Earlier this week, Google's Matt Cutts announced on Twitter that the company had sent warnings to 20,000 Web sites that they may have been hacked and infected with malware.

"The team has noticed that these sites are performing 'weird' redirections, possibly leading to malicious web sites," writes Tom's Guide's Kevin Parrish. "The team also warned that server configuration files may have been compromised as well."

"The search giant isn't required to notify webmasters that their sites are vulnerable; however, as it trawls the web to index sites, it is able to discover which sites have malicious content, and it chooses to notify the sites affected," writes ZDNet's Michael Lee. "The email advises webmasters to check their source code for unfamiliar JavaScript and files that contain known malicious code."

"Webmasters often don't notice that their site has been compromised because the malicious redirects are triggered only when a visitors comes to the site by clicking on the results of a Google search -- something that site owners or operators never do," notes Help Net Security's Zeljka Zorz.

"It is not the first time Google has warned website owners to look for malware infections, Google spokesman Mark Jansen said in an email," writes Computerworld's Loek Essers. "'It's part of our ongoing mission to be transparent with webmasters and do our bit to help prevent spam,' he said. 'In fact this isn't a new phenomenon; we communicate very openly with webmasters and always have done.'"