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OAuth Working Group                                           W. Denniss
Internet-Draft                                                    Google
Intended status: Standards Track                           June 28, 2018
Expires: December 30, 2018

                  OAuth 2.0 Incremental Authorization


   OAuth 2.0 authorization requests that include every scope the client
   might ever need can result in over-scoped authorization and a sub-
   optimal end-user consent experience.  This specification enhances the
   OAuth 2.0 authorization protocol by adding incremental authorization,
   the ability to request specific authorization scopes as needed, when
   they're needed, removing the requirement to request every possible
   scope that might be needed upfront.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on December 30, 2018.

Copyright Notice

   Copyright (c) 2018 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   include Simplified BSD License text as described in Section 4.e of

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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Notational Conventions  . . . . . . . . . . . . . . . . . . .   3
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Incremental Auth for Confidential Clients . . . . . . . . . .   3
   5.  Incremental Auth for Public Clients . . . . . . . . . . . . .   4
   6.  Usability Considerations  . . . . . . . . . . . . . . . . . .   4
     6.1.  Handling Denials  . . . . . . . . . . . . . . . . . . . .   4
   7.  Alternative Approaches  . . . . . . . . . . . . . . . . . . .   5
     7.1.  Alternative for Public Clients  . . . . . . . . . . . . .   5
     7.2.  Alternative for Confidential Clients  . . . . . . . . . .   5
   8.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .   5
     8.1.  Requesting Authorization In Context . . . . . . . . . . .   5
     8.2.  Preventing Overbroad Authorization Requests . . . . . . .   6
     8.3.  Authorization Correlation . . . . . . . . . . . . . . . .   6
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
     9.1.  Public Client Impersonation . . . . . . . . . . . . . . .   6
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
     10.1.  OAuth Parameters Registry  . . . . . . . . . . . . . . .   7
   11. Normative References  . . . . . . . . . . . . . . . . . . . .   7
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .   8
   Appendix B.  Document History . . . . . . . . . . . . . . . . . .   8
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   8

1.  Introduction

   OAuth 2.0 clients may offer multiple features that requiring user
   authorization, but commonly not every user will use each feature.
   Without incremental authentication, applications need to either
   request all the possible scopes they need upfront, potentially
   resulting in a bad user experience, or track each authorization grant
   separately, complicating development.

   The goal of incremental authorization is to allow clients to request
   just the scopes they need, when they need them, while allowing them
   to store a single authorization grant for the user that contains the
   sum of the scopes granted.  Thus, each new authorization request
   increments the scope of the authorization grant, without the client
   needing to track a separate authorization grant for each group of

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2.  Notational Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in Key
   words for use in RFCs to Indicate Requirement Levels [RFC2119].  If
   these words are used without being spelled in uppercase then they are
   to be interpreted with their normal natural language meanings.

3.  Terminology

   In addition to the terms defined in referenced specifications, this
   document uses the following terms:

   "OAuth"  In this document, OAuth refers to OAuth 2.0 [RFC6749].

4.  Incremental Auth for Confidential Clients

   For confidential clients, such as web servers that can keep secrets,
   the authorization endpoint SHOULD treat scopes that the user already
   granted differently on the consent user interface.  Typically such
   scopes are hidden for new authorization requests, or at least there
   is an indication that the user already approved them.

   By itself, this property of the authorization endpoint enables
   incremental authorization.  The client can track every scope they've
   ever requested, and include those scopes on every new authorization

   To avoid the need for confidential clients to re-request already
   authorized scopes, authorization servers MAY support an additional
   "include_granted_scopes" parameter in the authorization request.
   This parameter, enables the client to request tokens during the
   authorization grant exchange that represent the full scope of the
   user's grant to the application including any previous grants,
   without the app needing to track the scopes directly.

   The client indicates they wish the new authorization grant to include
   previously granted scopes by sending the following additional
   parameter in the OAuth 2.0 Authorization Request (Section 4.1.1 of
   [RFC6749].) using the following additional parameter:

   include_granted_scopes  OPTIONAL.  Either "true" or "false".  When
      "true", the authorization server SHOULD include previously granted
      scopes for this client in the new authorization grant.

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5.  Incremental Auth for Public Clients

   Unlike with confidential clients, it is NOT RECOMMEND to
   automatically approve OAuth requests for public clients without user
   consent (see Section 10.2 of OAuth 2.0 [RFC6749], and Section 8.6 of
   OAuth 2.0 [RFC8252]), thus authorization grants shouldn't contain
   previously authorized scopes in the manner described above for
   confidential clients.

   Public clients (and confidential clients using this technique) should
   instead track the scopes for every authorization grant, and only
   request yet to be granted scopes during incremental authorization.
   In the past, this would result in multiple discrete authorization
   grants that would need to be tracked.  To enable incrementing a
   single authorization grant for public clients, the client supplies
   their existing refresh token during the authorization code exchange,
   and receives new authorization tokens with the scope of the previous
   and current authorization grants.

   The client sends the previous refresh token in the OAuth 2.0 Access
   Token Request (Section 4.1.3 of [RFC6749].) using the following
   additional parameter:

   existing_grant  OPTIONAL.  The refresh token from the existing
      authorization grant.

   When processing the token exchange, in addition to the normal
   processing of such a request, the token endpoint MUST verify that
   token provided in the "existing_grant" parameter is unexpired and
   unrevoked, and was issued to the same client id and relates to the
   same user as the current authorization grant.  If this verification
   succeeds, the new refresh token issued in the Access Token Response
   (Section 4.1.4 of ) SHOULD include authorization for the scopes in
   the previous grant.

6.  Usability Considerations

6.1.  Handling Denials

   A core principle of OAuth is that users may deny authorization
   requests for any reason.  This remains true for incremental
   authorization requests.  In the case of incremental authorization,
   clients may already have a valid authorization and receive a denial
   for an incremental authorization request (that is, an "access_denied"
   error code as defined in Section of OAuth 2.0 [RFC6749]).
   Clients should SHOULD handle such errors gracefully and not discard
   any existing authorization grants if the user denies an incremental
   authorization request.  Clients SHOULD NOT immediately request the

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   same incremental authorization again, as this may result in an
   infinite denial loop (and the end-user feeling badgered).

7.  Alternative Approaches

7.1.  Alternative for Public Clients

   It is possible for OAuth clients to maintain multiple authorizations
   per user for feature-specific scopes without needing the feature
   documented in this specification.  For example, an app could maintain
   an authorization for the contacts and one for calendar, and store
   them separately.

   This specification offers a convenience that a single authorization
   grant can be managed that represents all the scope granted so far,
   rather than needing to maintain multiple, however it does require
   that all grants are made from a single end-user account (as
   authorization servers cannot typically combine grants from multiple
   users).  Apps where users may wish to authorize separate end-user
   accounts for different features should consider using the alternative
   documented above.

7.2.  Alternative for Confidential Clients

   An alternative incremental auth design for confidential clients is to
   ask for authorization scopes as they are needed and keep a running
   record of all granted scopes.  In this way each incremental
   authorization request would include all scopes granted so far, plus
   the new scope needed.  Authorization servers can see the existing
   scopes and only display the new scopes for approval (and likely to
   inform the user of the existing grants).  This approach can be
   performed using RFC 6749 without additions, but requires the client
   to keep track of every authorization grant.

   Confidential clients can also use the alternative documented for
   public clients in Section 7.1.

8.  Privacy Considerations

8.1.  Requesting Authorization In Context

   The goal of incremental authorization is to enhance end-user privacy
   by allowing clients to request only the authorization scopes needed
   in the context of a particular user action, rather than asking for
   ever possible scope upfront.  For example, an app may offer calendar
   and contacts integration, and an extension of OAuth like OpenID
   Connect for sign-in.  Such an app should first sign the user in with
   just the scopes needed for that.  If later the user interacts with

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   the calendar or contacts features then, and only then, should the
   requires scopes be requested.  By using this specification, apps can
   improve the privacy choices of end-users by only requesting the
   scopes they need in context.

   Clients authorizing the user with an authorization server that
   supports incremental auth SHOULD ask for the minimal authorization
   scope for the user's current context, and use this specification to
   add authorization scope as required.

8.2.  Preventing Overbroad Authorization Requests

   When this specification is implemented, clients should have no
   technical reason to make overbroad authorization requests (i.e.
   requesting every possible scope, even ones they don't immediately
   need).  It is therefore RECOMMENDED for authorization servers to
   limit the authorization scope that can be requested in a single
   authorization to what would reasonably be needed by a single feature.

8.3.  Authorization Correlation

   Incremental authorization is designed for use-cases where it's the
   same user authorizing each request, and thus all incremental
   authorization grants are correlated to that one user (by being merged
   into a single authorization grant).  For applications where users may
   wish to connect different user accounts for different features (e.g.
   contacts from one account, and calendar from another) it is
   RECOMMENDED to instead allow multiple unrelated authorizations, as
   documented in Section 7.1.

   The goal of this specification is to improve end-user privacy by
   giving them more choice over which scopes they grant access to.
   Previously many apps would request an overly large number of scopes
   upfront (typically for all the features of the app, rather than the
   subset that the user is currently wishing to use).  The scopes in
   such authorization grants are necessarily correlated with the same
   user as they are contained in a single authorization grant.
   Implementing this specification doesn't change that attribute, but it
   does improve user privacy overall by empowering the user to grant
   access in a more granular way.

9.  Security Considerations

9.1.  Public Client Impersonation

   As documented in Section 8.6 of RFC 8252 [RFC8252], some public
   clients are susceptible to client impersonation, depending on the
   type of redirect URI used.  If the "include_granted_scopes" feature

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   documented in Section 4 is used by an impersonating client, it may
   receive a greater authorization grant than the user specifically
   approved for that client.  For this reason, the
   "include_granted_scopes" feature MUST NOT be enabled for such public
   client requests.

   Note that there is no such restriction on the use of "existing_grant"
   feature documented in Section 5.  While it is designed for public
   clients, it MAY be supported for all client types.

10.  IANA Considerations

   This specification makes a registration request as follows:

10.1.  OAuth Parameters Registry

   This specification registers the following parameters in the IANA
   OAuth Parameters registry defined in OAuth 2.0 [RFC6749].

   o  Parameter name: include_granted_scopes

   o  Parameter usage location: authorization request

   o  Change controller: IESG

   o  Specification document(s): this document

   o  Parameter name: existing_grant

   o  Parameter usage location: token request

   o  Change controller: IESG

   o  Specification document(s): this document

11.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC6749]  Hardt, D., Ed., "The OAuth 2.0 Authorization Framework",
              RFC 6749, DOI 10.17487/RFC6749, October 2012,

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   [RFC8252]  Denniss, W. and J. Bradley, "OAuth 2.0 for Native Apps",
              BCP 212, RFC 8252, DOI 10.17487/RFC8252, October 2017,

Appendix A.  Acknowledgements

   This document was produced in the OAuth working group under the
   chairpersonship of Rifaat Shekh-Yusef and Hannes Tschofenig with
   Benjamin Kaduk, and Eric Rescorla serving as Security Area Directors.

   The following individuals contributed ideas, feedback, and wording
   that shaped and formed the final specification:

   Yanna Wu, Marius Scurtescu, Jason Huang, Nicholas Watson, and Breno
   de Medeiros.

Appendix B.  Document History

   [[ to be removed by the RFC Editor before publication as an RFC ]]


   o  Now a working group draft.


   o  Added usability, privacy, and security considerations.

   o  Documented alternative approaches.


   o  Initial draft based on the implementation of incremental and
      "appcremental" auth at Google.

Author's Address

   William Denniss
   1600 Amphitheatre Pkwy
   Mountain View, CA  94043

   Email: wdenniss@google.com
   URI:   https://wdenniss.com/incremental-auth

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