The proliferation of wireless technologies in a licensed manner has resulted in the scarcity of the radio spectrum. Therefore, spectrum availability has become a challenge for new and existing wireless technologies. Federal Communications Commission (FCC) has proposed a new spectrum sharing strategy, i.e., dynamic spectrum access (DSA), to opportunistically access the underutilized spectrum resources of licensed users and to fairly share the unlicensed spectrum with others. Cognitive Radio (CR) works as a promising technology to enable this opportunistic or dynamic spectrum access. Moreover, DSA and CR also work as fundamental building blocks of future spectrum coexistence, which aims to improve spectrum equity. This dissertation serves as a means to achieve secure and fair coexistence of different wireless technologies in the same spectrum—whether licensed or unlicensed.In this research, a novel attack surface, off-sensing interval, is introduced. It highlights a novel room of vulnerabilities in state-of-the-art channel sensing processes. This vulnerability illustrates how an attacker can ingeniously avoid the sensing interval of the victim and can intelligently interfere with the transmission or reception of the victim to trick it into believing that it is interfering with a licensed user. Such a scenario pushes the victim to perform a spectrum handoff and influences its spectrum utilization. It is named an off-sensing attack. Furthermore, a cross-layer attack in CR-based wireless mesh networks is proposed, where the attacker deploys the offsensing attack as an auxiliary attack to influence the traffic flow around the victim to divert traffic through a target node. It is named an off-sensing and route manipulation attack. Then, a strategy to thwart the off-sensing attack is proposed, where the defender hops through different channels in the spectrum band to confuse the attacker. The interaction between attackers and a defender is modeled as a Markovdecision process to assist the defender in making optimal decisions.In addition, another attack surface is discovered in state-of-the-art rendezvous and spectrum handoff processes in infrastructure-less CR-based networks. As infrastructure-less networks operate without a central entity, the lack of vigilance in current spectrum handoff strategies engenders this vulnerability where a selfish SU can trigger an early handoff to reserve the best available channel sooner than benign SUs. This research helps to design secure spectrum handoff processes.Finally, as the rapid commercialization of Internet-of-Things (IoT) is taking place, the density of spectrum hungry devices are increasing. This dense deployment of IoT devices—that may follow different wireless technologies—in the shared spectrum creates a new challenge to solve: secure coordination among co-located IoT devices from different IoT networks. This dissertation sheds light on this unique challenge and introduces a novel security vulnerability where an attacker can pose as a hidden terminal from a different network and compromise the victim device, namely hidden terminal emulation (HTE) attack. As the dense deployment of IoT devices will naturally aggravate such hidden terminal interference, it facilitates the HTE attacker with plausible deniability to interfere with its alleged hidden counterparts. This dissertation assesses this issue and proposes detection and defense measures, namely Third Eye and Jump and Wobble, deceptively.In summary, dynamic spectrum access promises to be one of the significant ideas to solve the spectrum scarcity problem. This research is essential to conduct an in-depth security assessment of spectrum sharing operations—which are unique to the dynamic spectrum access—before the commercialization of such technologies.