University College London research student Shah Mahmood and Chair of Information Communication Technology Yvo Desmedt have told a conference of what they call a “zero day privacy loophole” in Facebook.
Details of the loophole, which the pair name “Deactivated Friend Attack” was presented at the IEEE International Workshop on Security and Social Networking SESOC 2012 in Lugano, Switzerland on March 19th.
The pair say the attack works like this:
“Our deactivated friend attack occurs when an attacker adds their victim on Facebook and then deactivates her own account. As deactivation is temporary in Facebook, the attacker can reactivate her account as she pleases and repeat the process of activating and deactivating for unlimited number of times. While a friend is deactivated on Facebook, she becomes invisible. She could not be unfriended (removed from friend’s list) or added to any specific list.”
Complicating mattters is the fact that, the pair say, Facebook users aren't told when friends de-activate or re-activate accounts.
That means trouble if the account is re-activated, as the newly-re-activated friend regains access to anything their connections have posted. Once they've rummaged around, they can de-activate the account again and their friends will almost certainly not know what has happened or that they've shared information.
The pair label this behaviour “cloaking” and cannot resist explaining it with a Star Trek metaphor, writing “Badass Blink or Jem’Hadar has to uncloak (be visible), even if only for a moment, to open fire.”
The extended abstract of the talk asserts cloaking is a problem because many Facebook users aren't very discriminating about whom they befriend on the service. Some could therefore Friend members whose only intention is to “cloak” their accounts and then “... activate her account at the moment least likely to be detected and crawl her victims profile for information, keeping an updated record."
That's bad because, the pair say, "Various groups of information aggregators including marketers, background checking agencies, governments, hackers, spammers, stalkers and criminals would find this attractive as a permanent back door to the private information of a Facebook user.”
The user would never know of that information-gathering effort, unless they happened to be paying attention to the temporarily uncloaked account.
To prove the approach works, the pair say the conducted a lengthy experiment in which a dummy account acquired many friends and conducted frequent cloaking and uncloaking without attracting much attention.
The fix, the pair say, is for Facebook to notify users of de-activations and re-activations, so that odd behaviour can be spotted. Flagging of accounts that cloak is another option, as is removing re-activation features altogether. ®.