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. ®