We present a formal approach based on mutation techniques for the modelling of cybersecurity attacks and its application to 5G networks. We introduce formal definitions of the main concepts of network protocols, mutation operators, flow of network packets and network traffic. We design a formal approach based on different mutation operators that allows to design models that can be assimilated with known and unknown attacks. This approach has been implemented in our open source 5G network traffic fuzzer, 5Greplay, and has been applied to two use cases that are representative of attacks against 5G networks.