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Dropbox OAuth2 Issue: Scope Parameter Handling

According to OAuth2 Authorization documentation, the scope parameter is nullable, and as per RFC 6749, parameters without a value must be treated as omitted, with unrecognized parameters ignored.

However, Dropbox's OAuth2 implementation returns the following error when the scope parameter is included:

Error:

"error": "invalid_request",

"error_description": "unknown field \"scope\""

This behavior violates OAuth2 standards, as unrecognized parameters should not cause a failure.

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Comments

DB-Des

Hi @Vivek_Yadav,

Including the scope parameter without value in the Auth URL does omit it and allows authorization to continue.

For example, the following two Auth URLs work without issues:

In order to further investigate the error you have reported, please reply with:

the steps to reproduce the issue, including relevant code snippet(s), but don't include any access or refresh token(s)
the full text of any error or unexpected output

Vivek_Yadav

Hi @DB-Des

This happens when we send the token request with grant type as authorization code and empty scope parameter i.e. without a value.
Based on the RFC, Dropbox should be ignoring parameters they don't recognize as valid, not failing the request.
Last para of RFC 6749 - The OAuth 2.0 Authorization Framework

Vivek_Yadav

DB-Des

Hi @Vivek_Yadav

The oauth2/token endpoint allows the scope parameter only with a refresh_token because it aligns with OAuth 2.0 standards and best practices.

When using a refresh token, the scope parameter lets you request a subset of the originally granted permissions, ensuring least-privilege access and better security. For other grant types (such as authorization_code in your example), the scope is determined during the initial authorization process, based on user consent. Allowing the scope parameter in these cases could bypass this consent, potentially introducing security risks.

Further, RFC 6749 § 4.1.3, does not require the scope parameter, nor does it list it as optional, when using authorization_code as the grant type.

This restriction ensures compliance with the OAuth specification and helps maintain secure, user-approved access to resources.

Let us know if you have further concerns.

Vivek_Yadav

In that case Dropbox should ignore the unrecognized parameter, right??

I'm trying through setup the OAuth for an application which sends the scope on the token request for all provider, and according to them Dropbox should omit the unrecognized parameter.

DB-Des

@Vivek_Yadav,

I have submitted a request for the team to further investigate the reported issue. We'll follow up here once there is an update.

Vivek_Yadav

Hi @DB-Des, Any update on this?

DB-Des

@Vivek_Yadav,

Thank you for following up. At this time, we don’t have any updates to share. Please rest assured that we’ll reach out to you as soon as we have any developments to report.

DB-Des

@Vivek_Yadav,

Just following up to let you know this change request has been implemented. Sending a request to /oatuh2/token endpoint, with an empty scope parameter should no longer return an error.