Fast Flux Taken To The Next Level With Zbot Botnet

As attackers seek to maximize their profits and minimize downtime, fast flux networks have grown into a pivotal technique for bad guys seeking to evade detection. And according to a new report out today from RiskAnalytics, there's perhaps no better example of the evolution of fast fluxing techniques than the architecture that's arisen through the highly successful Zbot botnet.

"Much like businesses, it's all about uptime and making money. For the criminals, this is the evolution of that," says Wayne Crowder, director of threat intelligence for RiskAnalytics, referring to rise of Zbot in the underground economy based on its effective fast-flux techniques.

"It's harder to take down, it's harder to track and to control, and they're all about using this to provide uptime and resources for the criminal services sold, ensuring a higher-tier service in the criminal underground," he says.

Fast fluxing is a means of upping the frustration of security team's whack-a-mole hunt for malicious traffic by hiding the infrastructure that hosts malware, phishing, and other nasty sites on a constantly-changing rotation of compromised hosts acting as proxies. The technique has been around for years, but Zbot has taken it to the next level in the advancement of techniques and the scope at which it runs.

"Participating domains return a set of 10 IP addresses for each query with a varying DNS cache time-to-live (TTL) of less than 150 seconds, forcing the addresses to be refreshed after no more than two and a half minutes," explained Crowder and his report co-author Noah Dunker, director of security labs for RiskAnalytics. "Over time, hundreds or thousands of IP addresses are used. This technique is designed to bypass IP address blocking solutions while still maintaining the advantages of a highly-available network." 

One of the techniques that makes Zbot so notable is its use of double fast-fluxing, Dunker says.

"I think what really caught my attention here was that not only did we have fast flux, but we had double flux, which is where the authoritative DNS for the botnet is hosted inside the botnet itself and it is also a moving target," he says.

Additionally, Zbot stands out in its makers' business acumen and their ability to leverage these techniques to build out its level of services, making it a nexus of crimeware campaigns. 

"We weren’t dealing with a single hostname or a tiny group of hostnames or the same kind of behavior on this fast flux botnet," says Dunker. "We were running into a very diverse list of products running on this."

The laundry list included banking Trojans, carder sites, ransomware, spam sent from within the network, click fraud, and more. Overall, Dunker says the takeaway for security professionals is that this is yet another sign of how the criminals continue to hone their trade craft.

"One of the big takeaways is that the criminals are not relying on the old guard of bulletproof hosting providers overseas," he says. "They’re building their own networks from scratch with these botnet tools."

What this means for infosec practices is that not only are the networks resilient, but it is lowering the effectiveness of a lot of endpoint antivirus solutions.

"We highlighted some really low detection rates [in the report] in fact some [Zbot] samples were completely undetected on the first day that we saw the samples. And these are things that pull down multiple different types of malware and then install them on the infected computer," he says.

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