18 February 2003 Hackers have gained access to the accounts of five million MasterCard and Visa credit card holders in an attack disclosed late last night.

The hack was made against an unnamed company that processes credit card transactions in the US. Both MasterCard and Visa have so far refused to disclose how the attack happened, its scale or the level of penetration achieved by the hacker, but have asserted that the compromised credit card details have not been fraudulently used.

“Visa’s fraud team immediately notified all affected card issuing financial institutions and is working with the third-party payment card processor to protect against the threat of a future intrusion,” claimed Visa in a statement to Reuters.

MasterCard says that it informed banks of the security breach at the beginning of February. One US bank, New England-based Citizens Bank, has admitted that it has closed the account of 8,800 MasterCard holders whose credit card details had been compromised in the hack.

There are more than 560 million Visa and MasterCard credit cards in circulation in the US. Credit card users in the US are also covered by a zero liability guarantee, which means that they will not lose out if the compromised card details are fraudulently used.

The US Federal Bureau of Investigation (FBI) has been called in to lead the criminal investigation.