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Cybercrime's bulletproof hosting exposed
Zeus botnets' tangled web
Researchers at RSA have identified the network framework that endows some of the worlds most notorious botnets with always-on connections that are virtually immune from takedowns.
At the network's heart are the servers that shepherd tens of thousands of infected PCs so they continue to send spam, spread malware and stay updated with the latest bot software. By maintaining multiple conduits between these master control channels and the outside world, malware gangs are able to create highly redundant networks that are extremely difficult for authorities and whitehats to shut down.
"What they've worked really hard to do for themselves is build a spiderweb of connections to the outer ring if the outer ring were the internet at large," Sean Brady, manager of RSA's identity protection and verification group, told The Register. "As you start picking off threads, they work to reroute, to crawl along different threads."
The analysis explains how an ISP - or in internet parlance, an autonomous system - known as AS-Troyak was able to reconnect to the internet shortly after two of its upstream providers severed its ties. Such bulletproof connectivity is a chief selling point to botnet operators, who are willing to pay premiums to ensure they don't lose contact with the machines they worked so hard to infect.
Near the center of the spiderweb are eight networks dedicated to keeping ISPs such as Troyak connected no matter what may happen. According to RSA, they are:
- Citygame, AS12604
- Vishclub, AS50369
- Smila, AS50390
- Mariam UA, AS42229
- Prombuddetal, AS44107
- VVPN, AS49934
- Vesteh, AS47560, and
- Bogonet, AS47821
Surrounding these networks are the ISPs that host the command and control channels. Besides Troyak, they include:
- Profitlan, AS12383
- Taba, AS8287
- Smallshop, AS31366, and
- Ya, AS44051
These ISPs are connected to nine upstream ISPs that are believed to be law-abiding businesses. If for any reason, these legitimate ISPs pull the plug - as Ihome and Oversun-Mercury are believed to have done last week - Troyak and the other four networks can fall back on the bulletproof hosts, which have their own upstream providers.
That appears to be what Troyak did some 36 hours after it lost connectivity. As a result, many of the Zeus-related bots that had been orphaned were able to find their way home again.
The resilience of bot-friendly hosts has to be more than a little discouraging for the whitehats who have spent so much time trying to take the menaces offline. But RSA's Brady said there are only a finite number of upstream providers for the ISPs to rely on.
"You get enough of them and eventually, you're going to knock this whole thing offline."
What's more, security blogger Dancho Danchev wrote here, the outages, even if temporary, challenge a core value proposition of the providers: that their bulletproof services are immune to costly interruptions.
"Not only does this mean disruption of their operations, but most importantly, loss of confidence on behalf of their customers in Troyak-AS's ability to stay online," he wrote. "What the cybercriminals are forgetting, is the fact that every time they attempt to obtain access to the botnets, they sacrifice their OPSEC (operational security)."
RSA's analysis is here. ®