We cryptanalyze Kim et al’s one-time proxy signature scheme used in mobile agents, and then a successful forgery is introduced. It is showed that a dishonest customer can successfully forge a valid one-time proxy signature by impersonating the server. Furthermore, he can request the server with responsibility for the forged bidding information. We cryptanalyze Kim et al’s one-time proxy signature scheme used in mobile agents, and then a successful forgery is introduced. It is showed that a dishonest customer can successfully forge a valid one-time proxy signature by impersonating the server. request the server with responsibility for the forged bidding information.