The ESXi hypervisor architecture has many built-in security features such as CPU isolation, memory isolation, and device isolation. You can configure additional features such as lockdown mode, certificate replacement, and smart card authentication for enhanced security.
An ESXi host is also protected with a firewall. You can open ports for incoming and outgoing traffic as needed, but should restrict access to services and ports. Using the ESXi lockdown mode and limiting access to the ESXi Shell can further contribute to a more secure environment. Starting with vSphere 6.0, ESXi hosts participate in the certificate infrastructure. Hosts are provisioned with certificates that are signed by the VMware Certificate Authority (VMCA) by default.
See the VMware white paper Security of the VMware vSphere Hypervisor for additional information on ESXi security.