SANS ISC: Maldoc Submitted and Analyzed - SANS Internet Storm Center

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SANS ISC InfoSec Forums

Reader Jason submitted a malicious document he received via email. Although it contains VBA code with string obfuscation that is not too complex, it has a very low VirusTotal detection score.

Let's start the analysis with oledump.py:

We have 2 streams with VBA code: A3 and A4

The for loop and the Chr$, Asc and Mid functions are clear indications that function sierra is a decoding function.

Let's look at stream A4:

And here we see a call to function sierra with 2 long strings which is executed when the document is closed. One string looks like encoded text, and the second string is a chain of digits. The decoding is actually simple. From each character in the first string, we substract the digit in the second string: f - 3 = c, n - 1 = m, g - 3 = d, ... That spells out as cmd...

It's clear that this is the payload command, here I decode it with a line of Python code:

The payload uses command "waitfor /t 3 hUZM" to wait for 3 seconds (hUZM is a signal that will never be received, /t 3 is a timeout after 3 seconds).

Then bitsadmin is used to download a file (cc.bin) and write it to disk as an .exe, which is then executed via start.

Finally, PowerShell is used to download a second file, but nothing is done with the download. It must be a tracker.

File cc.bin is indeed an executable:

And the second download is very small:

 

 

 

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

Thank you. The break down is truly appreciated.
Great job with the tools also.

Can you comment on how you knew is was -3. Is that just from experience or was there something in the code that gave it away.

Thanks.

You're welcome.

The 3 is from the second long string: 31301300