Multi-application smart card technology facilitates applications to securely share their data and functionality. The security enforcement and assurance in application sharing is provided by the smart card firewall. The firewall mechanism is well defined and studied in the Issuer Centric Smart Card Ownership Model (ICOM), in which a smart card is under total control of its issuer. However, it is not analysed in the User Centric Smart Card Ownership Model (UCOM) that delegates the smart card control to their users. In this paper, we present UCOM's security requirements for the firewall mechanism and propose a generic framework that satisfies them.