Symantec identified that global spam volumes fell by 33.6 percent between March 15 and 17 following legal action against command and control hosts used by the Rustock botnet. In the days after the take down of Rustock, spam accounted for 33 billion emails per day, compared with an average of 52 billion per day in the previous week.

“It remains to be seen whether the criminals behind Rustock will be able to recover from this coordinated effort against what has become one of the most technically sophisticated botnets in recent years,” said MessageLabs Intelligence Senior Analyst, Symantec.cloud, Paul Wood.

“Rustock has been a significant part of the botnet and malware landscape since January 2006, much longer than many of its contemporaries.”

By the end of 2010, Rustock had been responsible for as much as 47.5% of all spam, sending approximately 44.1 billion e-mails per day.

Also in March, Symantec’s MessageLabs Intelligence analyzed the spam traffic from the top ten major spam sending botnets. Since the end of 2010, the more-active Bagle botnet has sent approximately 8.31 billion spam emails each day, the majority linking back to pharmaceutical products. Bagle does not have as many bots under its control, or spikes of traffic as large and dominating as Rustock, but its output has been more consistent.

In March 83.1% of global spam was sent from botnets, an increase of 6.1 percentage points compared with the 77% at the end of 2010. During 2010, botnets sent an average of 88.2% of global spam.

“Botnets have been and remain a destructive resource for cyber criminals and through the years have become the spammers’ air-supply, without which it would be very difficult for them to operate,” said Wood.

“Botnets are also used for other purposes such as launching distributed denial of service attacks, hosting illegal web site content on infected computers (known as bots), harvesting personal data from them and installing spyware to track the activities of their users.”

While Rustock has reduced the output of individual bots by 65 percent, it has increased the number of active bots by 300 percent, making up for the decreased output. Meanwhile, Cutwail has reduced in size to 600,000 bots down from 2 million bots in May 2009 and is now responsible for only 4 percent of all spam. Rustock remains the largest spam-sending botnet responsible for 32.8 percent of all spam.

“Affected by the closure of ISP Real Host in August 2009, Cutwail likely lost the ability to update some of its bots causing its numbers to diminish greatly without the ability to recover,” said MessageLabs Intelligence Senior Analyst, Paul Wood.

“As a result, Rustock has taken over significant volumes from spammers by undercutting the market with greater capacity and lower operational costs.”

Also in April, MessageLabs Intelligence analyzed passive fingerprinting (PF) signatures of spam email traffic to learn the types of operating systems that were running on the infected spam-sending computers. Many of the infected machines were running Windows and the percentage of spam with a PF signature was similar to the Windows share of the operating system market.

“Spam is more commonly sent from computers running Windows than from those running other operating systems,” Wood said. “However, spam not identified as coming from botnets was seen in lower proportions coming from Windows machines than from known botnets.”