Microsoft has been granted a court order to cut off 277.com domains associated with the notorious Waledac botnet.
This will effectively knock the brains of Waledac off the internet, by removing the command-and-control servers that criminals use to send commands to hundreds of thousands of infected machines.
Thought to be used by Eastern European spammers, Waledac has been a major source of computer infections and spam over the past year.
Microsoft believes the botnet can send over 1.5 billion spam messages daily.
In a lawsuit against the unknown spammers behind Waledac, Microsoft argues that Verisign, which manages the .com domain, is a choke-point for the botnet.
The court has apparently ordered Verisign to remove the botnet's command-and-control domains from the internet.
"This action has quickly and effectively cut off traffic to Waledac at the '.com' or domain registry level, severing the connection between the command and control centers of the botnet and most of its thousands of zombie computers around the world," Microsoft said in a blog.
Microsoft designed its lawsuit so the court order would sever the control ties to the botnet before its controller had time to react. "That unplugging of the Internet connection had to be done without him knowing," said Richard Boscovich, a senior attorney at Microsoft's digital crimes unit, in a video on the blog post.
Many of the affected domains already appeared to be gone after Microsoft's announcement, but others still appeared to be up.
Verisign could not immediately be reached for comment.
Because Waledac uses peer-to-peer techniques to control hacked boxes as well, Microsoft has more work to do, however.
"It's a busy night tonight and tomorrow is probably going to be a busy day as well," said Jeff Williams, director of Microsoft's Malware Protection Centre.
Williams didn't provide details on what Microsoft was doing to further attack Waledac, but in its blog posting the company said it is "taking additional technical countermeasures to downgrade much of the remaining peer-to-peer command and control communication within the botnet."
Microsoft expects to "continue to work with the security community to mitigate and respond to this botnet", the post states.
Known internally as Operation b49, Microsoft's takedown operation "was the result of months of investigation and the innovative application of a tried and true legal strategy", Microsoft said.
Microsoft tried to strike a blow against Waledac last April, by adding detection for the infection to its Malicious Software Removal tool. But that didn't stop the botnet, and spam levels have remained high.
"They didn't kill it," said Paul Ferguson , a researcher with Trend Micro. "I've been getting a boat-load of Waledac spam lately."
The majority of the domains ordered cut off are listed as having owners with contact details in China. The domains were registered with a small number of Chinese domain registrars, according to the Microsoft complaint, including one that was recently ordered by China's domain name overseer to improve its verification of customer information used to register domains.