New social engineering attacks are tricking Facebook users into exposing anti-CSRF tokens associated with their sessions. These security codes allow attackers to make unauthorized requests through the victim's browser.
Cross-site request forgery (CSRF) is an attack technique that abuses the trust relationship between websites and authenticated users. Because of the way the Web works, a page can theoretically force a visitor's browser to issue a request to a third-party site where the user is authenticated, thus piggybacking on their active session.
In order to prevent this from happening, websites embed unique authorization codes known as anti-CSRF tokens into forms. Since these are not available to attackers, rogue requests can no longer be triggered successfully.
However, security researchers from Symantec have detected a new type of Facebook attack in which victims are tricked into handing over such tokens manually by going through a fake verification process.
The scams start with spam messages that promote interesting videos being posted on the Facebook walls of already compromised users by already compromised users. These messages contain links to third-party pages that spoof YouTube.
When they arrive at these attack websites, victims are prompted with rogue dialogs that instruct them to paste randomly generated code allegedly generated by an anti-spam mechanism.
In reality, this piece of code is obtained by making a request to a Facebook script in the background and contains the anti-CSRF token assigned by the social networking website to the user.
Therefore, pasting it into the dialog box gives the attackers everything they need to make authorized requests on the victim's behalf. In the example presented by Symantec, the token was used to propagate the scam by posting the original spam message on the user's wall.
This social engineering trick is similar to that used in so-called self-XSS attacks, which involve users pasting JavaScript code into their browser's address bar. It's not clear why attackers adopted the CSRF method, as both attacks are similarly hard to pull off; however, it might have something to do with the fact that Facebook implemented security mechanisms earlier this year to detect and block self-XSS.
According to Symantec, the social networking giant said that it's working with browser vendors on solutions to these attacks and is constantly monitoring accounts for suspicious behavior.
"Attackers are using some really innovative social engineering techniques to trick their victims. We advise users to keep their security software up-to-date and not click on any links that seem suspicious," the Symantec security researchers advised.