Cis Azure Compute 20.3

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).

Rationale

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.

Impact

NOTE: You must have your key vault set up to utilize this. Encryption is available only on Standard tier VMs. This might cost you more.

Utilizing and maintaining Customer-managed keys will require additional work to create, protect, and rotate keys.

Audit Procedure

Using Azure Portal

1. Go to Disks
2. Click on Add Filter
3. In the filter field select Disk state
4. In the Value field select Unattached
5. Click Apply
6. For each disk listed ensure that Encryption type in the encryption blade is Encryption at-rest with a customer-managed key

Using Azure CLI

Ensure command below does not return any output.

az disk list --query '[? diskstate == `Unattached`].{encryptionSettings:encryptionSettings, name: name}' -o json

Expected Result

All unattached disks should have encryption type set to Encryption at-rest with a customer-managed key. The encryptionSettings should not be null.

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.

References

1. https://docs.microsoft.com/en-us/azure/security/fundamentals/azure-disk-encryption-vms-vmss
2. https://docs.microsoft.com/en-us/azure/security-center/security-center-disk-encryption?toc=%2fazure%2fsecurity%2ftoc.json
3. https://docs.microsoft.com/en-us/rest/api/compute/disks/delete
4. https://docs.microsoft.com/en-us/cli/azure/disk?view=azure-cli-latest#az-disk-delete
5. https://docs.microsoft.com/en-us/rest/api/compute/disks/update#encryptionsettings
6. https://docs.microsoft.com/en-us/cli/azure/disk?view=azure-cli-latest#az-disk-update
7. https://learn.microsoft.com/en-us/security/benchmark/azure/mcsb-data-protection#dp-5-use-customer-managed-key-option-in-data-at-rest-encryption-when-required

Profile

Level 2 | Automated