Original URL: http://www.theregister.co.uk/2010/08/18/facebook_login_info_leak/

Facebook login page still leaks sensitive info

Security malpractice 101

By Dan Goodin

Posted in Security, 18th August 2010 16:50 GMT

Facebook's login system continues to spill information that can be helpful to phishers, social engineers and other miscreants attempting to scam the more than 500 million active users of the social networking site.

When a legitimate email address is entered along with an incorrect password, the authentication system returns an error that reads: “Please re-enter your password. The password you entered is incorrect. Please try again (make sure your caps lock is off).” When an email address that doesn't belong to a Facebook user is entered, the response is: “Incorrect Email. The email you entered does not belong to any account.”

The difference in the wording makes it possible for anyone to discern whether a given email address is registered on Facebook, even when the corresponding password is unknown. It was flagged to us by Reg reader Ben Prescott, a security analyst for EMC Corporation's Critical Incident Response Center, who calls it “one of the oldest security malpractices in the book.”

The configuration makes it possible to verify the validity of huge numbers of email addresses. It has been in place since last week, when Facebook developers fixed a much more serious bug that allowed attackers to match unknown email addresses with users' pictures and full names. It worked even for accounts that were configured to be private. It came to light after a researcher published a simple script that could quickly scrape large numbers of names and pictures that corresponded to email addresses.

While the new setup offers considerably less information, it could still be helpful for people trying to guess the email addresses of people in a given company or organization. And there's no reason this hack couldn't be automated, either.

It wouldn't be surprising if Facebook changed this setting soon. ®