It was only a matter of time before the bad guys figured out how to exploit Facebook's third-party Platform Applications, aka "widgets": The first spyware disguised as a Facebook app is now spreading around the social networking site, according to Fortinet's research team.

The app, called Secret Crush, sends the victim an invitation to find out who has a secret "crush" on him or her, and attempts to lure them into installing and running the Secret Crush app. But the app instead spreads Zango spyware via an iFrame.

Then the attack gets even craftier: "Before you can find out who might have a crush on you, you need to invite at least 5 friends!" That basically sews up the attack as a social worm, according to Fortinet researchers. Like other Facebook Platform Applications, the terms of service for Secret Crush say users' personal information can be disclosed.

"Since users have freely chosen to install the widget at the cost of disclosing their personal information, psychologically speaking it is difficult for them to stop the process at that point. Therefore, most of them will invite at least 5 friends to complete the process," according to Fortinet.

Bottom line: The attackers get the hits and revenue they want for their sites, and the victim never gets a crush revelation. But the Zango iFrame exploit is relatively tame compared to the potential for an attacker to instead insert an MPack or other harmful malware attack, Fortinet points out.

Fortinet has alerted Facebook of the spyware, and as of Fortinet's latest posting, over 1 million Facebook users were already using the infected app. That's about 3 percent of the Facebook user community.

Facebook suffered a previous Zango intrusion when it first launched its ad program, which the social networking site quickly cleaned up. The new widget approach with Zango is more sophisticated and "drives it to the next level," says Guillaume Lovet, manager of Fortinet's Threat Response Team.

— Kelly Jackson Higgins, Senior Editor, Dark Reading