Kaspersky have warned of a new worm that's profligating itself via social networking sites. The worm turns infected computers into botnet zombies and spreads itself each time the user logs onto Facebook and MySpace

According to Kaspersky : Net-Worm.Win32.Koobface.a spreads when a user accesses his/her MySpace account. The worm creates a range of commentaries to friends' accounts. Net-Worm.Win32.Koobface.b, which targets Facebook users, creates spam messages and sends them to the infected users' friends via the Facebook site. The messages and comments include texts such as "Paris Hilton Tosses Dwarf On The Street"; "Examiners Caught Downloading Grades From The Internet"; "Hello; You must see it!!! LOL. My friend catched you on hidden cam"; "Is it really celebrity? Funny Moments" and many others.Messages and comments on MySpace and Facebook include links to http://youtube.[skip].pl. If the user clicks on this link, s/he is redirected to http://youtube.[skip].ru, a site which purportedly contains a video clip. If the user tries to watch it, a message appears saying that s/he needs the latest version of Flash Player in order to watch the clip. However, instead of the latest version of Flash Player, a file called codecsetup.exe is downloaded to the victim machine; this file is also a network worm. The result is that users who have come to the site via Facebook will have the MySpace worm downloaded to their machines, and vice versa.

Read the full announcement here on Kespersky's website : Kaspersky's lab detects new worms attacking MySpace and Facebook