Over 360,083 credit card accounts of Citigroup bank customers were affected as a result of a compromise of its card account management website in May, the bank said in an update.
These were accounts issued in the US, the bank said.
Citigroup first disclosed publicly the compromise of Citi Account Online last week, when it said that about 210,000 accounts had been affected. The Attorney General of the State of Connecticut, George Jepsen, said that Citigroup's disclosures about the data breach failed to explain how it occurred, and what is being done to protect affected customers from potential financial fraud.
Customers are not liable for any unauthorised use of their accounts, Citigroup said in a statement that may be a response to Jepsen's demand for information and additional data.
The main cards processing systems and other consumer banking online systems were not compromised, the bank said.
The customers' account information such as name, account number and contact information, including email address were viewed by the hackers. But data that is critical to commit fraud, such as the customers' social security number, date of birth, card expiration date and card security code (CVV), was not compromised, Citigroup said.
It said that for the security of its customers, and because of the ongoing law enforcement investigation, it could not disclose further details of how the data breach occurred. The bank said it has implemented "enhanced procedures" to prevent a recurrence of an event of this type.
A total of 217,657 accounts were reissued credit cards along with a notification letter. Some accounts were not reissued credit cards if the account is closed, or has already received new credit cards as a result of other card replacement practices. These accounts continue to receive heightened monitoring for suspicious activity, the bank said.
Citigroup has been criticised for delaying in communicating to customers that their personal data had been compromised. The details released confirm that Citibank issued notification letters to customers on June 3, over 20 days after it detected a data breach.
Tracing the chronology of events, Citigroup said that on May 10, a compromise to Citi Account Online was found as part of routine monitoring and immediately rectified.
Internal fraud alerts and enhanced monitoring were also placed on all accounts deemed at risk. The bank simultaneously began rigorous analysis to determine the precise accounts and type of information accessed, Citigroup said.
The majority of accounts impacted were identified within seven days of discovery. By May 24, the bank confirmed the full extent of information accessed on 360,069 accounts. An additional 14 accounts were confirmed subsequently. To determine the cardholder impact required analysis of millions of pieces of data, Citigroup said.
While the investigation was underway, preparations began to notify customers and, as appropriate, replace affected customers' credit cards, the bank said. As of May 24, it began the process of preparing notification packages including customer letters and manufacturing replacement cards, as well as preparing its customer service teams. Notification letters were sent beginning June 3, the majority of which included reissued credit cards.