Threat Level: green Handler on Duty: Brad Duncan

SANS ISC: InfoSec Handlers Diary Blog - Office Maldoc: Let's Focus on the VBA Macros Later... InfoSec Handlers Diary Blog


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Office Maldoc: Let's Focus on the VBA Macros Later...

Published: 2016-07-19
Last Updated: 2016-07-19 18:12:10 UTC
by Didier Stevens (Version: 1)
6 comment(s)

I received another malicious Office document.

oledump.py shows it contains VBA macros, but also a userform (A4 - A7).

Before we look at the VBA macros, we'll take a look at the values in the userform (A7 .../o).

It looks like it contains BASE64 text. Let's use a plugin to take a look at the values:

When we use option -q, we see just the output from the plugin:

That output can be piped into base64dump.py to see it we detect BASE64 text:

Not all text is recognized as valid BASE64. Let's try if concatenating all text produces a different result. We do this with option -w to ignore all whitespace:

It's clear now that this is valid BASE64, and that the decoded text starts with %COMSPEC% ...

So let's do a ASCII dump of that BASE64 text:

Now it's clear that this is a downloader using PowerShell. We can also dump the code:

When we analyze the VBA macros, we will find code that references the userform to concatenate the BASE64 text. It then decodes it and executes it.

But this time, just by poking a bit at the BASE64 text, we were able to recover the malicious payload.

 

 

Didier Stevens
Microsoft MVP Consumer Security
blog.DidierStevens.com DidierStevensLabs.com

Keywords: maldoc office
6 comment(s)
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