Selective forwarding is a major problem in wireless sensor networks (WSNs). The nature of sensor environments and the sensitivity of collected measurements in some fields such as war fields increase the need to prevent, detect, or mitigate the problem. One of the most used countermeasures for such problem is the use of voting system based on watchdogs' votes. However, this approach is not applicable in the case of mobile sensors. Mobile WSNs (MWSNs) is growing immensely due to the exposure of applications of mobile computing, vehicular networks, and Internet of things. This exposure has shed light on the security of using mobile sensors and raises the need to set appropriate methods for securing MWSNs against many attacks such as selective forwarding attacks. This paper introduces the problem of selective forwarding in MWSNs and discusses how the voting system used for mitigation; this problem in WSNs is not applicable in handling the problem in MWSNs due to sensors mobility. Therefore, the paper proposes a model that provides a global monitoring capability for tracing moving sensors and detecting malicious ones. The model leverages the infrastructure of fog computing to achieve this purpose. In addition, the paper suggests using software defined systems to be used along with the proposed model, which generalize the model to be used to secure MWSNs against other types of attacks easily and flexibly. The paper provides a complete algorithm, a comprehensive discussion and experiments that show the correctness and importance of the proposed approach.