In his analysis of a malicious document, Xavier mentions an unknown VBA function he calls func_FormatDocument. This function changes the format of the document, and in this diary I will explain what the purpose of this function is.

This function is part of a social engineering trick: in a normal situation (default settings), Microsoft Office macros are disabled for documents received as e-mail attachments. The attack fails. Unless the attacker can trick the user into enabling macros.

One popular social engineering trick to entice users to enable macros, is to make the user believe that the document contains secret or confidential information, and that the user needs to take action to reveal this information. The Word document will contain a message that the content is hidden (or encoded, or encrypted, ...) and that the user needs to enable the content (or the macros) to visualize it. And this VBA function func_FormatDocument is part of such an elaborate social engineering trick. This function will change the font color from white to black (thereby "revealing" the hidden information) and remove the header that instructs the user to enable the content. The goal of this is to make the user believe that the content was decoded/decrypted, and that everything is normal, while in reality VBA macros were enabled and that the malicious payload executed.

There should be a saying: "a video is worth a thousand pictures". Because what I try to explain here with words, is much easier to understand when you see it in action. That's why I made this video:

Didier Stevens
SANS ISC Handler
Microsoft MVP Consumer Security
blog.DidierStevens.com DidierStevensLabs.com
IT Security consultant at Contraste Europe.