Hundreds of computers that caused a wave of outages on US and South Korean government websites last July launched new attacks on the same sites, but no major damage was reported, police said on Thursday.
More than 460 computers infected with malicious computer codes assaulted 25 websites, including that of South Korea's presidential Blue House, on Wednesday as they are programmed to attack every July 7, said Jeong Seok-hwa, a police officer handling investigations on the cyberattacks.
"But the attacks were so weak that there were no problems in accessing the sites," he said.
An analysis of an infected computer in Seoul showed that it was programmed to attack every July 7 and the malicious computer codes used were identical to those mobilized last year, said Hyun Jae-sub, another police official.
Hyun said the attacks are traced to computers which were not vaccinated after the last year's attacks. He said about 270,000 infected computers were involved in last year's assaults.
The malicious computer codes, called malware, are used in triggering so-called denial of service attacks, in which large numbers of computers try to connect to a site at the same time to overwhelm the server.
Vaccine programs can repair the infected computers and prevent denial of service attacks, according to AhnLab, a top South Korean cybersecurity company.
AhnLab said, it provided free vaccine programs to computer users following last year's attacks as part of efforts to prevent their recurrence.
Jeong said some 430 infected computers are in South Korea and others are traced to the US, Britain, China and Japan.
Hyun said the Internet Protocol addresses (the Web equivalent of a street address or phone number) showed attacks are traced to computers in South Korea but the authorities have not yet located all the infected computers in the country.
He added that those who own the computers would not even know whether their computers were infected with malware.
Police cautioned that there could be other attacks on Thursday and Friday.
The latest attack came a month after two South Korean government websites were struck with denial of service attack that officials said, was traced to China.
Last July, government websites in South Korea and the US were paralyzed by cyberattacks.
South Korean officials believed those attacks were conducted by North Korea, but US officials have largely ruled out North Korea as the origin, according to cybersecurity experts. Experts say there is no conclusive evidence that North Korea, or any other nation, orchestrated it.
South Korean media have reported that North Korea runs an Internet warfare unit aimed at hacking into US and South Korean military networks to gather information and disrupt service.