Botnets are now responsible for distributing 87.9% of all spam, an increase of 2.9% since Q2 2009. With approximately 151 billion unsolicited messages each day being distributed by compromised computers, understanding who is responsible for such unprecedented levels is always of interest as, much like the threat landscape, the botnet landscape is ever changing. As highlighted in the latest analysis from MessageLabs Intelligence, the largest botnet now appears to be Rustock with an estimated 1.3 million to 1.9 million compromised computers in its control. However, estimated at half Rustock’s size, the most active botnet in terms of spam distribution is now the little-known botnet, Grum.

Both Grum and another botnet called Bobax have overtaken Cutwail as the most active spam-sending botnets, currently responsible for 23.2% and 15.7% of all spam respectively. Although significant in their own rights, their size and power highlight the dominance that Cutwail had in June...