Retailer Best Buy was one of many firms reporting that customer details had been exposed to hackers following a breach of the database of email marketing outfit Epsilon.
Dallas-based Epsilon announced on Friday that a "subset" of the firm's clients' customer data was involved an an 'unauthorized entry' into the firm's email system.
The firm went on to notify the affected clients which in turn have begun to issue their own warnings to their customers. In addition to retailers, banks including JPMorgan are known to be affected.
Most of the companies affected appear to be likewise based in North America.
The marketing outfit said the data is limited to email addresses and customer names, limiting the damage to an increased incidence of spam and phishing attacks.