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Facebook scrambles to close hole exposing private data

Gives attacker almost as much control as user

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Facebook engineers are finishing a patch for a critical vulnerability that exposed user birthdays and other sensitive data even when they were designated as private, a security researcher said Wednesday.

The bug could be exploited by prompting a user to click on a link while logged into the social networking site, said M.J. Keith, a senior security analyst with Alert Logic, a provider of cloud-based intrusion detection systems. Attackers could then read, delete, or alter a victim's profile page, including pictures and data that are set to be viewed only by trusted friends.

"I would assume that every single Facebook user [could] have [had] their Facebook page defaced or have exposed things about them," Keith told El Reg. The bug "gives the attacker almost as much control as the user."

At time of writing, much of the CSRF (cross-site request forgery) bug appeared to have been patched, Keith said. However, as noted earlier by IDG News, attackers still could exploit the flaw to control a user's "like" functions, which are used to endorse ads and other types of content.

Facebook representatives didn't respond to an email asking about the status of the bug fix.

The flaw involved a piece of code Facebook engineers dubbed "post_form_id," which is used to ensure that commands can be issued only by browsers that have previously logged into the website. Keith discovered a simple way to bypass the security token: by omitting it altogether, Facebook servers no longer attempted to validate browsers.

It's at least the second glitch to compromise Facebook user privacy this month. Nine days ago, Facebook had to temporarily disable the site's live chat function to contain a bug that allowed users to eavesdrop on their friends' conversations.

Keith's advisory is here. ®

Update

A Facebook spokesman sent us a statement that included the following:

"Alert Logic reported a CSRF vulnerability to us, and we worked quickly to resolve it. We're in the process of confirming that all endpoints have been patched. We maintain a strong relationship with security experts around the world and work closely with them in the rare instances in which they find vulnerabilities."

He also noted that people can report Facebook vulnerabilities here.

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