Key Performance Indicators (KPIs) are a higher-level characterization of the performance of a network slice, meant to be assessable at any time. Bodies such as the GSM Alliance have proposed the use of KPIs, including, but not limited to, latency, throughput, power consumption, and security. However, while latency, throughput, and power consumption are mensurable universally, security is much harder to measure. In this article, we propose using a Moving Target Defense (MTD) approach and measurable network properties to establish a new straightforward network security metric for underlying resilience against network-centric attacks. We called it DynSec, a comprehensive model for basic network security within the network slice. Monte Carlo experimentation showed that DynSec is accurate and suitable as a KPI.