With the rapid involvment of IoT networks in almost all aspects of our daily life, the security concern is being more challenging to protect the enormous amount of the data exchanged over theses networks. Statisitcs reveal that threats against IoT networks are more and more growing , while DoS attacks are taking the lead. However, mitigation solutions against attacks targetting IoT networks must be developped while taking into consideration the network scalability, but also they should fit with the limited resources of IoT devices in terms of processing, storage, energy efficiency, and connectivity-loss. For thi aim, the present work proposes a lightweight detection and mitigation solution against the version number attack "VNA" which is considered as one among the haitful DoS attacks against IoT networks. The proposed decentralized algorithm targets the edge part of the netowrk. Upon an illigitimate incremented version number the sink node is informed via an alert DAO message by the malicious node parent, and it disseminates this information through the overall network to advertise the malicois originator. Beside its efficiency, the proposed algorithm does not involve any encryption algorithm, it does not generale any heavyweight computation, nor a specific extra node is needed for monitoring the network. Simulations were carried out to demonstrate the effectiveness of the solution in terms of control overhead, energy efficiency, latency, and memory footrpint.