South Korea's National Intelligence Service said North Korea was behind a denial of service attack in 2009 that crippled dozens of websites, including that of the presidential office. Seoul also believes the North was responsible for cyberattacks on servers of Nonghyup bank in 2011 and Joongang Ilbo, a national daily newspaper, in 2012.
North Korea blamed South Korea and the United States for cyberattacks in March that temporarily disabled Internet access and websites in North Korea, where a small number of people can go online.
Though Wednesday's findings were from an interim investigation report, the final conclusions were not likely to change much, said Lim Chae-ho, a professor of network security at the Korea Advanced Institute of Science and Technology.
"Future evidence will strengthen the case rather than reverse it," Lim said. "It is worrisome that the North's cyberattacks are getting increasingly severe."
Experts believe North Korea trains large teams of cyber warriors and that the South and its allies should be prepared against possible attacks on key infrastructure and military systems. If the inter-Korean conflict were to move into cyberspace, South Korea's deeply wired society would have more to lose than North Korea's, which largely remains offline.