Potential Regsvr32 Commandline Flag Anomaly

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

Detects a potential command line flag anomaly related to "regsvr32" in which the "/i" flag is used without the "/n" which should be uncommon.

Known false positives

Administrator typo might cause some false positives

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: Potential Regsvr32 Commandline Flag Anomaly
id: b236190c-1c61-41e9-84b3-3fe03f6d76b0
status: test
description: Detects a potential command line flag anomaly related to "regsvr32" in which the "/i" flag is used without the "/n" which should be uncommon.
references:
    - https://twitter.com/sbousseaden/status/1282441816986484737?s=12
author: Florian Roth (Nextron Systems)
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

Stages and Predicates

Stage 0: `condition`

selection and not 1 of filter_main_*

Stage 1: `selection`

selection:
    Image|endswith: '\regsvr32.exe'
    CommandLine|contains|windash: ' -i:'

Stage 2: `not filter_main_flag`

filter_main_flag:
    CommandLine|contains|windash: ' -n '

Exclusions

The rule actively suppresses these predicates.

Field	Kind	Excluded values	Search
`CommandLine`	match	`-n`	`excludes:CommandLine` `field:"CommandLine" value:" -n "`

Indicators

These rows show field, operator, and value matches.

Field	Kind	Values	Search
`CommandLine`	match	`-i:` transforms: `windash` corpus 2 (sigma 1, elastic 1)	`field:"CommandLine" kind:match value:" -i:"`
`Image`	ends_with	`\regsvr32.exe` corpus 64 (sigma 64)	`field:"Image" kind:ends_with value:"\regsvr32.exe"`