Nobody can deny that a ‘Delay Tolerant Network (DTN)' is a wireless mobile network capable of withstanding intermittent connectivity and long delays. Furthermore, DTN is a partitioned network that ensures data delivery via the Store and Forward strategy. However, due to resource scarcity (limited storage capacity, limited power, and energy...), DTN can be vulnerable to various types of attacks that can pose a serious threat. Among these types of attacks is selfish behavior, which can potentially ruin the network's integrity, authenticity, confidentiality, and availability. A selfish node tries to maximize its own assets by refusing to transfer other nodes' messages and only forwarding its own. We attempt to investigate the impact of selfish behavior on the existing flooding and forwarding-based routing DTN protocols in this paper. Following that, a comprehensive overview of existing solutions to the selfish attack is presented. To address this threat, we proposed a security mechanism called CETA, which is based on an effective proposed novel algorithm (COOPERATION ENFORCEMENT and TRUST ALGORITHM). The proposed mechanism involves encouraging DTN nodes to collaborate in message forwarding in order to improve delivery probability. Using the ONE simulator, we assessed the performance of our proposed algorithm CETA in DTN under selfish attack. Through simulation tests, CETA efficiency was demonstrated, resulting in promising selfish behavior in terms of delivery probability, overhead ratio, Latency average, and hop count.