Cyber attacks on the biggest U.S. banks, including JPMorgan Chase & Co. and Wells Fargo & Co., have breached some of the nation's most advanced computer defenses and exposed the vulnerability of its infrastructure, said cybersecurity specialists tracking the assaults.

The attack, which a U.S. official yesterday said was waged by a still-unidentified group outside the country, flooded bank websites with traffic, rendering them unavailable to consumers and disrupting transactions for hours at a time.

Such a sustained network attack ranks among the worst-case scenarios envisioned by the National Security Agency, according to the U.S. official, who asked not to be identified because he isn't authorized to speak publicly. The extent of the damage may not be known for weeks or months, said the official, who has access to classified information.