Google's AdSense service has come under attack from a Trojan virus that replaces the text inside an infected ad and redirects the user to ads run by a different provider.

Romanian antivirus company BitDefender discovered the virus, which it says causes damages to users, publishers and advertisers alike.

In a statement, Google said it takes such threats very seriously.

"We have cancelled customer accounts that display ads redirecting users to malicious sites or that advertise a product violating our software principles," Google said. "We actively work to detect and remove sites that serve malware in both our ad network and in our search results. We have manual and automated processes in place to detect and enforce these policies."