Research describes a 150-year-old series of Geneva Conventions relating to cyberwar. No one has actually defined the term “cyberwar” and reaching broad agreement on a definition seems problematic at best. Two key provisions were added in the 1990s to an international treaty signed and ratified by almost every country that constrain the conditions under which a nation could adversely affect the networks, services and equipment in another nation. There is so little wiggle room that a “defensive measures” clause may emerge the next time this treaty is reviewed.
Source: https://thehackernews.com/2010/10/is-cyber-war-lawful.html