Hugely popular Nexon MMORPG Maple Story suffered a security breach last week, risking the personal information of three-quarters of its Korean player base.
The Korea Herald reports 13.2 million out of of 18 million South Korean accounts were affected, compromising user IDs and names.
Passwords and residential registration numbers - an important social identifier equivalent to the US's social security number - were also affected, but were encrypted. Nexon has urged users to change their passwords regardless, especially if used for multiple logins.
International players are not at risk thanks to regional server locks.
"We operate separate servers in each country. Only Korean users' information was affected," a Nexon representative said.
Nexon discovered the hack on Thursday, but has not revealed when it took place.
Maple Story is a free to play, 2D sidescroller, with over 100 million users.
Nexon is preparing to makes its initial public offering, and is expected to bring in almost $1.3 billion.