We describe a formal approach to the analysis of security aspects of an identity federation protocol for web services in convergent networks. This network protocol was proposed by Telecom Italia as a solution to allow end users to access services on the web through different access networks without explicitly providing any credentials, while the service providers can trust the user?s identity information provided by the access networks and access some user data. As a first step towards a full-blown formal security analysis of the protocol, we specify three user scenarios in the process algebra Crypto-CCS and verify the vulnerability of one of these specifications w.r.t. a man-in-the-middle attack with the model checker PaMoChSA.