In this paper, the vulnerabilities and the security attacks against vehicular networks, SDN architecture and some security solutions for SDVN are studied. Secondly, a complete and improved version of the secure HSDN-GRA routing protocol based on a distributed trust model is proposed where public keys of the vehicles are managed in a distributed way. Besides, in this approach, a weight is assigned to every vehicle, which is calculated fromits freeload and its trust values per those of the neighbors. The trust value is deduced from the historical interactions stored in a log of communication errors. To measure the trust value, three unreliable behaviors occurred in a time interval are considered. The vehicle with the role of cluster head hosts a list of each vehicles misbehaviors forming a log of communication errors. A vehicle will be chosen as the next relay according to its weight. As a positive result of this complete proposed approach, the following security requirements are achieved: the vehicle’s authentication and the data integrity are guaranteed by a signature mechanism, whereas an encrypted function is used to ensure the confidentiality of the exchanged data. The goal is to protect the routing process against malicious and unstable nodes. Finally, the implementation details and simulation analysis are given, and a comparative study between the secure and the insecure HSDN-GRA is presented in thepresence of a percentage of malicious nodes in the network. The perspective of managing non-collaborative vehicles is briefly introduced as future work.