Security experts are warning users of Facebook, MySpace and other social networking communities to be on guard against a new strain of the “Koobface” worm, which spreads by tricking users into responding to a message apparently sent from one of their friends. The latest version of Koobface arrives as an invitation from a user’s friend or contact, inviting the recipient to click on a link and view a video at a counterfeit YouTube site. Visitors are told they need need to install an Adobe Flash plug-in to view the video. The bogus plug-in instead installs a Trojan horse program that gives Koobface author(s) control over the infected user’s computer, according to security firm Trend Micro, which documented the new strain on its blog. In addition, the worm also hijacks the victim’s social networking account, by sending out additional invites in order to spread the worm to the victim’s friends and