A Microsoft-led operation resulted in the takedown of key servers associated with the infamous ZeuS and SpyEye banking Trojan botnets on Friday.

Months of investigation culminated in the coordinated seizure of command-and-control servers associated with the botnets and hosted in Scranton, Pennsylvania, and Lombard, Illinois.

The takedown came after Microsoft filed suit against 39 unnamed parties on Monday (16 March) asking for permission to sever the command-and-control structures of these ZeuS botnets. The action follows the same tactics as previous successful takedowns of Waledac, Rustock and Kelihos spam-distribution botnet networks.

In a statement, Microsoft described the ZeuS takedown as its most complex to date. It said the action had the "limited and achievable" aim of disrupting the operations of ZeuS-related cybercrime operations rather than decapitating a zombie network, as in previous operations.

Cybercriminals have built hundreds of botnets using variants of ZeuS malware. For this action – codenamed Operation b71 – we focused on botnets using ZeuS, SpyEye and Ice-IX variants of the ZeuS family of malware, known to cause the most public harm and which experts believe are responsible for nearly half a billion dollars in damages. Due to the unique complexity of these particular targets, unlike our prior botnet takedown operations, the goal here was not the permanent shutdown of all impacted targets. Rather, our goal was a strategic disruption of operations to mitigate the threat in order to cause long-term damage to the cybercriminal organization that relies on these botnets for illicit gain.

ZeuS and SpyEye are essentially cybercrime toolkits for the creation of customised banking Trojans. These crimeware kits sell for anywhere between $700 to $15,000, depending on the version and features of the kit. Many cybercrime gangs use ZeuS as the launchpad for banking fraud so there are many different zombie networks at play.

Microsoft has detected more than 13 million suspected infections of ZeuS and SpyEye-related malware worldwide, with more than 3 million in the United States alone.

Friday's takedown action follows months of work by investigators at Microsoft in co-operation with officers from the Financial Services – Information Sharing and Analysis Center (FS-ISAC) and the National Automated Clearing House Association, the US electronic payments association. Net security firm F-Secure is credited with providing a major help in analysing the malware that feature in the operation. US Federal Marshals accompanied investigators in raids on two hosting firms, during which servers was seized for subsequent analysis. Microsoft's statement fails to clarify this point but other reports strongly suggest the hosting firms involved were unwittingly playing host the key infrastructure resources associated with the ZeuS botnets rather than acting as accomplices in cybercrime.

The operations resulted in the dismantling of two IP addresses behind the ZeuS ‘command-and-control’ structure. Microsoft has started monitoring 800 domains secured in the operation, helping it to identify thousands of ZeuS-infected computers. ®