Internet of things (IoT), also referred to as the internet of objects, is a dynamic global information network consisting of internet — connected objects. Wireless sensor networks (WSN) and RFID enabled Internet of Things finds a plethora of applications in almost all the fields such as education, health, agriculture, transportation and entertainment. Compact smart devices constitute an essential part of IoT. These compact smart devices range widely in size, energy, use, capacity and computation power. On the other hand, the integration of these smart things into the standard internet introduces several security challenges because the majority of internet technologies and protocols were not designed to support IoT. Though number of researchers have explored about such security challenges and open problems in IoT, there is a lack of a systematic study of the security challenges in the IoT landscape. This paper briefs the motivation for IoT, secured IoT layered architecture, IoT applications, Security issues with various attacks in each layer of the IoT and existing methods for providing security solutions and their limitations.