Unauthorized serial devices must be disconnected on the virtual machine (VM).

Ensure no device is connected to a virtual machine if it is not required. For example, floppy, serial, and parallel ports are rarely used for virtual machines in a datacenter environment, and CD/DVD drives are usually connected only temporarily during software installation.

From the vSphere Client, right-click the Virtual Machine and go to "Edit Settings".

Review the VM's hardware and verify no serial devices exist.

or

From a PowerCLI command prompt while connected to the ESXi host or vCenter server, run the following command:

Get-VM | Where {$_.ExtensionData.Config.Hardware.Device.DeviceInfo.Label -match "serial"}

If a virtual machine has a serial device present, this is a finding.

The VM must be powered off to remove a serial device.

From the vSphere Client, right-click the Virtual Machine and go to "Edit Settings".

Select the serial device, click the circled "X" to remove it, and click "OK".