Recently, a significant increase has been witnessed by the world in the amount of drones being used, with a continuous global rise in demand due to their multiple and interdisciplinary applications. Drones provide people with a hawk eye that can not only be used anywhere but can also be activated as and when required. However, the malicious use of unmanned aerial vehicles and drones began emerging among cyber-criminals. The frequency and probability of these attacks is not only high but their impact has devastating and dangerous effects. Therefore, the requirement for preventive, protective and detective countermeasures is urgently needed. This chapter hence focuses on the emerging threats on unmanned aerial systems and drones in particular, such as cyber threats along with an overview of drone technology and its security requirements. This chapter further discusses various route optimisation algorithms as they have a major role to play in efficient and safe drone data delivery. Factors like optimal route selection, obstacle avoidance, better heuristics and collision avoidance have a huge impact on safe path planning. Finally, this work presents open challenges and future research directions for enhancing the security of unmanned aerial systems.