A key establishment solution for heterogeneous Machine to Machine (M2M) communications is proposed. Decentralization in M2M environment leads to situations where highly resource-constrained nodes have to establish end-to-end secured contexts with powerful remote servers, which would normally be impossible because of the technological gap between these classes of devices. This paper proposes a novel collaborative session key exchange method, wherein a highly resource-constrained node obtains assistance from its more powerful neighbors when handling costly cryptographic operations. Formal security analysis and performance evaluation of this method are provided; they confirm the safety and efficiency of the proposed solution.