Facebook has been forced to take its chat system offline after users found a way to spy on friends' personal information.
The security flaw meant users could view other people's live chats and pending friend requests — processes which are normally meant to be private.
The weak spot was flagged up by a technology blogger who said he achieved the feat with “a few mouse clicks”.
A spokeswoman for the social networking site said that engineers acted “promptly” to fix the bug yesterday afternoon.
She said users would have been performing a specific action with their privacy setting to be exposed and that hackers could only see pending friend requests — they could not accept them.
Blogger for TechCrunch Steve O'Hear wrote about the security breach yesterday afternoon.
He wrote: “There is a major security flaw in the social networking site that, with just a few mouse clicks, enables any user to view the live chats of their friends.
“Using what sounds like a simple trick, a user can also access their friends' latest pending friend requests and which friends they share in common.
“That's a lot of potentially sensitive information.”