This is the second security issue of the day for Google, following the vulnerability with the Google Toolbar, although this one is probably more serious. Anti-malware firm BitDefender has discovered that some of Google's web adverts are being hijacked by Trojans which replace the intended text with ads from another, potentially malicious, provider. The ads in question are those displayed by Google's AdSense service, which shows adverts linking to page content. Google is working on a solution to the problem, saying:

We have cancelled customer accounts that display ads redirecting users to malicious sites or that advertise a product violating our software principles. 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.

At present BitDefender is describing the Trojan as causing medium damage and spreading at a low level.