Grant limited access to Azure Storage resources using shared access signatures (SAS)
A shared access signature (SAS) provides secure delegated access to resources in your storage account without compromising the security of your data. With a SAS, you have granular control over how a client can access your data. You can control what resources the client may access, what permissions they have on those resources, and how long the SAS is valid, among other parameters.
Types of shared access signatures
Azure Storage supports three types of shared access signatures:
- User delegation SAS (preview). A user delegation SAS is secured with Azure Active Directory (Azure AD) credentials and also by the permissions specified for the SAS. A user delegation SAS applies to Blob storage only. To create a user delegation SAS, you must first request a user delegation key, which is used to sign the SAS. For more information about the user delegation SAS, see Create a user delegation SAS (REST API).
- Service SAS. A service SAS is secured with the storage account key. A service SAS delegates access to a resource in only one of the Azure Storage services: Blob storage, Queue storage, Table storage, or Azure Files. For more information about the service SAS, see Create a service SAS (REST API).
- Account SAS. An account SAS is secured with the storage account key. An account SAS delegates access to resources in one or more of the storage services. All of the operations available via a service or user delegation SAS are also available via an account SAS. Additionally, with the account SAS, you can delegate access to operations that apply at the level of the service, such as Get/Set Service Properties and Get Service Stats operations. You can also delegate access to read, write, and delete operations on blob containers, tables, queues, and file shares that are not permitted with a service SAS. For more information about the account SAS, Create an account SAS (REST API).
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 for access to Blob storage, use Azure AD credentials to create a user delegation SAS when possible for superior security.
A shared access signature can take one of two forms:
- Ad hoc SAS: When you create an ad hoc SAS, the start time, expiry time, and permissions for the SAS are all specified in the SAS URI (or implied, if start time is omitted). Any type of SAS can be an ad hoc SAS.
- Service SAS with stored access policy: A stored access policy is defined on a resource container, which can be a blob container, table, queue, or file share. The stored access policy can be used to manage constraints for one or more service shared access signatures. When you associate a service SAS with a stored access policy, the SAS inherits the constraints—the start time, expiry time, and permissions—defined for the stored access policy.
A user delegation SAS or an account SAS must be an ad hoc SAS. Stored access policies are not supported for the user delegation SAS or the account SAS.
How a shared access signature works
A shared access signature is a signed URI that points to one or more storage resources and includes a token that contains a special set of query parameters. The token indicates how the resources may be accessed by the client. One of the query parameters, the signature, is constructed from the SAS parameters and signed with the key that was used to create the SAS. This signature is used by Azure Storage to authorize access to the storage resource.
You can sign a SAS in one of two ways:
With a user delegation key that was created using Azure Active Directory (Azure AD) credentials. A user delegation SAS is signed with the user delegation key.
To get the user delegation key and create the SAS, an Azure AD security principal must be assigned a role-based access control (RBAC) role that includes the Microsoft.Storage/storageAccounts/blobServices/generateUserDelegationKey action. For detailed information about RBAC roles with permissions to get the user delegation key, see Create a user delegation SAS (REST API).
With the storage account key. Both a service SAS and an account SAS are signed with the storage account key. To create a SAS that is signed with the account key, an application must have access to the account key.
The SAS token is a string that you generate on the client side, for example by using one of the Azure Storage client libraries. The SAS token is not tracked by Azure Storage in any way. You can create an unlimited number of SAS tokens on the client side. After you create a SAS, you can distribute it to client applications that require access to resources in your storage account.
When a client application provides a SAS URI to Azure Storage as part of a request, the service checks the SAS parameters and signature to verify that it is valid for authorizing the request. If the service verifies that the signature is valid, then the request is authorized. Otherwise, the request is declined with error code 403 (Forbidden).
Here's an example of a service SAS URI, showing the resource URI and the SAS token:
When to use a shared access signature
Use a SAS when you want to provide secure access to resources in your storage account to any client who does not otherwise have permissions to those resources.
A common scenario where a SAS is useful is a service where users read and write their own data to your storage account. In a scenario where a storage account stores user data, there are two typical design patterns:
Clients upload and download data via a front-end proxy service, which performs authentication. This front-end proxy service has the advantage of allowing validation of business rules, but for large amounts of data or high-volume transactions, creating a service that can scale to match demand may be expensive or difficult.
A lightweight service authenticates the client as needed and then generates a SAS. Once the client application receives the SAS, they can access storage account resources directly with the permissions defined by the SAS and for the interval allowed by the SAS. The SAS mitigates the need for routing all data through the front-end proxy service.
Many real-world services may use a hybrid of these two approaches. For example, some data might be processed and validated via the front-end proxy, while other data is saved and/or read directly using SAS.
Additionally, a SAS is required to authorize access to the source object in a copy operation in certain scenarios:
- When you copy a blob to another blob that resides in a different storage account, you must use a SAS to authorize access to the source blob. You can optionally use a SAS to authorize access to the destination blob as well.
- When you copy a file to another file that resides in a different storage account, you must use a SAS to authorize access to the source file. You can optionally use a SAS to authorize access to the destination file as well.
- When you copy a blob to a file, or a file to a blob, you must use a SAS to authorize access to the source object, even if the source and destination objects reside within the same storage account.
Best practices when using SAS
When you use shared access signatures in your applications, you need to be aware of two potential risks:
- If a SAS is leaked, it can be used by anyone who obtains it, which can potentially compromise your storage account.
- If a SAS provided to a client application expires and the application is unable to retrieve a new SAS from your service, then the application's functionality may be hindered.
The following recommendations for using shared access signatures can help mitigate these risks:
- Always use HTTPS to create or distribute a SAS. If a SAS is passed over HTTP and intercepted, an attacker performing a man-in-the-middle attack is able to read the SAS and then use it just as the intended user could have, potentially compromising sensitive data or allowing for data corruption by the malicious user.
- Use a user delegation SAS when possible. A user delegation SAS provides superior security to a service SAS or an account SAS. A user delegation SAS is secured with Azure AD credentials, so that you do not need to store your account key with your code.
- Have a revocation plan in place for a SAS. Make sure you are prepared to respond if a SAS is compromised.
- Define a stored access policy for a service SAS. Stored access policies give you the option to revoke permissions for a service SAS without having to regenerate the storage account keys. Set the expiration on these very far in the future (or infinite) and make sure it's regularly updated to move it farther into the future.
- Use near-term expiration times on an ad hoc SAS service SAS or account SAS. In this way, even if a SAS is compromised, it's valid only for a short time. This practice is especially important if you cannot reference a stored access policy. Near-term expiration times also limit the amount of data that can be written to a blob by limiting the time available to upload to it.
- Have clients automatically renew the SAS if necessary. Clients should renew the SAS well before the expiration, in order to allow time for retries if the service providing the SAS is unavailable. If your SAS is meant to be used for a small number of immediate, short-lived operations that are expected to be completed within the expiration period, then this may be unnecessary as the SAS is not expected to be renewed. However, if you have client that is routinely making requests via SAS, then the possibility of expiration comes into play. The key consideration is to balance the need for the SAS to be short-lived (as previously stated) with the need to ensure that the client is requesting renewal early enough (to avoid disruption due to the SAS expiring prior to successful renewal).
- Be careful with SAS start time. If you set the start time for a SAS to now, then due to clock skew (differences in current time according to different machines), failures may be observed intermittently for the first few minutes. In general, set the start time to be at least 15 minutes in the past. Or, don't set it at all, which will make it valid immediately in all cases. The same generally applies to expiry time as well--remember that you may observe up to 15 minutes of clock skew in either direction on any request. For clients using a REST version prior to 2012-02-12, the maximum duration for a SAS that does not reference a stored access policy is 1 hour, and any policies specifying longer term than that will fail.
- Be specific with the resource to be accessed. A security best practice is to provide a user with the minimum required privileges. If a user only needs read access to a single entity, then grant them read access to that single entity, and not read/write/delete access to all entities. This also helps lessen the damage if a SAS is compromised because the SAS has less power in the hands of an attacker.
- Understand that your account will be billed for any usage, including via a SAS. If you provide write access to a blob, a user may choose to upload a 200 GB blob. If you've given them read access as well, they may choose to download it 10 times, incurring 2 TB in egress costs for you. Again, provide limited permissions to help mitigate the potential actions of malicious users. Use short-lived SAS to reduce this threat (but be mindful of clock skew on the end time).
- Validate data written using a SAS. When a client application writes data to your storage account, keep in mind that there can be problems with that data. If your application requires that data be validated or authorized before it is ready to use, you should perform this validation after the data is written and before it is used by your application. This practice also protects against corrupt or malicious data being written to your account, either by a user who properly acquired the SAS, or by a user exploiting a leaked SAS.
- Know when not to use a SAS. Sometimes the risks associated with a particular operation against your storage account outweigh the benefits of using a SAS. For such operations, create a middle-tier service that writes to your storage account after performing business rule validation, authentication, and auditing. Also, sometimes it's simpler to manage access in other ways. For example, if you want to make all blobs in a container publicly readable, you can make the container Public, rather than providing a SAS to every client for access.
- Use Azure Monitor and Azure Storage logs to monitor your application. You can use Azure Monitor and storage analytics logging to observe any spike in authorization failures due to an outage in your SAS provider service or to the inadvertent removal of a stored access policy. For more information, see Azure Storage metrics in Azure Monitor and Azure Storage Analytics logging.
Get started with SAS
To get started with shared access signatures, see the following articles for each SAS type.
User delegation SAS
- Create a user delegation SAS for a container or blob with PowerShell (preview)
- Create a user delegation SAS for a container or blob with the Azure CLI (preview)
- Create a user delegation SAS for a container or blob with .NET (preview)