Updated A bug in Facebook's login system allows attackers to match unknown email addresses with users' first and last names, even when they've configured their accounts to make that information private.

The information leak can be exploited by social-engineering scammers, phishers, or anyone who has ever been curious about the person behind an anonymous email message. If the address belongs to any one of the 500 million active users on Facebook, the social-networking site will return the full name and picture associated with the account.

"Facebook users have no control over this, as this works even when you have set all privacy settings properly," Atul Agarwal of Secfence Technologies wrote Wednesday on the Full-disclosure security listserve. "Harvesting this data is very easy, as it can be easily bypassed by using a bunch of proxies."

Exploiting the vulnerability is as easy as entering the email address into the Facebook sign-on page, typing a random password and hitting enter. To streamline the attack, Agarwal has written a PHP script that works with large lists of email addresses.

Over the past few years, Facebook has come under criticism for revealing too much information about its users. The data — which can include users' birthdays, home towns and personal friends — can then be used by marketers, stalkers, and other ne'er-do-wells to invade the users' privacy. The social-networking site has responded by giving users more control over who gets to see select pieces of user information.

Evidently, the name-to–email address extraction bug has been overlooked. We wouldn't be surprised to see this fixed in short order. ®

Update

Indeed, at 8 pm Wednesday California time, about 10 hours after this article was published, the exploit no longer worked.