After having disrupted the operation of the Waledac and Rustock botnets, Microsoft has set its sights on a smaller one that is thought to be an attempt to rebuild the Waledac botnet.

Microsoft has used the same tactics it employed in the previous cases - it asked a U.S. court for permision to shut down the Internet domains/command-and-control servers for the botnet.

But what makes this case unique is the fact that for the first time, a defendant was named in the suit and was notified of the action.

"In the complaint, Microsoft alleges that Dominique Alexander Piatti, dotFREE Group SRO and John Does 1-22 of owning a domain cz.cc and using cz.cc to register other subdomains such as lewgdooi.cz.cc used to operate and control the Kelihos botnet," writes Richard Boscovich, Senior Attorney with the Microsoft Digital Crimes Unit. "Our investigation showed that while some of the defendant’s subdomains may be legitimate, many were being used for questionable purposes with links to a variety of disreputable online activities."

The botnet - dubbed Kelihos - is rather small. Nevertheless, it is capable of sending out nearly 4 billion spam emails per day from approximately 41,000 computers located worldwide.

"Kelihos infected Internet users’ computers with malicious software which allowed the botnet to surreptitiously control a person’s computer and use it for a variety of illegal activities, including sending out billions of spam messages, harvesting users’ personal information (such as e-mails and passwords), fraudulent stock scams and, in some instances, websites promoting the sexual exploitation of children," Boscovich shares, and adds that the defendants' cz.cc domain has previously been known for hosting subdomains serving various malware, including the MacDefender rogueware.

After having been granted the request for a temporary restraining order, Microsoft has moved to sever the connections between the C&C centers and the bots. Immediately after the takedown Dominique Alexander Piatti and dotFREE Group SRO were served with the notice of the lawsuit. Microsoft is still trying to do the same for the other John Doe defendants.

"Through this case, we hope to demonstrate that if domain owners don’t hold themselves accountable for knowing their customers, they will be held accountable for what is happening on their infrastructure," points out Boscovich. "Our goal is for this case to spur an industry-wide discussion for more public and accountable subdomain registration practices to enable a safer, more secure Internet for all users."

As in the previous instances, Microsoft will take part of the cleanup action by adding the Win/32 Kelihos family to its Malicious Software Removal Tool.