Internet Engineering Task Force (IETF) B. Campbell
Request for Comments: 8707 Ping Identity
Category: Standards Track J. Bradley
ISSN: 2070-1721 Yubico
H. Tschofenig
Arm Limited
February 2020
Resource Indicators for OAuth 2.0
Abstract
This document specifies an extension to the OAuth 2.0 Authorization
Framework defining request parameters that enable a client to
explicitly signal to an authorization server about the identity of
the protected resource(s) to which it is requesting access.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8707.
Copyright Notice
Copyright (c) 2020 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Provisions Relating to IETF Documents
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described in the Simplified BSD License.
Table of Contents
1. Introduction
1.1. Requirements Notation and Conventions
1.2. Terminology
2. Resource Parameter
2.1. Authorization Request
2.2. Access Token Request
3. Security Considerations
4. Privacy Considerations
5. IANA Considerations
5.1. OAuth Parameters Registration
5.2. OAuth Extensions Error Registration
6. References
6.1. Normative References
6.2. Informative References
Acknowledgements
Authors' Addresses
1. Introduction
Several years of deployment and implementation experience with the
OAuth 2.0 Authorization Framework [RFC6749] has uncovered a need (in
some circumstances, such as an authorization server servicing a
significant number of diverse resources) for the client to explicitly
signal to the authorization server where it intends to use the access
token it is requesting.
Knowing the protected resource (a.k.a. resource server, application,
API, etc.) that will process the access token enables the
authorization server to construct the token as necessary for that
entity. Properly encrypting the token (or content within the token)
to a particular resource, for example, requires knowing which
resource will receive and decrypt the token. Furthermore, various
resources oftentimes have different requirements with respect to the
data contained in (or referenced by) the token, and knowing the
resource where the client intends to use the token allows the
authorization server to mint the token accordingly.
Specific knowledge of the intended recipient(s) of the access token
also helps facilitate improved security characteristics of the token
itself. Bearer tokens, currently the most commonly utilized type of
OAuth access token, allow any party in possession of a token to get
access to the associated resources. To prevent misuse, several
important security assumptions must hold, one of which is that an
access token must only be valid for use at a specific protected
resource and for a specific scope of access. Section 5.2 of
[RFC6750], for example, prescribes including the token's intended
recipients within the token to prevent token redirect. When the
authorization server is informed of the resource that will process
the access token, it can restrict the intended audience of that token
to the given resource such that the token cannot be used successfully
at other resources.
OAuth scope, from Section 3.3 of [RFC6749], is sometimes overloaded
to convey the location or identity of the protected resource,
however, doing so isn't always feasible or desirable. Scope is
typically about what access is being requested rather than where that
access will be redeemed (e.g., "email", "admin:org", "user_photos",
"channels:read", and "channels:write" are a small sample of scope
values in use in the wild that convey only the type of access and not
the location or identity).
In some circumstances and for some deployments, a means for the
client to signal to the authorization server where it intends to use
the access token it's requesting is important and useful. A number
of implementations and deployments of OAuth 2.0 have already employed
proprietary parameters toward that end. Going forward, this
specification aspires to provide a standardized and interoperable
alternative to the proprietary approaches.
1.1. Requirements Notation and 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
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
1.2. Terminology
This specification uses the terms "access token", "refresh token",
"authorization server", "resource server", "authorization endpoint",
"authorization request", "authorization response", "token endpoint",
"grant type", "access token request", "access token response", and
"client" defined by The OAuth 2.0 Authorization Framework [RFC6749].
2. Resource Parameter
In requests to the authorization server, a client MAY indicate the
protected resource (a.k.a. resource server, application, API, etc.)
to which it is requesting access by including the following parameter
in the request.
resource
Indicates the target service or resource to which access is being
requested. Its value MUST be an absolute URI, as specified by
Section 4.3 of [RFC3986]. The URI MUST NOT include a fragment
component. It SHOULD NOT include a query component, but it is
recognized that there are cases that make a query component a
useful and necessary part of the resource parameter, such as when
one or more query parameters are used to scope requests to an
application. The "resource" parameter URI value is an identifier
representing the identity of the resource, which MAY be a locator
that corresponds to a network-addressable location where the
target resource is hosted. Multiple "resource" parameters MAY be
used to indicate that the requested token is intended to be used
at multiple resources.
The parameter value identifies a resource to which the client is
requesting access. The parameter can carry the location of a
protected resource, typically as an https URL or a more abstract
identifier. This enables the authorization server to apply policy as
appropriate for the resource, such as determining the type and
content of tokens to be issued, if and how tokens are encrypted, and
applying appropriate audience restrictions.
The client SHOULD provide the most specific URI that it can for the
complete API or set of resources it intends to access. In practice,
a client will know a base URI for the application or resource that it
interacts with, which is appropriate to use as the value of the
"resource" parameter. The client SHOULD use the base URI of the API
as the "resource" parameter value unless specific knowledge of the
resource dictates otherwise. For example, the value
"https://api.example.com/" would be used for a resource that is the
exclusive application on that host; however, if the resource is one
of many applications on that host, something like
"https://api.example.com/app/" would be used as a more specific
value. Another example is when an API has multiple endpoints, e.g.,
when System for Cross-domain Identity Management (SCIM) [RFC7644] has
endpoints such as "https://apps.example.com/scim/Users",
"https://apps.example.com/scim/Groups", and
"https://apps.example.com/scim/Schemas". The client would use
"https://apps.example.com/scim/" as the resource so that the issued
access token is valid for all the endpoints of the SCIM API.
The following error code is provided for an authorization server to
indicate problems with the requested resource(s) in response to an
authorization request or access token request. It can also be used
to inform the client that it has requested an invalid combination of
resource and scope.
invalid_target
The requested resource is invalid, missing, unknown, or malformed.
The authorization server SHOULD audience-restrict issued access
tokens to the resource(s) indicated by the "resource" parameter.
Audience restrictions can be communicated in JSON Web Tokens
[RFC7519] with the "aud" claim and the top-level member of the same
name provides the audience restriction information in a Token
Introspection [RFC7662] response. The authorization server may use
the exact "resource" value as the audience or it may map from that
value to a more general URI or abstract identifier for the given
resource.
2.1. Authorization Request
When the "resource" parameter is used in an authorization request to
the authorization endpoint, it indicates the identity of the
protected resource(s) to which access is being requested. When an
access token will be returned directly from the authorization
endpoint via the implicit flow (Section 4.2 of OAuth 2.0 [RFC6749]),
the requested resource is applicable to that access token. In the
code flow (Section 4.1 of OAuth 2.0 [RFC6749]) where an intermediate
representation of the authorization grant (the authorization code) is
returned from the authorization endpoint, the requested resource is
applicable to the full authorization grant.
For an authorization request sent as a JSON Web Token (JWT), such as
when using the JWT Secured Authorization Request [JWT-SAR], a single
"resource" parameter value is represented as a JSON string while
multiple values are represented as an array of strings.
If the client omits the "resource" parameter when requesting
authorization, the authorization server MAY process the request with
no specific resource or by using a predefined default resource value.
Alternatively, the authorization server MAY require clients to
specify the resource(s) they intend to access and MAY fail requests
that omit the parameter with an "invalid_target" error. The
authorization server might use this data to inform the user about the
resources the client is going to access on their behalf, to apply
policy (e.g., refuse the request due to unknown resources), and to
determine the set of resources that can be used in subsequent access
token requests.
If the authorization server fails to parse the provided value(s) or
does not consider the resource(s) acceptable, it should reject the
request with an error response using the error code "invalid_target"
as the value of the "error" parameter and can provide additional
information regarding the reasons for the error using the
"error_description".
An example of an authorization request where the client tells the
authorization server that it wants an access token for use at
"https://api.example.com/app/" is shown in Figure 1 below (extra line
breaks and indentation are for display purposes only).
GET /as/authorization.oauth2?response_type=token
&client_id=example-client
&state=XzZaJlcwYew1u0QBrRv_Gw
&redirect_uri=https%3A%2F%2Fclient.example.org%2Fcb
&resource=https%3A%2F%2Fapi.example.com%2Fapp%2F HTTP/1.1
Host: authorization-server.example.com
Figure 1: Implicit Flow Authorization Request
Below in Figure 2 is an example of an authorization request using the
"code" response type where the client is requesting access to the
resource owner's contacts and calendar data at
"https://cal.example.com/" and "https://contacts.example.com/" (extra
line breaks and indentation are for display purposes only).
GET /as/authorization.oauth2?response_type=code
&client_id=s6BhdRkqt3
&state=tNwzQ87pC6llebpmac_IDeeq-mCR2wLDYljHUZUAWuI
&redirect_uri=https%3A%2F%2Fclient.example.org%2Fcb
&scope=calendar%20contacts
&resource=https%3A%2F%2Fcal.example.com%2F
&resource=https%3A%2F%2Fcontacts.example.com%2F HTTP/1.1
Host: authorization-server.example.com
Figure 2: Code Flow Authorization Request
2.2. Access Token Request
When the "resource" parameter is used on an access token request made
to the token endpoint, for all grant types, it indicates the target
service or protected resource where the client intends to use the
requested access token.
The resource value(s) that is acceptable to an authorization server
in fulfilling an access token request is at its sole discretion based
on local policy or configuration. In the case of a "refresh_token"
or "authorization_code" grant type request, such policy may limit the
acceptable resources to those that were originally granted by the
resource owner or a subset thereof. In the "authorization_code" case
where the requested resources are a subset of the set of resources
originally granted, the authorization server will issue an access
token based on that subset of requested resources, whereas any
refresh token that is returned is bound to the full original grant.
When requesting a token, the client can indicate the desired target
service(s) where it intends to use that token by way of the
"resource" parameter and can indicate the desired scope of the
requested token using the "scope" parameter. The semantics of such a
request are that the client is asking for a token with the requested
scope that is usable at all the requested target services.
Effectively, the requested access rights of the token are the
cartesian product of all the scopes at all the target services. To
the extent possible, when issuing access tokens, the authorization
server should downscope the scope value associated with an access
token to the value the respective resource is able to process and
needs to know. (Here, "downscope" means give fewer permissions than
originally authorized by the resource owner.) This further improves
privacy as a list of scope values is an indication that the resource
owner uses the multiple various services listed; downscoping a token
to only that which is needed for a particular service can limit the
extent to which such information is revealed across different
services. As specified in Section 5.1 of [RFC6749], the
authorization server must indicate the access token's effective scope
to the client in the "scope" response parameter value when it differs
from the scope requested by the client.
Following from the code flow authorization request shown in Figure 2,
the below examples show an "authorization_code" grant type access
token request (Figure 3) and response (Figure 4) where the client
tells the authorization server that it wants the access token for use
at "https://cal.example.com/" (extra line breaks and indentation are
for display purposes only).
POST /as/token.oauth2 HTTP/1.1
Host: authorization-server.example.com
Authorization: Basic czZCaGRSa3F0Mzpoc3FFelFsVW9IQUU5cHg0RlNyNHlJ
Content-Type: application/x-www-form-urlencoded
grant_type=authorization_code
&redirect_uri=https%3A%2F%2Fclient.example.org%2Fcb
&code=10esc29BWC2qZB0acc9v8zAv9ltc2pko105tQauZ
&resource=https%3A%2F%2Fcal.example.com%2F
Figure 3: Access Token Request
HTTP/1.1 200 OK
Content-Type: application/json
Cache-Control: no-cache, no-store
{
"access_token":"eyJhbGciOiJFUzI1NiIsImtpZCI6Ijc3In0.eyJpc3MiOi
JodHRwOi8vYXV0aG9yaXphdGlvbi1zZXJ2ZXIuZXhhbXBsZS5jb20iLCJzdWI
iOiJfX2JfYyIsImV4cCI6MTU4ODQyMDgwMCwic2NvcGUiOiJjYWxlbmRhciIs
ImF1ZCI6Imh0dHBzOi8vY2FsLmV4YW1wbGUuY29tLyJ9.nNWJ2dXSxaDRdMUK
lzs-cYIj8MDoM6Gy7pf_sKrLGsAFf1C2bDhB60DQfW1DZL5npdko1_Mmk5sUf
zkiQNVpYw",
"token_type":"Bearer",
"expires_in":3600,
"refresh_token":"4LTC8lb0acc6Oy4esc1Nk9BWC0imAwH7kic16BDC2",
"scope":"calendar"
}
Figure 4: Access Token Response
A subsequent access token request, using the refresh token, where the
client tells the authorization server that it wants an access token
for use at "https://contacts.example.com/" is shown in Figure 5 below
with the response shown in Figure 6 (extra line breaks and
indentation are for display purposes only).
POST /as/token.oauth2 HTTP/1.1
Host: authorization-server.example.com
Authorization: Basic czZCaGRSa3F0Mzpoc3FFelFsVW9IQUU5cHg0RlNyNHlJ
Content-Type: application/x-www-form-urlencoded
grant_type=refresh_token
&refresh_token=4LTC8lb0acc6Oy4esc1Nk9BWC0imAwH7kic16BDC2
&resource=https%3A%2F%2Fcontacts.example.com%2F
Figure 5: Access Token Request
HTTP/1.1 200 OK
Content-Type: application/json
Cache-Control: no-cache, no-store
{
"access_token":"eyJhbGciOiJFUzI1NiIsImtpZCI6Ijc3In0.eyJpc3MiOi
JodHRwOi8vYXV0aG9yaXphdGlvbi1zZXJ2ZXIuZXhhbXBsZS5jb20iLCJzdWI
iOiJfX2JfYyIsImV4cCI6MTU4ODQyMDgyNiwic2NvcGUiOiJjb250YWN0cyIs
ImF1ZCI6Imh0dHBzOi8vY29udGFjdHMuZXhhbXBsZS5jb20vIn0.5f4yhqazc
OSlJw4y94KPeWNEFQqj2cfeO8x4hr3YbHtIl3nQXnBMw5wREY5O1YbZED-GfH
UowfmtNaA5EikYAw",
"token_type":"Bearer",
"expires_in":3600,
"scope":"contacts"
}
Figure 6: Access Token Response
3. Security Considerations
An audience-restricted access token that is legitimately presented to
a resource cannot then be taken by that resource and presented
elsewhere for illegitimate access to other resources. The "resource"
parameter enables a client to indicate the protected resources where
the requested access token will be used, which in turn enables the
authorization server to apply the appropriate audience restrictions
to the token.
Some servers may host user content or be multi-tenant. In order to
avoid attacks where one tenant uses an access token to illegitimately
access resources owned by a different tenant, it is important to use
a specific resource URI including any portion of the URI that
identifies the tenant, such as a path component. This will allow
access tokens to be audience-restricted in a way that identifies the
tenant and prevents their use, due to an invalid audience, at
resources owned by a different tenant.
Although multiple occurrences of the "resource" parameter may be
included in a token request, using only a single "resource" parameter
is encouraged. If a bearer token has multiple intended recipients
(audiences), then the token is valid at more than one protected
resource and can be used by any one of those resources to access any
of the others. Thus, a high degree of trust between the involved
parties is needed when using access tokens with multiple audiences.
Furthermore, an authorization server may be unwilling or unable to
fulfill a token request with multiple resources.
Whenever feasible, the "resource" parameter should correspond to the
network-addressable location of the protected resource. This makes
it possible for the client to validate that the resource being
requested controls the corresponding network location, reducing the
risk of malicious endpoints obtaining tokens meant for other
resources. If the "resource" parameter contains an abstract
identifier, it is the client's responsibility to validate out of band
that any network endpoint to which tokens are sent are the intended
audience for that identifier.
4. Privacy Considerations
In typical OAuth deployments the authorization sever is in a position
to observe and track a significant amount of user and client
behavior. It is largely just inherent to the nature of OAuth, and
this document does little to affect that. In some cases, however,
such as when access token introspection is not being used, use of the
resource parameter defined herein may allow for tracking behavior at
a somewhat more granular and specific level than would otherwise be
possible in its absence.
5. IANA Considerations
5.1. OAuth Parameters Registration
This specification updates the following value in the IANA "OAuth
Parameters" registry [IANA.OAuth.Parameters] established by
[RFC6749].
Parameter name: resource
Parameter usage location: authorization request, token request
Change controller: IESG
Specification document(s): RFC 8707
5.2. OAuth Extensions Error Registration
This specification updates the following error in the IANA "OAuth
Extensions Error Registry" [IANA.OAuth.Parameters] established by
[RFC6749].
Error name: invalid_target
Error usage location: implicit grant error response, token error
response
Related protocol extension: resource parameter
Change controller: IESG
Specification document(s): RFC 8707
6. References
6.1. Normative References
[IANA.OAuth.Parameters]
IANA, "OAuth Parameters",
<https://www.iana.org/assignments/oauth-parameters>.
[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>.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, DOI 10.17487/RFC3986, January 2005,
<https://www.rfc-editor.org/info/rfc3986>.
[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>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
6.2. Informative References
[JWT-SAR] Sakimura, N. and J. Bradley, "The OAuth 2.0 Authorization
Framework: JWT Secured Authorization Request (JAR)", Work
in Progress, Internet-Draft, draft-ietf-oauth-jwsreq-20,
21 October 2019,
<https://tools.ietf.org/html/draft-ietf-oauth-jwsreq-20>.
[RFC6750] Jones, M. and D. Hardt, "The OAuth 2.0 Authorization
Framework: Bearer Token Usage", RFC 6750,
DOI 10.17487/RFC6750, October 2012,
<https://www.rfc-editor.org/info/rfc6750>.
[RFC7519] Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
(JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015,
<https://www.rfc-editor.org/info/rfc7519>.
[RFC7644] Hunt, P., Ed., Grizzle, K., Ansari, M., Wahlstroem, E.,
and C. Mortimore, "System for Cross-domain Identity
Management: Protocol", RFC 7644, DOI 10.17487/RFC7644,
September 2015, <https://www.rfc-editor.org/info/rfc7644>.
[RFC7662] Richer, J., Ed., "OAuth 2.0 Token Introspection",
RFC 7662, DOI 10.17487/RFC7662, October 2015,
<https://www.rfc-editor.org/info/rfc7662>.
Acknowledgements
This specification was developed within the OAuth Working Group under
the chairmanship of Hannes Tschofenig and Rifaat Shekh-Yusef with
Eric Rescorla, Benjamin Kaduk, and Roman Danyliw serving as Security
Area Directors. Additionally, the following individuals contributed
ideas, feedback, and wording that helped shape this specification:
Vittorio Bertocci, Sergey Beryozkin, Roman Danyliw, William Denniss,
Vladimir Dzhuvinov, George Fletcher, Dick Hardt, Phil Hunt, Michael
Jones, Benjamin Kaduk, Barry Leiba, Torsten Lodderstedt, Anthony
Nadalin, Justin Richer, Adam Roach, Nat Sakimura, Rifaat Shekh-Yusef,
Filip Skokan, Éric Vyncke, and Hans Zandbelt.
Authors' Addresses
Brian Campbell
Ping Identity
Email: brian.d.campbell@gmail.com
John Bradley
Yubico
Email: ve7jtb@ve7jtb.com
Hannes Tschofenig
Arm Limited
Email: hannes.tschofenig@gmx.net
