# Secure Cloud Infrastructure: A Survey on Issues, Current Solutions, and Open Challenges
## 1 Introduction  
The idea behind cloud computing is to provide all possible facilities such as software, IT infrastructure, and services to its customers for use over the internet. Cloud computing systems are large-scale, heterogeneous collections of autonomous systems and flexible computational architecture. This technology is emerging, as it is considered the first choice for businesses that do not want to deal with the in-house maintenance of systems and a development team [1]. Many businesses, such as Amazon AWS, Google, IBM, Sun, Microsoft, and many others, are developing efficient cloud products and technology [2]. In cloud technology, data are shared via virtual data centers from the customers and the organization [2].  
Cloud computing has evolved as a popular and universal paradigm for service-oriented computing where computing infrastructure and solutions are delivered as a service. The cloud has revolutionized the abstraction and use of computing infrastructure through its features (e.g., self-service on-demand, broad network access, resource pooling, etc.), making cloud computing desirable [3]. However, security is the biggest challenge, and concerns regarding cloud computing continue to arise as we witness an increasingnumber of new developments in cloud computing platforms [4]. In the post-COVID-19 world, it is clear that more people and businesses are adopting cloud services, software, and infrastructure, as they can be accessed anytime, and from anywhere. To handle security risks, several research works and developments, such as in [5; 6; 7; 8], have been proposed. Nonetheless, there are still more opportunities for new techniques to make the cloud more secure. Most of the existing techniques for securing the cloud do not focus on the new types of security risks that might face the cloud computing infrastructure. Hence, they cannot detect attacks or vulnerabilities that might come from the cloud service provider's side or the consumer's side. Furthermore, very few existing works have examined the different levels of cloud infrastructure altogether. Due to the high importance of investigating such issues, this paper conducted an extensive survey on the issues that the cloud computing infrastructure faces at different levels (application, host, network, and data level). It also presents the existing solutions used to mitigate these issues. Additionally, this paper highlights some open challenges that still need to be solved and suggest directions for future work. To the best of our knowledge, this study is the first effort to provide a systematic review of associated security issues and solutions based on cloud levels (application, host, network, and data level). The following are the main contributions of this study:  
1. Conducting a systematic evaluation of 103 articles on cloud infrastructure in connection with attacks and defenses.
2. Providing a new taxonomy for a systematic review of cloud infrastructure levels.
3. Investigating four levels that aim to cover all vulnerabilities that might come from the cloud service provider's side or the consumer's side.
4. Identifying the limitations of the examined studies and highlighting the open research challenges and proposed directions for future work.  
The rest of this paper is divided into eight sections: the methodology of this study is presented in Section 2. The background on cloud computing is given in Section 3. The current security issues that cloud infrastructure faces at different levels are investigated in Section 4. Section 5 presents the solutions proposed to solve the related security issues in the literature. The open challenges that still need to be solved are presented in Section 6. Section 7 suggests some future directions. The paper is concluded in Section 8.