The Iranian government has denied involvement.

The victimized banks, which have not released details on the attacks, are not the first to be struck this year by such highly sophisticated DDoS assaults, said Prolexic. Other large companies in the U.S., Middle East and Asia have been struck in the financial services, retail and online gaming industries.

Prolexic, which did not see any such attacks last year, expects the number to continue growing. "As the world gets more volatile, and we're seeing it right now, the propensity for these types of attacks, based on the past trends, is going to continue to go up," Hammack said.

in CSOonline's Malware/Cybercrime section.