Radio Frequency Identification (RFID) is taking place of barcodes in our lives, thanks to its remote identification capability. However, being transmitted via radio waves, the information is exposed to many possible attacks, which may endanger the security and privacy of relevant individuals and organizations. Fortunately, there have been efforts to construct an efficient and secure protocol in conformance with EPC Class 1 Generation 2 Standard (Gen 2), the most popular standard for RFID passive tags. Yeh et al. proposed a mutual authentication protocol conforming Gen 2. However, the proposed protocol is found to be vulnerable to an attack. Thus, in this paper, we show the vulnerability of Yeh et al.’s protocol and propose countermeasures to secure the protocol.