Wednesday, July 13, 2011

Binary C&C Over HTTP

A few weeks ago I gave a presentation at the CARO 2011 Workshop in Prague. Besides being set in a stunningly beautiful location, the conference was an excellent opportunity to meet malware researchers from around the world - a group who are, by and large, distinct from network security researchers.

Since I personally happen to think that the separation of these two groups is a shame (and, well, since I needed a topic that would get me out to Prague in the springtime), my presentation crossed the proverbial streams, by looking at malware-generated network traffic. Thanks to the malware sandbox we have running over here, I've got traffic like that coming out my ears.

Specifically, the presentation focused on the presence of pure binary C&C channels being sent over HTTP. After the Night Dragon trojan (SIDs 18458/18459 for those keeping score at home) created a big media stir back in February, I was struck by the realization that sending data without HTTP headers over port 80 was actually a pretty solid trick, and that other malware authors might be doing something similar. After all, basically every firewall on the planet will let you initiate an outbound connection to the Internet on that port, and net flow sure isn't going to do much good on the busiest port on any network. Where better to be a needle in a haystack?

Running through approximately 1.5 million PCAPs from the sandbox, I realized that not only was this sort of thing happening among other malware families - it was actually fairly common. In fact, a full 0.8% of those 1.5 million samples showed this sort of behavior - a number which seems small, until you realize just how much malware you could catch with extremely simple behavioral analysis.

For those interested in more details, you can read my slides here. We are willing to share samples with legitimate security researchers - provided you're willing to send relevant data back our way in return.

For those just interested in protecting their networks - we're currently working with the Snort team to find the best way of detecting traffic like this at a generic level. In the meantime, I highly suggest that you enable SID 18492 - which looks for DNS queries made by the most prevalent bit of malware displaying this behavior in our sandbox - and that you consider turning on the entirety of our blacklist.rules and botnet-cnc.rules categories, which is where we're adding most of the new rules pulled from data generated by the sandbox.
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