Unmanned Aerial Systems (UAS) and their associated technology is a rapidly growing and developing sector with employment across national infrastructure, conflict areas and commercial entities. As more governments, businesses and consumers acknowledge the benefits offered from intelligent, smart devices such as UAS, the risk of device compromise through cyber-attack could not only cause significant data breach, but also present a major risk to safety. However, their sought-after capabilities to provide real-time data often takes priority over the need for robust cyber security measures. Governmental procured Commercial Off The Shelf (COTS) UAS are required to operate with more strict security implementations including WiFi Protected Access 2 (WPA2), encryption and 802.11w implementation. This research investigates the current UAS cyber security landscape and demonstrates vulnerabilities within COTS UAS that can be exploited. Three stages of attacks were designed and conducted on three WiFi enabled COTS UAS using low cost equipment and open source programmes. The communication links were investigated and targeted for cyber attacks including Denial of Service (DoS), deauthentication, Password Recovery and Internal Access. An open source packet sniffing and injection suite of tools, named Aircrack-ng, and a USB WiFi antenna were used to monitor, inject and disrupt the UAS connections. The successful methodology and associated commands are outlined for the attacks and stages within the research. The extensive results documented vulnerabilities within the UAS including successful deauthentication and 4 way handshake capture against Protected Management Frame (PMF) capable systems. Finally, the research addresses a selection of the current UAS security challenges and provides mitigation for the issues identified, to enhance cyber security controls and management.