In general, internet users need to remember several IDs and passwords when they use diverse web sites. From an effective management perspective, SSO system was suggested to reduce user inconvenience. Kerberos authentication, which uses centralized system management, is a typical example of a broker-based SSO authentication model. However, further research is required, because the existing Kerberos authentication system has security vulnerability problems of password and replay attacks. In SSO authentication systems, a major security vulnerability is the replay attack. When user credentials are seized by attackers, an authorized session can be obtained through a replay attack. In this paper, an improved SSO authentication model based on the broker-based model and a secure lightweight SSO mechanism against credential replay attack is proposed.