According to researchers from Symantec, Facebook has leaked access to millions of users’ photographs, profiles and other personal information because of a years-old bug that overrides individual privacy settings.

The flaw, which the researchers estimate has affected hundreds of thousands of applications, exposed user access tokens to advertisers and others. The tokens serve as a spare set of keys that Facebook apps use to perform certain actions on behalf of the user, such as posting messages to a Facebook wall or sending RSVP replies to invitations. For years, many apps that rely on an older form of user authentication turned over these keys to third parties, giving them the ability to access information users specifically designated as off limits.

The Symantec researchers said Facebook has fixed the underlying bug, but they warned that tokens already exposed may still be widely accessible.

While many access tokens expire shortly after they’re issued, Facebook also supplies offline access tokens that remain valid indefinitely. Facebook users can close this potential security hole by changing their passwords, which immediately revokes all previously issued keys.

The flaw resides in an authentication scheme that predates the roll out of a newer standard known as OAUTH. Facebook apps that rely on the legacy system and use certain commonly used code variables will leak access tokens in URLs that are automatically opened by the application host. The credentials can then be leaked to advertisers or other third parties that embed iframe tags on the host’s page.

Facebook over the years has regularly been criticized for compromising the security of its users, which now number more than 500 million. The company has rolled out improvements, such as always-on web encryption, although users still must be savvy enough to turn it on themselves, since the SSL feature isn’t enabled by default.

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