The principle of proportionality is a legal test used to decide whether a measure is suitable for its aim and not excessive in relation to that aim. In practice, it asks three questions: does the measure help achieve the goal, is it necessary, and does it avoid unnecessary harm?
In cybersecurity, proportionality matters when defenders, platforms, or regulators choose controls that affect users and services. For example, a provider may need to screen accounts, block traffic, suspend a service, or restrict a payment path because of sanctions or abuse concerns. Proportionality helps ensure that the response matches the risk instead of causing wider disruption than needed. It is also important for audits and legal review, because operators may need to justify why a control was applied, how broad it was, and whether less intrusive options were available. Used well, proportionality supports security, compliance, and service continuity at the same time.