Multi-hop Device-to-Device (D2D) communication has emerged as a key enabler for public safety applications in 5G networks. A failure of these applications would be catastrophic since they control vital human-in-the-loop services. As a result, it is of utmost importance to identify and mitigate security risks prior to commercial deployment. To this end, we devise SEMUD, the first secure multi-hop D2D solution for 5G mobile networks. It enables robust and secure communication even in the absence of supporting infrastructure. We design SEMUD to be compliant with 3GPP Proximity-based Services, the standard to implement D2D communications. We implement SEMUD in the ns-3 simulator and our testbed. Via extensive simulation, we assert its resilience against strong adversaries and attacks. Furthermore, we show the energy efficiency and achievable throughput of our proposal on real devices.