It is well acknowledged that one of the key enabling factors for the realization of future 5G networks will be the small cell (SC) technology. Furthermore, recent advances in the fields of network functions virtualization (NFV) and software-defined networking (SDN) open up the possibility of deploying advanced services at the network edge. In the context of mobile/cellular networks this is referred to as mobile edge computing (MEC). Within the scope of the EU-funded research project SESAME we perform a comprehensive security modelling of MEC-assisted quality-of-experience (QoE) enhancement of fast moving users in a virtualized SC wireless network, and demonstrate it through a representative scenario toward 5G. Our modelling and analysis is based on a formal security requirements engineering methodology called Secure Tropos which has been extended to support MEC-based SC networks. In the proposed model, critical resources which need protection, and potential security threats are identified. Furthermore, we identify appropriate security constraints and suitable security mechanisms for 5G networks. Thus, we reveal that existing security mechanisms need adaptation to face emerging security threats in 5G networks.