Understanding OAuth2 Clients

Oct 13, 2025 by wbrum.org 115 views

Introduction

OAuth2 is a widely used authorization framework that enables secure access to protected resources on behalf of the resource owner. In an OAuth2 authorization flow, clients can be categorized into two types: public clients and confidential clients. While both types of clients can obtain an access token to access protected resources, there are significant differences in how they interact with the authorization server. In this article, we will explore why OAuth2 public clients cannot use a client secret and still achieve a secure workflow, and why client secrets are used for confidential clients.

Understanding OAuth2 Clients

Public Clients

Public clients are clients that are not capable of maintaining the confidentiality of their client credentials. This means that public clients cannot securely store their client secret and are therefore not allowed to use it in the authorization flow. Public clients are typically mobile or desktop applications that are installed on the user's device.

Confidential Clients

Confidential clients, on the other hand, are clients that are capable of maintaining the confidentiality of their client credentials. This means that confidential clients can securely store their client secret and are allowed to use it in the authorization flow. Confidential clients are typically web applications that are hosted on a server and have the ability to securely store their client secret.

Why Public Clients Can't Use Client Secret

So, why can't public clients use a client secret? The reason is that public clients are not capable of maintaining the confidentiality of their client credentials. If a public client were to use a client secret, it would be vulnerable to interception by an attacker. An attacker could intercept the client secret and use it to obtain an access token, which would allow them to access protected resources on behalf of the resource owner.

The Problem with Client Secret in Public Clients

If a public client were to use a client secret, it would introduce several security risks:

Interception: An attacker could intercept the client secret and use it to obtain an access token.
Replay: An attacker could replay the client secret to obtain multiple access tokens.
Man-in-the-middle: An attacker could position themselves between the public client and the authorization server and intercept the client secret.

The Solution: PKCE

To mitigate these security risks, OAuth2 introduces the Proof Key for Code Exchange (PKCE) extension. PKCE is a mechanism that allows public clients to securely obtain an access token without using a client secret. Instead of using a client secret, public clients use a code verifier and a code challenge to obtain an access token.

How PKCE Works

Here's an overview of how PKCE works:

Code Verifier: The public client generates a code verifier, which is a random string of characters.
Code Challenge: The public client generates a code challenge by hashing the code verifier using a hash function (such as SHA-256).
Authorization Request: The public client sends an authorization request to the authorization server, including the code challenge and the redirect URI.
Authorization Response: The authorization server responds with an authorization code, which the public client can use to obtain an access token.
Access Token Request: The public client sends an access token request to the token endpoint, including the authorization code and the code verifier.
Access Token Response: The authorization server responds with an access token, which the public client can use to access protected resources.

Why Confidential Clients Can Use Client Secret

So, why can confidential clients use a client secret? The reason is that confidential clients are capable of maintaining the confidentiality of their client credentials. Confidential clients can securely store their client secret and use it in the authorization flow.

The Benefits of Client Secret in Confidential Clients

Using a client secret in confidential clients provides several benefits:

Security: Confidential clients can securely store their client secret, which reduces the risk of interception or replay attacks.
Convenience: Confidential clients can use a client secret to obtain an access token, which eliminates the need for a code verifier and code challenge.
Flexibility: Confidential clients can use a client secret to obtain an access token, which provides more flexibility in the authorization flow.

Conclusion

In conclusion, OAuth2 public clients cannot use a client secret and still achieve a secure workflow. The reason is that public clients are not capable of maintaining the confidentiality of their client credentials. Instead, public clients use the Proof Key for Code Exchange (PKCE) extension to securely obtain an access token. Confidential clients, on the other hand, can use a client secret to obtain an access token, which provides more security, convenience, and flexibility in the authorization flow.

Best Practices for Implementing OAuth2

Here are some best practices for implementing OAuth2:

Use PKCE for Public Clients: Public clients should use the PKCE extension to securely obtain an access token.
Use Client Secret for Confidential Clients: Confidential clients should use a client secret to obtain an access token.
Implement Secure Storage: Clients should implement secure storage for their client secret and code verifier.
Implement Secure Communication: Clients should implement secure communication with the authorization server to prevent interception or replay attacks.

Q&A: OAuth2 Public Clients and Confidential Clients

Q: What is the main difference between OAuth2 public clients and confidential clients?

A: The main difference between OAuth2 public clients and confidential clients is that public clients are not capable of maintaining the confidentiality of their client credentials, while confidential clients are capable of maintaining the confidentiality of their client credentials.

Q: Why can't public clients use a client secret?

A: Public clients cannot use a client secret because they are not capable of maintaining the confidentiality of their client credentials. If a public client were to use a client secret, it would be vulnerable to interception by an attacker.

Q: What is the Proof Key for Code Exchange (PKCE) extension?

A: The Proof Key for Code Exchange (PKCE) extension is a mechanism that allows public clients to securely obtain an access token without using a client secret. Instead of using a client secret, public clients use a code verifier and a code challenge to obtain an access token.

Q: How does PKCE work?

A: Here's an overview of how PKCE works:

Code Verifier: The public client generates a code verifier, which is a random string of characters.
Code Challenge: The public client generates a code challenge by hashing the code verifier using a hash function (such as SHA-256).
Authorization Request: The public client sends an authorization request to the authorization server, including the code challenge and the redirect URI.
Authorization Response: The authorization server responds with an authorization code, which the public client can use to obtain an access token.
Access Token Request: The public client sends an access token request to the token endpoint, including the authorization code and the code verifier.
Access Token Response: The authorization server responds with an access token, which the public client can use to access protected resources.

Q: Why can confidential clients use a client secret?

A: Confidential clients can use a client secret because they are capable of maintaining the confidentiality of their client credentials. Confidential clients can securely store their client secret and use it in the authorization flow.

Q: What are the benefits of using a client secret in confidential clients?

A: Using a client secret in confidential clients provides several benefits:

Security: Confidential clients can securely store their client secret, which reduces the risk of interception or replay attacks.
Convenience: Confidential clients can use a client secret to obtain an access token, which eliminates the need for a code verifier and code challenge.
Flexibility: Confidential clients can use a client secret to obtain an access token, which provides more flexibility in the authorization flow.

Q: What are some best practices for implementing OAuth2?

A: Here are some best practices for implementing OAuth2:

Use PKCE for Public Clients: Public clients should use the PKCE extension to securely obtain an access token.
Use Client Secret for Confidential Clients: Confidential clients should use a client secret to obtain an access token.
Implement Secure Storage: Clients should implement secure storage for their client secret and code verifier.
Implement Secure Communication: Clients should implement secure communication with the authorization server to prevent interception or replay attacks.

Q: What are some common mistakes to avoid when implementing OAuth2?

A: Here are some common mistakes to avoid when implementing OAuth2:

Not using PKCE for public clients: Public clients should use the PKCE extension to securely obtain an access token.
Not using a client secret for confidential clients: Confidential clients should use a client secret to obtain an access token.
Not implementing secure storage: Clients should implement secure storage for their client secret and code verifier.
Not implementing secure communication: Clients should implement secure communication with the authorization server to prevent interception or replay attacks.

By following these best practices and avoiding common mistakes, you can ensure a secure and efficient OAuth2 authorization flow for your clients.