Exceptional characteristics of cloud computing have replaced all traditional computing. With reduced resource management and without advance investment, it has been victorious in making the IT world to migrate towards it. Microsoft announced its office package as Cloud, which can prevent people moving from Windows to Linux. As this drift is escalating in an exponential rate, the cloud environment has also become a target for attackers. Hence to acquire the best use of the cloud services, the security issues have to be dealt with first. Among various security threats faced by the cloud environment, this paper focuses on the most dangerous one, Distributed Denial of Service attack, DDoS. In DDoS attack, a victim is targeted simultaneously by numerous hosts distributed across the network. An attacker compromises the vulnerable systems in the network and infects those systems with malicious code which can trigger these systems, called zombies, to send requests in huge numbers to choke the target. This type of attack can happen both at network as well as service level. Network level attack is achieved by sending simultaneous invalid or incomplete requests, whereas in service level attacks, the target will be flooded with complete request for services provided by the CSP, but with malicious intent. These two types of attack trafficshave to be filtered out at different levels. In this paper, a three phase authentication scheme, REputation based Service for Cloud User Environment (RESCUE) has been proposed that can help the cloud service provider to detect and mitigate both the aforesaid types of DDoS attacks. RESCUE, the proposed scheme authenticates the users in three different phases. The simulation results presented here exhibits the strength of the proposed method in detection and prevention of DDoS attack in cloud computing environment.