An unidentified hacker gained access to 8 million Visa, MasterCard and American Express credit card account numbers by breaching the security of a company that processes transactions for merchants, the card companies said Tuesday.

More than 100,000 card numbers held by Canadians may be affected, including 60,000 Visa cards and 40,000 MasterCards.

The card holder associations issued a reminder that cardholders are protected against any unauthorized use. All Visa, MasterCard and American Express holders are protected under a zero-liability policy, which means unauthorized transactions on their accounts are not their responsibility.