Hackers are using Facebook's Wall messaging tool to target social networking users with malware.

The malware attack comes in the form of a Wall message supposedly posted by a friend that urges members to click on a link to view a video on a website supposedly hosted by Google, said Graham Cluley, senior technology consultant for Sophos.

However, the link takes users to a web page that isn't hosted by Google, where they are told they need a new version of Adobe's Flash player and are urged to download an executable file to watch the video.

The file is really a Trojan horse, Troj/Dloadr-BPL, that funnels other malicious code detected as Troj/Agent-HJX into users' machines. Once it has done that, it displays an image of a court jester sticking his tongue out.

While on the surface this might seem a practical joke from a friend, in reality it means the PC has been compromised and malicious hackers have gained control over it to use it for a variety of purposes, such as sending spam or distributing malware. "They now own your PC," Cluley said.

Malicious hackers have been employing this malware distribution technique for many years on email messages, so many users know to avoid these traps. However, people may be less vigilant in more closed and controlled environments such as social-networking sites.

For example, in this case, the malicious Wall message is masked as coming from someone on the user's list of Facebook friends, increasing the likelihood that the link will be clicked on. "Be very suspicious of Wall postings asking you to click on a link to go watch a video," he said.