arXiv:2610.33401v1 Announce Type: new Abstract: Aggregation platforms and smart-inverter control channels can allow a compromised coordination layer to alter active- and reactive-power commands across a geographically distributed DER fleet. This paper introduces the Coordinated Optimization-Based Attack (COBA), which uses network information and location-specific DER P/Q setpoints to maximize thermal loading on a selected transmission corridor. COBA is compared with low-information CAV baselines limited to a single uniform fleet command: all-off, all-on, maximum reactive absorption, or maximum reactive injection. The Hawaii Synthetic Grid-37 Buses case is evaluated with rooftop-photovoltaic-like capability aggregated at 20 selected 69-kV buses and verified using MATPOWER power flow. Under the case's single-reference-bus balancing convention, COBA overloads B86 (WAIALUA69-WAIANAE69) near 30% penetration and B51 (COCONUT69-PALM69) near 55%, while same-target uniform loadings remain lower and uniform commands primarily stress other natural weak lines. The results show that topology, target selection, and command capability determine DER-induced thermal risk beyond aggregate penetration alone.