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Facebook and MySpace have closed gaping security holes in their sites that gave attackers full access to accounts that had automatic-login features enabled.

The vulnerabilities, documented here by a Facebook application developer, were significant. Because the unauthorized access would be mapped to the victim's IP address and website cookie, the intrusions would be virtually untraceable. Attackers were then free to download photos and messages designated as private with no indication at all to the victim.

Facebook and MySpace closed the backdoors shortly after being notified, a marked improvement from the past, when the sites sometimes allowed serious security holes to persist for months. Still, it probably shouldn't have taken an outsider to discover the bug. This is the latest episode to demonstrate that the only sure way to ensure that data is private is to keep it off social networking sites altogether.

The backdoors were the result of a misconfiguration of a crossdomain.xml, a file websites use to share content using Adobe Flash across domains. Some of the domains that were accessible exposed authentication tokens for accounts that had the auto-login feature turned on.

Facebook developers had blocked access from the main domain, but didn't bother to notice the sensitive data was accessible when Facebook subdomains were used. MySpace similarly locked its front door but left a window at farm.sproutbuilder.com, which had full access to the data.

The holes could be exploited by luring victims to sites that had a Flash application installed designed to grab the authentication information, the developer said. ®

http://www.theregister.co.uk/2009/11..._backdoor_fix/