The authentication system of the PKI(public key infrastructure) provides the authenticity and security, accessibility, economic feasibility, and convenience to the service provider and users. Therefore the public and private companies in Korea widely use it as the authentication method of the web service. However, the safety client system is threatened by many vulnerable factors which possibly caused when using PKI-based authentication system. Thus, in this article vulnerable factors caused by using the PKI-based authentication system will be analyzed, which is expected to be the useful data afterwards for the construction of the new authentication system as well as performance improvement.