More than 10,000 websites have become unwitting hosts of malicious software, say security experts.
Those visiting the hijacked pages risk having keylogging software installed on their PC if it is not protected with the latest patches.
The webpages compromised are all legitimate sites devoted to subjects such as tax, jobs, tourism and cars.
The sites are thought to have been booby-trapped using a malware kit, called MPack, sold commercially online.
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