looking extension. The detection relies on Windows process creation telemetry, including the executable image/filename and the full command line suffix.
references:
  - https://thedfirreport.com/2021/11/29/continuing-the-bazar-ransomware-story/
  - https://blog.talosintelligence.com/2021/10/threat-hunting-in-large-datasets-by.html
  - https://guides.lib.umich.edu/c.php?g=282942&p=1885348
  - https://harfanglab.io/insidethelab/uac-0057-pressure-ukraine-poland/
  - https://github.com/SigmaHQ/sigma/blob/master/rules/windows/process_creation/proc_creation_win_regsvr32_susp_extensions.yml
author: Florian Roth (Nextron Systems), frack113, Huntrule Team
date: 2021-11-29
modified: 2025-08-27
tags:
  - attack.stealth
  - attack.t1218.010
logsource:
  category: process_creation
  product: windows
detection:
  selection_img:
    - Image|endswith: \regsvr32.exe
    - OriginalFileName: REGSVR32.EXE
  selection_cli:
    CommandLine|endswith:
      - .bin
      - .bmp
      - .cr2
      - .dat
      - .eps
      - .gif
      - .ico
      - .jpeg
      - .jpg
      - .log
      - .nef
      - .orf
      - .png
      - .raw
      - .rtf
      - .sr2
      - .temp
      - .tif
      - .tiff
      - .tmp
      - .txt
  condition: all of selection_*
falsepositives:
  - Unlikely
level: high
license: DRL-1.1

What it detects

This rule flags process creation events where REGSVR32.exe is executed and the command line ends with one of several file extensions commonly used for masquerading. Attackers may abuse regsvr32 to load a DLL via a crafted argument while disguising the payload filename with a benign-looking extension. The detection relies on Windows process creation telemetry, including the executable image/filename and the full command line suffix.

Known false positives

Unlikely

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