The Internet of Things (IoT) is in a continuous development, the basic notion of IoT is that each object within the global network is accessible and interconnected. In such an environment, Wireless Sensor Networks play a crucial role, since they support different applications domains. Nevertheless, security issues are the major obstacle for their deployment. Among these issues, authentication of the different interconnected entities and exchanged data confidentiality. In this paper, we propose a new ultra-lightweight authentication scheme for heterogeneous wireless sensor networks in the context of IoT. This scheme allows both of the sensor and the user to authenticate each other in order to secure the communication. The proposed scheme uses only nonces, exclusive-or, concatenation operations to achieve mutual authentication. Moreover, it terminates with a session key agreement between the sensor node and the user. To assess our scheme, we carry out a performance and security analysis. The obtained results show that the proposed scheme provides authentication with low energy consumption, and ensures a resistance against different types of attacks.