Cis Azure Storage 18.2

18.2 Ensure stored access policies (SAP) are used when generating shared access signature (SAS) tokens (Manual)

Description

Use stored access policies (SAP) when generating shared access signature (SAS) tokens in Azure to centrally manage permissions, expiration, and revocation settings for resource access. Stored access policies can be applied to blob containers, file shares, queues, and tables.

Rationale

Stored access policies provide centralized control over SAS token access, allowing administrators to update permissions or revoke access. This approach strengthens security by reducing the risk of unauthorized access to storage resources.

Impact

There is no cost for creating stored access policies, however there is some administrative overhead involved in managing these policies.

Audit Procedure

It is not currently possible to retrieve a list of all generated SAS tokens to check if they were associated with a SAP during creation.

An SAS token that has been created with a SAP will contain an si parameter that references the stored access policy identifier associated with the SAS, e.g. si=<stored-access-policy-identifier>.

The si parameter will be absent from an SAS token created without a SAP.

Expected Result

All SAS tokens should contain the si parameter referencing a stored access policy identifier. SAS tokens without the si parameter are not associated with a stored access policy and are non-compliant.

Remediation

Remediate from Storage Explorer

To create a SAP:

1. In Storage Explorer, expand Storage Accounts.
2. Expand a storage account.
3. Expand Blob Containers, File Shares, Queues, or Tables, and right-click a blob container, file share, queue, or table.
4. Click Manage Stored Access Policies....
5. Under Access Policies, click Add.
6. Modify the ID, Start time, Expiry time, and permissions appropriately.
7. Click Save.
8. Repeat steps 1-7 as needed to create SAP.

When generating SAS, select a SAP from the Access policy drop-down.

Remediate from Azure Portal

If SAS have been created without a SAP, the SAS can be revoked by regenerating the storage account access keys:

Note: Regenerating access keys can affect any applications or Azure services that are dependent on the storage account key.

1. Go to Storage accounts.
2. Click on a storage account.
3. Under Security + networking, click Access keys.
4. Next to each key, click Rotate key.
5. Click Yes to confirm.
6. Repeat steps 1-5 as needed to revoke SAS.

Default Value

By default, stored access policies are not associated with SAS. To use a stored access policy, it must be explicitly created and linked to the SAS at the time of creation.

References

1. https://learn.microsoft.com/en-us/azure/storage/common/storage-sas-overview#best-practices-when-using-sas
2. https://learn.microsoft.com/en-us/rest/api/storageservices/define-stored-access-policy
3. https://learn.microsoft.com/en-us/azure/storage/storage-explorer/vs-azure-tools-storage-explorer-blobs#manage-access-policies-for-a-blob-container

Additional Information

This recommendation is based on the recommendation Ensure stored access policies (SAP) are used when generating shared access signature (SAS) tokens, from the Common Reference Recommendations > Secrets and Keys > Shared Access Signatures section.

Profile

Level 1 | Manual