Internet DRAFT - draft-hardt-distributed-oauth
draft-hardt-distributed-oauth
Network Working Group D. Hardt
Internet-Draft Amazon
Intended status: Standards Track October 30, 2017
Expires: May 3, 2018
Distributed OAuth
draft-hardt-distributed-oauth-00
Abstract
The OAuth client credentials grant The Distributed OAuth profile
enables an OAuth client using the client credentials grant to
discover what authorization server to use for a given resource
server, and what attribute values to provide in the access token
request.
Status of This Memo
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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
funcationility, but each is in a different geopolitical region, which
requires authorization from authorization servers located in each
geopolitical region.
A priori knownledge 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 attributes 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.
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+--------+ +---------------+
| |--(A)-- Discovery Request ---->| Resource |
| | | Server |
| |<-(B)-- Discovery Response ----| |
| | +---------------+
| |
| | +---------------+
| |--(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 client
credential grant:
1) A discovery request (A) and discovery resopnse (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. The resource server responds
with a HTTP 401 response code and the "iss" attribute in the "WWW-
Authenticate" header. The client notes the "host" value from the TLS
2) An authorization request (C) to an authorization server listed in
the "iss" attribute and includes the "host" attribute. The
authorization servers provides an access token that includes the
"host" value.
3) An authenticated request (E) to the resource server that confirms
the "host" matches expected values.
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 makes an unauthenticated call to the
protected resource, with no HTTP "Authorization" request header field
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as defined in [RFC6750] section 2.1. The client notes the hostname
of the protected resource that was confirmed by the TLS connection,
and saves it as the "host" attribute.
Figure 1, step (B)
The resource server responds with the "WWW-Authenticate" HTTP header
that MAY incldue the "scope" and "realm" attribute per [RFC6750]
section 3, and MUST include the "iss" attribute. The "iss" attribute
has one or more space delimited URLs.
For example (with extra line breaks for display purposes only):
HTTP/1.1 401 Unauthorized
WWW-Authenticate: Bearer realm="example_realm",
iss="http://issuer.example.com/token",
scope="example_scope",
error="invalid_token"
3. Access Token Request
Figure 1, step (C)
The client makes an access token request to the issuer URL specified
by the "iss" attribute, or if more than one value, a randomly
selected URL from the list. If the client is unable to connect to
the issuer, 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
The client makes the access token request per [RFC6749] section 4.4,
Client Credentials Grant and MUST include the "host" attribute, for
example (with extra 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"
&host=resource.example.com
&realm="example_realm"
Figure 1, step (D)
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The authorization server returns an access token that MUST include
the "host" attribute or a reference that includes the "host"
attribute.
4. Accessing Protected Resource
Figure 1, step (E)
The client access the protected resource per [RFC6750] section 2.1.
The distributed oauth profile SHALL only use the authorization
request header field for passing the access token.
Figure 1, step (F)
The protected resource MUST verify the "host" attribute is the host
of the resource server the protected resource is hosted.
5. New Threats
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.
5.1. 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 inclusion of the "host" property in the acces
token, so that only the access token is only accepted at the intended
host.
5.2. 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 acces 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 rather than being declared by the resource server.
5.3. Malicious Issuer
A malicious resource server could redirect the client to a malicious
issuer, or the issuer may be malicious. The malicious issuer may
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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 possion
authentication mechanism with the issuer such as mutual TLS or a
signed token containing the issuer as the audience.
6. IANA Considerations
TBD.
7. Acknowledgements
TBD.
8. 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>.
[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>.
Appendix A. Document History
A.1. draft-hardt-distributed-oauth-00
o Initial version.
Author's Address
Dick Hardt
Amazon
Email: dick.hardt@gmail.com
Hardt Expires May 3, 2018 [Page 6]
