Connected and Autonomous Vehicles (CAVs) with their evolving data gathering capabilities will play a significant role in road safety and efficiency applications supported by Intelligent Transport Systems (ITS), such as Adaptive Traffic Signal Control (ATSC) for urban traffic congestion management. However, their involvement will expand the space of security vulnerabilities and create larger threat vectors. We perform the first detailed security analysis and implementation of a new cyber-physical attack category carried out by the network of CAVs on ITS, namely, coordinated Sybil attacks, where vehicles with forged or fake identities try to alter the data collected by the ATSC algorithms to sabotage their decisions. Consequently, a novel, game-theoretic mitigation approach at the application layer is proposed to minimize the impact of Sybil attacks. The devised minimax game model enables the ATSC algorithm to generate optimal decisions under a suspected attack, improving its resilience. Extensive experimentation is performed on a traffic dataset provided by the City of Montreal under real-world intersection settings to evaluate the attack impact. Our results improved time loss on attacked intersections by approximately 48.9%. Substantial benefits can be gained from the mitigation, yielding more robust control of traffic across networked intersections.