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Due to the financial and reputational costs of ‘traditional’ (in a current context – direct) warfare, not only do more and more states tend to use, but also will continue to utilize other methods, such as financial, logistical, and military support to proxy forces, to maintain their strategic interests. This paper explores the international legal obligations of the states, as third parties to the proxy warfare, stating that existing standards of attribution for Articles on State Responsibility for Internationally Wrongful Acts (ARSIWA) and corresponding case law have not sufficiently addressed sponsoring states' legal accountability. Therefore, it introduces the idea of the due diligence principle, otherwise known as ‘Duty of care’, to the mentioned framework. The study argues that the due diligence doctrine offers a more practical and preventive alternative, shifting the legal focus from control-based attribution to the obligation to prevent foreseeable harm. Relying on comparative analysis from international human rights, environmental, and cyber laws, the thesis proposes a tailored legal framework for applying due diligence in the context of proxy conflicts. To ensure the relevance and applicability of the proposed framework, three case studies, Russia’s actions in eastern Ukraine and Georgia, and the U.S.-led coalition’s support for Syrian opposition groups are examined. The study concludes that due diligence integration into the contemporary legal framework is both legally justifiable and necessary for plugging accountability gaps and enhancing international humanitarian law compliance.