title: Regsvr32 DLL Execution With Uncommon Extension
id: 50919691-7302-437f-8e10-1fe088afa145
status: test
description: Detects a "regsvr32" execution where the DLL doesn't contain a common file extension.
references:
    - https://app.any.run/tasks/34221348-072d-4b70-93f3-aa71f6ebecad/
author: Florian Roth (Nextron Systems)
date: 2019-07-17
modified: 2023-05-24
tags:
    - attack.privilege-escalation
    - attack.persistence
    - attack.stealth
    - attack.t1574
    - attack.execution
logsource:
    category: process_creation
    product: windows
detection:
    selection:
        - Image|endswith: '\regsvr32.exe'
        - OriginalFileName: 'REGSVR32.EXE'
    filter_main_legit_ext:
        CommandLine|contains:
            # Note: For better accuracy you might not want to use contains
            - '.ax'
            - '.cpl'
            - '.dll' # Covers ".dll.mui"
            - '.ocx'
    filter_optional_pascal:
        CommandLine|contains: '.ppl'
    filter_optional_avg:
        CommandLine|contains: '.bav'
    filter_main_null_4688:
        CommandLine: null
    filter_main_empty_4688:
        CommandLine: ''
    condition: selection and not 1 of filter_main_* and not 1 of filter_optional_*
falsepositives:
    - Other legitimate extensions currently not in the list either from third party or specific Windows components.
level: medium