Detectionhightest

Uncommon Userinit Child Process

Detects uncommon "userinit.exe" child processes, which could be a sign of uncommon shells or login scripts used for persistence.

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Tom Ueltschi, Tim SheltonCreated Sat Jan 12Updated Tue Nov 140a98a10c-685d-4ab0-bddc-b6bdd1d48458windows

Log Source

WindowsProcess Creation

ProductWindows← raw: windows

CategoryProcess Creation← raw: process_creation

Events generated when a new process is spawned on the system. Covers command-line arguments, parent/child relationships, and process metadata.

Detection Logic

detection:
    selection:
        ParentImage|endswith: '\userinit.exe'
    filter_main_explorer:
        Image|endswith: ':\WINDOWS\explorer.exe'
    filter_optional_logonscripts:
        CommandLine|contains:
            - 'netlogon.bat'
            - 'UsrLogon.cmd'
    filter_optional_windows_core:
        # Note: This filter is mandatory on Windows Core machines as the default shell spawned by "userinit" is "powershell.exe".
        # https://learn.microsoft.com/en-us/windows-server/administration/server-core/server-core-sconfig#powershell-is-the-default-shell-on-server-core
        CommandLine: 'PowerShell.exe'
    filter_optional_proquota:
        Image|endswith:
            - ':\Windows\System32\proquota.exe'
            - ':\Windows\SysWOW64\proquota.exe'
    filter_optional_citrix:
        Image|endswith:
            # As reported by https://github.com/SigmaHQ/sigma/issues/4569
            - ':\Program Files (x86)\Citrix\HDX\bin\cmstart.exe' # https://support.citrix.com/article/CTX983798/purpose-of-cmstart-command
            - ':\Program Files (x86)\Citrix\HDX\bin\icast.exe' # https://support.citrix.com/article/CTX983798/purpose-of-cmstart-command
            - ':\Program Files (x86)\Citrix\System32\icast.exe'
            - ':\Program Files\Citrix\HDX\bin\cmstart.exe' # https://support.citrix.com/article/CTX983798/purpose-of-cmstart-command
            - ':\Program Files\Citrix\HDX\bin\icast.exe' # https://support.citrix.com/article/CTX983798/purpose-of-cmstart-command
            - ':\Program Files\Citrix\System32\icast.exe'
    filter_optional_image_null:
        Image: null
    condition: selection and not 1 of filter_main_* and not 1 of filter_optional_*