BANGALORE, INDIA: Next time you see an unexpected photo viewing invite on Yahoo Messenger, do not click on the link. A worm is spreading via Yahoo Instant Messenger since the past two days 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.

With tags like 'photo or 'photos'  anong with a smiley face icon, the malicious link shows a link which is similar to a photo viewing request from Facebook or MySpace.

If you click on the link on a Macintosh system, an executable file will be downloaded, but no further action will take place. 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.