Vehicular Ad hoc Networks (VANETs) are emerging mobile ad hoc network technologies incorporating mobile routing protocols for inter-vehicle data communications to support intelligent transportation systems. Among others security and privacy are major research concerns in VANETs due to the frequent vehicles movement, time critical response and hybrid architecture of VANETs that make them different than other Ad hoc networks. Thus, designing security mechanisms to authenticate and validate transmitted message among vehicles and remove adversaries from the network are significantly important in VANETs. This paper presents several existing security attacks and approaches to defend against them, and discusses possible future security attacks with critical analysis and future research possibilities.