Threat Level: green Handler on Duty: Didier Stevens

SANS ISC: Not All Malware Samples Are Complex - SANS Internet Storm Center SANS ISC InfoSec Forums

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Not All Malware Samples Are Complex

Everyday we hear about new pieces of malware which implement new techniques to hide themselves and defeat analysts. But they are still people who write simple code that just "do the job". The sample that I’m reviewing today had a very short lifetime because it was quickly detected by most antivirus. Its purpose is to steal information from the infected computers like credentials. When the sample was submitted for the first time to VT, it reached a score of 11/59 which is not bad. Today, its score is 44/59[1].

Amongst actions like copying itself to C:\Users\%USER%\Temp\Skype\chrome.exe. It checks the victim’s computer location via hxxp:// and collects information about the victim. Then the malware creates a scheduled task to execute itself every minute:

C:\WINDOWS\system32\cmd.exe /c schtasks /create /tn "MOCLXG" /tr C:\DOCUME~1\Xavier\LOCALS~1\Temp\Skype\chrome.exe /sc minute /mo 1

The way it steals information from the victim in interesting in this case. People are often lazy so why reinvent the wheel? There already exists tools to collect credentials from applications like browsers, email clients, …

The network traffic generated by the malware is very interesting. The C2 is hosted behind a dynamic DNS host:[2]. The malware does not use the HTTP protocol but a simple TCP session via port TCP port 1340. The first things it does is to send information about the victim and the C2 return a PE file: 
We can see the location (country), date, IP address, logged user, OS, architecture and the resolution. I presume that the “No” strings indicate the presence of an antivirus and a firewall (which are both disabled in my sandbox).

The PE file is dropped on the file system, executed and the output is saved in a temporary file:

C:\WINDOWS\system32\cmd.exe /c Pl2.exe -f "Pl2.txt"

It captures passwords from well-known applications. Once completed, results are sent to the C2:

And, another tool is downloaded and executed using the same scenario:
And, the last one:
Everything is executed within a unique TCP session. This is quite simple and efficient if you don’t implement correct egress filtering.
Xavier Mertens (@xme)
ISC Handler - Freelance Security Consultant

314 Posts
ISC Handler

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