In this screenshot you can see such a setting. Actually I would recommend not to store any information on Facebook that nobody should see. But maybe you want an email address to login to Facebook that is separate from all other emails you use, or you need to add your company's email to add yourself to a Facebook network, or you just want to hide this temporarily or whatever.
When a new person joins Facebook, Facebook wants you to get your profile updated and to get new friends as soon as possible, so you get these advertisements that the person is new on Facebook and to suggest him or her new friends that he or she might also know. This is very useful to connect people and to get them quickly up to speed.
Unfortunately there is a slight glitch in this mechanism. When you suggest that the new person connects with somebody else and they make friends, you get a notification email. In this email you get notified that the two people joined. In this email you get a link to suggest the new person more contacts. It also contains your login email, so that you only have to enter your password. The problem now is that it doesn't contain your email, but errornously the one from the person new to Facebook. That way you can see the email of the person new to Facebook - independently of the security setting for this email.
So if you want to exploit this, you can suggest new friends to a person. If that person accepts any suggestion to join and you get a confirmation email, you can see the main Facebook email that the person uses. People often just accept friend requests if the people are from the same company, or other group, even if they don't really know them, so the chances are high that this way you could get new emails.
Here's an example of such an email. Black are my personal details (email, my name, user id or other information), red is the person that added the new yellow user. The first yellow mark is the full name, the second yellow mark is the first name only, and the right yellow mark is the secret email of the new user that should be my email actually.
- Exploiting this only works for friends, as you can only suggest new friends for your own friends.
- The person has to accept a friend suggestion.
- The only secret you can see is the person's main login email. In many cases this email is already known.
- The bug is fixed by Facebook in the meantime, but was open for several months - if not years.
- Somewhen between September 2009 and July 2010 Facebook changed the format of the link in this email. In the old version no email was visible in the link. It is unknown what email was displayed when clicked on the link, so possibly the bug was introduced at that time.
- 31-Dec-2010 After finding the bug, I immediately tweeted about it. Looking back now, I know I shouldn't have done this, but nobody noticed anyway.
- 31-Dec-2010 I thought about this again and decided to contact Facebook. Because I didn't find a security contact, I notified abuse support department and only told them that I found a security problem, without any details.
- 14-Jan-2011 Lillie from User Operations asked me for the details. A day later I replied her with the details.
- 19-Jan-2011 Lillie replied "Thanks for bringing this to our attention. We are investigating this matter and are working to get this issue resolved as soon as possible. We appreciate your report."
- 06-Feb-2011 I received a suggestion-confirmation email with the corrected link. Facebook never replied that it's solved.
- 12-Feb-2011 This blog was created and the problem made public.