In recent years, the number of automobiles on the road has increased tremendously. Due to high density and mobility of vehicles, possible threats and road accidents are increasing. Wireless communication allows sending safety and other critical information. Due to this inherent wireless characteristic and periodic exchange of safety packets, Vehicular Ad-hoc Network (VANET) is vulnerable to number of security threats like Sybil attack or temporal attack. In this paper, a detailed discussion has been done on both the type of attacks. With the help of already published works, some approaches have also been studied which have proved to be of significance in detection of these attacks.