KT has fallen victim to a major and prolonged data breach involving the theft of personal data on around 12 million subscribers.
South Korean authorities have arrested three suspects for allegedly hacking into KT's website and stealing the records, Yonhap Newsreported.
Police say the stolen data includes victims' names, resident registration numbers and bank account information. The theft allegedly took place over the span of a year, with attackers collecting up to 300,000 records in a single day.
South Korea's telecom ministry has announced plans to set up a team of government officials, online security specialists and members of telecom regulator KCC to investigate the incident. The ministry will also set up a 24-hour call center to allow victims to report on damages incurred from the breach.
KT has meanwhile been ordered to inform victims about the breach and set up a webpage to allow users to check if their information was taken.