Blogs, or online journals, have become a popular feature of the Internet, allowing readers to swap information and opinions. But a computer security firm warns that some blogs are also sharing nasty viruses.

Websense, which provides Internet management solutions for businesses, says blogs are increasingly being exploited as a means to distribute malicious code and keylogging software. To date, Websense Security Labs says it has discovered hundreds of instances of blogs involved in the storage and delivery of harmful code.

According to the firm, cyber-criminals are taking advantage of blog sites that allow users to easily publish their own web pages at no cost.

It says blogs can be attractive vehicles for hackers for several reasons, since blogs offer large amounts of free storage, they do not require any identity authentication to post information, and most blog hosting facilities do not provide anti-virus protection for posted files.

In some cases, the culprits create a blog on a legitimate host site, post viral code or keylogging software to the page, and attract traffic to the toxic blog by sending a link through spam email or instant messaging (IM) to a large number of recipients. In other cases, the blog can be used as a storage mechanism, keeping malicious code that can be accessed by a Trojan horse that has already been hidden on the user's computer.

For example, on March 23, 2005, Websense Security Labs issued an alert detailing a spoofed email message that attempted to redirect users to a malicious blog which would run a Trojan horse designed to steal banking passwords.

In this situation, the user received a message spoofed from a popular messaging service, offering a new version of their IM program. Upon clicking the link, the user was redirected to a blog page which was hosting a password-stealing keylogger. When predetermined banking websites were accessed, the keylogger (bancos.ju) logged keystrokes and sent them to a third party.

"These aren't the kind of blog websites that someone would stumble upon and infect their machine accidentally. The success of these attacks relies upon a certain level of social engineering to persuade the individual to click on the link," said Dan Hubbard, senior director of security and technology research for Websense, Inc. "In addition, the blogs are being utilized as the first step of a multi-layered attack that could also involve a spoofed email, Trojan horse, or a keylogger."