We present a formal analysis of the authentication protocol of Facebook Connect, the Single Sign-On service offered by the Facebook Platform which allows Facebook users to login to affiliated sites.
Formal specification and verification have been carried out using the specification language HLPSL and AVISPA, a state-of-the-art verification tool for security protocols. AVISPA has revealed two security flaws, one of which (previously unheard of, up to our knowledge) allows an intruder to impersonate a user at a service provider affiliated with Facebook. To address this problem, we propose a modification of the protocol, by adding a message authentication mechanism; this protocol has been verified with AVISPA to be safe from the masquerade attack. Finally, we sketch a JavaScript implementation of the modified protocol.