Regsvr32.EXE Calling of DllRegisterServer Export Function Implicitly

title: Regsvr32.EXE Calling of DllRegisterServer Export Function Implicitly
id: ce2c44b5-a6ac-412a-afba-9e89326fa972
related:
    - id: 0ba1da6d-b6ce-4366-828c-18826c9de23e
      type: similar
status: test
description: |
    Detects execution of regsvr32 with the silent flag and no other flags on a DLL located in an uncommon or potentially suspicious location.
    When Regsvr32 is called in such a way, it implicitly calls the DLL export function 'DllRegisterServer'.
references:
    - https://thedfirreport.com/2023/08/28/html-smuggling-leads-to-domain-wide-ransomware/
    - https://www.virustotal.com/gui/file/288fc4f954f98d724e6fab32a89477943df5c0e9662cb199a19b90ae0c63aebe/detection
    - https://learn.microsoft.com/en-us/windows/win32/api/olectl/nf-olectl-dllregisterserver
    - https://ss64.com/nt/regsvr32.html
author: Andreas Braathen (mnemonic.io), Nasreddine Bencherchali (Nextron Systems)
date: 2023-10-17
tags:
    - attack.stealth
    - attack.t1218
    - detection.threat-hunting
logsource:
    category: process_creation
    product: windows
detection:
    selection_image:
        - Image|endswith: '\regsvr32.exe'
        - OriginalFileName: 'REGSVR32.EXE'
    selection_cmdline:
        CommandLine|contains:
            - ' /s '
            - ' /e '
    filter_main_paths:
        - CommandLine|contains:
              - ':\Program Files (x86)'
              - ':\Program Files\'
              - ':\Windows\System32\'
              - ':\Windows\SysWOW64\'
        - CurrentDirectory|contains:
              - ':\Program Files (x86)'
              - ':\Program Files\'
              - ':\Windows\System32\'
              - ':\Windows\SysWOW64\'
    filter_main_other_flags:
        # Note: We filter other flags to keep the logic of the rule
        CommandLine|contains:
            - ' /i:'
            - '/U '
    filter_main_rpcproxy:
        ParentCommandLine|endswith: ':\Windows\System32\RpcProxy\RpcProxy.dll'
        CommandLine: 'regsvr32 /s rpcproxy.dll'
    condition: all of selection_* and not 1 of filter_main_*
falsepositives:
    - Legitimate usage as part of application installation, but less likely from e.g. temporary paths.
level: medium