Since we last reported a vulnerability to Facebook regarding the Mutual Friends List privacy settings, it appears little has changed. The vulnerability allows attackers to discover, or more precisely, reconstruct the private Friends List of any Facebook user.
The vulnerability exists in the “Mutual Friend” section, in the following URL: https://www.facebook.com/profile.name(Hidden Friends Profile)/friends?and=second.profile(Public Friends Profile)
So while the company's CEO prefers to keep his friends list hidden, Chris is OK with sharing his list with the public (you and me). Here's where it gets interesting - what if we wanted to see the friends both Chris and Mark share? No problem. Follow this link: https://www.facebook.com/zuck/friends?and=ChrisHughes
As we can see, Chris shares 61 mutual friends between with Mark (which of course means Mark has those 61 friends as well).
Official Facebook Response
Huh? Didn't Mark explicitly choose to keep his friends list hidden? Here's what Facebook had to say:
Basically, Facebook is saying: "You can edit your privacy settings, but they're not really privacy settings". Couldn't Facebook just call this "Display Settings"?
But I'm not an interface designer. My job is to figure out how attackers could access data they're not supposed to. So the next obvious step is to run a query that exposes Mark's entire friends list. Here's how I did it:
A simple graph search gathers a list of potential friends for Mark based on a common feature. For the case of User#4 (Mark Zuckerberg), we want to find people that work at Facebook and live in United States.
Create a list of all the related users who have their Friends List privacy settings configured as public. We'll call this the "Relate List"
Run a "brute force" search to cross-reference with Mark's Friends List, using the Mutual Friends URL for each of the related users.
Repeat the search for mutual friends, with the mutual friends accounts identified (in previous phase), whose Friends List is public.
To drive the point home, I wrote a POC tool that shows how such easy it is for anyone to hack your hidden friends list.
The tool automates the attack methodology described above (apart from the last recursive phase). It first creates a Related List of users, and in the second phase, searches through this list to find mutual friends, resulting in an output file with a complete friends list.
Here's a screen capture video of the entire process: