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Why You Should Always Use Access Tokens to Secure APIs

There are two types of tokens that are often talked about in relation to identity:

Identity Tokens

The ID Token is a JSON Web Token (JWT), and it is meant for use by the application only. For example, if there's a to-do app that uses Google to log in users and to sync their calendars, Google sends an ID Token to the to-do app that includes information about the user. The app then parses the token's contents and uses the information (including details like your name and your profile picture) to customize the user experience.

Be sure to validate an ID Token before using the information it contains! You can use a library to help with this task.

Access Tokens

Access Tokens (which aren't always a JWT) are meant for use by an API.

The Access Token's purpose is to inform the API that the bearer of the token has been authorized to:

  1. Access the API
  2. Perform a predetermined set of actions (these actions are specified by the scopes granted)

In the Google/to-do app example above, Google would have sent an Access Token to the to-do app after the user logged in and provided consent for the to-do app to read/write to their Google Calendar.

Whenever the to-do app wants to write to Google Calendar, it will send a request to the Google Calendar API, making sure to include the Access Token in the HTTP Authorization header.

Your applications should treat Access Tokens as opaque strings, since they are meant for APIs. Your application should not attempt to decode them or expect to receive tokens in a particular format.

How not to use tokens

Now that we've seen some ways in which we can use tokens, let's talk about when they should not be used.

  • Access Tokens must never be used for authentication. Access Tokens cannot tell us if the user has authenticated. The only user information the Access Token possesses is the user ID, located in the sub claim.

  • ID Tokens should not be used to gain access to an API. Each token contains information for the intended audience (which is usually the recipient). Per the OpenID Connect specification, the audience of the ID Token (indicated by the aud claim) must be the client ID of the application making the authentication request. If this is not the case, you should not trust the token. Conversely, an API expects a token with the aud value to equal the API's unique identifier. Therefore, unless you maintain control over both the application and the API, sending an ID Token to an API will generally not work. Since the ID Token is not signed by the API, the API would have no way of knowing if the application had modified the token (e.g., adding more scopes) if it were to accept the ID Token.

Compare the tokens

To better clarify the concepts we covered above, let's look at the contents of some sample ID and Access Tokens.

The (decoded) contents of our sample ID Token look like the following:

{
  "iss": "http://YOUR_DOMAIN/",
  "sub": "auth0|123456",
  "aud": "YOUR_CLIENT_ID",
  "exp": 1311281970,
  "iat": 1311280970,
  "name": "Jane Doe",
  "given_name": "Jane",
  "family_name": "Doe",
  "gender": "female",
  "birthdate": "0000-10-31",
  "email": "janedoe@example.com",
  "picture": "http://example.com/janedoe/me.jpg"
}

This token is meant to authenticate the user to the application. The audience (the aud claim) of the token is set to the application's identifier, which means that only this specific application should consume this token.

For comparison, let's look at the contents of an Access Token:

{
  "iss": "https://YOUR_DOMAIN/",
  "sub": "auth0|123456",
  "aud": [
    "my-api-identifier",
    "https://YOUR_DOMAIN/userinfo"
  ],
  "azp": "YOUR_CLIENT_ID",
  "exp": 1489179954,
  "iat": 1489143954,
  "scope": "openid profile email address phone read:appointments"
}

Note that the token does not contain any information about the user itself besides their ID (sub claim). It only contains authorization information about which actions the application is allowed to perform at the API (scope claim). This is what makes it useful for securing an API, but not for authenticating a user.

In many cases, you might find it useful to retrieve additional user information at the API, so the Access Token is also valid for calling the /userinfo API, which returns the user's profile information. The intended audience (indicated by the aud claim) for this token is both your custom API as specified by its identifier (such as https://my-api-identifier) and the /userinfo endpoint (such as https://YOUR_DOMAIN/userinfo).

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