Securing Your Symfony 5 Application with Authorization Code Grant and PKCE
The Problem:
Modern web applications require secure authentication and authorization. One popular method for achieving this is the OAuth 2.0 Authorization Code Grant flow. However, this flow has a security vulnerability when used with client-side JavaScript applications. This is where Proof Key for Code Exchange (PKCE) comes in.
Rephrased:
Imagine you're building a website where users can log in with their Google account. You need a secure way to handle user authentication and data access. Using OAuth 2.0, you can send users to Google for authorization, but if you're using a JavaScript-based application, there's a security risk. PKCE solves this by adding an extra layer of protection.
Scenario:
Let's say you have a Symfony 5 application that allows users to log in using their Google account. You are using the Authorization Code Grant flow for authentication.
Original Code:
// In your controller
$client = new \Google\Client();
$client->setAuthConfig('path/to/your/credentials.json');
$client->setRedirectUri('your-app-callback-url');
$client->setScopes(['email', 'profile']);
$authUrl = $client->createAuthUrl();
// Redirect user to Google for authorization
return new RedirectResponse($authUrl);
// After authorization, the user will be redirected back to your callback URL
$code = $_GET['code'];
// Exchange the code for an access token
$accessToken = $client->fetchAccessTokenWithAuthCode($code);
// Access user data using the access token
$service = new Google_Service_Oauth2($client);
$userData = $service->userinfo->get();
The Problem with the Original Code:
The above code is vulnerable to a man-in-the-middle attack because it uses the client secret directly in the frontend. This means a malicious actor could intercept the client secret and gain unauthorized access to your application.
Solution with PKCE:
PKCE adds an extra layer of protection by generating a unique code verifier on the client-side and sending its corresponding code challenge to the authorization server. This code challenge is then used to verify the code exchanged for the access token.
Implementation in Symfony 5:
- Install the
league/oauth2-clientpackage:
composer require league/oauth2-client
- Configure the OAuth2 client:
// In your config/services.yaml
services:
app.oauth2_client:
class: League\OAuth2\Client\Provider\Google
arguments:
- '%env(GOOGLE_CLIENT_ID)%'
- '%env(GOOGLE_CLIENT_SECRET)%'
- 'https://accounts.google.com/o/oauth2/auth'
- 'https://accounts.google.com/o/oauth2/token'
- ['email', 'profile']
- Implement the Authorization Code Grant with PKCE flow:
// In your controller
$client = $this->container->get('app.oauth2_client');
// Generate a code verifier and code challenge
$codeVerifier = bin2hex(random_bytes(32));
$codeChallenge = hash('sha256', $codeVerifier, true);
$codeChallengeMethod = 'S256';
// Build the authorization URL
$authUrl = $client->getAuthorizationUrl([
'state' => uniqid(),
'code_challenge' => base64_encode($codeChallenge),
'code_challenge_method' => $codeChallengeMethod
]);
// Redirect user to Google for authorization
return new RedirectResponse($authUrl);
// After authorization, the user will be redirected back to your callback URL
$code = $_GET['code'];
// Exchange the code for an access token
$accessToken = $client->getAccessToken('authorization_code', [
'code' => $code,
'code_verifier' => $codeVerifier
]);
// Access user data using the access token
// ...
Explanation:
- We generate a random code verifier and its corresponding code challenge.
- We include the code challenge and method in the authorization URL.
- When the user is redirected back to the callback URL, we exchange the code for an access token using the code verifier.
- This ensures that the code challenge used during authorization matches the code verifier provided during the access token request, preventing a man-in-the-middle attack.
Benefits of PKCE:
- Enhanced security by preventing attackers from exploiting client secrets.
- Improved user experience by simplifying the authentication process.
- Compatibility with JavaScript-based single-page applications.
Resources:
- OAuth 2.0 Authorization Code Grant Flow
- Proof Key for Code Exchange (PKCE)
- League OAuth2 Client
- Symfony documentation
By implementing PKCE in your Symfony 5 application, you can ensure that your application is secure and protected from vulnerabilities. This will provide peace of mind for both you and your users.
