Wireless Sensor Network (WSN) is an innovative technology with a broad range of applications and highly attractive benefits, such as low cost of implementation and data transmission, unmonitored access to the network, autonomous and long-term operation. With extensive demand for the advancement of related technologies (cloud computing, near-field communications and cellular mobile networks), the Internet of Things (IoT) is becoming a very exciting paradigm. By using communication technologies in sensors and sensing features in web devices, WSNs have begun interaction with the Iot devices. IoT provides access to a large amount of information gathered by WSNs. However, the security of WSN and IoT comes at a cost, mainly due to privacy management issues. Therefore, this paper offers a comprehensive analysis of security threats against WSN and IoT, along with the strategies for preventing, detecting and mitigating those threats. The related defense mechanisms can help in building a safe IoT expansion and widespread understanding by getting familiar with the details of these attacks. The aim of this paper is to address and demonstrate the impact of the security problems on WSNs from the viewpoint of the IoT and its applications. In the analysis carried out for this work, a classification of available attacks and threats against these requirements has also been included.