The zombie machines which formerly powered the infamous Rustock botnet are down to half their original number, according to Microsoft.
Redmond ran a successful takedown operation back in March that effectively knocked out Rustock's command and control nodes. That meant that infected PCs were no longer being sent spam templates or other instructions while doing nothing to alter the fact that they were contaminated with malware.
Subsequent efforts, including the addition of Rustock botnet disinfection agents to the Microsoft Malicious Software Removal tool, have reduced the number of infected hosts by more than half.
Worldwide Rustock infection rates are down from a zombie count of 1.6 million in the middle of March to 703,000 in the week ending 18 June, the latest available figures. India, the US and Turkey are the most infected countries, but the malware itself remains widely spread across the world, as explained in a blog post here.
Microsoft has pledged to continue its clean-up efforts. In the meantime it is chasing leads on the owners and operators of the botnet. The Rustock control servers were rented by two Russians - Vladimir Alexandrovich Shergin and Dmitri A Sergeev - who have each become key suspects in the case.
Rustock began operation in 2006 and, at its peak, accounted for more than 13.8 billion spam emails daily, most of which punted unlicensed pharmaceutical websites. The take down operation was the second of its type. Microsoft was also heavily involved in the takedown of the Waledac botnet in March 2010.
A Special Edition of Microsoft's Security Intelligence Report provides more information on the Rustock threat and can be found here. ®