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Steganography or encryption in bankers?

Dmitry Bestuzhev
Kaspersky Lab Expert
Posted November 10, 10:34  GMT
Tags: Obfuscation, Malware Descriptions, Malware Technologies, Malware Creators, Credit Cards
0.5
 

While looking over some potentially malicious links from Brazil, I came across an interesting group of files. They were of varying sizes but had similar structures.

First I thought this was some type of steganography. The files has a jpeg extension, but were in fact bmp files in structure.

It was evident that they contained encrypted malware and some additional data. After further analysis, I discovered that this was a block cipher. As far as I know, this is the first time it has been used by malware writers anywhere in Latin America. This is what the malicious program looked like after decryption:

By using this technique, the virus creators kill several birds with one stone. Firstly, it may cause automatic malware analysis systems to function incorrectly: the file would be downloaded and analyzed by the antivirus program, and given the all-clear; with time the link will be exempted from checks altogether. Secondly, the administrators of the sites where such encrypted malicious files are hosted won’t be able to identify them as malicious and will leave them as they are. Thirdly, some malware researchers may not have the time or necessary expertise to deal with them. All of this plays into the hands of the cybercriminals.
We have observed that the virus writers behind this specific attack publish new mirrors with the files and new malware every 2 days or so. So far, the encryption algorithm has been the same, but I’m sure it will be changed after this post is published. This is the decryption script for the current status:


8 comments

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RootkitResearch

2011 Nov 10, 16:29
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Threat details

Hi Dmitry Bestuzhev,

Very nice analysis. Could you please provide what is the name of this threat?

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Dmitry Bestuzhev

2011 Nov 10, 17:59
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Re: Threat details

Hi and thanks.

This particular sample is detected by us as Trojan-Banker.Win32.Delf.vh
But there are 7 malicious samples in the same chain.

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alecw

2011 Nov 10, 17:33
0
 

Here's an example of a binary hidden in a GIF...

http://wirewatcher.wordpress.com/2010/05/04/malware-in-a-bottle/

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EHackingNews

2011 Nov 10, 17:58
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Interesting

someone use the technique in 2010 itself?!

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alecw

2011 Nov 10, 18:14
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Re: Interesting

Yes, it looks like it. I've no idea what the hidden content actually was, though.

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mrhoward

2011 Nov 10, 19:24
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execution ?

Thank you for this great post. Yet I do not understand : the file is pushed to the victim, encrypted, but how is it decrypted/executed ? I mean, if the user double-clicks on the file, it will just do nothing, or print a bogus image, right ?

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Dmitry Bestuzhev

2011 Nov 10, 20:48
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Re: execution ?

Hi and thanks for the comment.
There is another binary .exe file (the eighth) which is responsible for downloading from the Web a config file, also encrypted but differently and containing keys for decryption.

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EHackingNews

2011 Nov 10, 23:25
-1
 

Re: Re: execution ?

So the .exe file won't look suspicious but it decrypts the malware and unleash it. kaspersky updated the virus definition?

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