South Korea's second-largest wireless service provider KT Corp apologised on Sunday after the personal data of millions of mobile phone subscribers was hacked.
It is the latest in a string of large-scale personal information hacking cases in one of the world's most wired countries.
Police said two computer programmers had been arrested for hacking personal data of about 8.7 million KT subscribers. KT claims a mobile service subscription membership of 16 million.
Police were also arrested, but did not detain, seven others suspected of having purchased and used the hacked KT data, which included names, resident registration numbers and phone numbers.
"We deeply apologise for worrying you," KT said in a statement.
The company said it had blocked any further illegal access to data. The hacking began in February.
The Next Web reported that the data was sold to telemarketers by two hackers, who had made $US880,000 by selling the information.
KT only contacted the Korean police on July 13 this month.
In November, more than 13 million subscribers of Nexon Korea Corp, a leading game developer, fell victim to a hacking attack.
Months before, the information of up to 35 million users of an internet portal and blogging site operated by SK Comms, the country's top mobile service provider, was attacked by hackers from China.