The resource-constrained sensors in mission-critical applications are subject to both random failures and intentional compromise by the adversary, which poses severe security threats in Wireless Sensor Networks (WSNs). The different types of security threats have been identified and addressed in an individual manner in the past. And most solutions are cryptography based. In this paper, the authors argue that cryptography alone is insufficient to fully address the insider attacks in the existence of both the compromised and faulty sensor nodes. They further propose a Proactive Data Security Framework (PDSF) to identify compromised and faulty nodes proactively and prohibit them from participating network activities in a dynamic manner.