# Secure Cloud Infrastructure: A Survey on Issues, Current Solutions, and Open Challenges
## 4 Security Issues in Cloud Computing Infrastructure
### Host-Level Issues
#### 4.4.1 Virtualization Issues  
The concept of the cloud is based on virtualization, in which many guest VMs share common physical hardware. Malicious actors can target the virtualization elements, such as the hypervisor and virtual machines [45]. Therefore, securing the hypervisor in cloud environments is necessary, as hackers can use it to compromise all VMs built based on it [46]. Many of the issues raised in the context of virtualization and hypervisor issues [21] can be summarized as follows:  
Hypervisor Vulnerabilities  
To run multiple guest VMs and applications simultaneously on a single host machine and to provide isolation between the guest VMs, a hypervisor or virtual machine manager (VMM) is generated. While hypervisors are supposed to be vigorous and stable, they are vulnerable to attacks. When attackers take control of the hypervisor, it will be under their full control, and all the VMs and data will be accessible to them. The greater control offered by the bottom layers in the virtual machine is another reason hackers consider the VMM a potential target. Compromising a VMM also allows attackers to access the underlying physical device and applications they host. Many of the well-known attacks (e.g., Hyperjacking, Bluepill, etc.) inject rootkits based on VMs that can mount or change the current rogue hypervisor to take full control of the environment. Since the hypervisor  
Figure 10: Host Level Issues. runs underneath the host OS, standard security measures make it difficult to detect these types of attacks [45]. Virtual machine (VM) theft is another virtualization-related attack that can hit cloud environments. For this attack to mount and run, an attacker copies a VM over the network or to a portable storage medium. This could be done by utilizing the migration utility on the hypervisor. Nevertheless, to trigger unauthorized migration of the guest VM to its cloud infrastructure, an intruder may also tamper with the VMM control panel that handles live migration [47]. As such, securing the hypervisor could help in mitigating the VM theft attack. VM Escape  
Virtual machines are intended to support a strong separation between hosts and VMs. However, the operating system's vulnerabilities running within the VM can help attackers to inject malware, which not only can affect the current VM but also bypass the VMM layer into the other VMs on the same hypervisor. This allows the malware to gain access to the host machine and initiate sustainable attacks, utilizing the set of backdoors they open [45]. Hyper jacking is an attack in which, inside a virtual machine (VM) host, a hacker takes control over the hypervisor that generates the virtual environment. This attack aims to target the operating system of the virtual machines. Therefore, the software of the attacker can run, and its existence will be completely hidden from the applications on the VMs [45]. Hyper-call attacks involve an intrusion into the VM using well-defined hyper-call interfaces by an unauthorized guest VM by exploiting vulnerabilities in the hyper-call handler of a VMM. Such attacks could lead to a shift in the functionality of the VMM or a \"host crash\" when a malicious code with VMM privileges is executed [47]. Guest-hopping attacks include any failure of separation between shared infrastructures. An attacker tries to access one virtual machine by accessing another virtual machine hosted on the same hardware. The Forensics and VM debugging instruments are one of the potential mitigations of the guest hopping attack to observe any attempts to manipulate VM [42]. Hypervisor Single Point of Failure  
The hypervisor technology that regulates the access of VMs to physical resources is the basis of current virtualized environments and is critical for the overall functionality of the system. Therefore, hypervisor failure due to the overuse of hardware or device faults leads to overall system collapse [45]. This is known as single-point-of-failure, which originates from lacking redundant hypervisor and/or underlying hardware.