Rogue hackers are switching their attention from servers to less secure routers, with potentially disastrous results, according to the US Computer Emergency Response Team (Cert).

And a new breed of tools to aid intruders has led to a stream of self-propagating worms such as Nimda and Code Red, some of which have been used for denial of service (DoS) attacks designed to jam website traffic.

"Evolution in intruder tools is a long-standing trend and it will continue," said a Cert report. "And DoS attacks by their very nature are difficult to defend against and will continue to be an attractive and effective form of attack."

Cert warned users to "evaluate how security policies, procedures and technologies may need to change to address the current trends".