The credit card details of almost half of all South Koreans have been stolen and sold to marketing firms. It is the nation’s largest theft of personal information involving credit-card holders.

The data was stolen by a computer contractor working for a company that produces credit scores. The names, social security numbers and credit card details of 20 million South Koreans were copied by the IT worker.

The stolen data includes email and residential addresses, phone numbers and resident-registration numbers. Prosecutors have indicted the contractor and two of the alleged buyers of the information, but say the stolen personal details has not been widely circulated.

Regulators have sought to reassure the public, but consumers are flooding call-centers to find out if their information has been stolen. The three bosses of the credit card firms involved has made a public apology for the security breach. Other executives at the three companies have offered to resign.