## 10 Challenges and Open Research Problems
### Security Challenges  
Security is a big challenge in UAV-aided communication. Due to the small form factor and capability of most UAVs, they are prone to both cyber and physical security threats, such as hijacking. Moreover, due to the broadcast nature of UAV communications, they are subject to eavesdropping, packet snooping, jamming, spoofing, denial of service attacks, and cyber attacks [19; 314]. The security of UAV-to-UAV communication against multiple eavesdroppers was investigated in [315] where the authors derived expressions for the statistical SNR as well as for the secrecy outage probability. In extreme cases, interceptors could potentially spoof the control signals and use them to gain control of the UAVs [19; 316], or jam such signals to prevent them from reaching the ground control station. Thus, authentication of users and operators is an issue in UAV-based networks [317]. UAV security issues are categorized under cyber detection, which focuses on identifying intruders within the UAV network, and cyber protection, which focuses on eliminating or reducing external threats to the UAV network [318]. The authors in [319] also distinguished different forms of security, such as information and software security, sensors security, and communications security. Information and software security is related to the security of the UAV operating systems (including its configuration files, mission-related data, and flight control files) as well as its collected data. Sensor security deals with the security of the various sensors used for the real-time maneuvering of the UAV, such as accelerometers, gyroscopes, GPS, barometers, etc. If attackers gain control of such subsystems, it could lead to the malfunctioning of the UAV or cause it to send erroneous data. Communications security is related to the security of the communications components of the UAV, including control commands, telemetry data, and transmission of the collected data. Since these communications happen over the air and some of the packets are unencrypted (e.g., data packets from simple sensors or even GPS data), they pose cybersecurity threats to the successful operation of UAV networks. Packet routing is another potential source of attacks on UAV communications. Here, malicious entities can disguise themselves as legitimate elements in the UAV network to steal, modify, or drop packets. There are three common types of routing-related attacks: wormhole, selective forwarding, and sink hole attacks [320]. Wormhole attacks involve a malicious entity that captures packets in one location within the network, then tunnels the packet to a third party in another location where the packets are modified and retransmitted into the network [321]. This attack is common in ad hoc networks that use reactive routing protocol such as shortest-path-first routing, where the routing metric is based on the hop count. Malicious intermediaries pretend to be neighbor nodes of genuine network nodes, and route packets through private tunnels. Tunneling ensures that the packets arrive at the destination node with shorter distances or smaller number of hops than genuinely multi-hop-routed packets, thereby misleading the receiver. Selective forwarding attacks occur when malicious nodes behave like genuine network nodes and correctly forward packets most times, but occasionally selectively drop sensitive packets that may be critical to the functioning of the network. In UAV networks, other UAVs or edge devices can be used to perpetrate selective forwarding attacks. Since wireless networks randomly drop packets (due to congestion or unreliable links), this type of attack is difficult to detect because it is hard to distinguish if packet drops are due to network faults or malicious attacks [322]. In sink hole attacks, adversary nodes advertise a false hop count to the sink, tricking its neighboring nodes into believing that they have found a shorter route to the sink and then forward their data packets via the adversary node [323].