Security is hard, we know quite well by now, but instead of getting easier it seems that, as time goes by, it is getting harder.
Consider Public Cloud: in a Public Cloud environment, the threat scenario is much more complex than in a dedicated, on-premises HW one. Assuming that in both cases the initial HW and SW configuration is secure, the threat scenario for services running on dedicated, on-premises HW consists of external attacks either directly from the network or mediated by the system users who could (unintentionally) download malware into the service. Instead the threat scenario in a Public Cloud environment must also include attacks from other services running on the same HW (other virtual machines, tenants, dockers etc.) and attacks from the infrastructure itself running the services (hypervisors on host machines).
Protecting virtual machines and cloud services from other machines and services running on the same HW and from the hypervisor is hard. New hardware features are needed to be able to effectively separate the guest services from each other and from the host. But even hardware features are not easy to design and implement to these purposes.
For example, Intel has introduced the SGX hardware extensions to create enclaves to manage in HW very sensitive data like cryptographic keys. In this paper it has been shown, as initially feared by Rutkowska, that these HW extensions both provide security features to the users but also to the attackers, who can exploit them to create practically invisible and undetectable malware. In the article it is actually shown in a particular scenario, how it is possible to recover some secret RSA keys used for digital signatures sealed in one enclave from another enclave. Since not even the hypervisor can see what there is in one enclave, the malware is practically undetectable.
IT security is a delicate balance between many factors, HW, SW, functionalities, human behaviour etc. and the more complex is this ecosystem, the easier it is to find loopholes in it and ways to abuse it.