Botnets went through a remarkable period of growth and change last year, and that trend shows no signs of stopping, according to a study published last week.

Accoding to Damballa's "Top 10 Botnet Threat Report -- 2010," the botnet landscape underwent radical change last year, both in its makeup and in its targets.

"At its peak in 2010, the total number of unique botnet victims grew by 654 percent, with an average incremental growth of eight percent per week," the report states.

Of the top 10 largest botnets in 2010, six did not exist in 2009, Damballa says. Only one (Monkif) was present, ranked among the 10 largest botnets of 2009. The top 10 largest botnets in 2010 accounted for approximately 47 percent of all botnet compromised victims -- down from 2009, when the top 10 botnets accounted for 81 percent of all victims.

The biggest botnet of 2010 -- one associated with the TDL Gang -- dramatically rose to international attention in the second half of the year, claiming nearly 15 percent of all unique infected victims in 2010. "It assumed the prime position and became twice as big as the next one on the list," says Gunter Ollmann, vice president of research at Damballa.

Botnets generally operate in three- to four-week cycles, Ollmann says. They grow quickly at first, and then the growth slows as signature-based antivirus tools begin to detect and prevent them from infecting other machines, he says. At the end of the four-week cycle, botnet operators typically launch a new campaign, using signatures that will not be immediately detected.

"In 2010, more than 35 percent of the machines we were monitoring had more than one botnet on them," Ollmann says.

While most security professionals continue to view botnets primarily as a means of spreading spam or DDoS attacks, many botnet operators now prefer to use the networks primarily as a means of stealing data, which is a more lucrative business than spam or DDoS, Ollmann observes.

"A newly infected machine that works well has a certain value," Ollmann explains. "As it's used and abused, it becomes less valuable and is sold down the chain. The machines used for spam and DDoS are usually the ones that have already been harvested for everything else."

Have a comment on this story? Please click "Comment" below. If you'd like to contact Dark Reading's editors directly, send us a message.