Rob Rachwald, director of security for Imperva, said an all-volunteer army launching such an attack is in unusual. Hacktivists often use a combination of supporters and botnets, he said. In addition, rather than try to overwhelm the bandwidth of a large bank, attackers often find a vulnerable component in the site first and target traffic to just that area.

While he hasn't monitored the recent attacks, Rachwald said he believes the attackers are much more sophisticated. An indication of that is the fact that the hactivists posted warnings in advance, naming the targeted banks. Nevertheless, the banks were unable to prevent disruption.

"It tells you that more than likely the attackers were pretty sophisticated," he said. "They're using some new technique, or variation of older techniques to bring the sites down."

None of the banks have given details of the attacks.

Ideologically motivated hacktivism was the primary motivation behind DDoS attacks last year, according to Arbor Networks' . The number of high-bandwidth DDoS attacks increased significantly, with 25% exceeding the total bandwidth into a data center.