Single Page Applications (SPAs) face unique challenges when implementing OAuth 2.0 authorization flows due to their inability to securely store client secrets. The Authorization Code Flow with PKCE provides a secure, modern approach to handle user authentication and authorization in SPAs while protecting against common attacks such as code interception.

Why Use Authorization Code Flow with PKCE for SPAs?

Unlike the traditional Implicit Flow, which exposes access tokens directly in the browser URL and has been deprecated by many providers, Authorization Code Flow with PKCE shifts token exchanges to a secure backend or a secure client-side mechanism. PKCE ensures that authorization codes cannot be intercepted or reused by attackers.

Step-by-Step Implementation Overview

  1. Generate Code Verifier and Code Challenge Before redirecting the user to the authorization endpoint, generate a cryptographically random code_verifier and derive the code_challenge using SHA-256 and base64-url encoding.

  2. Redirect User to Authorization Endpoint Include the code_challenge and the method (S256) in the authorization request URL.

  3. User Authenticates and Grants Consent The authorization server authenticates the user and returns an authorization code to the redirect URI.

  4. Exchange Authorization Code for Tokens The SPA uses the original code_verifier to exchange the authorization code for access and refresh tokens securely.

  5. Store Tokens Securely Tokens should be stored in secure, short-lived memory or protected storage mechanisms to minimize risk.

Sample Authorization Request URL

https://authorization-server.com/authorize?
response_type=code&
client_id=your_client_id&
redirect_uri=https://yourapp.com/callback&
scope=openid profile email&
code_challenge=E9Melhoa2OwvFrEMTJguCHaoeK1t8URWbuGJSstw-cM&
code_challenge_method=S256

Token Request Example

const tokenRequestBody = new URLSearchParams({
  grant_type: 'authorization_code',
  code: receivedAuthorizationCode,
  redirect_uri: 'https://yourapp.com/callback',
  client_id: 'your_client_id',
  code_verifier: originalCodeVerifier,
});

fetch('https://authorization-server.com/token', {
  method: 'POST',
  headers: { 'Content-Type': 'application/x-www-form-urlencoded' },
  body: tokenRequestBody,
})
.then(response => response.json())
.then(data => {
  // Use access token for API calls
})
.catch(error => {
  console.error('Token exchange failed:', error);
});

Security Tips for SPAs

  • Always use HTTPS to protect data in transit.
  • Avoid storing tokens in localStorage or sessionStorage if possible; consider in-memory storage.
  • Implement token refresh with rotating refresh tokens to minimize risk if a token is leaked.
  • Validate ID tokens and access tokens to ensure they are from trusted issuers.

Real-World Case

Spotify and Google APIs both recommend Authorization Code Flow with PKCE for their SPA integrations to protect user data and prevent token leakage.

Questions to Consider

  • How does your SPA handle token expiration and refresh securely?
  • Are you prepared to migrate from Implicit Flow if currently used?
  • How can you enhance your SPA’s security posture beyond OAuth flows?

Conclusion

Using Authorization Code Flow with PKCE is the recommended best practice for SPAs, offering robust security without the need for client secrets. Proper implementation protects users and applications while enabling seamless authentication experiences.

👉 Related: Understanding the Authorization Code Flow with PKCE in OAuth 2.0
👉 Related: Authorization Code Flow vs Implicit Flow: Which One Should You Use?