The Authorization Code Grant
Explore the OAuth 2.0 authorization code grant to understand its dual-channel architecture that separates user authentication from client authorization. Learn the step-by-step process, including secure token exchange, client classifications, and error handling essential for building secure OAuth 2.0 applications with Spring Security.
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The authorization code grant is the most secure and widely adopted OAuth 2.0 flow for modern applications. Unlike legacy patterns that mishandle user credentials, this grant strictly separates user authentication from client authorization. It accomplishes this through a two-step process that safely delegates access without ever exposing the user’s password to the client.
In this lesson, we break down the mechanics of this flow, correct common misconceptions about how tokens are exchanged, and examine how different client types authenticate to the authorization server.
Two distinct communication channels
The foundational security architecture of the authorization code grant relies on splitting the process across two different communication channels. We refer to these as the front channel and the back channel. The front channel occurs in the user’s browser, where redirects are visible and potentially interceptable. The back channel occurs directly between backend servers, providing a secure path for sensitive data.
This separation ensures that the highly sensitive access token is never exposed to the user’s browser, significantly reducing the risk of token theft. We will now examine the exact HTTP requests that power each step.
Step 1: The authorization request and state handling
The flow begins when the client application wants to access a protected resource. The client constructs an authorization URL and redirects the user’s browser to the authorization ...