Implicit Flow

Using the OAuth 2 Implicit Flow

An app that needs access to an OAuth-protected back-end API on behalf of a user must obtain an Access Token. If the app cannot keep a secret (i.e., it is deployed in an environment where users have access to its protected storage or communications), there is no added security in authenticating the app to the token endpoint as in the code flow. The Implicit Flow was specified exactly for this case, where the user is authenticated by the Authorization Server, and an Access Token is returned to the app directly in the fragment of a redirect URL.

Pre-requisites

This tutorial builds on the configuration setup described in the “First Configuration” and the “Configure an Authenticator” steps under the menu “Getting Started”. If you haven’t done those steps yet you can visit those guides here:

• First Configuration
• Setup a Username Password Authenticator

It’s possible to run this tutorial on a custom setup also, but the URLs may be different, as well as the capabilities configured in the profiles.

Overview

Authorization Endpoint

1. Browser redirects to the Authorize endpoint of the OAuth Server
2. If the user isn’t authenticated the OAuth Server redirect to the Authentication Service
3. The User authenticates, and is redirected back to the OAuth Server
4. The OAuth Server issues the Access Token immediately and redirects back to the client

Because the app is not capable of keeping a secret, there is no long-lived, Refresh Token issued in this flow. Also, the issued Access Token should have a limited lifetime. Because an Access Token is returned in the fragment of the URI, it should not be cached or stored by intermediate systems or proxies. The security of the process relies on this property, as an Access Token should not be exposed to any party except the app.

For reference, the Implicit Flow is documented in section 4.2 of the OAuth 2.0 Specification.

To learn more about the client parameters of the Implicit flow see OAuth Implicit Flow.

Setup in Curity

Visit the Profiles screen and click the Token Service. On the right select Clients and click New.

New Client

Give the client an ID (eg. spa for a single page website client).

Capabilities

Scroll down to the Capabilities section and click Add capabilities.

Select the Implicit Flow capability and click Next.

Redirect URL

The redirect URI is back at the client. If you don’t know what you will use just enter http://localhost/callback for now. This can be changed later. This tutorial will manually run the flow so localhost is fine. If you plan to use OAuth.tools for testing purposes click the Add Redirect URIs button to add the callback for oauth.tools.

User Authentication

For user authentication select the authenticator created in the authenticator tutorial.

Add the openid Scope

We will also run the OpenID Code flow, so add the openid scope to the client by scrolling down to the Permissions section of the client.

Commit

Make sure to remember to commit the changes in the Changes -> Commit menu.

Making Requests with the Client

With the default configuration we’ve setup, the server’s authorization endpoint is at https://localhost:8443/oauth/v2/oauth-authorize. If you are not using that configuration, determine the URL of your authorization endpoint, and adjust the following requests accordingly. To use the implicit flow, an app would create a URL to this endpoint, and redirect the user to it. The URL looks like this:

https://localhost:8443/oauth/v2/oauth-authorize?client_id=spa&response_type=token

You can pretend to be the app, and enter this URL in a browser yourself. As a result, you will be asked to authenticate, after which Curity will redirect you back to the app by adding the token response parameters to the configured redirect URI, e.g.:

https://localhost/callback#access_token=e161b107-7110-421f-b449-940659ab58c1&token_type=bearer&expires_in=300

In case you want to request an ID Token as well, it turns into an OpenID Connect request, and for that you will need to include the redirect_uri and nonce parameters in the request:

https://localhost:8443/oauth/v2/oauth-authorize?client_id=spa&redirect_uri=https://localhost/callback&response_type=id_token&scope=openid&nonce=538B0D2A-545A-4113-ACD7-A2C3D315C605

After doing so, you should receive an ID token on the fragment of the callback URI.

If the app needs an ID token and an access token, the response_type request parameter should include both id_token and token, like this:

https://localhost:8443/oauth/v2/oauth-authorize?client_id=spa&redirect_uri=https://localhost/callback&response_type=id_token+token&scope=openid&nonce=538B0D2A-545A-4113-ACD7-A2C3D315C607