The digitalization and decentralization of the electric power grid are key thrusts towards an economically and environmentally sustainable future. Towards this goal, distributed energy resources (DER), including rooftop solar panels, battery storage, electric vehicles, etc., are becoming ubiquitous in power systems, effectively replacing fossil-fuel based generation. Power utilities benefit from DERs as they minimize transmission costs, provide voltage support through ancillary services, and reduce operational risks via their autonomous operation. Similarly, DERs grant users and aggregators control over the power they produce and consume. Apart from their sustainability and operational objectives, the cybersecurity of DER-supported power systems is of cardinal importance. DERs are interconnected, interoperable, and support remotely controllable features, thus, their cybersecurity should be thoroughly considered. DER communication dependencies and the diversity of DER architectures (e.g., hardware/software components of embedded devices, inverters, controllable loads, etc.) widen the threat surface and aggravate the cybersecurity posture of power systems. In this work, we focus on security oversights that reside in the cyber and physical layers of DERs and can jeopardize grid operations. We analyze adversarial capabilities and objectives when manipulating DER assets, and then present how protocol and device -level vulnerabilities can materialize into cyberattacks impacting power system operations. Finally, we provide mitigation strategies to thwart adversaries and directions for future DER cybersecurity.