Oauth - Authorization Code Grant and Flow

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1 - About

The authorization code grant type (flow) works with an intermediate credential called a authorization code.

It is a indirect and redirection-based flow that is optimized for confidential clients.

The client asks first for an authorization code that it is then used to obtain an access tokens (an optionally a refresh tokens) to get access to the protected resources. More on the flow ? See authorization code flow details


3 - Pro and cons

Because the resource owner (end-user) only authenticates with the authorization server, the resource owner's credentials are never shared with the client (app)

The authorization code provides a few important security benefits, such as:

4 - Flow

The authorization code is obtained by using an authorization endpoint (authorization server component) as an intermediary between the client and resource owner (end-user).

Instead of requesting authorization directly from the resource owner,

The flow includes the following steps

4.1 - (A) Authorization request

The client (app) initiates the flow by directing the resource owner's user-agent (end user using a browser) to the authorization endpoint.

See authorization endpoint authorization request

The client includes in this request:

  • local state (query parameter and value that comes back with the response),
  • and a redirection URI to which the authorization server will send the user-agent back once access is granted (or denied).

4.2 - (B) Authorization

The authorization endpoint (authorization server component):

  • authenticates the resource owner (via the user-agent)
  • establishes whether the resource owner grants or denies the client's access request.

4.3 - (C) Redirection after grant

Assuming the resource owner grants access, the authorization endpoint (authorization server component) redirects the user-agent back to the client using the redirection URI.

The redirection URI includes:

4.4 - (D) Client request access token

The client requests an access token from the authorization server's token endpoint by including the authorization code received in the previous step.

When making the request, the client authenticates with the token_endpoint (authorization server).

The client (app) includes the redirection URI used to obtain the authorization code for verification.

4.5 - (E) Getting access token and, optionally, a refresh token

The token endpoint (authorization server component):

If valid, the token endpoint (authorization server component) responds back with an access token and, optionally, a refresh token.


5 - Documentation / Reference