A message went out from Google to the webmasters of 20,000 sites saying their sites may have suffered a hack, said the head of Google’s Webspam team, Matt Cutts.
In the email message, the company warns operators the sites look like attackers are using the sites to redirect visitors to a malicious site.
The email contains a link to Google’s Webmaster Tools support page with instructions designed to help web masters clean up their sites.
Google also asks the administrators to close the security hole exploited to infect the site. Google started warning webmasters in this way in late 2010. At the time, the company said it also intended to warn users about visiting infected sites in its search results.