Fog computing is an extension of cloud computing that brings processing power closer to the source of data, thereby enhancing Internet of Things (IoT) applications by reducing latency and improving efficiency. However, this distributed paradigm introduces significant security and privacy challenges due to the limited computing resources and physical accessibility of edge devices. Ensuring the secure development and deployment of fog services requires comprehensive frameworks that consistently integrate robust security mechanisms across various platforms. This paper addresses these challenges by proposing the Security Policies Model for the Fog (SPM4FOG), a novel solution that uses the Event-B formal method. The SPM4FOG model enables industrial designers to formally validate security requirements during the design phase, promoting the secure development of IoT applications by design. Leveraging mathematical foundations and formal logic, the SPM4FOG model rigorously verifies security policies, mitigates vulnerabilities, and enhances the security of fog computing environments.