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Detecting PE Files Created with Non-Standard Extensions

So far I haven't figured out how to stop these file creations, but this will only detect even if you set it to block. You can do standard extensions as well, but that is easy to do and block by extension.  This will help with things like the specific attacks used in CVE-2021-40444 where the dropped DLL was given a .inf extension.  

 

Rule {
	Process {
		Include OBJECT_NAME {
			-v "winword.exe"
			-v "powerpnt.exe"
			-v "excel.exe"
			-v "mshta.exe"
		}
	}
	Target {
		Match FILE {
			Include PE { -v "1" }
			Exclude OBJECT_NAME { 
				-v "**.dll"
				-v "**.exe"
			}
			Include -access "WRITE_ATTRIBUTE"
		}
	}
}

 

I used this code to test it in a Word macro:

Sub asdf()
Dim FSO As Object
Set FSO = CreateObject("Scripting.FileSystemObject")
Call FSO.CopyFile("c:\windows\system32\version.dll", "c:\pickaFolder\whatever.inf", True)
End Sub

Obviously, block Office from executing the various suspected child processes is hugely important itself, but this is one step in better overall coverage.  

2 Replies
Pravas
McAfee Employee
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Re: Detecting PE Files Created with Non-Standard Extensions

Hi @Daveb3d ,

Thanks for sharing your results with community.

Other alternative which Microsoft has mentioned is to disable installation of ActiveX. This should stop the attack as it leverages ActiveX within Internet Explorer.

For details please refer the link below.

https://msrc.microsoft.com/update-guide/vulnerability/CVE-2021-40444

Thanks

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Re: Detecting PE Files Created with Non-Standard Extensions

Unfortunately,  it looks like that has been defeated as a fix. 

 

https://twitter.com/GossiTheDog/status/1435570418623070210?s=19

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