Malware writers have created a new worm that installs a new browser and plays screeching music.
The annoyance starts with a link apparently sent by a friend in Yahoo's instant messaging program.
Instant messaging security company FaceTime Communications described the malware, which it calls "yhoo32.explr", as "insidious" in a security advisory Friday.
When the link is clicked, a worm installs the so-called "Safety Browser," a program that leads the user to pages mined with adware and viruses, FaceTime said. The Safety Browser uses an Internet Explorer logo to make it look more legitimate.
Malware spread through instant messaging programs is on the rise. However, FaceTime said this malware appeared to be the first to install a browser without the user's permission.
The bug also hijacks Internet Explorer's home page, directing users to the Safety Browser's Web site.
After it is launched, the worm sends itself to others on the user's instant messaging contact list.
The malware is engineered to overwrite instant messages typed by a user, FaceTime said. The infected message can also be changed on-the-fly, the company said.
The screeching music, however, is blocked by Microsoft's Windows XP Service Pack 2, FaceTime said.
FaceTime has posted screenshots of the infection process on its blog at http://blog.spywareguide.com/.