Software-defined vehicular cloud (SDVC) is a novel proposal to form a cloud consisting of vehicles and to share their underutilized resource and carry out collaborative jobs. Since the research on SDVC is in its initial stage, security and privacy issues are still challenging. In this paper, we propose a security mechanism for SDVC to satisfy fundamental security requirements including authentication, message integrity, non-repudiation, confidentiality, and privacy. We also present the preliminary analysis of the proposed security mechanism.