It was thought that hackers stole this information in order to set up new accounts in the online game "Lineage." The game's operator, software company NCSoft, was offering a free three-day pass to the game to anyone that signed up. It normally costs $31 a month to play.
NCSoft said that no financial damage was reported by affected users, although end users first reported that other people were using their details to play. The company said that the data was thought to have been leaked from an internet shopping website during December and January.
The company said police had been contacted and an investigation was underway.
The company requires users to register using their national identity numbers to stop people from creating multiple accounts. The South Korean government has warned companies against this practice, as the identity number could lead to hackers obtaining other information on the users, such as phone numbers, home addresses, car registration numbers and other family members.
"We can't force them, but we have advised the companies not to request the names and resident registration numbers. In most cases, they really don't need such data," Chung Yeon-soo, manager of the Korea Information Security Agency (KISA) told the Korean Times.
According to Kang Dal-cheon, manager of KISA's legal office, the hackers cannot be punished under Korean law because no money had been stolen from users.