Users have to require authentication with many times different a set of username and password which access various service providers and applications in their daily task and on social life. In this case, the user must need to memorize many pair of a set of username and password. This one is then to enforce the users using ordinary/same passwords or to keep note of passwords somewhere. It is a problem as a secret of private user information on social life which to generate more crucial problems for a business applications. To get rid of this problem, a single sign on (SSO) is suggested. SSO describing a set of username and password maintain multiply passwords to access for different service providers and applications. In this study, we argued out the prevalent and the current issue of SSO protocols in literature and CAS which is one of the SSO protocols, is used to examine a model of business application.