The software defined networks are being used in many scenarios where the ordinary or traditional network management becomes the real problem. Such networks are defined or managed with the SDN platform, which is used as the network programming rather than the network configuration. The problem of user legitimacy is a big issue in the cloud platforms. The user legitimacy assurance is quite important to protect the cloud platforms from several types of attacks. The user legitimacy assurance must be performed on two given events, one is pre-setup, second is post-setup. The existing models incorporate the post-setup phase authentication only, where the pre-setup phase is left immature, where the hackers can easily attack over. In this paper, we are proposing the model for the security of cloud by user legitimacy assurance during the pre-setup phase with the use of pre-shared information in the form or RUID (rigid user ID), which is provided to the user during the registration. The RUID will add the new layer of security by mitigating the threat of user session hijacking, which will make the cloud infrastructure highly secure in comparison with the existing models.