In recent years, the fast developments in hardware, software, networking and communication technologies have facilitated the big emergence of many technologies such as Internet of things. Measurement and collecting data from physical world, and then sending it to digital world is base of this technology. The transmitted data are stocked, processed and then possibly used to act upon the physical world. IoT adds intelligence and autonomy to many domains (e.g. health care, smart transportation and industrial monitoring). As a result, it makes human life more comfortable and simple. However, as all emerging technologies, IoT is suffering from several security challenges and issues, especially that most of IoT devices and sensors are resources- constrained devices. As security issues and attacks could put systems in dangerous and could threat human life too, this paper treats these problems. We will provide an overview about IoT technology, and we will present various security issues that target the perception and the network levels. Moreover, we will discuss how each layer is damaged by harmful and malicious purposes. Most of recent papers use the three layers architecture (which is an old architecture) to present security problems; but this paper uses one of the new reference architectures to study security threats and attacks.