The April 2010 MessageLabs Intelligence Report also revealed that Rustock has surpassed Cutwail as the biggest botnet both in terms of the amount of spam it sends and the amount of active bots under its control.

The report noted that Rustock has reduced the output of individual bots by 65 per cent but increased the number of active bots by 300 per cent, thus, making up for the decreased output. Meanwhile, Cutwail has reduced in size to 600,000 bots from two million bots in May 2009 and is now responsible for only four per cent of all spam. "Rustock remains the largest spam-sending botnet responsible for 32.8 per cent of all spam," the report read.

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