A survey on security issues and solutions at different layers of Cloud computing <s> Virtualization level security issues <s> Cloud computing is revolutionizing how information technology resources and services are used and managed but the revolution comes with new security problems. Among these is the problem of securely managing the virtual-machine images that encapsulate each application of the cloud. These images must have high integrity because the initial state of every virtual machine in the cloud is determined by some image. However, as some of the enefits of the cloud depend on users employing images built by third parties, users must also be able to share images safely. This paper explains the new risks that face administrators and users (both image publishers and image retrievers) of a cloud's image repository. To address those risks, we propose an image management system that controls access to images, tracks the provenance of images, and provides users and administrators with efficient image filters and scanners that detect and repair security violations. Filters and scanners achieve efficiency by exploiting redundancy among images; an early implementation of the system shows that this approach scales better than a naive approach that treats each image independently. <s> BIB001 </s> A survey on security issues and solutions at different layers of Cloud computing <s> Virtualization level security issues <s> Large distributed systems such as clouds are increasingly becoming targets of attacks. Virtualization can be leveraged to increase the security of such systems by protecting the integrity of guest components. This paper discusses the integrity protection problem in the clouds and sketches a novel architecture, Transparent Cloud Protection System (TCPS) for increased security of cloud resources. TCPS can be tailored to different cloud flavors to monitor the integrity of guests and infrastructure components while remaining transparent to virtual machines. <s> BIB002
In the virtualized (multi-tenant) environment, multiple OSs run concurrently on a host computer using hypervisor. Existing vulnerabilities [40] in a VM that are distributed throughout the physical and virtual enterprise resources allow cyber attacker, malware, or other threats to remotely exploit. VMs' collocation also increases the security risk. As the number of guest operating systems (OSs) running on a hypervisor increase, the security concerns with that newer guest OSs also increase. Because it is not possible to keep track of all guest OSs, and hence maintaining the security of those OSs is difficult. It may happen that a guest system tries to run a malicious code on the host system and bring the system down or take full control of the system and block access to other guest OSs. There are risks associated with sharing the same physical infrastructure between a set of multiple users, even one being malicious can cause threats to the others using the same infrastructure. If a hacker is able to get control over the hypervisor, he can make changes to any of the guest OSs and get control over all the data passing through the hypervisor. Isolation between two VMs is not completely adequate by current virtual machine monitors (VMMs). By compromising the lower layer hypervisor vulnerabilities, an attacker can gain control over installed VMs, for example, Bluepill, SubVirt, and DKSM are some well-known attacks on the virtual layer. This is still an open problem to prevent such threats. Virtualization based malware and rootkit: New generation of rootkits that benefit from the processor technology that allows an attacker to insert an additional hypervisor between the hardware and the software. The hypervisor takes control of the system and converts the original operating system into a virtual guest on the fly. In contrast to software-based virtualization, this kind of hijacking does not need a restart, and that makes it all the more difficult to detect the intrusion. Sharing of VM images in Cloud introduces security risks. The owner of an image is concerned about confidentiality (e.g. unauthorized accesses to the image). The user of an image is concerned about safety (e.g. a malicious image that is capable of corrupting or stealing the user's own private data). For example, instances running on Amazon's EC2 platform can be easily compromized by performing various attacks like the signature-wrapping attack, cross site scripting (XSS) attack, and DoS attack. This allows attackers to create, modify, and delete VM images, and change administrative passwords and settings that are put into instances with EC2 for S3 access. There is a risk of non-compliance (e.g. running unlicensed software or software with expired licenses). The administrator of Cloud is concerned with the security and compliance of the Cloud as a whole and the integrity of the images. There is a risk of damages caused by malware contained in any image stored in the repository. There should be a standard mechanism for checking integrity of guest VMs for successfully executing workload and avoiding interruption of computation, data loss, and misuse of resources. As shown in Fig. 6 [31] , host based transparent Cloud protection system (TCPS) monitors integrity of Cloud components. TCPS is placed between guest's kernel and the virtualization layers, which monitors guest VMs and protects them against intruders and attacks. It also addresses transparency problems in the Cloud. BIB002 Authors in BIB001 provided solution for securing virtual image repository and access control. As shown in Fig. 7 , access control mechanism is used to reduce risk of unauthorized access to publisher's VM images. Image filters are used to remove user's personal information for providing privacy at publishing and retrieving time, where the tracking system is used to disable malicious attempts by tracing versions of images and their operations. Also, image repository is maintained by periodically implementing virus scan and fixing vulnerabilities. Volokyta et al.  proposed a mechanism of monitoring of virtual machines to increase security of Cloud resources that can be affected by the attacks. Using a detector, the host OS is monitored for integrity checking. All the malicious activities are analyzed by this detector and logged into a log file. Using periodic checksum verification of executable file and libraries, the integrity of Cloud resources is checked by the virtual machine monitor (VMM). In this approach, all the monitoring activities are done through VMM. However, if hypervisor (or VMM) is compromised, guest OSs (running on that VMM) can be compromised.