A fully integrated CMOS security-enhanced passive (SEP) tag that compensates the security weakness of ISO/IEC 18000-6C is presented in this paper. For this purpose, we propose a security-enhanced protocol that provides mutual authentication between tag and reader. We show that the proposed protocol meets the security demands of the ongoing international standard for RFID secure systems, ISO/IEC 29167-6. This paper fabricates the SEP tag with a 0.18-${mu}m$ CMOS technology and suggests the optimal operating frequency of the CMOS SEP tag to comply with ISO/IEC 18000-6C. Furthermore, we measure the SEP tag under a wireless environment. The measured results show that communications between the SEP tag and reader are successfully executed in both conventional passive and SEP modes, which follow ISO/IEC 18000-6C and the proposed security enhanced protocol, respectively. In particular, this paper shows that the SEP tag satisfies the timing link requirement specified in ISO/IEC 18000-6C.