Abstract
Cloud computing is considered as a new paradigm shift in distributed computing. Due to its increasing popularity, it has gotten increasing attention in the research community. In the last few years, the usage of cloud computing has increased because of its user-friendly services. However, as the number of users increases in the cloud, the service complexity and new security challenges take place. Data security is one of the major issues, which blocks users from adopting cloud. In this paper, we conducted a detailed literature review divided into two categories i.e. Single/multi-cloud data security issues and data security techniques used to secure the data in cloud virtual environment. We worked hard to find out the limitations of the existing data security solutions. This effort may help the research community to analyze single and multi-cloud in more details.