We have already notified our regulators in the US, Canada and Europe, and we are in the process of notifying affected users via email. We appreciate the security researcher's report to our White Hat program, and have paid out a bug bounty to thank him for his efforts."
Facebook did not specify time period for which the user details were exposed but said the bug was reported only 'recently'. It has alerted its six million users whose personal details were exposed besides sending out an apology via email.
Here is the full text of the apology email:
Your privacy is incredibly important to everyone who works at Facebook, and we're dedicated to protecting your information. While many of us focus our full-time jobs on preventing or fixing issues before they affect anyone, we recently fell short of our goal and a technical bug caused your telephone number or email address to be accessible by another person.
The bug was limited in scope and likely only allowed someone you already know outside of Facebook to see your email address or telephone number. That said, we let you down and we are taking this error very seriously.
Describing what caused the bug can get pretty technical, but we want to explain how it happened. When people upload their contact lists or address books to Facebook, we try to match that data with the contact information of other people on Facebook in order to generate friend recommendations. Because of the bug, the email addresses and phone numbers used to make friend recommendations and reduce the number of invitations we send were inadvertently stored in their account on Facebook, along with their uploaded contacts. As a result, if a person went to download an archive of their Facebook account through our Download Your Information (DYI) tool, which included their uploaded contacts, they may have been provided with additional email addresses or telephone numbers.
Here is your contact Information (inadvertently accessible by at most 1 Facebook user):
[Email address 1]
[Email address 2]
We estimate that 1 Facebook user saw this additional contact info displayed next to your name in their downloaded copy of their account information. No other info about you was shown and it's likely that anyone who saw this is not a stranger to you, even if you're not friends on Facebook.
We recognize that mistakenly sharing contact info is unacceptable, even if you are acquainted with people who saw these details, and we've taken measures to prevent this from happening again. For more information on the bug, please read our blog post.
All of us at Facebook take this issue very personally. We appreciate your ongoing use of Facebook, and are working every day to deliver the level of service you expect and deserve.
The Facebook Team