The practice of greenwashing in corporate sustainability reports presents legal challenges because until now Positive Law in Indonesia has not established clear parameters to determine when sustainability claims are categorized as misleading information. The absence of these parameters creates the problem of legal certainty that has an impact on corporate accountability for investment decisions obtained based on the information submitted in the sustainability report whether it meets legally accountable standards. This study aims to analyze the normative basis in the assessment of greenwashing practices and reconstruct legal parameters to achieve legal certainty in corporate sustainability reporting. Using normative legal research methods that focus on statutory and conceptual approaches, this study shows that positive law in Indonesia has provided a normative basis for corporate sustainability reporting and information disclosure. Current rules are still fragmented and have not distinguished between legitimate sustainability claims and misleading claims. This study proposes legal reconstruction through three assessment parameters such as the threshold of material non-conformity, the methodology of proving the element of intentionality, and the measurable standard that can be audited consistently. These parameters are intended to operationalize the principle of transparency of sustainability information in order to strengthen legal certainty and ensure accountability for corporate sustainability claims in Indonesia.