The growing popularity of sustainable energy sources has seen a dramatic rise in the deployment of Distributed Energy Resources (DERs). Decentralised DERs such as roof-top solar panels, battery storage, and electric vehicles (EVs) provide a cost effective and efficient source of renewable energy. Integrating with smart grids (SGs) to realize full benefits requires a complex, bi-directional monitoring and control communication system to maintain stability and resiliency of the electrical grid. Typically installed in residential or small business premises, DERs contain embedded IoT controllers that require network connectivity which renders them vulnerable to a variety of cyber-attacks. The geographically diverse and heterogeneous nature of DERs present additional cyber-security challenges. This article explores the cyber-security implications of DERs within SG environments through a review of recent academic literature. Exploring cyber-physical security threats, risks and vulnerabilities, it will compile a taxonomy of practical network- and device-level attacks and present a framework of cyber-defensive risk mitigation strategies and countermeasures designed to protect DERs. Finally, the report will examine emerging trends and technologies in DER cyber-security and identify current challenges and gaps that might form the basis of future research.