International hacker group Anonymous on Thursday broke into Uriminzokkiri, a North Korean website that spreads propaganda from North Korea's official KCNA news agency.
The hacker collective released the personal information of the website's 9,001 subscribers, including their ID and password, name, date of birth and other details.
The South Korean National Intelligence Service said, "Many of the leaked details on the website match those of South Koreans."
Police are taking a keen interest. A National Police Agency spokesman said, "A considerable number of people who want to access information about North Korea are registered on Uriminzokkiri. If they have carried out any pro-North Korean activities such as spreading articles from the site or posting pro-North Korean comments, we will take actions since that violates the National Security Law."
The site, which is run by North Korea's Committee for the Peaceful Reunification of the Fatherland, is blocked in South Korea but can be accessed using proxy servers.
Anonymous in a statement on Tuesday urged North Korea to stop developing nuclear arms and threatening the world with nuclear weapons, and called for Kim Jong-un to step down. It also warned of a "cyber war."
Pyongyang did not react to the hacker attack, which also targeted other North Korean websites. A South Korean security official said, "Given the nature of North Korean system, where decision-making takes a long time, there's unlikely to be any official comment till the weekend."
But North Korea experts say the attack must have hurt. Besides the identity theft, the hackers also managed to keep a satirical image showing Kim Jong-un as a pig with a Mickey Mouse tattoo on the front page of the website all day.
North Korea also suffered cyber attacks from unknown sources on March 13 and 14. Access to websites with servers in North Korea including the Rodong Sinmun, North Korean Central News Agency and Naenara was disabled.