You are presented with multiple paper abstracts, alongside the associated literature review and chapter titles. Given the provided information, you are going to do a literature review. Please generate a coherent literature review that synthesizes the content, draws connections between different works, highlights major findings, and identifies any gaps in the existing research.

Literature Review Title: Advances in cognitive radio networks: A survey 
Chapter Title:  K. Security in CR Networks 
1. Abstract of Cited Paper (BIB001):  This paper discusses the topic of wireless security in cognitive radio networks, delineating the key challenges in this area. With the ever-increasing scarcity of spectrum, cognitive radios are expected to become an increasingly important part of the overall wireless networking landscape. However, there is an important technical area that has received little attention to date in the cognitive radio paradigm: wireless security. The cognitive radio paradigm introduces entirely new classes of security threats and challenges, and providing strong security may prove to be the most difficult aspect of making cognitive radio a long-term commercially-viable concept. This paper delineates the key challenges in providing security in cognitive networks, discusses the current security posture of the emerging IEEE 802.22 cognitive radio standard, and identifies potential vulnerabilities along with potential mitigation approaches. 
2. Abstract of Cited Paper (BIB002):  Distributed spectrum sensing (DSS) enables a Cognitive Radio (CR) network to reliably detect licensed users and avoid causing interference to licensed communications. The data fusion technique is a key component of DSS. We discuss the Byzantine failure problem in the context of data fusion, which may be caused by either malfunctioning sensing terminals or Spectrum Sensing Data Falsification (SSDF) attacks. In either case, incorrect spectrum sensing data will be reported to a data collector which can lead to the distortion of data fusion outputs. We investigate various data fusion techniques, focusing on their robustness against Byzantine failures. In contrast to existing data fusion techniques that use a fixed number of samples, we propose a new technique that uses a variable number of samples. The proposed technique, which we call Weighted Sequential Probability Ratio Test (WSPRT), introduces a reputation-based mechanism to the Sequential Probability Ratio Test (SPRT). We evaluate WSPRT by comparing it with a variety of data fusion techniques under various network operating conditions. Our simulation results indicate that WSPRT is the most robust against the Byzantine failure problem among the data fusion techniques that were considered. 
3. Abstract of Cited Paper (BIB003):  Cognitive radios sense spectrum activity and apply spectrum policies in order to make decisions on when and in what bands they may communicate. These activities go beyond what is done when traditional radios communicate. This paper examines the denial of service vulnerabilities that are opened by these additional activities and explores potential protection remedies that can be applied. An analysis of how vulnerable are victim cognitive radios to potential denial of service attacks is presented along different axis, namely the network architecture employed, the spectrum access technique used and the spectrum awareness model. The goal is to assist cognitive radio designers to incorporate effective security measures now in the early stages of cognitive radio development. 
4. Abstract of Cited Paper (BIB004):  This paper describes a new class of attacks specific to cognitive radio networks. Wireless devices that can learn from their environment can also be taught things by malicious elements of their environment. By putting artificial intelligence in charge of wireless network devices, we are allowing unanticipated, emergent behavior, fitting a perhaps distorted or manipulated level of optimality. The state space for a cognitive radio is made up of a variety of learned beliefs and current sensor inputs. By manipulating radio sensor inputs, an adversary can affect the beliefs of a radio, and consequently its behavior. In this paper we focus primarily on PHY-layer issues, describing several classes of attacks and giving specific examples for dynamic spectrum access and adaptive radio scenarios. These attacks demonstrate the capabilities of an attacker who can manipulate the spectral environment when a radio is learning. The most powerful of which is a self-propagating AI virus that could interactively teach radios to become malicious. We then describe some approaches for mitigating the effectiveness of these attacks by instilling some level of "common sense" into radio systems, and requiring learned beliefs to expire and be relearned. Lastly we provide a road-map for extending these ideas to higher layers in the network stack.

Literature Review:
Due to their new characteristics, such as the requirement on the awareness of the surrounding environment and internal state, reasoning and learning from observations and previous experience to recognize environment variations, adaptation to the environment, and coordination with other users/devices for better operation, CR networks face their unique security challenges. In BIB002 , awareness spoofing and its impact on different phases of a cognitive cycle has been studied. Through spoofing, the malicious attackers can cause an erroneously perceived environment, introduce biases to CR decision-making process, and manipulate secondary users' adaptation. In BIB001 , the authors have investigated the primary user emulation attack, where the cognitive attackers mimic the primary signal to prevent secondary users from accessing the licensed spectrum. Localization-based defense mechanism is proposed, which verifies the source of the detected signals by observing the signal characteristics and estimating its location from the received signal energy. The work in BIB003 has investigated the spectrum sensing data falsification attack, and proposed a weighted sequential probability ratio test to alleviate the performance degradation due to sensing error. In the proposed approach, individual sensing reports are compared with the final decision. Users whose reports are identical to the final decision will have high reputation values, and their reports will then carry more weight in future decision fusion. Several types of denial of service attacks in CR networks have been discussed in BIB004 , such as spectrum occupancy failures where secondary users are induced to interfere with primary users, policy failures that affect spectrum coordination, location failures, sensor failures, transmitter/receiver failures, compromised cooperative CR, and common control channel attacks. Simple countermeasures are also discussed. How to secure a CR network by understanding identity, earning and using trust for individual devices, and extending the usage of trust to networking has been discussed in BIB005 .