OAuth Working Group W. Denniss
Internet-Draft Google
Intended status: Standards Track May 3, 2020
Expires: November 4, 2020
OAuth 2.0 Incremental Authorization
draft-ietf-oauth-incremental-authz-04
Abstract
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
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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
6.2. Handling Scope Reductions . . . . . . . . . . . . . . . . 5
7. Alternative Approaches . . . . . . . . . . . . . . . . . . . 5
7.1. Alternative for Public Clients . . . . . . . . . . . . . 6
7.2. Alternative for Confidential Clients . . . . . . . . . . 6
8. Privacy Considerations . . . . . . . . . . . . . . . . . . . 6
8.1. Requesting Authorization In Context . . . . . . . . . . . 6
8.2. Preventing Overbroad Authorization Requests . . . . . . . 7
8.3. Authorization Correlation . . . . . . . . . . . . . . . . 7
8.4. Previously Granted Scopes . . . . . . . . . . . . . . . . 8
9. Discovery Metadata . . . . . . . . . . . . . . . . . . . . . 8
10. Security Considerations . . . . . . . . . . . . . . . . . . . 8
10.1. Public Client Impersonation . . . . . . . . . . . . . . 8
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
11.1. OAuth Parameters Registry . . . . . . . . . . . . . . . 9
11.2. OAuth Extensions Error Registration . . . . . . . . . . 9
11.3. OAuth 2.0 Authorization Server Metadata . . . . . . . . 9
12. Normative References . . . . . . . . . . . . . . . . . . . . 10
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 10
Appendix B. Document History . . . . . . . . . . . . . . . . . . 10
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 11
1. Introduction
OAuth 2.0 clients may offer multiple features that require 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
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needing to track a separate authorization grant for each group of
scopes.
2. Notational Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"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
request.
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 client 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 access and refresh tokens issued in the Access
Token Response (Section 4.1.4 of ) MUST include authorization for the
scopes in the previous grant, unless the authorization server is
exercising its prerogative to "fully or partially ignore the scope
requested by the client" per Section 3.3 of OAuth 2.0 [RFC6749].
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 4.1.2.1 of OAuth 2.0 [RFC6749]).
Clients should SHOULD handle such errors gracefully and not discard
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any existing authorization grants if the user denies an incremental
authorization request. Clients SHOULD NOT immediately request the
same incremental authorization again, as this may result in an
infinite denial loop (and the end-user feeling badgered).
6.2. Handling Scope Reductions
As specified by Section 5.1 of OAuth 2.0 [RFC6749], a successful
response may not always include all the scope that was asked for, a
fact indicated by the "scope" response parameter when it happens.
This is still true in the case of incremental auth. The success
response may include less scope than what was requested, or even less
scope than before the incremental authorization request (say, if the
user was given an opportunity to revise the grant down). Clients
MUST check for the "scope" parameter in success responses and react
accordingly.
For the purposes of an incremental auth request, a success response
to an incremental authorization request that contains the same scope
granted prior to the request being made, and an error response (for
example, in the case of a denial) can have the same effect: the
client retains a grant with the same scope as before. In the case of
the approved request but with the same scope, they have a new grant,
but with the same scope. In the case of the denied incremental
authorization request, they still have the old grant with the same
scope (although in some cases it may have been revoked or reduced in
scope out of band).
An incremental authorization request isn't the only time that scope
can be reduced for a grant. As specified by section 6 of OAuth 2.0
[RFC6749], scope can be reduced during a token refresh as well. So
it's a good practice for clients to retain the current scope of the
grant, update it during authorization, incremental authorization and
token refreshes, and take action at any time based on the current
scope by presenting an incremental authorization if a non-present
scope is needed.
7. Alternative Approaches
This non-normative section discusses some alternative ways to achieve
the incremental authorization result purely on the client side.
These options are somewhat more complex and burdensome to client
developers.
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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, a public client (such
as a mobile 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). Clients where users may wish to authorize separate end-user
accounts for different features should consider using the alternative
documented in this section.
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
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.
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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 they might ever expect need, rather
than ones related to the user's current activity). To improve
privacy, 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.
The authorization server MAY deny such authorization requests with
the following error code.
overbroad_scope
The scope of the request is considered overbroad by the
authorization server. Consult the documentation of your
authorization server to determine acceptable scope combinations,
and consider using [[ This Specification ]] to perform incremental
authorization requests in the context that the scope is needed.
Determining what constitutes an overbroad request is the purview of
the authorization server. As an example, say an authorization
supported "calendar" and "mail" scopes to access a user's calendar
and inbox respectively. They may decide that their users should have
the chance to grant such requests in context through incremental
authorization, rather then all at once upfront, and deny the request
for being overly broad.
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
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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.
8.4. Previously Granted Scopes
When the authorization server displays the list of scopes on page and
prompts the user to consent to sharing access, users may assume that
the displayed list of scopes on such a page is the full and complete
list being granted to the application. It may be desirable for such
a consent page to list previously granted scopes, provided that the
client is confidential, or one that cannot be impersonated.
9. Discovery Metadata
Support for the incremental authorization MAY be declared in the
OAuth 2.0 Authorization Server Metadata [RFC8414] with the following
metadata:
incremental_authz_types_supported
OPTIONAL. JSON array of OAuth 2.0 client types that are supported
for incremental authorization. The possible types are
"confidential", and "public".
Specifically, "confidential" indicates that the behavior documented
in Section 4 (Incremental Auth for Confidential Clients) is
supported, and "public" indicates that the behavior documented in
Section 5 (Incremental Auth for Public Clients) is supported.
A server which supports both forms of incremental auth documented in
this specification would declare support like so:
"incremental_authz_types_supported": ["confidential", "public"]
10. Security Considerations
10.1. 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
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.
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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.
11. IANA Considerations
This specification makes a registration request as follows:
11.1. 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
11.2. OAuth Extensions Error Registration
This specification registers the following values in the IANA "OAuth
Extensions Error Registry" registry [IANA.OAuth.Parameters]
established by [RFC6749].
Name: overbroad_scope
Usage Location: authorization code grant error response
Protocol Extension: [[ This Specification ]]
Change Controller: IETF
Reference: Section 8.2 of [[ This Specification ]]
11.3. OAuth 2.0 Authorization Server Metadata
This specification registers the following values in the IANA "OAuth
2.0 Authorization Server Metadata" registry [IANA.OAuth.Parameters]
established by [RFC8414].
o Metadata Name: incremental_authz_types_supported
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o Metadata Description: JSON array containing a list of client types
that support OAuth 2.0 Incremental Authorization [[ this
specification ]]. The possible types are "confidential", and
"public".
o Change controller: IESG
o Specification Document: Section 9 of [[ this specification ]]
12. 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,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC6749] Hardt, D., Ed., "The OAuth 2.0 Authorization Framework",
RFC 6749, DOI 10.17487/RFC6749, October 2012,
<https://www.rfc-editor.org/info/rfc6749>.
[RFC8252] Denniss, W. and J. Bradley, "OAuth 2.0 for Native Apps",
BCP 212, RFC 8252, DOI 10.17487/RFC8252, October 2017,
<https://www.rfc-editor.org/info/rfc8252>.
[RFC8414] Jones, M., Sakimura, N., and J. Bradley, "OAuth 2.0
Authorization Server Metadata", RFC 8414,
DOI 10.17487/RFC8414, June 2018,
<https://www.rfc-editor.org/info/rfc8414>.
[IANA.OAuth.Parameters]
IANA, "OAuth Parameters",
<http://www.iana.org/assignments/oauth-parameters>.
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, Breno de
Medeiros, Naveen Agarwal, Brian Campbell, and Aaron Parecki.
Appendix B. Document History
[[ to be removed by the RFC Editor before publication as an RFC ]]
01
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o Changed a SHOULD to a MUST in Section 5 regarding the protocol
behavior of incremental auth for public clients, while clarifying
that the authorization server retains the prerogative to do
whatever it wants.
o Defined an OAuth Metadata entry.
00
o Now a working group draft.
draft-wdenniss-oauth-incremental-auth-01
o Added usability, privacy, and security considerations.
o Documented alternative approaches.
draft-wdenniss-oauth-incremental-auth-00
o Initial draft based on the implementation of incremental and
"appcremental" auth at Google.
Author's Address
William Denniss
Google
1600 Amphitheatre Pkwy
Mountain View, CA 94043
USA
Email: wdenniss@google.com
URI: https://wdenniss.com/incremental-auth
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