A large network of hacked computers called Rustock, which was responsible for a great volume of spam, has shut down, perhaps as a result of another co-ordinated take-down by security researchers.
Rustock's inactivity, first reported by security writer Brian Krebs on his blog, started on Wednesday when analysts noted that its command-and-control servers were offline.
Richard DG Cox, the CIO for the spam-tracking organisation Spamhaus, said on Thursday that he was waiting for more information from US-based colleagues about why Rustock is not working.
He noted that while it is possible that Rustock's decline is the result of another joint action by the security community, it could be that Rustock's operators are just reorganising, since the botnet has slowed down before.
"We are aware something is happening," Cox said.
The security company M86 wrote on its blog that Rustock completely stopped spamming, and its command-and-control servers would not respond.
"It is unclear yet who or what caused the shutdown," wrote Phil Hay, an analyst with M86. "It's also possible it has been abandoned. Over the past three years, Rustock has been responsible for a huge amount of spam, at times representing half of all spam caught in our spam traps."
Hay said that Rustock's spam output dropped after the closure of Spamit.com, a Russian website for email spam affiliates linked to GlavMed, which ran the "Canadian Pharmacy" pharmaceutical spam campaigns.
Several botnets - Pushdo, Waledac and Bredolab - have come into the cross-hairs of security analysts and law enforcement. Microsoft marshalled a group of security researchers that attacked Waledac in February 2010.
Waledac used a complicated peer-to-peer communication system, but researchers were able to disrupt it and take control of some 60,000 computers. Microsoft also went to court to shut down domains used by the botnet to communicate. But botnets are difficult to completely stop, and there are signs that Waledac has come back.
Last October, the Dutch High Tech Crime Team, along with other organisations, shut down 143 servers used to control Bredolab, which sent an estimated 3.6 billion emails a day. Armenian authorities, acting on a request from Dutch police, arrested a 27-year-old man accused of being Bredolab's controller.
Botnets manage to come back from shutdowns as code is modified by hackers and more computers become infected.
"Previous attempts at botnet shutdowns have tended to be short-lived as the botnet herders simply regroup and start again," Hay wrote. "It's too early to say bye bye Rustock, but the thought is certainly nice."