By exploiting the website vulnerability, the hackers were able to bypass traditional safeguards and impersonate actual credit card holders before being discovered in May, according to the New York Times.
The hackers were able to penetrate the bank's defenses by logging on to the site reserved for its credit card customers, then using automated tools to insert thousands of different account numbers sequentially into a string of text found in the browser's address bar to capture confidential data.
On June 9, Citigroup confirmed that customers' names, account numbers and contact information, including e-mail addresses, were viewed, but said other information such as birth dates, social security numbers, card expiry dates and card security codes were not compromised.
Although the method was simple, security experts say the fact that the hackers knew of the vulnerability is an indicator of the expertise behind the attack.
Hackers use sophisticated methods of attack
Law enforcement officials report an increasing sophistication in breaches by hackers targeting credit card numbers and other confidential information to meet growing demand on the underground market.
Citigroup joins a growing list of high-profile companies to report security breaches in recent weeks, with other incidents involving Sony, Nintendo, Google's Gmail and Lockheed Martin.
This story was first published by Computer Weekly