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Malware threats on the Web grew at an exponential rate over the first half of 2008, security experts say.

During this period, there was a new Web page infected with malicious content every five seconds, according to Sophos. That's an increase from a rate of one every 14 seconds over the latter half of 2007.

Businesses were a major target of such attacks, as cyber-criminals used Web 2.0 channels of attack to infiltrate their corporate networks.

Two major security vendors have released reports on threats faced by Internet users everywhere.

The reports paint a grim picture of growing attacks against legitimate Web sites, many of them social networking sites containing user-generated content.

The number one host of malware was Google-owned Blogger. The popular blog-hosting service allows users to quickly set up their own personal Web page and post content. That makes it easy for hackers to take advantage of the service.

That's not a slur on Blogspot, according to Graham Cluely, senior technology consultant at Sophos. "The guys at Google are very good at rapidly shutting down infected Web pages. But even if you have a page shut down, it is very easy to start up a new one."

Out of the top 100 most popular Web sites, 60 have either hosted or been involved in malicious activity, according to a report from Websense. It's an example of hackers using legitimate Web sites to target unsuspecting victims.

"With their large user base, good reputations, and support of Web 2.0 applications, these sites provide malicious code authors with abundant opportunity," the report says.

Anecdotal evidence of the malware attacks reveals how cyber-criminals are becoming more cunning, relying on the trust users place in famous social network sites.

MySpace unknowingly ran an advertising banner that pushed malware on to unsuspecting users. A third-party Facebook application called "Secret Crush" was actually ad-serving software in disguise.

"A lot of our customers have been treating social networking with a draconian policy of 'no one can go there'," says Dan Hubbard, chief technology officer at Websense.

"But that binary yes or no policy doesn't work anymore. Web 2.0 services are becoming a part of doing business for many companies and as they open up, there are new risks."