Customers of Bank of Bermuda and Butterfield Bank may have had their cardholder data sent out to an unknown third party, according to Heartland Payment Systems.

The New Jersey-based vendor has advised banks across the globe, including the Bank of Bermuda and Butterfield Bank, of a data breach that has compromised a significant number of Visa and Mastercard cardholders.

Heartland Payment Systems, which providing payment processing for approximately 200,000 businesses worldwide, confirmed a data breach detected early last month. It discovered a hidden malware on its servers which enabled cardholder data to be transmitted to an unknown third party.

Lisa Fox, head of card services at the Bank of Bermuda said that the bank was working to safeguard its affected customers by contacting them directly, closing compromised accounts and issuing replacement cards.

"We strive to be proactive in these instances to ensure our customers are minimally affected and we remain focused on ensuring all areas under our control have the highest available security measures in place," she said.

Butterfield Bank said it was made aware of the problem, which may have affected some of the debit and credit cards it issued. In response, the bank has increased fraud monitoring on accounts that were potentially impacted by the security breach, prohibiting authorisations from certain merchant locations, while customers were contacted to verify any transactions attempted at these restricted locations. Butterfield Bank said that if fraud or attempted fraud was confirmed on a customer account, it would automatically issue a replacement card with a new account number.

Bank of Bermuda cardholders can report any suspicious activity to the bank's 24-hour call centre at 299-5518, while Butterfield Bank cardholders can call 295-1111 in Bermuda and (781) 756-8169 outside.