The personal details of as many as 20 million South Koreans may have been exposed after an employee at a credit ratings firm was arrested on suspicion of selling the records to marketing firms.

The temporary consultant, who worked at the Korea Credit Bureau (KCB), is suspected of lifting the data from the servers of KB Kookmin Card, Lotte Card, and NH Nonghyup Card, before selling it to phone marketing companies.

The data included customer names, social security numbers, credit card numbers and expiry dates, the Korean Financial Supervisory Service (FSS) said in a statement seen by AAP.

"The credit card firms will cover any financial losses caused to their customers due to the latest accident," the regulator said.

An investigation has also been launched into whether the security measures the affected firms had in place weren't up to scratch.

This kind of thing is not unheard of in South Korea.

In 2012, two hackers were arrested for illegally obtaining the personal details of 8.7 million KT mobile customers before selling it on to telemarketing firms.

A year earlier, a breach at South Korean game developer Nexon exposed data on 13 million subscribers, while local retailer Shinsegae and several others were hit in 2010 in an attack which stole 20 million customer account details.

However, the biggest to date remains the attacks on the Cyworld social networking website and the Nate web portal, which breached personal info on as many as 35 million users.

To put that in perspective, there are around 50 million people living in the north-east Asian nation today. ®