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Custom JWT Authentication

On this page

  • Overview
  • Configuration
  • Audience
  • Verification Method
  • Metadata Fields
  • JSON Web Tokens
  • JWT Header
  • JWT Payload
  • JWT Signature
  • Firebase JWT Authentication
  • Configure Firebase Authentication
  • Configure App Services JWT Authentication
  • Log In to App Services Using Firebase JWT with Client SDK
  • Examples
  • Walkthrough
  • Prerequisites
  • Procedure
  • Summary

The Custom JWT authentication provider allows users to authenticate with an authentication system that is independent from Atlas App Services. The external system must return a signed JSON Web Token that contains a unique ID value for the authenticated user.

App Services uses the JWT to identify your application's users and authenticate their requests but does not impose any restrictions on the external authentication system's requirements or authentication methods. For example, the system could require the user to perform two factor authentication, provide specific credentials, or otherwise identify themself.

Diagram of Custom JWT authentication architecture.

The Audience of a JWT specifies its intended recipient. JWTs describe their audience in the aud claim. By default, App Services expects aud to contain the App ID of the App for which the provider is configured.

If the external authentication system JWT specifies a different aud value, then you can configure the provider to use that value instead.

You can input a single audience or multiple audiences as a comma-separated list. If you add multiple audiences, a toggle appears containing the following options:

  • All of these audiences: the JWT must include every audience in the list.

  • Any one of these audiences: the JWT only needs to include one audience from the list.

The Verification Method configures how the provider determines which signing algorithm and signing keys the external authentication system must use to sign each JWT.

You can either manually specify signing keys or specify a JSON Web Key URI.

You can manually configure the signing algorithm and specify one or more signing keys that the external authentication system may use to sign JWTs.

Signing Algorithm

The cryptographic method that the external system uses to sign the JWT. Custom authentication supports JWTs signed using any of the following algorithms:

  • HS256

  • RS256

The Signing Algorithm configuration dropdown
Signing Key

A list of the names of up to three Secrets that each contain a signing key used by the external authentication system to sign JWTs. Each signing key Secret must be a string with length between 32 and 512 characters.

The Signing Keys configuration inputs


A Signing Key is a secret key and anyone with the key can issue valid user credentials for your app. Ensure that it's never stored in a publicly accessible location, such as a git repository, message board, or in your code.

Some external authentication systems provide a JSON Web Key Set that describes the signing algorithm and signing keys the system uses to sign JWTs. You can use the JWKS to configure the provider instead of manually specifying the signing algorithm and keys. When enabled, each token must include a kid header that specifies the Key ID of a key from the JWKS.

If true, configures App Services to use a signing algorithm and signing keys defined in a JWK or JWKS. The JWKS must be accessible at a URL that you specify.

A URL that hosts a JWK or JWKS that describes the signing method and signing keys the JWTs should use. The JWKS may specify up to three signing keys and must use the RS256 algorithm.

The JWK URI input

Metadata Fields are additional data that describe each user. App Services determines the value of each metadata field from the value of some field included in the JWT from the external authentication system. If you set the name field of a user, then App Services will use that field as the user's display name. App Services refreshes a user's metadata whenever they log in and exposes the fields in the data object of the user object.


2048 Character Limit

The length of a JWT token increases with the number of metadata fields in the token and the size of each field. App Services limits the length of a JWT token to 2048 characters. If you exceed this limit, App Services logs an error and the ticket is not processed.

If true , the metadata field is required for all users associated with the provider, i.e. the JWT returned by the external system must have a value assigned to the field designated by Path.

The name of a field in the JWT that contains the value for the metadata field. To specify a field in an embedded object, use dot notation.


Escaping periods in JWT keys

Use a backslash (\) to escape period (.) characters in JWT keys. Take this JSON object for example:

{ "valid.json.key": {
"nested_key": "val"

You could represent the "nested_key" in the path name as valid\.json\.key.nested_key.

Field Name

Optional. A name for the field in the user object's data document that exposes the metadata field value. If not specified, this defaults to the same name as the JWT field that contains the value. The metadata field name may contain a maximum of 64 characters.


An external authentication system returns JWTs that include additional information about each user in the user_data field:

"aud": "myapp-abcde",
"exp": 1516239022,
"sub": "24601",
"user_data": {
"name": "Jean Valjean",
"aliases": [
"Monsieur Madeleine",
"Ultime Fauchelevent",
"Urbain Fabre"

To include the values from the user_data field in each user's user object, you could specify the following metadata fields:

Field Name

We can now access the mapped values directly from the user object, which would resemble the following for the given JWT:

"id": "59fdd02846244cdse5369ebf",
"type": "normal",
"data": {
"name": "Jean Valjean",
"aliases": [
"Monsieur Madeleine",
"Ultime Fauchelevent",
"Urbain Fabre"
identities: [
"id": "24601",
"provider_type": "custom-token",
"data": {
"name": "Jean Valjean",
"aliases": [
"Monsieur Madeleine",
"Ultime Fauchelevent",
"Urbain Fabre"

The external authentication system must return a JSON web token that uniquely identifies the authenticated user. JSON web tokens are an industry standard (see RFC 7519) for securely representing claims between two parties. A JWT is a string that consists of three parts: a header, a payload and a signature and has the following form:


The header portion of the JWT consists of a Base64UrlEncoded document of the following form:

"alg": "HS256",
"typ": "JWT",
"kid": "<JWK Key ID>"

Required. A string representing the hashing algorithm being used.

App Services supports JWTs encoded with the following algorithms:

RSA Signature SHA-256
Required. The type of the token. App Services expects a JSON web token so the value should be "JWT".
Optional. The Key ID of a specific key to use from a set of keys in a JWK. You must include the kid header when you use a JWK URL.

The payload portion of the JWT consists of a Base64UrlEncoded document of the following form:

"aud": "<realm app id>"
"sub": "<unique user id>",
"exp": <NumericDate>,
"iat": <NumericDate>,
"nbf": <NumericDate>,
Required. The audience of the token. By default, App Services expects this value to be the App ID of your App. If your external authentication service returns a different aud value, you should specify that value instead.
Required. The subject of the token. The value should be a unique ID for the authenticated user from your custom-built authentication system.

Required. The Expiration date of the token. The value should be a NumericDate number indicating the time at which the token expires.


App Services will not accept expired authentication tokens.

Optional. The "issued at" date of the token. The value should be a NumericDate number that indicates the time after which the token is considered valid. This field is functionally identical to nbf.
Optional. The "not before" date of the token. The value should be a NumericDate number that indicates the time before which the token is considered invalid. This field is functionally identical to iat.


App Services maps certain values from the JWT the user object so that you can access them in your application:

  • The required fields listed in this section map to the property for their Custom JWT provider identity.

  • The metadata fields listed in the Custom JWT provider configuration map to the property.

  • Any optional fields not listed in the provider configuration are ignored and do not map to the user object.

The signature portion of the JWT is a hash of the encoded token header and payload. To form the signature, concatenate the encoded header and payload with a period and sign the result with the Signing Key specified in the authentication provider configuration using the hashing algorithm specified in the "alg" field of the header.

base64UrlEncode(header) + "." + base64UrlEncode(payload),

You can use the Firebase Authentication service to authenticate users into App Services Apps as well as Firebase apps. Use App Services Custom JWT authentication to access App Services with Firebase credentials.

To set up Firebase Authentication as your Custom JWT provider in App Services:


Follow the official docs to set up Firebase Authentication for your project.


For more details, refer to the JWT authentication configuration documentation.


Now that you've configured App Services to work with Firebase JWTs, you can log users in using Firebase and Realm SDKs in languages supported by both.

  1. Log the user into Firebase. See the relevant Firebase SDK documentation.

  2. Retrieve the user's Firebase JWT. See the documentation on using a client Firebase SDK to get a user's JWT.

  3. Pass the Firebase JWT to a Realm SDK Custom JWT authentication method. For more information how to use Realm SDKs to perform Custom JWT authentication, refer to Custom JWT Examples.

For code examples that demonstrate how to register and log in using Custom JWT authentication, see the documentation for the Realm SDKs:

In this walkthrough, you will implement Custom JWT Authentication for your App Services App. You will learn how to:

  • Enable JWT Authentication

  • Use a third-party JWT provider to generate a valid token

  • Authenticate against the backend from a client application

You can use any App Services App for this tutorial. If you do not have a test app to use, you can create an app or use one of the Template Apps.

This procedure is divided into three logical steps:

  1. Create a JWT token

  2. Enable and configure JWT Authentication in App Services

  3. Add authentication code to your client app

For this tutorial, we will use a third-party website,, to generate a JWT token.

Go to Scroll down until you see the Encoded and Decoded headers. In the Decoded column, you will see three sections that comprise a JWT token: Header, Payload, and Verify Signature areas, each of which is explained in the next sections.

The header of a JWT informs consuming applications what algorithm was used to encode the token. The default, which we will use for this tutorial, is HS256.


For more information on JWT algorithms, see JSON Web Token (JWT) Signing Algorithms Overview. App Services supports HS256 and RS256.

The Payload section contains the information that we need to configure for our App Services-specific needs. By default, three fields are included: sub, name, and iat. We will use these three, plus two more fields. The following table explains each field and the value expected:

The subject of the jwt, which you set according to your business needs. For more information, see RFC 7519 4.1.2.
<your email or any other unique value>
The name of the user to whom this JWT is assigned. If specified, App Services uses this value as the user's display name. Optional.
<your name>
"Issued At" is a Unix epoch timestamp of when the token was issued.
The current date-time in seconds since Unix epoch.
The date-time when this token will expire.
A future date-time (in seconds since Unix epoch) for when you want the token to expire. Depending on your business needs, this can be any duration.
The audience (consumer) of the token.
Your App Services App ID, which you can find in the App Services UI. If applicable, you can input multiple App IDs as a comma-separated list. When inputting multiple, you can specify whether the JWT requires all or any one of the listed audiences.

For this tutorial, we have created a JWT for a user named "Caleb". The token was issued at the time of writing and will expire in exactly 1 year, and is intended to be used by an App Services with an ID of "boiboi-cul8r". Our completed JSON payload looks like this:

"sub": "1234567890",
"name": "Caleb",
"iat": 1617313420,
"exp": 1648849420,
"aud": "boiboi-cul8r"

The final step in creating the JWT is to add a 256-bit secret with which the token is encoded. In this last section, find the field with the placeholder text of "your-256-bit-secret". Paste in any 256-bit string value into this field. If you are uncertain what value to use, consider visiting a random key generator website, like, and copy one of the 256-bit values that has been generated.


The key you use must only contain ASCII letters, numbers, underscores, and hyphens, and must be between 32 and 512 characters long.


Keep track of this secret, as you will need it in the next section when we configure App Services authentication.

After pasting in your key, check the secret base64 encoded check box.

At this point, you have generated a JWT key that can be used with — and only with — the App you specified. Copy this key from the Encoded box and temporarily save it in a text document. Your JWT will be 3 sections of values, separated by periods, and looks something like the following (your token will not match this!):

eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9. ⤶
eyJzdWIiOiIxMjM0NTY3ODkwIiwibmFtZSI6IkNhbGViIiwiaWF0IjoxNjE3MzEzNDIwLCJle ⤶
HAiOjE2NDg4NDk0MjAsImF1ZCI6ImJvaWJvaS1jeWFsOHIifQ. ⤶

If you are not currently logged in to App Services do so now and navigate to the App that you specified when generating the JWT in the previous section.

  1. In the left-hand navigation, under Data Access, select Authentication.

  2. Select the Authentication Providers tab.

  3. Near the bottom of the list of providers, select Custom JWT Authentication.

  4. Use the following table to set the properties on this page:

    Provider Enabled
    Select to enable
    Verification Method
    Manually specify signing keys
    Signing Algorithm
    Signing Key (Secret Name)

    Provide a name that will have some meaning to you (for example, "newKey"), and then click the box that appears immediately below the name that reads "Create <key name>".

    In the resulting Signing Key text box, paste in the 256-bit key that you used when generating the JWT in the previous section. Do not click the "Add Signing Key" button.


    A Signing Key is a secret key and anyone with the key can issue valid user credentials for your app. Ensure that it's never stored in a publicly accessible location, such as a git repository, message board, or in your code.

    Metadata Fields (Optional)
    Leave blank
    Audience (Optional)
    Leave blank.
  5. When you have set all of the values, click Save. If you are prompted at the top of the screen to do so, review and deploy your changes.

The final step in this tutorial is to update your client application to use JWT authentication. The a Realm SDKs make this process straightforward, and while each language has its own specific implementation, the pattern they each use is the same: pass the generated token to a login call:

  • Custom JWT authentication allows you to identify your application's users via a JSON Web Token issued outside of App Services.

  • App Services expects JWT audience values to be the App ID of the App unless otherwise configured in the application.

  • App Services can verify JWTs using either manually specified keys and signing algorithms or a JSON Web Key URI.

  • You can pass custom user data in your JWT that App Services refreshes whenever the user refreshes their token.

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