Yesterday, ZDNet reported, Advertisements placed by Google in Web pages are being hijacked by so-called Trojan horse software that replaces the intended text with ads from a different provider, Romanian antivirus company BitDefender says.

The Trojan redirects queries meant to be sent to Google servers to a rogue server, which displays ads from a third party instead of ads from Google, BitDefender said in a statement.

Google said on Wednesday: "We have cancelled customer accounts that display ads redirecting users to malicious sites or that advertise a product violating our software principles."