Authorization Code Flow

Back-Channel Isolation

The Authorization Code Flow is the industry-standard implementation for secure server-side applications. It creates a robust, multi-stage “Handshake” that ensures sensitive tokens remain exclusively on your secure backend server, never touching the user’s browser in their final, usable form. By decoupling the initial user consent from the final token issuance, this flow effectively eliminates the risk of token exposure through browser history, referrer headers, or local storage.

AUTH-CODE

Handshake Process

Core Mission

Secret-Based Delegation. Protecting the connection between the application and the identity provider via a private, server-to-server exchange.

Like a Bank Safe Receipt: At the teller window (The Browser), you present your ID and receive a small, serialized receipt (The Auth Code). This code by itself has no value—it's not money. You then take that receipt to a secure back office (The Backend), where you swap it for the actual cash (The Access Token). Because the cash never crosses the lobby, an observer in the lobby can never steal your funds.

Server-Side Web Apps / Confidential Clients / Legacy Migration

The Handshake Sequence

The security of the process depends on the strict separation of the Front-Channel (User’s Browser) and the Back-Channel (Server-to-Server).

sequenceDiagram
    participant User
    participant Browser
    participant Server as Client Backend
    participant IdP as Auth Server

    User->>Browser: Click "Login"
    Browser->>IdP: GET /authorize (state, redirect_uri)
    IdP->>User: Authenticate & Consent
    User-->>IdP: Approved
    IdP-->>Browser: Redirect with code & state
    Browser->>Server: Deliver code to Redirect URI
    Server->>IdP: POST /token (code, client_secret)
    IdP-->>Server: Issue Access Token

1

Initiate (Front-Channel)

The client redirects the user to the Authorization Server. Crucially, it includes a `state` parameter—a unique, unguessable string—that the server will return to prevent Cross-Site Request Forgery (CSRF).

2

Redirect (Front-Channel)

After consent, the Auth Server redirects the user back to the application with a short-lived "Authorization Code." At this stage, the Client Backend must verify that the `state` returned matches the one sent.

3

Exchange (Back-Channel)

The Backend performs a server-to-server POST request to the Token endpoint. It presents the Auth Code and its private `client_secret`. The Auth Server validates both and returns the final Access Token directly to the backend.

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Parameter Governance Matrix

Every parameter in an Authorization Code request is a critical security control. Improper configuration results in high-risk vulnerabilities.

Strategic Parameter Grid

Parameter	Purpose	Strategic Requirement
state	CSRF Protection.	Unique per-session, unguessable nonce.
redirect_uri	Delivery Target.	Must be pre-registered; exact match.
client_secret	Backend Auth.	NEVER expose to the frontend/browser.
code	Swap Key.	Single-use only; 5-10 minute lifespan.

Technical Implementation Guide

Implementing the exchange requires a robust, error-handling HTTP client on your backend.

Token Exchange (JavaScript Example)

// Server-Side Token Exchange Implementation
async function exchangeAuthCode(code, state) {
    // 1. Verify CSRF state from session
    if (state !== session.oauth_state) throw new Error("State mismatch");

    // 2. Perform back-channel POST
    const response = await fetch("https://auth.example.com/token", {
        method: "POST",
        body: new URLSearchParams({
            grant_type: "authorization_code",
            code: code,
            redirect_uri: process.env.REDIRECT_URI,
            client_id: process.env.CLIENT_ID,
            client_secret: process.env.CLIENT_SECRET // Secure env var
        })
    });

    return await response.json();
}

OAuth Implementation Guides

Master the technical details of the Authorization Code flow and its extensions.

PKCE Enforcement

Hardening the Auth Code flow for mobile and single-page apps (SPAs).

Attack Mitigations

Specific defenses against code injection and redirect URI manipulation.

Token Lifecycle

Managing the storage and rotation of tokens received during the exchange.

OpenID Connect

Adding identity layers (ID Tokens) to the standard OAuth code flow.

Next Steps

• Explore State Management Patterns for resilient CSRF protection.
• Review Confidential vs. Public Clients to understand secret requirements.
• Check Authorization Server Metadata for automating endpoint discovery.