Smart city, cloud, IoT are some buzzwords in recent times. Study shows that more than two billion connected things are deployed in the smart cities across the world. But such a vast number also exposes these systems to vulnerabilities that can be exploited by hackers and other malicious actors. The smart city offers technology where service providers use various ICT (Information and communication technologies) to create more effective urban organizations that can improve the quality of human life. The emerging IoT based architecture is the foundation of a smart city. IoT enabled sensors, networks, and interfaces also make vulnerability window wider. This paper explores current challenges considering the characteristics of smart city, threat vectors that can lead systems in the fatal situation and the security approach that can be considered while building smart cities.