Once again, social networking site Facebook has been hit by the Koobface worm, which has been used to attack it several times since July. This latest attack, discovered by security vendor Fortinet, sends a message to users' Facebook friends urging them to click to view a video uploaded to either Google's Picasa photo-sharing site or to a shared video in Google Reader RSS feed aggregation site.

When victims try to do so, an error message pops up, asking them to download a new version of "Video ActiveX Object" so they can view the video. The Video ActiveX Object is a known malware application that helps spread other Trojans.

The disclosure comes as the latest high-profile attack built around major, widely regarded sites. The fact that hackers are using Google's (NASDAQ: GOOG) Reader and Picasa sites is by design, in an effort to make the worm more likely to spread.