A security researcher found an easy-to-exploit Facebook vulnerability that could have enabled users to overtake anybody's Facebook page thanks to the social networking service's Mobile Texts feature.
Facebook has since patched the flaw and paid British researcher fin1te $20,000 through its bug bounty program.
To exploit the vulnerability, fin1te first texted the letter F to 32665, Facebook's SMS shortcode in the U.K. and the United States, with the goal of activating mobile texts for his account, which allows users to receive and respond to Facebook notifications from their phone.
He next received a confirmation code to his mobile phone, which he entered into a Facebook web form under Mobile Settings as part of the setup process.
Then, he modified the form's source code to input a different user's profile ID, a numeric string that easily can be found on the web for any Facebook member. Next, he submitted the form, which sent a confirmation text to his phone saying that he had successfully installed the Mobile Texts capability.
Fin1te was now able reset the target user's account password because there is an option to be texted a code to do this, which was sent to his phone despite the fact that he was acting as a different user (the one whose profile ID Facebook had accepted).
"We enter this code into the [password reset] form, choose a new password, and we're done," Fin1te wrote. "The account is ours."
Five days after being alerted of the vulnerability, Facebook patched the issue by "no longer accepting the profile_id parameter from the user."
This isn't the first time researchers have taken advantage of a social networking provider's SMS functionality.