Researchers have discovered what appears to be the first example of a worm that installs its own web browser as it infects a PC.
With a degree of irony, the worm calls its malware browser the "Safety Browser".
The worm, called yhoo32.explr, is spreading via Yahoo Messenger and affects Windows, according to FaceTime Security Labs, which discovered the malware on Friday.
Initially, users receive a link sent via instant messaging, which promises "locational technology" offering content appropriate to the user's region. In fact the site installs a malicious command file and the so-called Safety Browser.
The browser directs the user to a special home page, called Demoplanet, and the malware also makes this the homepage of Internet Explorer. The browser then urges the user to click on a series of advertisements, which in turn install more spyware and adware on the PC, according to FaceTime.
The Safety Browser's icon changes to the Internet Explorer icon, in order to further fool users. The browser appears to be a "shell" for IE, according to FaceTime.
The malware also causes looped music to play every time the PC starts up.
Finally, the worm has a complicated system for sending infected links to the user's Yahoo Messenger contacts. A file on the PC contacts a URL which continually modifies the way the infection is spread, tailoring the message for Yahoo Messenger, IRC and other messaging systems, according to FaceTime.
"It even randomly overtypes some of your IM messages as you hit the send button," said FaceTime researcher Chris Boyd in an online commentary on the worm.
"This looks like an all-new area of hijacking that will (of course) be built upon and continue to grow," Boyd said.