Fast Flux Botnets – Catch Me If You Can

Thursday, August 30, 2007

Cybercriminals are increasingly using an advanced method of hiding and sustaining their malicious Websites and botnet infrastructures — dubbed “fast-flux” — that could make them more difficult to detect, researchers say.  Criminal organizations behind two infamous malware families — Warezov/Stration and Storm — in the past few months have separately moved their infrastructures to so-called fast-flux service networks…..Fast-flux is basically load-balancing with a twist.  It’s a round-robin method where infected bot machines (typically home computers) serve as proxies or hosts for malicious Websites.  These are constantly rotated, changing their DNS records to prevent their discovery by researchers, ISPs, or law enforcement.  “The purpose of this technique is to render the IP-based block list — a popular tool for identifying malicious systems — useless for preventing attacks,” says Adam O’Donnell, director of emerging technologies at security vendor Cloudmark.  Fast-flux helps cybercriminals hide their content servers, including everything from fake online pharmacies, phishing sites, money mules, and adult content sites, Logan says…..The bad guys like fast-flux — not only because it keeps them up and running, but also because it’s more efficient than traditional methods of infecting multiple machines, which were easily discovered.  “The ISP would shut down my 100 machines, and then I’d have to infect 100 more to serve my content and relay my spam,” Logan says.  Fast-flux, however, lets hackers set up proxy servers that contact the “mother ship,” which serves as command and control.  It uses an extra layer of obfuscation between the victim (client) and the content machine, he says.  A domain has hundreds or thousands of IP addresses, all of which are rotated frequently — so the proxy machines get rotated regularly, too — some as often as every three minutes — to avoid detection.  “It’s not a bunch of traffic to one node serving illegal code,” Logan says.  “I send you a phishing email, you click on — but it’s really taking you to Grandma’s PC on PacBell, which wakes up and says ‘it’s my turn now.’  You’d have 100 different users coming to Grandma’s PC for the next few minutes, and then Auntie Flo’s PC gets command-and-controlled” next, Logan explains.  The home PC proxies are infected the usual way, through spam email, viruses, or other common methods, Logan says…..

What can be done about fast flux?  ISPs and users should probe suspicious nodes and use intrusion detection systems; block TCP port 80 and UDP port 53; block access to mother ship and other controller machines when detected; “blackhole” DNS and BGP route-injection; and monitor DNS, the report says…..”I don’t think there’s any question that the FFers know they are being monitored,” says Nicholas Bourbaki, an independent researcher and fast-flux expert.  That poses a few obvious challenges to an investigation, including the perpetrators turning on you: Bourbaki goes only by this pseudonym for fear of reprisal from botnet operators that he tracks and fights.  “If you are not a Trend Micro, Symantec, [or] McAfee, you are in the position of being wiped off of the map anytime you get too annoying to these criminals.”…..Researchers have witnessed a major jump in the use of this method.  Bourbaki says, in December of 2006, he and other researchers logged about 2,500 unique host names “fast fluxing” on over 18,000 unique IPs, and so far in August, there are over 14,000 host names on over 36,000 unique IPs.  And this represents just a snapshot of data on fast flux.  Bourbaki says one underlying problem is that most network equipment does not filter by host names.  “Our defense mechanisms need to be able to segment traffic and treat certain traffic with more scrutiny than others.  And they need to be able to block, or just do reputation-based [filtering] based on name server host name, not just IP,” he says.  Still, IP blacklists are essential to the fight against botnets because they keep botnets on the move, and their movement is one way to detect them.  “They either move, or they are on a blacklist,” he says.  “And if they move, that movement is detectable, and the difference between the way malicious resources move versus the way legitimate [ones do] can be used to protect the enterprise and the user.”

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