Internet of things (IoT) is gaining popularity now-a-days as it is revolutionizing the world of internet and physical systems in a more advanced and technical way. IoT consists of physical things in which sensing, processing and communication capabilities are added. These devices have restricted resources and processing competences. The networks formed from these miniature devices have a lot of scope and applications that include healthcare, industrial automation, military surveillance, forest fire detection, flood alarming system, smart homes, smart cities etc. Most of these applications demand secure transmission of sensed information from source IoT node to gateway node or broker. Thus, it is imperative to pay attention to the security of these networks as they are highly susceptible to risks because of wireless medium used for communication and the constrained nature of these devices. In this paper, we have presented a variety of attacks that can harm the fidelity of transmitted information in IoT, thereby generating unauthorized effects. Furthermore, various counter measures against these possible attacks that have been proposed in the literature with their merits and demerits are presented, together with possible research opportunities for future work.