Network Working Group J. Richer
Internet-Draft The MITRE Corporation
Intended status: Standards Track September 07, 2012
Expires: March 11, 2013
A Method of Bearer Token Redelegation and Chaining for OAuth 2
draft-richer-oauth-chain-00
Abstract
This document provides a method for a resource server to present a
token that it has received from a client back to its authorization
server for the purposes of receiving a derivative token for use on
another resource server in order to chain together service requests.
Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
Status of this Memo
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provisions of BCP 78 and BCP 79.
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Table of Contents
1. Service Chaining . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Abbreviations Used In This Document . . . . . . . . . . . . 3
2. Protocol Description . . . . . . . . . . . . . . . . . . . . . 4
3. Redelegation Grant Type . . . . . . . . . . . . . . . . . . . . 5
3.1. Redelegate Request . . . . . . . . . . . . . . . . . . . . 6
3.2. Access Token Response . . . . . . . . . . . . . . . . . . . 6
3.3. Error Response . . . . . . . . . . . . . . . . . . . . . . 6
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7
5. Security Considerations . . . . . . . . . . . . . . . . . . . . 7
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 7
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 7
7.1. Normative References . . . . . . . . . . . . . . . . . . . 7
7.2. Informative References . . . . . . . . . . . . . . . . . . 7
Appendix A. Standardization of Scopes . . . . . . . . . . . . . . 7
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 8
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1. Service Chaining
The OAuth2 Authorization protocol provides methods for clients to
request tokens from authorization servers on behalf of resource
owners for use at resource servers. However, there are no provisions
for a resource server to act as a client itself for another resource
server, a practice known generally as service chaining. Typically,
the services involved in the chain are within a single security
domain, and with OAuth they would be using a single Authorization
Server.
For services using the OAuth2 Bearer token profile, it is possible
for anyone holding the token to call any other service that accepts
the token. While this is functional, it is bad practice since the
token is knowingly being re-used by someone other than the client to
which it was issued. Since the same token is used in each step, this
approach also does not allow for attenuation of rights as the chain
progresses.
Using a new form of grant_type, this specification presents such
chained resource servers with an alternative approach that takes
advantage of the simplicity and structure of the OAuth protocol by
providing a means for any resource server to present the token it has
been accessed with back to the authorization server in order to
exchange it for a token of equal or lesser strength for use with
another resource server. In this way, the original access token
which has been delegated to the client can be redelegated to a
secondary service.
This approach differs slightly from the Refresh Token described in
the OAuth 2 Core. With a Refresh Token, the Client presents the
token to the authorization server to get a new Access Token without
involving the Resource Owner. With a redelegated Access Token, as
described in this document, the Resource Server presents the Access
Token which was provided to it by a Client in order to get a
secondary Access Token.
1.1. Abbreviations Used In This Document
AS Authorization Server
C Client
RO Resource Owner
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RS1 Primary Resource Server, initially called by C on behalf of RO
RS2 Chained Resource Server, called by RS1 to fulfill request from C
AT1 Bearer Access Token granted by AS to C to access RS1 on behalf
of RO
AT2 Bearer Access Token granted by AS to RS1 to access RS2 on behalf
of RO
2. Protocol Description
The process begins with any standard OAuth2 protocol flow, where the
client obtains AT1 from the AS.
The beginning of the process is standard OAuth2 S.1.2 using any legal
OAuth2 grant type to obtain the AT1.
+--------+ +-------------+
| |--(A)->| Resource |
| | | Owner |
| |<-(B)--| (RO) |
| | +-------------+
| |
| | +---------------+
| |--(C)----------------------->| Authorization |
| Client | | Server |
| |<-(D)------------------------| (AS) |
| | | |
| | | |
| | +-------------+ | |
| |--(E)->| Primary |--(F)->| |
| | | Resource | | |
| | | Server |<-(G)--| |
| | | (RS1) | +---------------+
| | | |
| | | | +---------------+
| | | |--(H)->| Chained |
| | | | | Resource |
| | | | | Server |
| | | |<-(I)--| (RS2) |
| |<-(J)--| | +---------------+
+--------+ +-------------+
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(A) Client requests authorization from Resource Owner
(B) Client receives authorization from the Resource Owner using any
valid OAuth2 grant type
(C) Client requests AT1 from the AS by authenticating with the AS
and presenting the authorization grant obtained in (B)
(D) AS authenticates the Client and issues access token AT1 for use
at RS1
(E) Client presents access token AT1 to RS1 to access a protected
resource
(F) RS1 needs to access RS2 to fulfill this request, makes a call to
the Token Endpoint on the AS using the redelegate grant_type
(G) AS validates AT1 and issues a token AT2 for use by RS1 against
RS2, where the rights assigned to AT2 are a subset of those
assigned to AT1
(H) RS1 presents AT2 to RS2 to access a protected resource
(I) RS2 validates token AT2 and returns the protected resource to
RS1
(J) Client receives protected resource from RS1, including
information sourced from RS2
Steps A-E and J are standard OAuth2 and OAuth2 Bearer tokens
involving token AT1. As such, the Client MAY make use of any OAuth2
grant type, such as authorization_code, implicit, client_credentials,
password, assertion, or even the redelegation protocol defined in
this document. Steps F-G are described in section 3 of this
document. Steps H-I are standard OAuth2 Bearer token usage, but
using the delegated token AT2.
The means by which the Resource Servers validate the Access Tokens is
out of scope of this specification. At the time of this writing,
there are two main approaches found in practice: token introspection
and structured tokens.
3. Redelegation Grant Type
The Resource Server RS1 makes a request using the Access Token that
was presented to it in order to obtain a new Access Token.
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3.1. Redelegate Request
To access RS2, RS1 makes a POST request to the Authorization Server's
Token Endpoint with the following parameters:
grant_type REQUIRED. Value MUST be set to
"urn:ietf:params:oauth:grant_type:redelegate".
token REQUIRED The token that was presented to the resource server
by the client, referred to as AT1 in the protocol flow, an OAuth2
Bearer token
scope OPTIONAL a space-separated list of strings as described in
OAuth 2. If present, this scope list MUST be equal to or lesser
than the scopes incorporated in AT1. The AS MUST issue a token of
equal or lesser scope than the token above.
The Authorization Server MAY require RS1 to have registered as a
client on its own behalf. In this case, RS1 MUST present its client
credentials as described in OAuth2 Core.
3.2. Access Token Response
If the request is valid and authorized, the AS issues an access
token, referred to as AT2 in the protocol flow, as described in
OAuth2 Core. As this access token is bound to an existing access
token, the authorization server MUST NOT issue a refresh token. If
the request failed, the authorization server returns an error
response as described in OAuth2 Core.
HTTP/1.1 200 OK
Content-Type: application/json;charset=UTF-8
Cache-Control: no-store
Pragma: no-cache
{
"access_token":"2YotnFZFEjr1zCsicMWpAA",
"token_type":"example",
"expires_in":3600,
"example_parameter":"example_value"
}
3.3. Error Response
If the token request is not valid, such as the access token presented
does not allow for redelegation, the AS returns an error response as
described in OAuth2 Core.
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4. IANA Considerations
[Registration into the OAuth registry for
urn:ietf:params:oauth:grant_type:redelegate]
Note to RFC Editor: this section may be removed on publication as an
RFC.
5. Security Considerations
A resource server engaging in service chaining and token redelegation
SHOULD request a redelegated token with only the minimum set of
scopes necessary for calling downstream services.
A resource server MUST indicate in service documentation the full set
of scopes required for accessing the full service chain. A
redelegation request MUST NOT request escalated privileges without
involving the resource owner in a new authorization grant.
6. Acknowledgements
This work has grown from discussions with Paul Nguyen and Stephen
Moore, both of MITRE.
7. References
7.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
7.2. Informative References
[InfRef] "", 2004.
Appendix A. Standardization of Scopes
The OAuth2 specification explicitly leaves the definition of scopes
to the Authorization Server and Protected Resource to agree upon.
However, in the course of redelegation, it is sometimes desirable to
have a scope value related to the redelegation permission itself. It
is RECOMMENDED to use a scope value of "redelegate" if possible.
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Author's Address
Justin Richer
The MITRE Corporation
202 Burlington Rd.
Bedford, Massachusetts 01821
USA
Phone: +1-781-271-8176
Fax:
Email: jricher@mitre.org
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