Cyber attack against Critical National Infrastructure is a developing capability in state arsenals. The onset of this new instrument in national security has implications for conflict thresholds and military ethics. To serve as a legitimate tool of policy, cyber attack must operate in accordance with moral concerns. To test the viability of cyber attack, this paper provides a new perspective on cyber ethics. Cyber attack is tested against the criteria of the Common Good. This involves identifying the four core components of the common good from a conflict perspective: respect for the person; social wellbeing; peace and security; and solidarity. The fate of these components is assessed in relation to the six key characteristics of cyber attack from a moral standpoint: security; the role or absence of violence; discrimination; proportionality; cyberharm; and the threshold of conflict. It is concluded that the common good must be incorporated into developing state cyber strategies.