Antivirus software has been implemented on the guest operating system only is the observation that an IS auditor would consider the greatest risk when conducting an audit of a virtual server farm for potential software vulnerabilities. A virtual server farm is a collection of servers that run multiple virtual machines (VMs) on a single physical host using a software layer called a hypervisor. A guest operating system is the operating system installed on each VM. Antivirus software is a software program that detects and removes malicious software from a computer system. If antivirus software has been implemented on the guest operating system only, it means that the hypervisor and the host operating system are not protected from malware attacks, which could compromise the security and availability of all VMs running on the same host.
Therefore, antivirus software should be implemented on both the guest and host operating systems as well as on the hypervisor. References: CISA Review Manual, 27th Edition, page 378