"With the frequency of widely reported malicious cyber operations carried out by states steadily increasing, more and more policymakers and scholars have started deliberating how states should be allowed to, respond when their critical infrastructures orother legally protected interests fall victim to a significant cybersecurity incident triggered by an adversarial actor's conduct. Focusing on 'active cyber defences' such as 'hacking back' and other unilateral remedies, ranging from publicly calling outanother state for having engaged in malicious cyber conduct to imposing sanctions, the book closely examines three unilateral remedies found in customary international law that a targeted state may invoke in order to justify its response: self-defence, countermeasures, and necessity. Taking into account the pervasive problem of attributing cyber operations to the responsible actor in a reliable and above all timely manner, the study seeks to unfold the principal legal challenges that states face when applying rules of current international law to questions of transnational cybersecurity. The book concludes with some principled suggestions for future norm development for cyberspace"--