Citigroup have said that hackers accessed the credit card information of tens of thousands of its North American customers.
The bank said it recently discovered during routine monitoring that account information for about 1% of customers was viewed.
Citi has more than 21 million credit card customers in North America, according to its 2010 annual report. The New York-based bank did not say exactly how many accounts were breached.
The hackers were able to gain access to Citi's Account Online service to view customer names, their account numbers and contact information including email addresses.
They were not able to gain access to social security numbers, birth dates, card expiry dates or card security codes.
The Citigroup online security breach comes on the heels of the high profile hacker attacks against multiple Sony sites. An attack against the PlayStation Network site affected more than 100 million online accounts and forced the site to temporarily shut down.
Sean Kevelighan, a spokesman for Citi's North America Consumer Banking division, said in a statement that it is contacting the customers affected and putting in procedures to prevent the security breach happening again.
"For the security of these customers, we are not disclosing further details," he said.
The hacker attack was first reported by the Financial Times.