Using Refresh Tokens With PKCE For Secure Authentication

Aug 1, 2025 by Rajiv Sharma 57 views

Can You Use Refresh Token Grant Type with PKCE? A Deep Dive

Hey everyone! Let's dive into a super interesting question today: can we actually use the refresh token grant type with PKCE (Proof Key for Code Exchange)? This is a common head-scratcher, especially when you're building modern, secure applications, like single-page apps (SPAs), and want to handle token refreshing safely. We'll break down the concepts, explore the possibilities, and look at how to implement this in a Spring Boot OAuth 2.0 authorization server.

Understanding the Basics: OAuth 2.0, PKCE, and Refresh Tokens

Before we jump into the specifics, let's make sure we're all on the same page with the key concepts. OAuth 2.0 is the industry-standard protocol for authorization, allowing applications to access resources on behalf of a user without ever knowing the user's credentials. This is huge for security and user experience. Instead of asking users for their passwords directly, your app can request permission to access specific resources, like their profile information or photos.

Now, what's PKCE? PKCE is a security extension to the authorization code grant flow in OAuth 2.0. It's designed to protect public clients, like SPAs or mobile apps, from authorization code interception attacks. In simple terms, it adds an extra layer of security by ensuring that only the client that initiated the authorization request can exchange the authorization code for an access token. This prevents malicious actors from hijacking the authorization code and gaining unauthorized access.

Finally, refresh tokens are long-lived tokens that can be used to obtain new access tokens without requiring the user to re-authenticate. Access tokens, on the other hand, are short-lived and have a limited lifespan. Refresh tokens are essential for maintaining a smooth user experience because they allow your application to automatically renew access tokens in the background, so users don't have to constantly log in. Imagine having to re-enter your password every 15 minutes – not a great experience, right?

The authorization code grant with PKCE is particularly relevant here. It's a secure flow where the client first obtains an authorization code from the authorization server and then exchanges it for an access token. The PKCE part ensures that the exchange is secure, even if the authorization code is intercepted. This is the recommended approach for public clients because they can't securely store client secrets.

In the traditional authorization code flow, the client would exchange the authorization code for an access token using its client secret. However, public clients can't keep a secret, so PKCE introduces a code verifier and a code challenge. The client generates a code verifier (a random string) and derives a code challenge from it. The code challenge is sent to the authorization server along with the authorization request. When the client exchanges the authorization code for an access token, it also sends the code verifier. The authorization server then verifies that the code verifier matches the code challenge, ensuring that the client performing the exchange is the same one that initiated the request.

The Core Question: Refresh Tokens and PKCE – Can They Coexist?

So, here's the million-dollar question: can we use refresh tokens with PKCE? The short answer is a resounding yes! PKCE is a security mechanism that enhances the authorization code grant, and refresh tokens are a mechanism for obtaining new access tokens. They are not mutually exclusive. In fact, they work very well together to provide a secure and user-friendly authentication experience.

The beauty of this combination is that you get the security benefits of PKCE for the initial authorization code exchange, and you can then use refresh tokens to keep the user logged in without requiring them to re-authenticate frequently. This is especially important for SPAs and mobile apps, where you want to avoid exposing credentials and provide a seamless experience.

The OAuth 2.0 specification doesn't prevent the use of refresh tokens with PKCE. It's a perfectly valid and recommended approach for public clients. The key is to implement the flow correctly and ensure that your authorization server supports both PKCE and refresh token grants.

However, there's a crucial point to consider: security. While refresh tokens are designed to be long-lived, they are still sensitive credentials. If a refresh token is compromised, an attacker could use it to obtain new access tokens and potentially gain unauthorized access to resources. Therefore, it's essential to protect refresh tokens and implement appropriate security measures.

One common technique is to implement refresh token rotation. This involves issuing a new refresh token each time the client uses the old one. The old refresh token is then invalidated, reducing the window of opportunity for an attacker if a refresh token is compromised. This is a best practice for enhancing the security of your OAuth 2.0 implementation.

Another important consideration is refresh token storage. For SPAs, storing refresh tokens securely can be challenging because the client-side environment is inherently less secure than a server-side environment. Avoid storing refresh tokens in local storage or cookies, as these are susceptible to cross-site scripting (XSS) attacks. Instead, consider using the Browser's IndexedDB API or HTTP-only cookies with appropriate security flags to mitigate the risk of token theft.

Creating a Public Client That Supports Refresh Tokens with PKCE

Now, let's get practical. How do you actually create a public client that supports refresh token grant type with PKCE? The key is to configure your OAuth 2.0 authorization server correctly. In a Spring Boot environment, this typically involves configuring your AuthorizationServerConfigurerAdapter or using the newer Spring Authorization Server project.

First, you'll need to ensure that your client registration is set up as a public client. This means that you won't be using a client secret. Instead, you'll rely on PKCE to secure the authorization code exchange. You'll also need to grant the client the refresh_token grant type.

Here's an example of how you might configure a client in your Spring Boot authorization server:

@Configuration
@EnableAuthorizationServer
public class AuthorizationServerConfig extends AuthorizationServerConfigurerAdapter {

    @Override
    public void configure(ClientDetailsServiceConfigurer clients) throws Exception {
        clients.inMemory()
            .withClient("my-public-client")
            .authorizedGrantTypes("authorization_code", "refresh_token")
            .redirectUris("http://localhost:8080/callback")
            .scopes("read", "write")
            .accessTokenValiditySeconds(3600) // 1 hour
            .refreshTokenValiditySeconds(2592000) // 30 days
            .autoApprove(true);
    }
}

In this example, we're configuring an in-memory client named my-public-client. We've specified the authorization_code and refresh_token grant types, which means the client can use both the authorization code flow with PKCE and the refresh token flow. We've also set the redirect URIs, scopes, and token validity periods. The autoApprove(true) setting is for testing purposes and should not be used in production.

Next, you'll need to ensure that your authorization endpoint supports PKCE. Spring Authorization Server provides excellent support for PKCE, so this is typically enabled by default when you use the authorization code grant type. You can customize the PKCE settings if needed, but the defaults are usually sufficient.

On the client side, you'll need to implement the PKCE flow. This involves generating a code verifier and a code challenge, sending the code challenge to the authorization server, and then sending the code verifier when exchanging the authorization code for an access token. There are many libraries available that can help you with this, such as jsrsasign for JavaScript clients.

When you receive an access token and a refresh token from the authorization server, you'll need to store the refresh token securely. As mentioned earlier, avoid using local storage or cookies. Consider using the Browser's IndexedDB API or HTTP-only cookies with appropriate security flags. Then, when the access token expires, you can use the refresh token to obtain a new access token without requiring the user to re-authenticate.

Here's a simplified example of how you might refresh an access token using a refresh token in JavaScript:

function refreshAccessToken(refreshToken) {
    const tokenEndpoint = 'YOUR_TOKEN_ENDPOINT'; // e.g., /oauth/token
    const clientId = 'my-public-client';
    
    const params = new URLSearchParams();
    params.append('grant_type', 'refresh_token');
    params.append('refresh_token', refreshToken);
    params.append('client_id', clientId);

    return fetch(tokenEndpoint, {
        method: 'POST',
        headers: {
            'Content-Type': 'application/x-www-form-urlencoded'
        },
        body: params.toString()
    })
    .then(response => response.json())
    .then(data => {
        // Handle the new access token and refresh token
        console.log('New access token:', data.access_token);
        console.log('New refresh token:', data.refresh_token);
        // Store the new tokens securely
    });
}

In this example, we're sending a POST request to the token endpoint with the grant_type set to refresh_token and the refresh_token and client_id included in the request body. The authorization server will then validate the refresh token and issue a new access token (and possibly a new refresh token) if everything is valid.

Refreshing Tokens in Single Page Applications: Best Practices

Now, let's focus specifically on refreshing tokens in single-page applications (SPAs). SPAs present unique challenges when it comes to security because they run entirely in the browser, which is a less secure environment than a traditional server-side application.

The key to securely refreshing tokens in an SPA is to minimize the risk of token theft. Here are some best practices to follow:

Use PKCE: As we've discussed, PKCE is essential for protecting the authorization code flow in public clients like SPAs.
Store Refresh Tokens Securely: Avoid using local storage or cookies for refresh tokens. Instead, consider using the Browser's IndexedDB API or HTTP-only cookies with appropriate security flags.
Implement Refresh Token Rotation: Rotate refresh tokens to limit the impact of a compromised token.
Use HTTP-Only Cookies (with caution): HTTP-only cookies are not accessible via JavaScript, which can help prevent XSS attacks. However, they are still vulnerable to CSRF attacks, so you'll need to implement appropriate CSRF protection.
Consider the Backend for Frontend (BFF) Pattern: The BFF pattern involves creating a server-side component that acts as an intermediary between your SPA and the authorization server. This can help you to securely store and manage refresh tokens on the server side, reducing the risk of token theft in the browser. The BFF can handle the token refresh logic and provide the SPA with access tokens as needed.
Monitor and Revoke Tokens: Implement monitoring and logging to detect suspicious activity, such as multiple refresh token requests from the same IP address. Provide a mechanism for users to revoke tokens, such as a "logout all sessions" feature.

By following these best practices, you can significantly enhance the security of your SPA's authentication and authorization process.

Addressing the Initial Question: Public Clients and Refresh Tokens

Let's revisit the original question: Is there any way to create a public client that supports the refresh token grant type? The answer, as we've established, is a resounding yes. PKCE makes it possible to create secure public clients that can use refresh tokens.

However, it's crucial to understand the implications of using refresh tokens in a public client. You need to take extra precautions to protect the refresh token, as it's a long-lived credential that can be used to obtain new access tokens. This is why the best practices we discussed earlier are so important.

If you're not comfortable with the risk of storing refresh tokens in the browser, you might consider alternative approaches, such as using short-lived access tokens and prompting the user to re-authenticate more frequently. However, this can negatively impact the user experience. Another option is to use the BFF pattern, which allows you to securely manage refresh tokens on the server side.

Conclusion: Securely Combining Refresh Tokens and PKCE

In conclusion, you absolutely can and should use refresh tokens with PKCE in your applications, especially in SPAs and mobile apps. This combination provides a secure and user-friendly authentication experience. PKCE protects the authorization code exchange, and refresh tokens allow you to maintain user sessions without requiring frequent re-authentication.

However, it's crucial to implement appropriate security measures to protect refresh tokens. This includes using refresh token rotation, storing refresh tokens securely, and considering the BFF pattern. By following these best practices, you can build secure and scalable applications that leverage the power of OAuth 2.0, PKCE, and refresh tokens.

I hope this deep dive has cleared up any confusion and given you a solid understanding of how to use refresh tokens with PKCE. If you have any further questions, feel free to ask! Keep building awesome and secure applications, guys!