The cyber security cooperation for confidence-building measures (CBMs) in ASEAN faces several challenges in its implementation. This situation is caused by a capacity gap among members having different perceptions and an imbalance of cyber capacity in terms of system and technology. This research aims to scrutinize the current situation of cyber security cooperation in the ASEAN Defense Ministers’ Meeting/Plus (ADMM/ADMMPlus) with all of its challenges, analyze Indonesia’s cyber security capacity as a member of ADMM, and formulate Indonesia’s course of action in defense diplomacy to address the challenges. This research used a quasiqualitative method by applying SWOT and descriptive analysis with a phenomenological approach to examine the problems. A semi-structured interview and literature studies are conducted to obtain related data. Among the theories used are nation defense, international relations and cooperation in the defense context, defense diplomacy concept, cyber security, and CBMs. The research concluded that cyber security for CBMs cooperation is not performing effectively in ADMM/ADMM-Plus due to the capacity gap and Singapore’s domination factor. Indonesia as a member has a strong position for interfering or defining the cooperation by utilizing defense diplomacy pragmatically. The research also found that Indonesia was still struggling with internal issues such as regulation and inter-agencies coordination in cyber security. In conclusion, Indonesia has to use its leadership capacity in defense diplomacy as chairman of ASEAN in 2023 through ADMM/ADMM Plus.