One in 10 web pages scrutinised by search giant Google contained malicious code that could infect a user’s PC.
Researchers from the firm surveyed billions of sites, subjecting 4.5 million pages to “in-depth analysis”.
About 450,000 were capable of launching so-called “drive-by downloads”, sites that install malicious code, such as spyware, without a user’s knowledge.
A further 700,000 pages were thought to contain code that could compromise a user’s computer, the team report.
To address the problem, the researchers say the company has “started an effort to identify all web pages on the internet that could be malicious”.