Security requirements for the integration of emerging M2M networks in future internet of things are addressed. The heterogeneous nature of M2M devices raises new security challenges that existing proposals could not fulfill. Two entities may not be able to establish a secure end-to-end communication because of the technological gap between them and the resulting inconsistencies in their cryptographic primitives. This paper proposes a novel approach for establishing session keys for highly resource-constrained sensor nodes encountered in these M2M environments with an external server. The proposed system exploits collaboration between heterogeneous nodes by delegating heavy asymmetric cryptographic operations to a set of assisting nodes. A security analysis is conducted to verify that the proposed solution accomplishes safely and efficiently its objective.