Abstract The trend of technological advances in the vehicle industry illustrates that future cars would have added functionalities with smart features, better connectivity and autonomous behaviour. These naturally involve a higher number of Electronic Control Units (ECUs) being connected using existing conventional in-vehicle network protocols such as Controller Area Network (CAN). In this context, security of systems is now becoming a major concern while industry’s primary interest in the manufacturing of cars is reliability and safety. It is now in daily news that smart cars are being hacked due to weaknesses in their embedded electronics that provides ways of hardware attacks [1] [2]. Hardware Trojan (HT) is the threat that has been recently recognised as one of the primary sources of backdoor access that enables hackers to attack systems. As trouble, HT remains silent until a rare function/event triggers it for activation. This paper contributes to the challenge of demonstration of disruption in CAN buses raised from hidden Hardware Trojan. In this regard, it is presented how just a small size Hardware Trojan disrupts the CAN bus communication without an adversary having physical access to the bus. The attack is neither detectable via frame analysis, nor can be prevented via network segmentation; additionally, a rare triggering mechanism activates HT to process untraceable faults.