Why Are There Two Steps to Get an Access Token in OAuth 2.0?

Understanding OAuth 2.0: Why Two Steps and Why Redirects?

OAuth 2.0’s Authorization Code Flow is one of the most secure ways to authorize third-party applications. But it raises a couple of common questions:

• Why are there two steps to get the access token?
• Why is a browser redirect required in the flow?

✅ Why Two Steps to Get an Access Token?

1. Security via Redirect (Step 1: Authorization Code)

After a user logs in and grants permission to a third-party app, the authorization server issues a temporary authorization code. This code is passed back to the application via a browser redirect.

This approach ensures that the access token is not exposed in the browser or in URLs, reducing the risk of interception or leakage.

2. Secure Token Retrieval (Step 2: Token Exchange)

The app then makes a backend request — using its client ID and secret — to exchange the authorization code for an access token. This request happens over a secure server-to-server connection and ensures that only authenticated apps can obtain tokens.

🔒 Benefits of the Two-Step Authorization Code Flow

Reason	Description
Keeps Token Secret	Access token is never exposed in browser or URL, reducing token theft risk.
Client Authentication	The app proves its identity using the client ID/secret during token exchange.
Mitigates Replay Attacks	Authorization codes are single-use and short-lived, limiting misuse if intercepted.
PKCE Support	For public clients (like SPAs or mobile apps), PKCE protects the flow without a client secret.

🔁 OAuth Flow Recap

1. User is redirected to the authorization server and logs in.
2. User grants permission, and a short-lived authorization code is sent back via browser redirect.
3. The app backend exchanges the code (plus its credentials) for an access token.

🔑 Analogy

Think of it like getting access to a secure building:

• You go to the reception and ask for a visitor pass (authorization code).
• The receptionist gives you a temporary slip.
• You show this slip at the security gate with your ID (client secret) to get a key card (access token).

🌐 Why Is Browser Redirect Required?

1. User Login at the Identity Provider

The identity provider (e.g., Google, Microsoft, Okta) hosts a secure login form. A browser redirect ensures the user is taken to this official page, not an impersonated or fake login screen hosted by a third-party app.

2. Granting Consent

OAuth requires user consent before an application can access data on their behalf. The browser redirect brings the user to a consent screen where they can approve or deny the request.

3. Secure Code Delivery

After successful login and consent, the authorization code is delivered back to the app via a redirect URI. This keeps the flow traceable and ensures the app gets a time-limited, one-time-use code.

4. Keeps Credentials Safe

Redirecting to the identity provider ensures that user credentials are only ever entered at the provider’s secure login screen — not handled or stored by third-party apps.

✅ Summary: Why Redirect is Essential

Purpose	Why Redirect Is Used
User Login	To authenticate at a trusted identity provider’s login page.
Consent UI	To allow the user to explicitly grant or deny permissions.
Secure Return	To send the authorization code back to the app after login.
Credential Protection	Ensures user passwords are never shared with the app.