Implementing OAuth 2.1 PKCE Authentication in Flutter

Introduction

OAuth 2.1 with PKCE (Proof Key for Code Exchange) is the recommended pattern for securing native mobile apps. Flutter, as a cross-platform mobile framework, must implement PKCE correctly to avoid exposing client secrets and to maintain user privacy. This tutorial walks through a practical, secure implementation pattern: dependencies and setup, the PKCE flow overview, building the authorization + redirect handling, exchanging the code and storing tokens securely, then hardening considerations for production.

Setup And Dependencies

Use packages that minimize custom network and browser logic. Recommended packages: url_launcher for opening the system browser, uni_links or a platform-specific callback handler for app redirects, http for token requests, and flutter_secure_storage for storing tokens. Add these to pubspec.yaml and configure your Android intent filters and iOS URL schemes (or universal links) to receive the redirect.

Essential configuration items from your OAuth provider: authorization endpoint, token endpoint, client_id (public/native), redirect URI, and requested scopes. Do not embed a client secret in the app.

PKCE Flow Overview

PKCE augments the authorization code flow so public/native clients can securely exchange an auth code for tokens without a secret. Flow summary:

• App creates a random code_verifier and derived code_challenge (SHA256, base64url).

• App opens the system browser to the authorization endpoint with client_id, redirect_uri, code_challenge, and code_challenge_method=S256.

• User authenticates and provider redirects back to the app with an authorization code.

• App POSTs that code, along with the original code_verifier, to the token endpoint to receive tokens.

Keep the code_verifier ephemeral in-memory until the token exchange completes.

Request, Redirect And Exchange

Generate the PKCE values and build the authorization URL. Use the system browser (not a WebView) to leverage platform SSO and security. Example generator for code verifier and challenge:

import 'dart:convert';
import 'dart:math';
import 'package:crypto/crypto.dart';

String generateCodeVerifier(int length) {
  final rand = Random.secure();
  final bytes = List<int>.generate(length, (_) => rand.nextInt(256));
  return base64UrlEncode(bytes).replaceAll('=', '');
}

String codeChallengeFromVerifier(String v) =>
    base64UrlEncode(sha256.convert(utf8.encode(v)).bytes).replaceAll('=', '');

Open the authorization URL built with the code_challenge. Handle the incoming deep link or universal link with uni_links or platform channels to capture the redirect URL and extract the authorization code.

After you receive the code, perform the token exchange with code_verifier. Use a simple POST with application/x-www-form-urlencoded content-type.

import 'package:http/http.dart' as http;

Future<http.Response> exchangeCode(String tokenEndpoint, String code, String verifier, String clientId, String redirectUri) {
  final body = {
    'grant_type': 'authorization_code',
    'code': code,
    'code_verifier': verifier,
    'client_id': clientId,
    'redirect_uri': redirectUri,
  };
  return http.post(Uri.parse(tokenEndpoint), body: body);
}

Check and parse the token response for access_token, id_token (if requested), and refresh_token. Validate ID token claims locally if using OpenID Connect.

Secure Storage And Best Practices

Store tokens in platform-provided secure storage: flutter_secure_storage on both Android and iOS uses Keychain or Keystore. Avoid storing refresh tokens in plaintext or in shared preferences. Keep the following in mind:

• Limit token lifetime with short-lived access tokens and use refresh tokens guarded by device-level storage.

• Invalidate refresh tokens on logout and revoke tokens on the server when possible.

• Always perform token requests over HTTPS and validate server TLS.

• Do not embed secrets or hard-coded keys in the app binary.

When refreshing tokens, apply the same care: perform refresh requests from the app with the refresh_token and rotate refresh tokens if supported.

Security Considerations

• Use the system browser: it prevents credential capture and allows SSO.

• Use S256 code_challenge_method always; plain is insecure and unsupported by many providers.

• Resist using custom embedded browsers (WebView) for OAuth flows.

• Validate redirect URIs strictly and register platform-specific schemes or universal links to avoid interception.

• Implement telemetry for auth errors and monitor for anomalous token requests.

Test your implementation against common providers (Auth0, Okta, Google, Microsoft) and with PKCE enforced. Use automated tests to simulate the redirect and token exchange logic.

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Conclusion

Implementing OAuth 2.1 with PKCE in Flutter requires careful orchestration: generate and keep the code_verifier ephemeral, use the system browser for login, capture the authorization code via deep links, exchange securely for tokens, and store tokens in secure storage. Following these steps and security best practices yields a robust, production-ready authentication flow suitable for mobile development in Flutter.