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A Facebook bug that would allow attackers to take over user accounts with minimal effort has netted $US20,000 for a UK-based security researcher.

As detailed on his blog, Jack Whitten, writing as fin1te, found that accounts could be traversed by exploiting a bug in how Facebook linked user accounts to mobile phones. The registration code sent to users linking phones to accounts arrives with an editable profile_id field, which Whitten demonstrated could be changed to another user.

“The flaw lies in the /ajax/settings/mobile/confirm_phone.php end-point. This takes various parameters, but the two main are code, which is the verification code received via your mobile, and profile_id, which is the account to link the number to,” he writes.

Facebook allows the registration code sent to the attacker to be used to access the victim's account – meaning that the victim's account is then linked to the attacker's phone. Whitten then requested a password reset on the target account, received the changed password form, and took over the account.

According to Whitten's account, Facebook fixed the flaw five days after he reported it. The fix is simple enough: “Facebook responded by no longer accepting the profile_id parameter from the user,” he writes.

“The bounty assigned to this bug was $20,000, clearly demonstrating the severity of the issue,” he concluded. ®

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