A U.K.-based researcher has netted $20,000 for spotting a very serious flaw in Facebook that could have allowed an attacker to take over anyone's account with minimal effort.
The flaw was fixed by Facebook about a month ago, wrote Jack Whitten, an application security engineer who posted a post-mortem on his blog.
Whitten found it was possible to reset the password of anyone's account by exploiting an error in how Facebook lets users link their mobile phone to their account for purposes such as receiving updates over SMS.
Facebook sends a verification code by SMS when a user registers a phone. But Whitten found it was possible to modify the "profile_id" field within the mobile confirmation form to that of a different user.
Facebook then asks for a password but not the one of the potential victim, Whitten wrote. The account of the victim is then inadvertently linked with the attacker.
Whitten then initiated a password reset on the victim's account. The code needed to reset that password was sent to Whitten's phone, along with a link to a password reset page.
"We enter this code into the form, choose a new password, and we're done. The account is ours," Whitten wrote.
A flaw such as the one found by Whitten would have had value among cyberattackers seeking to abuse Facebook accounts for spam or for the theft of personal data.
"The bounty assigned to this bug was $20,000, clearly demonstrating the severity of the issue," he wrote.