Which security model operates on the principle that no one can be trusted by default?

The security model that operates on the principle that no one can be trusted by default is the Zero Trust model. This approach emphasizes that both internal and external network requests must be verified, regardless of their origin. In a Zero Trust framework, every user and device is authenticated, authorized, and continuously evaluated for trustworthiness before granting access to any network resources.

This model is especially relevant in today's digital environments, where threats can come from anyone, even those inside the organization. Instead of assuming that users within a network perimeter are safe, Zero Trust suggests a proactive stance towards security, acknowledging that breaches can happen at any moment. By implementing strict access controls and not trusting any user or device by default, organizations can significantly minimize their attack surface and better protect sensitive data.

The other choices represent different security concepts that do not inherently abide by the Zero Trust principle. For example, firewall protection typically relies on predefined rules to allow or deny traffic, which can assume certain levels of trust within a network. Public Key Infrastructure helps in managing digital keys for encryption and is based on trust relationships among entities, while network segmentation involves dividing a network into segments to improve performance and security but does not encapsulate the trust model inherent in Zero Trust.