Cooperative and automated vehicular platoons enhance road safety and reduce traffic congestion by enabling vehicles to travel closely together and maneuver in a synchronized manner. This synchronization relies on vehicle-to-vehicle (V2V) communications, which, while beneficial, also introduce vulnerabilities to potential cyberattacks. In this paper, we introduce a new cooperative and secure protocol for platoon departures, focusing specifically on the departure phase. We demonstrate that, without security measures, a vehicle attempting to leave the platoon could exploit the leave messages of the platoon protocol and introduce attacks that may disrupt the formation of the platoon or even jeopardize its stability. To mitigate this risk, wepropose data consistency measures that protect both the stability and integrity of the platoon. Simulations conducted using Plexe simulator validate the security of the proposed protocol through rigorous security assessments.