About 1% of the bank's card customers were affected by the breach, according to Citi's description to Reuters.
Customers' names, account numbers and contact information, including email addresses, were illegally accessed. Hackers failed to get other information including birth dates, Social Security numbers, card security codes and expiration dates.
"We are contacting customers whose information was impacted," a spokesman told Reuters. "Citi has implemented enhanced procedures to prevent a recurrence of this type of event."
[Related Article: Obama Proposes New Rule for Data Breaches]
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