Analysis The unidentified criminals behind the infamous Rustock botnet were paying at least $10,000 a month for US-based command and control servers prior to a successful takedown operation last week.
Instead of using bulletproof hosting outfits (rogue ISPs normally based in eastern Europe) that ignore takedown notices, the botherders behind Rustock attempted to hide in plain sight...
Alex Lanstein, a security analyst at FireEye Malware Intelligence Labs, who worked with Microsoft on the successful takedown operation, credits the tactic as being the main reason why Rustock stayed in operation for more than five years.
Rustock, which spread largely via drive-by download from compromised websites, turned compromised machines into spam-spewing monsters. At its peak, the botnet was responsible for almost the worldwide production of spam. Much of this junk mail promoted sites selling unlicensed pharmaceutical drugs, such as Viagra. With the closure of the world's largest spam affiliate program – spamit.com – back in October, this volume of junk mail dropped slightly. This was essentially because one of the botnet owner's main clients had shut up shop, but Rustock was still responsible for at least 30 per cent of global spam.
The dismantling of the core network of around 26 servers that were used to control infected clients meant that this spam torrent suddenly trickled away to nothing last Wednesday.
Rustock: My part in its downfall
The operation to mount the takedown began nine months previously, when Microsoft held a conference call with security partners. During the call, Rustock emerged as one of the primary threats facing Redmond's customers. A loose-knit committee focusing on dismantling Rustock was formed which, over time, divided itself into four organisations as other security researchers dropped out to tackle other projects. The four groups included representatives from Microsoft itself, along with staff from pharmaceutical giant Pfizer, as well as security researchers from FireEye and computer scientists from the University of Washington.
Lanstein, who had previously worked on the takedown of the Mega-D and Szerbi botnets, told El Reg that the pooling of resources was necessary because Rustock was a more sophisticated threat and everyone had only part of the overall jigsaw. "Everyone learned from each other," he explained.
Compromised machines were infected with malware packed via custom encryption techniques that made it look like a .rar compressed archive file. Each machine communicated with control nodes via posts that, to a casual observer, would look like posts to a bulletin board or web forum. In addition, the command and control servers were all US-based and mostly located with small hosting firms who had no idea that anything was amiss... and not the specialist cybercrime-friendly hosting facilities in eastern Europe beloved by most cybercrime gangs.
Using this approach, the Rustock botnet herders were largely able to keep their systems off the radar of collaborative anti-spam and net security efforts run by the likes of Spamhaus and Shadowserver.
FireEye was able to fingerprint what communication between zombie clients and the command nodes of the Rustock network looked like. In this way it was able to identify the main command nodes and backup servers used by Rustock.
Microsoft is operating sinkhole servers for Rustock command and control domains so they are able to identify PCs compromised with botnet software, providing more useful raw intelligence on the zombie network in the process.
The project members then turned their energies towards creating a plan to decapitate these systems in one fell swoop. A piecemeal operation that took out command nodes one by one would not work because the cybercrooks would have an opportunity to move control systems, update code and instruct zombie clients where to look.
Instead legal authority was sought to mount a series of co-ordinated raids, seizing equipment at key hosting facilities. This kit was often not owned by the hosting providers themselves, creating a legal hurdle to the action.
"Microsoft filed the case early this year but there was no supporting case law," Lanstein explained.
Microsoft had to show that the extra workload of sending spam that passed through Hotmail from machines in the Rustock botnet was costing it money before connecting these zombies back to their control nodes and showing violation of US federal anti-spam laws (specifically the CAN spam Act).
The effort allowed the seizure of third-party kit and sets an important legal precedent that may become useful in future cases, Lanstein told El Reg.
Lanstein said he is still not sure who was behind Rustock, although "all indications point to eastern Europe". He reckons no more than three cybercrooks managed the botnet, issuing (encrypted) instructions and spam templates via intermediate servers linked to the two-dozen-or-so main command nodes of the botnet network that compromised clients polled for instructions.
"The money they must have been making was off the chart," Lanstein, adding that the criminals had been spending $10,000 a month on hosting costs alone.
Other botnet operators (such as the criminals behind the infamous Mariposa botnet) have given themselves away in the effort to re-establish control of their botnets following takedown operations. This did not happen in the case of Rustock.
Compromised machines are still infected, but are now seeking instructions from sinkholes controlled by Microsoft. Redmond has also pre-registered backup domains that Rustock zombies are programmed to contact in future.
Rustock cybercrooks should be checking every shadow
Re-establishing control at this stage would be difficult, but it is not completely impossible. Lanstein reckons it is far more likely that the cybercrooks behind Rustock will move onto other projects. They will likely use their skills to build another spam bot, or morph the current code base to develop a data stealer or DDoS bot, he warns.
Microsoft and a team of forensic experts are examining the seized servers in a bid to find clues that might lead towards Rustock's ultimate controllers. Ultimately, as Lanstein notes in a detailed technical write-up of the bot and the takedown operation, only the placement of a "pair of shiny silver bracelets" on the wrists of the botherders behind Rustock is likely to take them away from the cybercrime scene.
"If I were the operator, I would not be sleeping well knowing that the full resources of the Microsoft Corporation were available to the investigators trying to track me down," he concludes. ®