Network Function Virtualization (NFV) provides new opportunities for efficient and low-cost security solutions. Real-time traffic monitoring and fast security threat detection is a challenge to reduce the risk of great damages. In this paper, we propose a virtualized network function in an Open Source Platform for providing a real-time threat detection service. Our function combines cloud computing and distributed stream processing techniques to accurately and quickly detect threats. The proposed virtualized network function shows a good elasticity shrinking and scaling accordingly to the required load. The results show that the proposed function is able to scale dynamically, analyzing more than five million messages per second. In addition, the function easily migrates sensor elements to reduce latency, allowing the sensor to be located as near as possible to the client.