In today's increasingly interconnected and fast-paced digital ecosystem, mobile networks, such as 5G and future generations such as 6G, play a pivotal role and must be considered as critical infrastructures. Ensuring their security is paramount to safeguard both individual users and the industries that depend on these networks. An essential condition for maintaining and improving the security posture of a system is the ability to effectively measure and monitor its security state. In this work we address the need for an objective measurement of the security state of 5G and future networks. We introduce a state machine model designed to capture the security life cycle of network functions and the transitions between different states within the life cycle. Such a model can be computed locally at each node, or hierarchically, by aggregating measurements into security domains or the whole network. We identify three essential security metrics -- attack surface exposure, impact of system vulnerabilities, and effectiveness of applied security controls -- that collectively form the basis for calculating the overall security score. With this approach, it is possible to provide a holistic understanding of the security posture, laying the foundation for effective security management in the expected dynamic threat landscape of 6G networks. Through practical examples, we illustrate the real-world application of our proposed methodology, offering valuable insights for developing risk management and informed decision-making strategies in 5G and 6G security operations and laying the foundation for effective security management in the expected dynamic threat landscape of 6G networks.