Vehicular ad hoc networks VANETs are being increasingly advocated for traffic control, accident avoidance, and management of parking lot and public areas. In particular, communication devices are being installed in more and more cars and roadside infrastructure components. At the same time, they will have direct access to a fixed roadside network infrastructure with information flowing both ways Security and privacy are two major concerns in VANETs. Unfortunately, in VANETs, most Privacy-preserving schemes are vulnerable to Sybil attacks, whereby a malicious user can pretend to be multiple other vehicles. The detection of Sybil attacks in this manner does not require any vehicle in the network to disclose its identity; hence privacy is preserved at all times. I present a light weight and scalable protocol to detect Sybil attacks. In this protocol, a malicious user pretending to be multiple other vehicles can be detected in a distributed manner through passive overhearing by s set of fixed nodes called road-side boxes RSBs.In this paper, we are concerned with this problem and in particular, how to achieve security pseudonym - based authentication. Simulation results are presented.