Thousands of Facebook applications have inadvertently leaked access tokens to third parties like advertisers, security giant Symantec has warned.
The flaw may have let hundreds of thousands of apps leak "millions" of access tokens to third parties over the years, Symantec estimated.
As of April 2011, close to 100,000 applications were leaking the tokens, potentially enabling third parties to get hold of user data and post messages on profiles, the security firm said.
Users are supposed grant apps access tokens to allow the applications to carry out certain tasks, such as posting on profile pages.
We fear a lot of these tokens might still be available in log files of third-party servers or still being actively used by advertisers.
The indirect leaks were seen by Symantec during the sign-up phase for apps, at the point when the application used a client-side redirect designed to send the user to a permissions dialog box.
"This indirect leak could happen if the application uses a legacy Facebook API and has the following deprecated parameters, return_session=1' and session_version=3,' as part of their redirect code," the security firm explained in a blog post.
"If these parameters are used, Facebook subsequently returns the access token by sending an HTTP request containing the access tokens in the URL to the application host."
Once this happened, the Facebook app could have leaked access tokens to third parties, possibly on purpose.
Facebook has now fixed the problem, after Symantec informed the social networking giant of the issue.
Yet the security firm raised fears third parties could still be using the tokens for their own gains.
"We fear a lot of these tokens might still be available in log files of third-party servers or still being actively used by advertisers," Symantec added.
"Concerned Facebook users can change their Facebook passwords to invalidate leaked access tokens. Changing the password invalidates these tokens and is equivalent to 'changing the lock' on your Facebook profile."
Facebook's security practices have been under heavy scrutiny in recent times.
Last month, Sophos issued a public letter to Facebook, calling for the company to enforce some security changes.
In particular, Sophos wanted to see HTTPS used for everything on the site.
