Security in Wireless Sensor Networks (WSNs) is commonly bootstrapped by cryptographic key distribution among nodes. In this scenario, one of the greatest research challenges is to ensure secure keys storage by sensors due to their constrained resources and major exposure to tampering attempts. The main contribution of this work is to propose and analyze a mechanism for secure symmetric key distribution solving the stored key exposure problem. Additionally, a tampering attempt alarm for sensors is designed in order to isolate compromised nodes or keys from the network. Hence, this approach aims to increase the system's security against the tampering and compromising of sensor nodes.