New Fast-Flux Botnet Unmasked

SAN FRANCISCO -- RSA Conference 2011 -- A researcher has discovered a new botnet that uses the rare fast-flux method to stay alive and evade takedown.

Joe Stewart, director of malware research for Dell SecureWorks Counter Threat Unit, here yesterday showed a sample of the botnet's malware he had reverse-engineered, with evidence that the botnet uses fast-flux. Fast-flux is basically load-balancing with a twist: It's a round-robin method where infected bot machines serve as proxies or hosts for malicious sites and are constantly rotated, changing their DNS records to prevent discovery by researchers.

The now-defunct Storm and Warezov/Stration botnets were the first major ones to use fast-flux, but despite worries by researchers that this evasion method would catch on, it has remained rare. Avalanche/RockPhish and Warezov also used fast-flux to keep a low profile. "When fast-flux first came out, it was thought everybody was going to use it. That never materialized," Stewart said.

That's because an extra level of expertise and effort is required to design the botnet this way, he said. Stewart dubbed this new fast-flux botnet "Wibimo," after a moniker "Wild Big Money" he found associated with the code in an online forum.

"Fast-flux is an individual choice made by [botnet operators] based on how much takedown they can tolerate," he said.

Stewart first spotted evidence of Wibimo while working on his new top spamming botnets report, which was released this week. He finished reverse-engineering the bot samples he acquired while here at the RSA Conference and was able to confirm it was a fast-flux botnet. "I don't think it's [a] huge [botnet]," he said. "But if feels like a new botnet: It doesn't mesh with what we've seen" with existing botnets.

Wibimo pushes fake pharmaceutical spam, and uses a pay-per-install model, likely with Virut, Stewart said. And its author appears to like the No. 10, Stewart said: For example, every 10 seconds, it connects to a fake pharmaceutical site, and it uses a 10-round encryption style for its communications and downloads.

"It runs the same encryption algorithm [RC4] over the entire message 10 times," he said. Most typically use just one round with RC4, he notes. It's still possible to reverse-engineer it, but it makes crypto-analysis of the code more difficult. "He's just trying to make it a little more frustrating to researchers," Stewart saidof the botnet's malware author.

The botnet, which appears to initially infect its victims via a malicious link in email, also is packaged with four modules: a proxy Trojan, a DNS proxy, a reverse-HTTP proxy, and one that gathers information on the infected system. "In this case, it's hard to say if the modules are a marketing thing [for selling the botnet package], or if it's just more convenient for them or for encouraging other development of it," Stewart said. Stewart said the botnet operators posted the modules on a free file-hosting site, disguising them as font files, and its author appears to more sophisticated than the average botnet author. "Fast-flux is harder to pull off … you have to be at a slightly higher programmer level," he said.

The only clues to Wibimo's authors is that character sets embedded into the reverse proxy and where the command and control domains are registered indicate someone who speaks Russian, he said.

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