Create a user delegation SAS for a container or blob with .NET (preview)

A shared access signature (SAS) enables you to grant limited access to containers and blobs in your storage account. When you create a SAS, you specify its constraints, including which Azure Storage resources a client is allowed to access, what permissions they have on those resources, and how long the SAS is valid.

Every SAS is signed with a key. You can sign a SAS in one of two ways:

  • With a key created using Azure Active Directory (Azure AD) credentials. A SAS signed with Azure AD credentials is a user delegation SAS.
  • With the storage account key. Both a service SAS and an account SAS are signed with the storage account key.

This article shows how to use Azure Active Directory (Azure AD) credentials to create a user delegation SAS for a container or blob with the Azure Storage client library for .NET.

About the user delegation SAS (preview)

A SAS token for access to a container or blob may be secured by using either Azure AD credentials or an account key. A SAS secured with Azure AD credentials is called a user delegation SAS, because the OAuth 2.0 token used to sign the SAS is requested on behalf of the user.

Microsoft recommends that you use Azure AD credentials when possible as a security best practice, rather than using the account key, which can be more easily compromised. When your application design requires shared access signatures, use Azure AD credentials to create a user delegation SAS for superior security. For more information about the user delegation SAS, see Create a user delegation SAS.

Note

The user delegation SAS preview is intended for non-production use only.

Caution

Any client that possesses a valid SAS can access data in your storage account as permitted by that SAS. It's important to protect a SAS from malicious or unintended use. Use discretion in distributing a SAS, and have a plan in place for revoking a compromised SAS.

For more information about shared access signatures, see Grant limited access to Azure Storage resources using shared access signatures (SAS).

Authenticate with the Azure Identity library (preview)

The Azure Identity client library for .NET (preview) authenticates a security principal. When your code is running in Azure, the security principal is a managed identity for Azure resources.

When your code is running in the development environment, authentication may be handled automatically, or it may require a browser login, depending on which tools you're using. Microsoft Visual Studio supports single sign-on (SSO), so that the active Azure AD user account is automatically used for authentication. For more information about SSO, see Single sign-on to applications.

Other development tools may prompt you to login via a web browser. You can also use a service principal to authenticate from the development environment. For more information, see Create identity for Azure app in portal.

After authenticating, the Azure Identity client library gets a token credential. This token credential is then encapsulated in the service client object that you create to perform operations against Azure Storage. The library handles this for your seamlessly by getting the appropriate token credential.

For more information about the Azure Identity client library, see Azure Identity client library for .NET.

Assign RBAC roles for access to data

When an Azure AD security principal attempts to access blob data, that security principal must have permissions to the resource. Whether the security principal is a managed identity in Azure or an Azure AD user account running code in the development environment, the security principal must be assigned an RBAC role that grants access to blob data in Azure Storage. For information about assigning permissions via RBAC, see the section titled Assign RBAC roles for access rights in Authorize access to Azure blobs and queues using Azure Active Directory.

Install the preview packages

The examples in this article use the latest preview version of the Azure Storage client library for Blob storage. To install the preview package, run the following command from the NuGet package manager console:

Install-Package Azure.Storage.Blobs -IncludePrerelease

The examples in this article also use the latest preview version of the Azure Identity client library for .NET to authenticate with Azure AD credentials. To install the preview package, run the following command from the NuGet package manager console:

Install-Package Azure.Identity -IncludePrerelease

Add using directives

Add the following using directives to your code to use the preview versions of the Azure Identity and Azure Storage client libraries.

using System;
using System.IO;
using System.Threading.Tasks;
using Azure.Identity;
using Azure.Storage;
using Azure.Storage.Sas;
using Azure.Storage.Blobs;
using Azure.Storage.Blobs.Models;

Get an authenticated token credential

To get a token credential that your code can use to authorize requests to Azure Storage, create an instance of the DefaultAzureCredential class.

The following code snippet shows how to get the authenticated token credential and use it to create a service client for Blob storage:

string blobEndpoint = string.Format("https://{0}.blob.core.windows.net", accountName);

BlobServiceClient blobClient = new BlobServiceClient(new Uri(blobEndpoint),
                                                     new DefaultAzureCredential());

Get the user delegation key

Every SAS is signed with a key. To create a user delegation SAS, you must first request a user delegation key, which is then used to sign the SAS. The user delegation key is analogous to the account key used to sign a service SAS or an account SAS, except that it relies on your Azure AD credentials. When a client requests a user delegation key using an OAuth 2.0 token, Azure Storage returns the user delegation key on behalf of the user.

Once you have the user delegation key, you can use that key to create any number of user delegation shared access signatures, over the lifetime of the key. The user delegation key is independent of the OAuth 2.0 token used to acquire it, so the token does not need to be renewed so long as the key is still valid. You can specify that the key is valid for a period of up to 7 days.

Use one of the following methods to request the user delegation key:

The following code snippet gets the user delegation key and writes out its properties:

UserDelegationKey key = await blobClient.GetUserDelegationKeyAsync(DateTimeOffset.UtcNow,
                                                                   DateTimeOffset.UtcNow.AddDays(7));

Console.WriteLine("User delegation key properties:");
Console.WriteLine("Key signed start: {0}", key.SignedStart);
Console.WriteLine("Key signed expiry: {0}", key.SignedExpiry);
Console.WriteLine("Key signed object ID: {0}", key.SignedOid);
Console.WriteLine("Key signed tenant ID: {0}", key.SignedTid);
Console.WriteLine("Key signed service: {0}", key.SignedService);
Console.WriteLine("Key signed version: {0}", key.SignedVersion);

Create the SAS token

The following code snippet shows create a new BlobSasBuilder and provide the parameters for the user delegation SAS. The snippet then calls the ToSasQueryParameters to get the SAS token string. Finally, the code builds the complete URI, including the resource address and SAS token.

BlobSasBuilder builder = new BlobSasBuilder()
{
    ContainerName = containerName,
    BlobName = blobName,
    Permissions = "r",
    Resource = "b",
    StartTime = DateTimeOffset.UtcNow,
    ExpiryTime = DateTimeOffset.UtcNow.AddMinutes(5)
};

string sasToken = sasBuilder.ToSasQueryParameters(key, accountName).ToString();

UriBuilder fullUri = new UriBuilder()
{
    Scheme = "https",
    Host = string.Format("{0}.blob.core.windows.net", accountName),
    Path = string.Format("{0}/{1}", containerName, blobName),
    Query = sasToken
};

Example: Get a user delegation SAS

The following example method shows the complete code for authenticating the security principal and creating the user delegation SAS:

async static Task<Uri> GetUserDelegationSasBlob(string accountName, string containerName, string blobName)
{
    // Construct the blob endpoint from the account name.
    string blobEndpoint = string.Format("https://{0}.blob.core.windows.net", accountName);

    // Create a new Blob service client with Azure AD credentials.  
    BlobServiceClient blobClient = new BlobServiceClient(new Uri(blobEndpoint), 
                                                            new DefaultAzureCredential());

    // Get a user delegation key for the Blob service that's valid for seven days.
    // Use the key to generate any number of shared access signatures over the lifetime of the key.
    UserDelegationKey key = await blobClient.GetUserDelegationKeyAsync(DateTimeOffset.UtcNow,
                                                                       DateTimeOffset.UtcNow.AddDays(7));

    // Read the key's properties.
    Console.WriteLine("User delegation key properties:");
    Console.WriteLine("Key signed start: {0}", key.SignedStart);
    Console.WriteLine("Key signed expiry: {0}", key.SignedExpiry);
    Console.WriteLine("Key signed object ID: {0}", key.SignedOid);
    Console.WriteLine("Key signed tenant ID: {0}", key.SignedTid);
    Console.WriteLine("Key signed service: {0}", key.SignedService);
    Console.WriteLine("Key signed version: {0}", key.SignedVersion);

    // Create a SAS token that's valid a short interval.
    BlobSasBuilder sasBuilder = new BlobSasBuilder()
    {
        ContainerName = containerName,
        BlobName = blobName,
        Permissions = "r",
        Resource = "b",
        StartTime = DateTimeOffset.UtcNow,
        ExpiryTime = DateTimeOffset.UtcNow.AddMinutes(5)
    };

    // Use the key to get the SAS token.
    string sasToken = sasBuilder.ToSasQueryParameters(key, accountName).ToString();

    // Construct the full URI, including the SAS token.
    UriBuilder fullUri = new UriBuilder()
    {
        Scheme = "https",
        Host = string.Format("{0}.blob.core.windows.net", accountName),
        Path = string.Format("{0}/{1}", containerName, blobName),
        Query = sasToken
    };

    Console.WriteLine("User delegation SAS URI: {0}", fullUri);
    return fullUri.Uri;
}

Example: Read a blob with a user delegation SAS

The following example tests the user delegation SAS created in the previous example from a simulated client application. If the SAS is valid, the client application is able to read the contents of the blob. If the SAS is invalid, for example if it has expired, Azure Storage returns error code 403 (Forbidden).

private static async Task ReadBlobWithSasAsync(Uri sasUri)
{
    // Try performing blob operations using the SAS provided.

    // Create a blob client object for blob operations.
    BlobClient blobClient = new BlobClient(sasUri, null);

    // Download and read the contents of the blob.
    try
    {
        // Download blob contents to a stream and read the stream.
        BlobDownloadInfo blobDownloadInfo = await blobClient.DownloadAsync();
        using (StreamReader reader = new StreamReader(blobDownloadInfo.Content, true))
        {
            string line;
            while ((line = reader.ReadLine()) != null)
            {
                Console.WriteLine(line);
            }
        }

        Console.WriteLine();
        Console.WriteLine("Read operation succeeded for SAS {0}", sasUri);
        Console.WriteLine();
    }
    catch (StorageRequestFailedException e)
    {
        // Check for a 403 (Forbidden) error. If the SAS is invalid, 
        // Azure Storage returns this error.
        if (e.Status == 403)
        {
            Console.WriteLine("Read operation failed for SAS {0}", sasUri);
            Console.WriteLine("Additional error information: " + e.Message);
            Console.WriteLine();
        }
        else
        {
            Console.WriteLine(e.Message);
            Console.ReadLine();
            throw;
        }
    }
}

Resources for development with .NET

The links below provide useful resources for developers using the Azure Storage client library for .NET.

Azure Storage common APIs

Blob storage APIs

.NET tools

See also