denialbydefault

Denial by default, or default-deny, is a security posture in which access to resources is blocked unless an explicit grant is configured. The approach contrasts with permissive or default-allow models, where access is permitted unless it is explicitly denied.

In practice, denial by default is implemented through various access control mechanisms such as access control lists (ACLs), role-based access control (RBAC), attribute-based access control (ABAC), and policy languages. Common patterns include deny-by-default with allow-by-exception, explicit deny rules that take precedence over allows, and configuration baselines that require explicit permissions to be added before a resource can be accessed.

Benefits of this approach include a smaller attack surface, stronger adherence to the principle of least privilege, easier auditing and compliance, and a reduction in accidental exposure. It also helps enforce predictable security behavior, since anything not explicitly permitted is blocked.

Challenges involve policy complexity and maintenance: accurately enumerating all legitimate permissions can be difficult, and misconfigurations can inadvertently block legitimate users or services. It may introduce initial friction during deployment and require tooling for testing, monitoring, and change management to avoid legitimate access disruptions.

Applications of denial by default span operating systems, network security, and application design. OS file systems and SELinux/AppArmor contexts, firewall rules and cloud IAM policies, and secure development practices often employ deny-by-default strategies to minimize unintended exposure and encourage explicit authorization.