According to the report, "gigabytes of data" including customer and employee information was accessed during the attack, that took place earlier this month. None of the banks have commented publicly on the incident, while the FBI has been unable to provide further details due to its continuing investigation.
Bloomberg said that the attack appeared to be "far beyond the capability of ordinary criminal hackers", who are more likely to attack businesses with less sophisticated defences. In one case, the hackers used a software flaw known as a 'zero-day vulnerability', undetected by the affected bank, to gain access to the data through the affected bank's website, it said.
"Companies of our size unfortunately experience cyber attacks nearly every day," said a JP Morgan spokeswoman in an email to the site. "We have multiple layers of defence to counteract any threats and constantly monitor fraud levels."