In this paper we analyze the security of SEAS protocol. The only security goal of this protocol is to authenticate the RFID tag to the RFID reader which, in this paper, we show that the protocol does not satisfy this property. Hence, we do not recommend this protocol to be employed in any application. In this paper we present a tag impersonation attack against it. Tag impersonation attack is a forgery attack in which the reader authenticates the attacker as a legitimate tag. Our tag impersonation attack’s success probability, which is the first attack against the SEAS protocol to the best of our knowledge, is “1” and its complexity is only two runs of protocol.