Resource Owner Password Credentials Grant

Oct 13, 2025 by wbrum.org 122 views

Introduction

When working with OAuth 2.0, it's essential to understand the correct usage of the token endpoint, particularly when exchanging an authorization code for an access token. However, a recent observation has raised concerns about the token endpoint accepting a scope parameter for the authorization_code grant type, which seems to contradict the OAuth 2.0 specifications.

The OAuth 2.0 Specifications

According to the OAuth 2.0 specification (RFC 6749), the scope parameter is used in specific grant types, such as:

Client Credentials Grant: The scope parameter is used to specify the scope of the access token requested by the client. (Section 4.4.2)
Implicit Grant: The scope parameter is used to specify the scope of the access token requested by the client. (Section 4.2.1)
Resource Owner Password Credentials Grant: The scope parameter is used to specify the scope of the access token requested by the client. (Section 4.3.2)
Refresh Token Flow: The scope parameter is used to specify the scope of the access token requested by the client. (Section 6)

However, the scope parameter is not used in the Authorization Code Grant flow (Section 4.1.3).

The Token Endpoint and the Authorization Code Grant

When using the token endpoint to exchange an authorization code for an access token, it appears that the endpoint accepts a scope parameter. This is concerning, as it seems to contradict the OAuth 2.0 specifications.

Potential Issues

Accepting a scope parameter in the authorization_code grant flow may lead to unintended side-effects, such as:

Scope Mismatch: If the client requests a scope that is not authorized by the resource owner, the token endpoint may issue an access token with an incorrect scope, leading to security vulnerabilities.
Scope Overlap: If multiple clients request overlapping scopes, the token endpoint may issue an access token with an incorrect scope, leading to security vulnerabilities.

Mastodon Documentation

The Mastodon documentation appears to get this wrong, stating that the token endpoint accepts a scope parameter for the authorization_code grant type. This may be a bug or an unintended side-effect.

Conclusion

In conclusion, the token endpoint appears to accept a scope parameter for the authorization_code grant type, which seems to contradict the OAuth 2.0 specifications. This may lead to unintended side-effects, such as scope mismatch or scope overlap. It's essential to review the OAuth 2.0 specifications and ensure that the token endpoint is implemented correctly to avoid security vulnerabilities.

Recommendations

To avoid potential issues, we recommend the following:

Review the OAuth 2.0 Specifications: Ensure that the token endpoint is implemented according to the OAuth 2.0 specifications.
Avoid Using the scope Parameter: Do not use the scope parameter in the authorization_code grant flow.
Implement Correct Scope Handling: Implement correct scope handling to avoid scope mismatch or scope overlap.

Q&A: Token Endpoint and Scope Parameter

Q: What is the correct usage of the token endpoint?

A: The token endpoint is used to exchange an authorization code for an access token. However, when using the token endpoint, it's essential to understand the correct usage of the scope parameter.

Q: What is the `scope` parameter used for in OAuth 2.0?

A: The scope parameter is used in specific grant types, such as:

Client Credentials Grant: The scope parameter is used to specify the scope of the access token requested by the client.
Implicit Grant: The scope parameter is used to specify the scope of the access token requested by the client.
Resource Owner Password Credentials Grant: The scope parameter is used to specify the scope of the access token requested by the client.
Refresh Token Flow: The scope parameter is used to specify the scope of the access token requested by the client.

However, the scope parameter is not used in the Authorization Code Grant flow.

Q: Why is the `scope` parameter not used in the authorization_code grant flow?

A: The scope parameter is not used in the authorization_code grant flow because the authorization code is already tied to a specific scope. The client can request a specific scope when requesting the authorization code, and the authorization code is then tied to that scope.

Q: What are the potential issues with accepting a `scope` parameter in the authorization_code grant flow?

A: Accepting a scope parameter in the authorization_code grant flow may lead to unintended side-effects, such as:

Scope Mismatch: If the client requests a scope that is not authorized by the resource owner, the token endpoint may issue an access token with an incorrect scope, leading to security vulnerabilities.
Scope Overlap: If multiple clients request overlapping scopes, the token endpoint may issue an access token with an incorrect scope, leading to security vulnerabilities.

Q: What should I do if I'm using the Mastodon documentation and it says that the token endpoint accepts a `scope` parameter for the authorization_code grant type?

A: If you're using the Mastodon documentation and it says that the token endpoint accepts a scope parameter for the authorization_code grant type, you should review the OAuth 2.0 specifications and ensure that the token endpoint is implemented correctly. You should also avoid using the scope parameter in the authorization_code grant flow.

Q: How can I ensure that the token endpoint is implemented correctly?

A: To ensure that the token endpoint is implemented correctly, you should:

Review the OAuth 2.0 Specifications: Ensure that the token endpoint is implemented according to the OAuth 2.0 specifications.
Avoid Using the scope Parameter: Do not use the scope parameter in the authorization_code grant flow.
Implement Correct Scope Handling: Implement correct scope handling to avoid scope mismatch or scope overlap.

By following these recommendations, you can ensure that the token endpoint is implemented correctly and avoid potential security vulnerabilities.

Conclusion

Recommendations

To avoid potential issues, we recommend the following:

Review the OAuth 2.0 Specifications: Ensure that the token endpoint is implemented according to the OAuth 2.0 specifications.
Avoid Using the scope Parameter: Do not use the scope parameter in the authorization_code grant flow.
Implement Correct Scope Handling: Implement correct scope handling to avoid scope mismatch or scope overlap.

By following these recommendations, we can ensure that the token endpoint is implemented correctly and avoid potential security vulnerabilities.