AhnLab Inc., South Korea's leading computer security company, said the presidential office, the Ministry of Foreign Affairs and Trade and Kookmin Bank were among the 29 institutions whose Web sites were attacked with malicious computer codes beginning at around 10 a.m. Some were shut down temporarily by the distributed denial-of-service attack, Yonhap News Agency reported.
"There was a DDoS attack, but no damage was done," the president's office said.
AhnLab said more attacks began at around 6:30 p.m. but there had been no major damage so far.
"There is no sign of abnormality as of now as state and corporate organizations prepared for the additional attacks and zombie computers were cured by vaccines," a company official said.
The Web site of the Financial Services Commission, South Korea's financial regulation agency, was overloaded for a time and a brokerage's online stock trading operation was briefly shut down, but officials said those operations had returned to normal.
The Korea Communications Commission, the country's telecommunications regulator, issued its third-highest alert.
AhnLab said the hackers attacked two domestic peer-to-peer file-sharing Web sites Thursday and inserted malicious codes into files that were downloaded onto PCs, triggering the cyberattack.
An estimated 4,300 to 11,000 personal computers were infected by malware that caused the attack, the South Korean news agency said. The security company is distributing free software to correct the breach.
The National Police Agency said it has opened an investigation to trace the origin of the attack.