When we request an access token using 3 scopes (scope1, scope2, scope3).
Then will receive a refresh token (refresh_token1) with the access token.
After that will request another access token with refresh_token1 and provide the scope list as scope1 and scope2 (Narrow down scopes).
Similarly, get another refresh token (refresh_token2) with the access token.
Now if we request another access token with refresh_token2, we cannot request scope3, instead, we can either request both scope1 and scope2 or one of them.
But in the specification, didn't able to find anything related to narrow-down scopes with refresh token.
From Spec
1.5. Refresh Token - Refresh tokens are issued to the client by the authorization server and are used to obtain a new access token when the current access token becomes invalid or expires or to obtain additional access tokens with identical or narrower scope (access tokens may have a shorter lifetime and fewer permissions than authorized by the resource owner).
6. Refreshing an Access Token
The scope of the access request as described by Section 3.3. The requested scope MUST NOT include any scope not originally granted by the resource owner, and if omitted is treated as equal to the scope originally granted by the resource owner.
https://datatracker.ietf.org/doc/html/rfc6749
IMO, from a security aspect, the current behaviour is much more secure because it is designed to maintain the principle of least privilege, where it updates the refresh token authorised scopes based on the requested ones.
What should be the correct behaviour?
narrow-down scope refresh token should also be able to request access token with original scope list?
