Wireless ad hoc networks are dynamic networks in which nodes can move freely in the network. A new type of Vehicular Ad Hoc Network (VANET) that allows smart transport system to provide road security and reduces traffic jams through automobile-to-automobile and automobile-to-roadside communication. In this, vehicles rely on the integrity of received data for deciding when to present alerts to drivers. Because of wireless network the VANET messages are vulnerable to many attacks and the security concerns are also major issues. So, with respect to these methods, this article will discuss the Denial of Service (DoS) attack, masquerading, and their vulnerabilities. Also, it classifies the securities and their prevention mechanisms in overcoming these security issues in VANET and Cognitive Radio VANET perspectives.