Worried about the American Express data breach? Here’s what to do.

A hack at a third-party company may have exposed the account information of American Express cardholders, American Express Co. said in a notice to Massachusetts regulators last week.

AmEx hasn’t named the hacked company or the number of people who may be affected, though the credit card issuer advised customers in a letter to review their accounts for fraudulent charges. The breach exposed account numbers, card expiration dates and customer names, according to the filing.