Those behind the attack gained access to Citibank's online banking platform, Citi Account Online, and viewed customer account numbers and contact information, including email addresses, a Citibank spokesman said in a statement send to SCMagazineUS.com on Thursday.
Social Security numbers, birth dates, card expiration dates and card security code (CVV) were not compromised.
The intrusion affected about one percent of Citibank's North American customers, the company said in its statement.
Citibank has about 21.2 million customers in North America, according to its Annual Report for 2010, meaning roughly 210,000 individuals were impacted by the breach.
The bank discovered the unauthorized access during “routine monitoring,” the statement said. The bank did not reveal how the breach occurred. According to reports, the intrusion was discovered last month.
The bank is notifying affected individuals. In addition, it has implemented “enhanced procedures” to ensure a similar incident does not recur.
Affected customers should be cautious for phishing attacks purporting to be from Citibank in the wake of the breach, Chet Wisniewski, senior security adviser at Sophos, said in a blog post Thursday.
Attackers will be able to craft realistic-looking scams using customers' names, account numbers and other personal information that was stolen, he said. These phishing scams could lead to the attackers pillaging even more sensitive information from victims.
“While Citi customers aren't likely to have fraudulent charges against their accounts as a result of this breach, they are likely to encounter social engineering attempts to enable further crime,” Wisniewski wrote.
The breach follows a rash of attacks affecting other major corporations, including Sony, Google and Lockheed Martin.