Internet DRAFT - draft-ietf-oauth-distributed
draft-ietf-oauth-distributed
OAuth Working Group D. Hardt
Internet-Draft Amazon
Intended status: Standards Track B. Campbell
Expires: April 22, 2019 Ping Identity
N. Sakimura
NRI
October 19, 2018
Distributed OAuth
draft-ietf-oauth-distributed-01
Abstract
The Distributed OAuth profile enables an OAuth client to discover
what authorization server or servers may be used to obtain access
tokens for a given resource, and what parameter values to provide in
the access token request.
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
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on April 22, 2019.
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
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
Hardt, et al. Expires April 22, 2019 [Page 1]
Internet-Draft Distributed OAuth October 2018
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
1. Introduction
In [RFC6749], there is a single resource server and authorization
server. In more complex and distributed systems, a clients may
access many different resource servers, which have different
authorization servers managing access. For example, a client may be
accessing two different resources that provides similar
functionality, but each is in a different geopolitical region, which
requires authorization from authorization servers located in each
geopolitical region.
A priori knowledge by the client of the relationships between
resource servers and authorizations servers is not practical as the
number of resource servers and authorization servers scales up. The
client needs to discover on-demand which authorization server to
request authorization for a given resource, and what parameters to
pass. Being able to discover how to access a protected resource also
enables more flexible software development as changes to the scopes,
realms and authorization servers can happen dynamically with no
change to client code.
1.1. Notational Conventions
In this document, the key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
and "OPTIONAL" are to be interpreted as described in BCP 14,
[RFC2119].
1.2. Terminology
Issuer: the party issuing the access token, also known as the
authorization server.
All other terms are as defined in [RFC6749] and [RFC6750]
1.3. Protocol Overview
Figure 1 shows an abstract flow of distributed OAuth.
Hardt, et al. Expires April 22, 2019 [Page 2]
Internet-Draft Distributed OAuth October 2018
+--------+ +---------------+
| |--(A)-- Discovery Request ---->| Resource |
| | | Server |
| |<-(B)-- Discovery Response ----| |
| | +---------------+
| |
| | (client obtains authorization grant)
| |
| | +---------------+
| |--(C)- Authorization Request ->| Authorization |
| Client | | Server |
| |<-(D)----- Access Token -------| |
| | +---------------+
| |
| | +---------------+
| |--(E)----- Access Token ------>| Resource |
| | | Server |
| |<-(F)--- Protected Resource ---| |
+--------+ +---------------+
Figure 1: Abstract Protocol Flow
There are three steps where there are changes from the OAuth flow:
1) A discovery request (A) and discovery response (B) where the
client discovers what is required to make an authenticated request.
The client makes a request to the protected resource without
supplying the Authorization header, or supplying an invalid access
token. The resource server responds with a HTTP 401 response code
and links of relation types "resource_uri" and the
"oauth_server_metadata_uri". The client confirms the "host" value
from the TLS connection is contained in the resource URI per
[RFC6125] section 6, and fetches each OAuth Server Metadata URI and
per [OASM] discovers one or more authorization server end point URIs.
The client then obtains an authorization grant per one of the grant
types in [RFC6749] section 4.
2) An authorization request (C) to an authorization server and
includes the "resource_uri" link as a resource parameter per [OARI]
2.1. The authorization servers provides an access token that is
associated to the "resource_uri" value.
3) An authenticated request (E) to the resource server that confirms
the "resource_uri" linked to the access token matches expected value.
Hardt, et al. Expires April 22, 2019 [Page 3]
Internet-Draft Distributed OAuth October 2018
2. Authorization Server Discovery
Figure 1, step (A)
To access a protected resource, the client needs to learn the
authorization servers or issuers that can issue access tokens that
are acceptable to the protected resource. There may be one or more
issuers that can issue access tokens for the protected resource. To
discover the issuers, the client attempts to make a call to the
protected resource URI as defined in [RFC6750] section 2.1, except
with an invalid access token or no HTTP "Authorization" request
header field. The client notes the hostname of the protected
resource that was confirmed by the TLS connection per [RFC6125]
section 6, and saves it as the "host" attribute.
Figure 1, step (B)
The resource server responds with the "WWW-Authenticate" HTTP header
that includes the "error" attribute with a value of "invalid_token"
and MAY also include the "scope" and "realm" attribute per [RFC6750]
section 3, and a "Link" HTTP Header per [RFC8288] that MUST include
one link of relation type "resource_uri" and one or more links of
type "oauth_server_metadata_uri".
For example (with extra spaces and line breaks for display purposes
only):
HTTP/1.1 401 Unauthorized
WWW-Authenticate: Bearer realm="example_realm",
scope="example_scope",
error="invalid_token"
Link: <https://api.example.com/resource">;
rel="resource_uri",
<https://as.example.com/.well-known/oauth-authorization-server>;
rel="oauth_server_metadata_uri"
The client MUST confirm the host portion of the resource URI per
[RFC6125] section 6, as specified in the "resource_uri" link,
contains the "host" attribute obtained from the TLS connection in
step (A). The client MUST confirm the resource URI is contained in
the protected resource URI where access was attempted. The client
then retrieves one or more of the OAuth Server Metadata URIs to learn
how to interact with the associated authorization server per [OASM]
and create a list of one or more authorization server token endpoint
URLs.
Hardt, et al. Expires April 22, 2019 [Page 4]
Internet-Draft Distributed OAuth October 2018
3. Authorization Grant
The client obtains an authorization grant per any of the mechanisms
in [RFC6749] section 4.
4. Access Token Request
Figure 1, step (C)
The client makes an access token request to the authorization server
token endpoint URL, or if more than URL is available, a randomly
selected URL from the list. If the client is unable to connect to
the URL, then the client MAY try to connect to another URL from the
list.
The client SHOULD authenticate to the issuer using a proof of
possession mechanism such as mutual TLS or a signed token containing
the issuer as the audience.
Depending on the authorization grant mechanism used per [RFC6749]
section 4, the client makes the access token request and MUST include
"resource" as an additional parameter with the value of the resource
URI. For example, if using the [RFC6749] section 4.4, Client
Credentials Grant, the request would be (with extra spaces and line
breaks for display purposes only):
POST /token HTTP/1.1
Host: issuer.example.com
Authorization: Basic czZCaGRSa3F0MzpnWDFmQmF0M2JW
Content-Type: application/x-www-form-urlencoded
grant_type=client_credentials
&scope=example_scope
&resource=https%3A%2F%2Fapi.example.com%2Fresource
Figure 1, step (D)
The authorization server MUST associate the resource URI with the
issued access token in a way that can be accessed and verified by the
protected resource. For JWT [RFC7519] formatted access tokens, the
"aud" claim MUST be used to convey the resource URI. When Token
Introspection [RFC7662] is used, the introspection response MUST
containe the "aud" member with the resource URI as its value.
Hardt, et al. Expires April 22, 2019 [Page 5]
Internet-Draft Distributed OAuth October 2018
5. Accessing Protected Resource
Figure 1, step (E)
The client accesses the protected resource per [RFC6750] section 2.1.
The Distributed OAuth Profile MUST only use the authorization request
header field for passing the access token.
Figure 1, step (F)
The protected resource MUST verify the resource URI in or referenced
by the access token is the protected resource's resource URI.
6. Security Considerations
Three new threats emerge when the client is dynamically discovering
the authorization server and the request attributes: access token
reuse, resource server impersonation, and malicious issuer.
6.1. Access Token Reuse
A malicious resource server impersonates the client and reuses the
access token provided by the client to the malicious resource server
with another resource server.
This is mitigated by constraining the access token to a specific
audience, or to a specific client.
Audience restricting the access token is described in this document
where the resource URI is associated to the access token by inclusion
or reference, so that only access tokens with the correct resource
URI are accepted at a resource server.
Sender constraining the access token can be done through [MTLS],
[OATB], or any other mechanism that the resource can use to associate
the access token with the client.
6.2. Resource Server Impersonation
A malicious resource server tells a client to obtain an access token
that can be used at a different resource server. When the client
presents the access token, the malicious resource server uses the
access token to access another resource server.
This is mitigated by the client obtaining the "host" value from the
TLS certificate of the resource server, and the client verifying the
"host" value is contained in the host portion of the resource URI,
Hardt, et al. Expires April 22, 2019 [Page 6]
Internet-Draft Distributed OAuth October 2018
rather than the resource URI being any value declared by the resource
server.
6.3. Malicious Issuer
A malicious resource server could redirect the client to a malicious
issuer, or the issuer may be malicious. The malicious issuer may
replay the client credentials with a valid issuer and obtain a valid
access token for a protected resource.
This attack is mitigated by the client using a proof of possession
authentication mechanism with the issuer such as [MTLS] or a signed
token containing the issuer as the audience.
7. IANA Considerations
Pursuant to [RFC5988], the following link type registrations will be
registered by mail to link-relations@ietf.org.
o Relation Name: oauth_server_metadata_uri
o Description: An OAuth 2.0 Server Metadata URI.
o Reference: This specification
o Relation Name: resource_uri
o Description: An OAuth 2.0 Resource Endpoint specified in [RFC6750]
section 3.2.
o Reference: This specification
8. Acknowledgements
TBD.
9. Normative References
[MTLS] Jones, M., Campbell, B., Bradley, J., and W. Denniss,
"OAuth 2.0 Mutual TLS Client Authentication and
Certificate Bound Access Tokens", October 2018,
<https://datatracker.ietf.org/doc/draft-ietf-oauth-mtls/>.
[OARI] Campbell, B. and J. Bradley, "OAuth Resource Indicators",
October 2018, <https://datatracker.ietf.org/doc/
draft-ietf-oauth-resource-indicators/>.
Hardt, et al. Expires April 22, 2019 [Page 7]
Internet-Draft Distributed OAuth October 2018
[OASM] Jones, M., Campbell, B., and J. Bradley, "OAuth 2.0
Authorization Server Metadata", October 2018,
<https://datatracker.ietf.org/doc/
draft-ietf-oauth-discovery/>.
[OATB] Jones, M., Campbell, B., Bradley, J., and W. Denniss,
"OAuth 2.0 Token Binding", October 2018,
<https://datatracker.ietf.org/doc/
draft-ietf-oauth-token-binding/>.
[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>.
[RFC5988] Nottingham, M., "Web Linking", RFC 5988,
DOI 10.17487/RFC5988, October 2010,
<https://www.rfc-editor.org/info/rfc5988>.
[RFC6125] Saint-Andre, P. and J. Hodges, "Representation and
Verification of Domain-Based Application Service Identity
within Internet Public Key Infrastructure Using X.509
(PKIX) Certificates in the Context of Transport Layer
Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March
2011, <https://www.rfc-editor.org/info/rfc6125>.
[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>.
[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>.
[RFC7662] Richer, J., Ed., "OAuth 2.0 Token Introspection",
RFC 7662, DOI 10.17487/RFC7662, October 2015,
<https://www.rfc-editor.org/info/rfc7662>.
[RFC8288] Nottingham, M., "Web Linking", RFC 8288,
DOI 10.17487/RFC8288, October 2017,
<https://www.rfc-editor.org/info/rfc8288>.
Hardt, et al. Expires April 22, 2019 [Page 8]
Internet-Draft Distributed OAuth October 2018
Appendix A. Document History
[[ to be removed by the RFC Editor before publication as an RFC ]]
draft-ietf-oauth-distributed-01
o added reference to OAuth Resource Indicators draft
o added reference to RFC 6125 for matching host
draft-ietf-oauth-distributed-00
o Initial version adopted from draft-hardt-oauth-distributed-02
Authors' Addresses
Dick Hardt
Amazon
Email: dick.hardt@gmail.com
Brian Campbell
Ping Identity
Email: brian.d.campbell@gmail.com
Nat Sakimura
NRI
Email: n-sakimura@nri.co.jp
Hardt, et al. Expires April 22, 2019 [Page 9]
