Wednesday's Internet attacks on government Websites originated from some of the same zombie computers used in massive hacker attacks a year ago that were automatically reactivated on the same day, police said.
Korea's government Websites, including those of the presidential office and the foreign ministry, came under attack again a year after the so-called distributed denial-of-service (DDoS) shut down 25 Internet sites for hours, including 11 in Korea and 14 in the United States.
Intelligence authorities initially suspected North Korea of mounting cyber warfare, but no conclusions have been made as to who is responsible.
"We found yesterday's DDoS attacks traced to zombie computers that were used in last year's attacks and left untreated, which became active on the same hour and same day after lying dormant for a year," the Cyber Terror Response Center of the National Police Agency said in a press release on Thursday.
Zombie computers refer to those contaminated with a virus.
These computers are believed to be programmed to resume the attacks on July 7 every year, the center said.
Police said a total of 462 computers were used in Wednesday's attacks, which is very small scale in comparison to last year's that mobilized 270,000 computers.
Similar to the 2009 case, there can be second or third rounds of attacks until the third day, they said, asking computer users to download and install vaccine programs available on Internet portals to cope with further attacks.