The Firebase Admin SDK allows you to integrate your own servers with Firebase Authentication. You can use the Admin SDK to manage your users or to manage authentication tokens. There are a number of reasons you would want to do this:
It is not always convenient to have to visit the Firebase console in order to manage your Firebase users. The admin user management API provides programmatic access to those same users. It even allows you to do things the Firebase console cannot, such as retrieving a user's full data and changing a user's password, email address or phone number.
You can integrate an external user system with Firebase. For example, you may already have a pre-existing user database or you may want to integrate with a third-party identity provider that Firebase Authentication doesn’t natively support.
To do this, you can create custom tokens with arbitrary claims identifying the user. These custom tokens can then be used to sign into the Firebase Authentication service on a client application and assume the identity described by the token's claims. This identity will then be used when accessing other Firebase services, such as the Firebase Realtime Database and Cloud Storage.
Firebase Authentication is primarily used to identify users of your app in order to restrict access to other Firebase services, such as the Firebase Realtime Database and Cloud Storage. But you can also use the service to identify these users on your own server. This lets you securely perform server-side logic on behalf of users that have signed in with Firebase Authentication.
To do this, you can retrieve an ID token from a client
application signed in with Firebase Authentication and include the token in a request
to your server. Your server then verifies the ID token and extracts the claims
that identify the user (including their
uid, the identity provider they
logged in with, etc.). This identity information can then be used by your
server to carry out actions on behalf of the user.
The Firebase Admin SDK provides methods for accomplishing the authentication tasks above by enabling you to manage your users, generate custom tokens, and verify ID tokens.
Custom User Claims
In some cases, you may want to implement fine-grained access control for users already signed in with one of the supported Firebase auth providers such as Email/Password, Google, Facebook, phone, etc. A combination of custom user claims and application security rules provides this capability. For example, a user signed in with Firebase Auth's Email/Password provider can have access control defined using custom claims.
The Firebase Admin SDK provides an API for managing your Firebase users with elevated privileges. The admin user management API gives you the ability to programmatically retrieve, create, update, and delete users without requiring a user's existing credentials and without worrying about client-side rate limiting.
Custom token creation
The primary use for creating custom tokens is to allow users to authenticate against an external or legacy authentication mechanism. This could be one you control, such as your LDAP server, or a third-party OAuth provider which Firebase does not natively support, such as Instagram or LinkedIn.
The Firebase Admin SDK has a built-in method for creating custom tokens. You can also programmatically create custom tokens in any language using third-party JWT libraries.
Your server should create a custom token with a unique identifier (
pass that token to a client app, which will use it to sign in to Firebase. See
Create custom tokens for code samples and more
details about the custom token creation process.
ID token verification
If your Firebase client app communicates with your backend server, you might need to identify the currently signed-in user on your server so you can perform server-side logic on their behalf. You can do this securely by using ID tokens, which are created by Firebase when a user signs into a Firebase app. ID tokens conform to the the OpenID Connect spec and contain data to identify a user, as well as some other profile and authentication related information. ID tokens are used under the covers by Firebase services (such as the Firebase Realtime Database and Cloud Storage) to authorize user access. You can also send, verify, and inspect these tokens from your own backends. This allows you to securely identify the currently signed in user and authorize them into your own backend resources.
The Admin SDK has a built-in method for verifying ID tokens. You can also programmatically verify ID tokens in any language using third-party JWT libraries. See Verify ID tokens for more details and code samples about the ID token verification process.
Custom user claims
The Admin SDK lets you set custom attributes on user accounts. With custom user claims, you can give users different levels of access (roles), which are then enforced in an application's security rules.
After custom claims are modified on a user via the Admin SDK, they are propagated to the authenticated users on the client side via their ID tokens. The ID token is a trusted mechanism for delivering these custom claims, and all authenticated access must validate the ID token before processing the associated request.