Windows regsvr32 Usage of /i Without /n Flag

Alerts on regsvr32.exe invocations using /i: without the usually paired /n flag.

FreeUnreviewedSigmamediumv1

windows-regsvr32-usage-of-i-without-n-flag-b236190c

title: Windows regsvr32 Usage of /i Without /n Flag
id: d3f53070-2316-4310-bb16-aa1888304c30
status: test
description: This rule flags Windows process creation events where the image path ends with regsvr32.exe and the command line contains the /i flag while the /n flag is not present. Attackers may use regsvr32 with atypical flag combinations to execute actions stealthily, so unusual parameter usage can be an early signal. The detection relies on process creation telemetry including the process image name and the full command line.
references:
  - https://twitter.com/sbousseaden/status/1282441816986484737?s=12
  - https://github.com/SigmaHQ/sigma/blob/master/rules/windows/process_creation/proc_creation_win_regsvr32_flags_anomaly.yml
author: Florian Roth (Nextron Systems), Huntrule Team
date: 2019-07-13
modified: 2024-03-13
tags:
  - attack.stealth
  - attack.t1218.010
logsource:
  category: process_creation
  product: windows
detection:
  selection:
    Image|endswith: \regsvr32.exe
    CommandLine|contains|windash: " -i:"
  filter_main_flag:
    CommandLine|contains|windash: " -n "
  condition: selection and not 1 of filter_main_*
falsepositives:
  - Administrator typo might cause some false positives
level: medium
license: DRL-1.1
related:
  - id: b236190c-1c61-41e9-84b3-3fe03f6d76b0
    type: derived

What it detects

This rule flags Windows process creation events where the image path ends with regsvr32.exe and the command line contains the /i flag while the /n flag is not present. Attackers may use regsvr32 with atypical flag combinations to execute actions stealthily, so unusual parameter usage can be an early signal. The detection relies on process creation telemetry including the process image name and the full command line.

Known false positives

Administrator typo might cause some false positives

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