Infrastructure as a Service (IaaS) is a cloud model in which the cloud service provider (CSP) offers the basic computing resources, such as servers, storage, network, and virtualization, as a service over the internet. The cloud service buyer (CSB) is responsible for installing, configuring, managing, and securing the operating systems, applications, data, and middleware on top of the infrastructure. Therefore, the CSB assumes the most security responsibility in the IaaS model, as it has to protect the confidentiality, integrity, and availability of its own assets and information in the cloud environment.

In contrast, in the other cloud models, the CSP takes over more security responsibility from the CSB, as it provides more layers of the service stack. In Disaster Recovery as a Service (DRaaS), the CSP offers the replication and recovery of the CSB’s data and applications in the event of a disaster. In Platform as a Service (PaaS), the CSP offers the development and deployment tools, such as programming languages, frameworks, libraries, and databases, as a service. In Software as a Service (SaaS), the CSP offers the complete software applications, such as email, CRM, or ERP, as a service. In these models, the CSB has less control and visibility over the underlying infrastructure, platform, or software, and has to rely on the CSP’s security measures and contractual agreements.

References = CISM Review Manual, 16th Edition, Chapter 3: Information Security Program Development and Management, Section: Information Security Program Management, Subsection: Cloud Computing, page 140-1411