There has been an increasing number of unauthorized modifications to websites that lead visitors to other sites that then infect their computers with viruses, a computer security association said.
According to JPCERT, an incorporated association providing computer security information, more than 4,000 websites have been found since April to have been altered by outsiders without their administrators being aware of the changes.
About 20 per cent of the websites were not fixed even after site administrators were informed of the alteration.
"Administrators can be both victims and perpetrators of spreading a virus," an expert said. "They should immediately announce the manipulation when they become aware of it."
"I didn't know my computer had been exposed to a virus infection," said a 33-year-old company owner in the Tokyo metropolitan area. "Why didn't the administrator reveal that fact to visitors?"
He was surprised that Yucasee Media, an information website that he has periodically checked, had been tampered with.
The website is operated by Abraham Group Holdings Co., which has companies including a consultant firm on investment in foreign funds under its umbrella. Visitors to its site include many company owners. Someone secretly modified the website to include a section that ushered visitors to a different site, whereupon their computers were infected with information-stealing viruses.
The change became a rumour on the Internet in March. The website was closed once, but reopened two weeks later. In July, a domestic security company found the site had been doctored, and reported it to JPCERT, but the web administrator did not reveal the fact. More visitors may have been infected before the site was repaired in August.