Control: 8.4 Ensure that 'Unattached disks' are encrypted with 'Customer Managed Key' (CMK)

Description

Ensure that unattached disks in a subscription are encrypted with a Customer Managed Key (CMK).

Managed disks are encrypted by default with Platform-managed keys. Using Customer-managed keys may provide an additional level of security or meet an organization's regulatory requirements. Encrypting managed disks ensures that its entire content is fully unrecoverable without a key and thus protects the volume from unwarranted reads. Even if the disk is not attached to any of the VMs, there is always a risk where a compromised user account with administrative access to VM service can mount/attach these data disks, which may lead to sensitive information disclosure and tampering.

Remediation

If data stored in the disk is no longer useful, refer to Azure documentation to delete unattached data disks at:

- https://docs.microsoft.com/en-us/rest/api/compute/disks/delete
- https://docs.microsoft.com/en-us/cli/azure/disk?view=azure-cli-latest#az-disk-delete

If data stored in the disk is important, to encrypt the disk refer to azure documentation at:

- https://docs.microsoft.com/en-us/azure/virtual-machines/disks-enable-customer-managed-keys-portal
- https://docs.microsoft.com/en-us/rest/api/compute/disks/update#encryptionsettings

Default Value

By default, managed disks are encrypted with a Platform-managed key.