Network function virtualization (NFV) yields numerous benefits, particularly the possibility of a cost-efficient transition of telco hardware functionalities on the software platform to break the vendor lock-in problem. These benefits come at the price of some security flaws. Indeed, with NFV, virtual mobile networks become vulnerable to a number of security threats. These threats can be leveraged using some available mitigation techniques and also through other emerging solutions. This article presents critical security threats that exist in the NFV infrastructure, proposes best security practices to protect against them.