Worm infecting PCs via Yahoo Messenger

A worm is spreading via Yahoo Instant Messenger that tricks people into downloading what they think is a photo from a friend but is instead malware that installs a backdoor on Windows systems and spreads to a victim's IM contacts.

The worm, seen spreading on Thursday, arrives via a message from a contact with the word "photo" or "photos" and a smiley face icon, along with a link to a Web site resembling a Facebook page, MySpace page, or some other page where photos might reside.

If the user clicks on the link on a Macintosh system, an executable file will be downloaded, but no further action will occur. On a Windows system, the executable will download and if the user runs the file, the computer will become infected and the malicious message will be distributed to all of the IM contacts.

Yahoo said in a blog post that it was aware of the issue and working to address it. "We recently learned of an issue where some users have received spam messages from their contact list. Yahoo Messenger has quickly worked to resolve the situation," the post said.

For more on this story, read Worm spreading via Yahoo Messenger on CNET News.