Botnets are getting smaller as criminals seek to fly in under the radar of corporate network defences, security experts warned today.

Security firms Finjan and F-Secure warned that criminal gangs are splitting their botnets into smaller groups in a bid to create multi-swarm attacks that can escape detection.

"F-Secure's assertion is in line with our own trends analysis," said Yuval Ben-Itzhak, chief technology officer at Finjan.

"Our latest quarterly security trends report suggests that there are numerous new attack vectors that raise the number of Trojan infections that create botnets.

"In fact viruses have barely changed over the past year and are usually a slight variation of a previous version which is then disguised using code obfuscation techniques.

"The focus has now moved on to the crimeware toolkits that generate the infections more easily and with greater force. The resultant botnet swarm potential from such infections is significant."

Ben-Itzhak warned that botnets are being rented out for as little as $100 for a few hours.

"By escaping detection in this way, criminals can effectively fly their rented botnets under the security radar, and ensure that the swarm hits the relevant websites with devastating results. This is a potentially serious evolution in the world of botnets," he added.