Credential chains in the Azure Identity client library for JavaScript

The Azure Identity client library provides credentials which are public classes that implement the Azure Core library's TokenCredential interface. A credential represents a distinct authentication flow for acquiring an access token from Microsoft Entra ID. These credentials can be selected individually or chained together to form an ordered sequence of authentication mechanisms to be attempted.

• Individual credentials provide speed and certainty. If they fail, you know the credential wasn't authenticated.
• Chains provide fallbacks. When the credential fails to authenticate, the next credential in the chain is attempted.

Design your authentication flows

When you use Azure SDK client libraries, the first step is to authenticate to Azure. There are many options of how to authenticate to consider, such as tools and IDEs used in the development team, automation workflows such as testing and CI/CD, and hosting platforms such as Azure App Service.

Choose from the following common progressions for your authentication flow:

• Use the DefaultAzureCredential for teams whose developers use various IDEs and CLIs to authenticate to Azure. This allows the greatest flexibility. This flexibility is provided at the cost of performance to validate the credentials in the chain until one succeeds.

  • The fallback from credential to credential is selected on your behalf based on the detected environment.
  • To determine which credential was selected, turn on debugging.

• Use the ChainedTokenCredential for teams which have a strict and scoped selection of tools. For example, they all authenticate in and use the same IDE or CLI. This allows the team to select the exact credentials and the order which still provides flexibility but at a reduced performance cost.

  • You select the fallback path from credential to credential regardless of the environment it's run in.
  • To determine which credential was selected, turn on debugging.

• For teams with certainty of credentials in all the environments, a control flow statement such as if/else, allows you to know which credential was chosen in each environment.

  • There's no fallback to another credential type.
  • You don't need to debug to determine which credential was chosen because it was specified.

How a chained credential works

At runtime, a credential chain attempts to authenticate using the sequence's first credential. If that credential fails to acquire an access token, the next credential in the sequence is attempted, and so on, until an access token is successfully obtained. The following sequence diagram illustrates this behavior:

Use DefaultAzureCredential for flexibility

DefaultAzureCredential is an opinionated, preconfigured chain of credentials. It's designed to support many environments, along with the most common authentication flows and developer tools. In graphical form, the underlying chain looks like this:

The order in which DefaultAzureCredential attempts credentials follows.

In its simplest form, you can use the parameterless version of DefaultAzureCredential as follows:

import { DefaultAzureCredential } from "@azure/identity";
import { BlobServiceClient } from "@azure/storage-blob";

// Acquire a credential object
const credential = new DefaultAzureCredential();

const blobServiceClient = new BlobServiceClient(
    "https://<my_account_name>.blob.core.windows.net",
    credential
);

Credentials are global to the environment

DefaultAzureCredential checks for the presence of certain environment variables. It's possible that someone could add or modify these environment variables at the system level on the host machine. Those changes apply globally and therefore alter the behavior of DefaultAzureCredential at runtime in any app running on that machine.

Use ChainedTokenCredential for granularity

ChainedTokenCredential is an empty chain to which you add credentials to suit your app's needs. For example, the following example adds a ManagedIdentityCredential instance, then an AzureCliCredential instance.

import { 
    ChainedTokenCredential, 
    ManagedIdentityCredential, 
    AzureCliCredential 
} from "@azure/identity";

const credential = ChainedTokenCredential(
    ManagedIdentityCredential({ clientId: "<YOUR_CLIENT_ID>" }),
    AzureCliCredential()
);

The preceding code sample creates a tailored credential chain comprised of two credentials. The user-assigned managed identity variant of ManagedIdentityCredential is attempted first, followed by AzureCliCredential, if necessary. In graphical form, the chain looks like this:

Debug a chained credential

To debug a credential chain, enable Azure SDK logging.

More resources

• Azure CLI