Malware on mainstream Web sites are becoming an increasingly larger problem as hackers are finding ways to infiltrate them, and place code that infects whomever visits the site.

StopBadware.org co-director Jonathan Zittrain called the trends worrisome, especially considering many webmasters are doing little -- if anything -- to stop it.

In its report released this month, the group said that a significant number of the new sites reported as containing badware were actually 'benign' sites that had been hacked without the knowledge of the website owner.

Two common ways to launch these attacks in the first half of 2007 were by loading a malicious IFRAME into a benign page, or through the use of JavaScript exploits, StopBadware said.

This is made possible through the rise in what the group calls "drive-by downloads," where the installation of the software occurs just by visiting an infected site.

Additional attacks seemed to be coming via social networking through the rise in spam profiles. StopBadware said many of these come with links to malicious sites, or attempt to place malicious code within the pages.

Currently StopBadware's list of malicious websites contains some 237,800 URLs.