In lightweight mobile ad hoc networks, both probabilistic and deterministic key management schemes are fragile to node fabrication attacks. Our simulation results show that the Successful Attack Probability (SAP) can be as high as 42.6% with the fabrication of only 6 copies from captured nodes comprising only 3% of all nodes. In this paper, we propose two low-cost secure-architecture- based techniques to improve the security against such node fabrication attacks. Our new architectures, specifically targeted at the sensor-node platform, protect long-term keys using a root of trust embedded in the hardware System-on-a-Chip (SoC). This prevents an adversary from extracting these protected long-term keys from a captured node to fabricate new nodes. The extensive simulation results show that the proposed architecture can significantly decrease the SAP and increase the security level of key management for mobile ad hoc networks.