Update: VBA Maldoc & UTF7 (APT-C-35)

In diary entry "VBA Maldoc & UTF7 (APT-C-35)", I analyze a malicious document with VBA code that injects shellcode into the host process. That shellcode is UTF7 encoded.

I did the UTF7 decoding with Python, but that resulted in an error. Still, I instructed Python to ignore that error, and I was able to recover a URL, but it was a bit corrupted (HTTP protocol):

As can be seen, the decoded URL starts with htt//

I took a second look this weekend at this sample, and this time, I used WIN32 API function MultiByteToWideChar (from Python), just like the VBA code does.

I wrote a small decoding function for translate.py:

from ctypes import *

def DecodeUTF7(data):
    result = bytes(10000)
    windll.kernel32.MultiByteToWideChar(65000, 0, data, len(data), result, len(result))
    return result

And then I used this to decode the UTF7 payload. And this time, I deobfuscated the decoded shellcode & strings with xorsearch.

This approach worked, both for the 32-bit and 64-bit shellcode:

This approach (calling WIN32 API from Python) only works on Windows machines.

And you have to be careful not to execute malicious code accidentaly. Here I call a decoding function (MultiByteToWideChar), but if I would call another function that is used in the VBA code (Internal_EnumUILanguages), I would actually execute the shellcode.

 

Didier Stevens
Senior handler
Microsoft MVP
blog.DidierStevens.com

DidierStevens

677 Posts
ISC Handler
Aug 29th 2022

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