Former security advisor to the White House Richard Clarke said the number of zombie networked machines has risen to 15 times greater than last year's figure of 2,000 compromised PCs.
"The number of botnets has gone in the last year from 2,000 to about 30,000," said Clarke, now chairman of Good Harbor Consulting. "I don't know what the average number of machines is per botnet, but you can bet it's in the thousands. The only thing I know they are good for is denial-of-service attacks [DoS]. Even if people aren't reporting it, you know they are having it."
Clarke said he thought ISPs would start taking more responsibility in helping customers to defend from DoS attacks: "We are going to see companies asking their ISPs to do more. All of the DoS attacks could be prevented if ISPs cooperated with each other."
Earlier today, Clarke said that decent civil liberties could be an integral part of good security practice. He highlighted the piloting of the trusted-traveller programme - a US two-factor identification scheme that uses iris scanning and smart cards.
"If this is mishandled, this would reduce our privacy," he said. "We have to be careful how we use this. Technology is a tool. It's neither good nor bad. There are ways of using it without eroding liberty, but you need technology to secure access to data."
Clarke was speaking at the RSA Conference in Barcelona.