Industrial Internet of Things (IIoT) applications connect machines, sensors and actuators in high-stake manufacturing industries. Industrial systems are using the potential of IoT to reduce the unnecessary operational cost and increase the usability and reliability of the industrial assets to achieve more profits. However, such smart Industries need connectivity and interoperability to enhance performance which makes them susceptible to various attacks. Recent attacks on Cyber-physical systems raise a strong security concern as such attacks causes a huge property loss and may also lead to life threatening situations. In this paper we discuss the potential security threats to the Industries adapting to IIoT and study the various attacks that are possible on the components in the layered IIoT architecture and some of the preventive measures. Finally, we propose IIoT attack taxonomy which would help in mitigating the risks of the attacks.