Implementing the Authorization Code Flow with PKCE

In this lesson, you will learn how to secure your single-page application (SPA) by preventing authorization code interception using the Proof Key for Code Exchange (PKCE) extension to the OAuth2 flow.

The Problem: Interception

In a standard Authorization Code flow, your app receives a temporary code from the authorization server, which it then exchanges for an access token. For mobile or browser-based apps, this code could theoretically be intercepted by malicious software on the user’s device. Since the browser doesn't have a secure way to store a "client secret," an attacker who grabs that code could potentially exchange it for a token themselves.

The Solution: PKCE

PKCE (pronounced "pixie") adds a layer of cryptographic verification. Instead of relying on a static client secret, your application creates a dynamic secret for every login attempt:

1. Code Verifier: A high-entropy random string generated by your SPA.
2. Code Challenge: A hashed version of that verifier (usually SHA-256).

When your app initiates the login, it sends the code_challenge to the authorization server. The server stores this challenge temporarily. Later, when your app exchanges the authorization code for an access token, it sends the original code_verifier. The server hashes the verifier you provided and compares it to the code_challenge it received earlier. If they don't match, the request is rejected.

Implementation Concept

In your Node.js or browser-based frontend logic, you perform the generation before redirecting to the login endpoint:

// 1. Create a random code verifier
const verifier = generateRandomString(64);

// 2. Hash it to create the challenge (using SubtleCrypto in browsers)
const challenge = await sha256(verifier);

// 3. Send the challenge in the initial auth request
// GET /authorize?response_type=code&code_challenge=...&code_challenge_method=S256

By requiring the actual verifier during the token exchange, you ensure that the party requesting the token is the exact same one that initiated the request, rendering any intercepted code useless to an attacker who lacks the original secret verifier.

Key Takeaway: PKCE replaces the need for a static client secret in insecure environments by requiring a dynamically generated, hashed secret for every single authentication attempt.