The vast range of IP addresses used by banks also makes defense more difficult, as hackers try different attack techniques against applications and ports, testing for latency, or how long it takes the bank's systems to respond.

"It's not like protecting mom and pop's ABC hardware store with a single IP [address] and a couple of ports," Hammack said.

Prolexic executives won't speculate on the motivation for the attacks or what group may be responsible, but Hammack said he is "frustrated when people say this is a dumb attack by some kid in an apartment in Brooklyn."