Facebook has fixed a serious bug in its password reset feature that gave an attacker endless opportunities to guess a six-digit code and reset an account's password.
The social network has paid researcher Anan Prakash $15,000 for reporting what he says was a “simple vulnerability” but one that could also have let him easily hack into anyone’s Facebook account, view messages, payment card details, and basically any content or information the rightful owner can.
Prakash found Facebook hadn't implemented rate-limiting in its password reset procedure on the beta sites, beta.facebook.com and mbasic.beta.facebook.com.
The researcher explained in a blogpost today that he discovered the bugs while testing the security of Facebook’s password reset feature on its main site.
In the event a user forgets their Facebook password, they can click the “Forgotten your password” link. Facebook provides the option to reset a password via a six-digit code sent by SMS or email to the user’s account. Once the SMS code is entered the user can then reset their password.
Prakash found the main Facebook login page blocked him after 10 invalid attempts, however, the beta sites didn’t impose any limit on the number of wrong attempts.
Given Facebook’s bug-bounty policy against harming other users, Prakash used his discovery to hack his own account. He's posted a proof of concept video on YouTube demonstrating a script he wrote in a web application pen-testing tool which ran through about 900 possible six-digit codes until it identified the right one. This allowed him to set a new password and then log into his account.
As he explained, he found a vulnerable POST request in the beta pages, which featured a snippet “lsd=AVoywo13&n=XXXXX”
“Brute forcing the ’n’ successfully allowed me to set new password for any Facebook user,” Prakash noted.
Prakash said he reported the bug to Facebook on February 22 and that he was able to verify the issue was fixed the next day.
The $15,000 reward suggests Facebook's security team does consider the bug to have been been a high impact one. Last year Facebook's average payout to security researchers was $1,780.
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