JP Morgan has warned 465,000 customers using its prepaid cash cards that personal information may have been compromised after it was targeted in a cyber attack earlier this year.
The prepaid UCards are used by US companies to pay employees, and by government agencies to pay benefits such as unemployment compensation.
A bank spokesperson told Reuters that data relating to card holders was stolen in July as attackers targeted www.ucard.chase.com web servers, but did not reveal the methods used to breach its network.
The bank first detected the hack in September, before fixing the issue and informing authorities. In the months since discovering the hack, JP Morgan investigated which accounts had been accessed and what data had been taken.
Although the card holder information is usually encrypted, personal data temporarily appeared in plain text files.
The bank said that only a small amount of data was retrieved in the attack, and that the information did not include important information such as social security numbers, birth dates or email addresses.
It is not thought that any funds were stolen as a result of the breach, though the data could be used to set up fraudulent card accounts.
JP Morgan said that it has offered customers a year's free credit card monitoring services, but would not be issuing replacement UCards following the hack.
The bank said that it does not know who was behind the attacks, but the issue is currently being investigated by the FBI.
Earlier this year JP Morgan's consumer banking website was taken down following a distributed denial-of-service (DDoS) attack. It was later speculated that the attack was launched by hacktivist group Martyr Izz ad-Din al-Qassam Cyber Fighters, as part of a number of attempts made to compromise US banks' security.