The latest hack has reportedly exposed the names, account numbers and contact information of thousands of the bank's customers.
Citibank went on to specify that it did not lose certain key bits of information like customers birth-dates or card security codes.
The bank is the latest in a number of high profile companies to suffer an attack on its networks. Since the revelation that the cyber attack had in fact happened in May this year, Citibank has since come under heavy scrutiny for failing to report the attack earlier.
Speaking to Reuters a Citi representative commented, "We are contacting customers whose information was impacted. Citi has implemented enhanced procedures to prevent a recurrence of this type of event."
The company has stated that only around one per cent of its 21 million account holders were affected by the attack.
The company is yet to confirm how the attack was carried out.