defaultdeny

Defaultdeny, or default-deny, is a security posture in which access to resources is denied by default and only explicitly authorized access is allowed. This approach follows an allow-by-exception model: policy rules specify which actions are permitted, while all other actions are blocked unless explicitly allowed.

In practice, default-deny is commonly applied in network protection and access control. In firewalls and network ACLs, administrators start with blocking all traffic and then add specific allow rules for the traffic that is permitted. In cloud and container environments, default-deny is used through network policies, security groups, and IAM configurations to restrict traffic between workloads and services, requiring explicit allowances for legitimate communication. The principle is also reflected in secure software design, where access to sensitive resources is granted only to verified principals with explicit permissions.

Benefits of default-deny include a reduced attack surface, stronger enforcement of least privilege, and a clearer boundary for what is considered allowed behavior. Challenges include potential service disruption if legitimate traffic is not whitelisted, increased policy complexity as environments grow, and ongoing maintenance overhead. Effective implementation requires careful policy design, testing, and governance to avoid inadvertently blocking essential operations.

Best practices include establishing a baseline of default-deny and then layering narrowly scoped allow rules, using infrastructure as code to manage policies, and testing changes in staging before production. Regular auditing, monitoring, and documentation help sustain the posture, and temporary exceptions should have explicitly defined expiration and revocation processes. Related concepts include allow-listing, firewalls, access control lists, least privilege, and zero-trust security.