On Tuesday, researchers at unified threat management vendor Fortinet that a program similar to the Koobface worm had started using the Google Reader and Picasa Web sites to spread. In the attack, criminals host images that look like YouTube videos on the Google sites in hopes of tricking victims into downloading malicious Trojan software.

Hackers initially unleashed Koobface in late July, but Facebook's security team soon by blocking the Web sites that were hosting the malicious Trojan software.

That has prompted the criminals to change tactics, according to Guillaume Lovet, a senior research manager with Fortinet. In this latest attack they have hosted files that appear to be YouTube videos on Picasa and Google Reader and used Facebook to send them to victims.

The links appear safe because they go to Google.com Web sites, but once the victim arrives on the Google Reader or Picasa page, he is invited to click on a video or a Web link. The victim is then told he needs to download special codec decompression software to view the video. That software is actually a malicious Trojan Horse program, which is blocked by most antivirus programs, according to Facebook.

Lovet believes the cyber-criminals behind Koobface have deliberately misspelled their Facebook messages to further help them evade detection by filters.