A contractor working for the Korea Credit Bureau, a firm that produces credit scores, allegedly used the company's access to databases containing information on three South Korean credit card firms to copy the data to a USB drive, according to the BBC.
The contractor, who has been arrested, allegedly sold the stolen information to marketing firms, the BBC reports. Compromised information includes names, Social Security numbers and credit card details.
The three credit card firms affected by the breach are KB Kookmin Card, Lotte Card and NH Nonghyup Card. Regulators are looking into the security measures in place at the three companies, according to the BBC report.
The Financial Supervisory Service did not immediately respond to a request for comment.