Vehicular Ad Hoc Network (VANET) is a pillar of the envisioned Intelligent Transport System (ITS) and a subset of Mobile Ad Hoc Network that grants the communication in between the vehicles alongside with the absence of established communication infrastructure. Exposure to vulnerabilities of Vehicular Ad-Hoc Network (VANET) has been shown to be related to its nature of the environmental. For this reason, VANET security becomes a critical challenge that need to be resolved. In this paper, we assess the VANET security issues and discuss the challenges in VANET. Equally important, we comparatively review the security requirements, the type of attacks and capabilities of attackers present in VANET.