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Facebook disclosed last Friday that 50 million accounts had been breached, and forcibly logged out 90 million affected users. It appears the hackers could have accessed sensitive profile information, purchase histories and private messages. Most disturbingly, since Facebook logins can be used on other sites, companies using that Facebook Connect feature are now rushing to figure out whether their sites were breached.

Why it matters: Single-sign-on login systems do not make a hack more likely. But they do affect what a hacker can access from inside a system. While Facebook reports there is no evidence third-party apps were accessed, this incident should cause consumers to re-evaluate whether to link accounts in the first place.

Single-sign-on systems allow hackers to get more information in one sweep. So, for third-party apps that contain sensitive data, it’s important to compartmentalize. If data held on the third-party site — medical records, for example — would be more sensitive if linked to a Facebook account, it should be kept separate. Similarly, if two third-party sites contain data that would be more sensitive if accessed together — say, credit card information and upcoming travel plans — those shouldn’t be linked either.

Yes, but: Facebook Connect–style login systems are still useful where the third-party app does not contain sensitive information. For sites without payment information or personal data, using Facebook Connect is convenient and poses limited risk. Because such systems can be easier to reset, they also can prevent hackers’ long-term access.

The bottom line: Even the companies best at protecting consumer data will not get it right all the time. All it takes is a handful of flaws — in this case, three — for a hacker to enter a system. Consumers need to be wary of linking information that collectively make them more vulnerable. Information that must be kept private is best left offline.

Betsy Cooper is joining the Aspen Institute's Technology and Cybersecurity Program this month as policy director. She is also a senior advisor at Albright Stonebridge Group.

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