In order to take countermeasures properly under customary international law, states must attribute the triggering internationally wrongful act to the perpetrator state accurately. International law tolerates no mistake or error in such attributions, in essence holding states to a standard of proof of “beyond reasonable doubt” for a countermeasure to be lawful. However, in the potentially more consequential context of self-defence — in which, unlike with countermeasures, military force is authorised — a notably less stringent standard of “reasonableness” applies and errors in attribution are accepted. The author proposes that standards of proof applicable to peacetime cyber attribution should be more stringent as the severity of the action in response increases. According to the new Tallinn Paper, a more balanced approach would subject attribution of internationally wrongful cyber operations giving rise to countermeasures to a preponderance of the evidence standard. At the same time, any response taken by a state in self-defence should require attribution based on clear and convincing evidence before it is deemed “reasonable”.