Citigroup Inc. says computer hackers breached the bank's network and accessed data on hundreds of thousands of bank card holders in the latest of a string of cyber attacks on high-profile companies.
Citigroup said about 1 per cent of its card customers were affected by the breach, which a report in the Financial Times said had been discovered in May during routine monitoring.
The names of customers, account numbers and contact information, including e-mail addresses, were viewed, Citi said.
However, it said other information such as birth dates, social security numbers, card expiration dates and card security codes were not compromised.
"We are contacting customers whose information was impacted. Citi has implemented enhanced procedures to prevent a recurrence of this type of event," Sean Kevelighan, a U.S.-based spokesman, said by e-mail.
Citigroup joins a growing list of companies that have suffered cyber attacks.
Data storage firm EMC Ltd. offered to replace millions of electronic keys after hackers used data from its RSA security division to break into the network of arms supplier and information technology provider Lockheed Martin .
Electronics giant Sony has reported several attacks, including one in which hackers accessed the personal information on 77 million PlayStation Network and Qriocity accounts.
Sony was criticized for a delay in telling account holders that their information had been stolen by hackers.
Google Inc. revealed a major attack on its Gmail accounts targeting, among others, senior U.S. government officials that it said appeared to originate in China.
Washington has scrambled to assess if security had been compromised by the raid on Google's Gmail system, reflecting increasing concerns among global policymakers about cyber security.
Citi said it had discovered the unauthorized access at Citi Account Online, an online banking service, through routine monitoring.
In April, Citigroup global enterprise payments head Paul Galant, who previously ran the bank's credit card unit, said security breaches are a fact of life for financial institutions.
"Security breaches happen, they're going to continue to happen ... the mission of the banking industry is to keep the customer base safe and customers feeling secure about their financial transactions and payments," he told Reuters in an interview.