XDocCrypt/Dorifel – Document encrypting and network spreading virus

Another day, another malware, and today it was an unknown Delphi application which encrypts your office documents on your non-root and non-CD/DVD drives and prepends it with a copy of itself, turning a document into an executable. Great, everybody loves these kinds of things as your entire IT dependant organization will grind to a complete halt if it hits your organization. So, how did people get infected by this? Well as it seems it was not a drive-by exploit on a large news site or some compromised advertisement server which caused this malware to run, but instead an already existing ZeuS variant named Citadel which downloaded and executed “a.exe”.

Should you worry about all those encrypted document files on your network…, what you should really worry about, is that there apparently was/is a trojan (ZeuS/Citadel) on your network that was doing active C&C communications and has been leaking all kinds of information from your organization for days, weeks or perhaps even months. And apparently none of your IT security defenses has removed it, has blocked it and neither has signaled you that there was something wrong on that system. If you were hit, you will likely start asking yourself some questions now… A properly configured IDS would have picked up the attack earlier and you would have been notified of the event.

Communication to the following IP addresses might indicate malicious behavior on your system:

184.82.162.163

184.22.103.202

So the big question is, why did it encrypt files… because it is not ransomware as it has no ransom note, the answer is, because they can. Interesting are also the references to the FGint library and RSA crypto, which was probably a failed experiment on behalf of the author. But RSA was not used for the encrypted documents, no it was something much more common in malicious software, actually the most common crypto apart from plain xor… RC4.

So, how would you recover such a file? Well the best method would be to use the RC4 key and use the standard RC4 implementation and decrypt the crypted document. You can find this from the separator “[+++scarface+++]” at offset 0x24a00 or 0x25000 depending on the infector version, and not forgetting to skip the last 7 0-bytes, otherwise Office will likely complain when opening the file. The RC4 key appears to be consistent in all versions: \x0d\x0a\x05\x0f\x59\x7b\x38\x5a\x5b\x36\x31\x69\x7e\x0d\x0d\x09

An alternative method for the less adventurous, or perhaps more adventurous users, if you actually run the file, it will decrypt and save a copy of the document to the same filename with “–.doc” appended to it and open it in the default application for that file type. In case nothing else works, or perhaps in case of modified crypto in newer versions and failing tools, this might be a quick way to recover an important file. Note: we do not advise this, but if you have to do this, please do it in a virtual machine without network connectivity. The actual function in the executable used for this is:

You can see in the function the separator of the file scarface, but encrypted with another common encryption function in malicious software, rot13. Other interesting strings are “SayHellotomyLittleFriend” a quote from the movie Scarface and “BreakingBad” a TV series.

The big question is of course, what is the purpose of this Trojan, one might suspect it is ransomware, but without a ransom note I guess that would be a no go. The fact that it infects shares means that it will spread to other systems that open the infected ‘documents’ on a share. Additionally HTTP based connection functionality suggests that the Trojan has additional download tasks and likely executes additional payloads on systems that have been infected. Given the Modus Operandi of this operation, it is likely that it downloads the Citadel Trojan and this entire attack was just to increase the size of the botnet through spreading of network shares. Currently however there appears to be no task defined and no additional malware is downloaded.

The interesting thing with this attack is that it appears to target NL pretty badly with over 2200 infections during the night, with the majority of the infections taking place in The Netherlands and only around 100 in Denmark and only few in other countries. The loader panel can be observed below:

All together it is a pretty interesting attack, which is obviously very visible due to the fallout with encrypted documents. And also due to the large amount of public sector organizations which were heavily affected by this attack.

Thanks to SurfRight for infected office document samples and the blog at http://www.damnthoseproblems.com/ for listing a lot of the information during the attack shared with the public.

Update:

SurfRight has provided a decryptor.

McAfee extra.dat file:  https://www.medusoft.eu/w32xdoccrypt-a/#.UCNwk03N-Ao

TrendMicro has detections for Dorifel / QUERVAR here.