Tuesday, June 7, 2016

Understanding VBA from an Invoice Email

This malicious email contained a microsoft word attachment with VB code in it. Here is a link to the full original macro code.


The code was interesting because it seemed amateurish in terms of it's obfuscation. Sure there were tons of random looking letters, variable names, etc. But in general, this was poorly obfuscated, if at all. Here are a few examples:


The above variable was used for doing bitwise masking of a string value, yet the attacker literally named it "HighMask" so you know exactly what it's used for.

Public Function Encode64(sString As String) As String

The above function literally performs base64 encoding of a string, so the name Encode64 makes sense and makes life simpler for the security researcher.

Public Sub Wipedir(pppppppppppppppppppppppppp As String)

The above function Wipredir calls the VBA deletefolder method in it, so again it's clearly deleting the evidence afterwards, thus the obfuscation here is poor.

So de-obfuscating this was quite simple then. One of the easier ways to quickly de-obfuscate VBA code like this is to put it into a word document, comment out the malicious lines, replace them with MsgBox statements, and execute the code, let the code do the work for you. For example:

Comment out this code with a tick

'MkDir (decryptString(Encode64(b)))

And replace it with this code

MsgBox ("MkDir=" + decryptString(Encode64(b)))

Comment out this code with a tick

'ChDrive (vEnd988888527)

And replace it with this code

MsgBox ("ChDrive=" + vEnd988888527)

Comment out this code with a tick

'Open vEnd3491963883 For Binary As vEnd1400215006

And replace it with this code

MsgBox ("OpenFile=" + vEnd3491963883)

Comment out this code with a tick

' OBsGG = Shell(vbHH, 1)

And replace it with this code

MsgBox ("Shell=" + vbHH)

Comment out this code with a tick

'OOO.deletefolder pppppppppppppppppppppppppp

And replace it with this code

MsgBox ("deletefolder=" + pppppppppppppppppppppppppp)

And like magic, you'll get a bunch of message boxes that tell you exactly what this code is trying to do. In this case
1.) Create a new folder C:\ProgramData\Memsys
2.) Navigate to that folder C:\ProgramData\Memsys
3.) Open and Write to a file C:\ProgramData\Memsys\ms.exe
4.) Execute that file from the command prompt C:\ProgramData\Memsys\ms.exe
5.) Delete that folder C:\ProgramData\Memsys

Of course always do this in a sandbox, not connected to the Internet, in case you accidentally execute malicious code that you didn't mean to and infect yourself.

More about neonprimetime

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