This book explores the current impasse that global regulators face in the digital sphere. Computer technology has advanced human civilization tenfold, but the freedom to interact with others in cyberspace has made individuals, discrete communities, organizations and governments more vulnerable to abuse. In consequence, political decision-makers are seriously considering granting limited legal immunity to victims who decide to ‘hack- back. ’ Many victims frustrated by the slow pace of law enforcement in cyberspace have chosen to ‘take the law into their own hands,’ retaliating against those who have stolen valuable data and damaged network operations. Political deliberations about limited immunity for hackbacks usually ignore global justice and moral justifications for ‘active defense’ policies. Typically, cyber security policies balance deterrence against two different understandings of morality and the ‘good life’ : fairness or welfare. This book proposes a third moral rationale for cyber security policies : capability theory, developed principally by Amartya Sen and Martha Nussbaum. Properly formulated, a capability-based defense of retaliatory hackbacks can minimize attribution and cyber-escalation risks, deter bad behavior by casual computer users, disingenuous security experts, big tech companies, criminals and rogue governments, and satisfy calls for more retributive and distributive justice in the ‘open world’. This book will appeal to legal theorists, political philosophers, social activists, investors, international relations scholars and businesspeople in the tech community.