South Korea has just witnessed one of its largest ever cybersecurity breaches, after the credit card details of a whopping 20 million people – almost half of the its population – were stolen from one of its leading credit agencies.

Data breaches in South Korea are nothing new – indeed, hacking is rife in the country. Only last year, Yonhap reported that the country sees in excess of 100,000 cases of cybercrime every single year, with the majority of data breaches the result of hacking or theft from company employees.

However, the latest case is simply staggering, even by South Korean standards. The BBC reports that more than 20 million customers from three credit card firms were affected, in a country of just 50 million people. It adds that an employee of the personal credit rating firm Korea Credit Bureau has already been arrested, and is accused of stealing the data whilst working for the company as temporary consultant.

Citing the Financial Supervisory Service (FSS), the BBC reports that the stolen data includes the names, phone numbers, credit card numbers, expiration dates and social security numbers of each card holder. It’s believed that this data was then sold to a number of phone marketing companies, whose managers were arrested earlier this month. KB Kookmin Card, Lotte Card and NH Nonghyup Card were named as the three credit card companies affected.

Regulators have now launched an investigation into the affected company’s security protocols, says the FSS, adding that any financial losses suffered as a result of the breach will be covered by the credit card companies.

The breach is just the latest in a series of high profile data thefts to affect South Koreans. Just last month, the data of more than 34,000 Citibank Korea customers was compromised, in another apparent ‘inside job’. Before that, in 2012, two South Korean hackers were arrested for stealing the data of around 8.7 million customers at the nation’s second-biggest mobile operator, while in November 2011, South Korea’s biggest online games developer Nexon saw the personal information of 13 million of its users stolen by hackers.

However the biggest data breach to date remains the July 2011 hack of Cyworld, South Korea’s most popular social networking site. Back then, hackers successfully hacked into its servers and stole personal data from a staggering 35 million people.