OAuth Working Group M. Jones
Internet-Draft Microsoft
Intended status: Standards Track J. Bradley
Expires: March 11, 2017 B. Campbell
Ping Identity
September 7, 2016
OAuth 2.0 Token Binding
draft-ietf-oauth-token-binding-00
Abstract
This specification enables OAuth 2.0 implementations to apply Token
Binding to Access Tokens and Refresh Tokens. This cryptographically
binds these tokens to the TLS connections over which they are
intended to be used. This use of Token Binding protects these tokens
from man-in-the-middle and token export and replay attacks.
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 http://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 March 11, 2017.
Copyright Notice
Copyright (c) 2016 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
(http://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
Jones, et al. Expires March 11, 2017 [Page 1]
Internet-Draft OAuth 2.0 Token Binding September 2016
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Notation and Conventions . . . . . . . . . . 3
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2. Token Binding for Refresh Tokens . . . . . . . . . . . . . . 3
3. Token Binding for Access Tokens . . . . . . . . . . . . . . . 4
3.1. Initial Access Tokens . . . . . . . . . . . . . . . . . . 5
3.2. Refreshed Access Tokens . . . . . . . . . . . . . . . . . 5
3.3. Resource Server Token Binding Validation . . . . . . . . 5
3.4. Representing Token Binding in JWT Access Tokens . . . . . 5
4. Phasing in Token Binding and Preventing Downgrade Attacks . . 6
5. Token Binding Metadata . . . . . . . . . . . . . . . . . . . 7
5.1. Token Binding Client Metadata . . . . . . . . . . . . . . 7
5.2. Token Binding Authorization Server Metadata . . . . . . . 7
6. Security Considerations . . . . . . . . . . . . . . . . . . . 7
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
7.1. OAuth Parameters Registration . . . . . . . . . . . . . . 8
7.1.1. Registry Contents . . . . . . . . . . . . . . . . . . 8
7.2. OAuth Dynamic Client Registration Metadata Registration . 8
7.2.1. Registry Contents . . . . . . . . . . . . . . . . . . 8
7.3. OAuth Authorization Server Discovery Metadata
Registration . . . . . . . . . . . . . . . . . . . . . . 8
7.3.1. Registry Contents . . . . . . . . . . . . . . . . . . 9
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
8.1. Normative References . . . . . . . . . . . . . . . . . . 9
8.2. Informative References . . . . . . . . . . . . . . . . . 10
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 10
Appendix B. Open Issues . . . . . . . . . . . . . . . . . . . . 11
Appendix C. Document History . . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11
1. Introduction
This specification enables OAuth 2.0 [RFC6749] implementations to
apply Token Binding The Token Binding Protocol Version 1.0
[I-D.ietf-tokbind-protocol] Token Binding over HTTP
[I-D.ietf-tokbind-https] to Access Tokens and Refresh Tokens. This
cryptographically binds these tokens to the TLS connections over
which they are intended to be used. This use of Token Binding
protects these tokens from man-in-the-middle and token export and
replay attacks.
Jones, et al. Expires March 11, 2017 [Page 2]
Internet-Draft OAuth 2.0 Token Binding September 2016
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 RFC
2119 [RFC2119].
1.2. Terminology
This specification uses the terms "Access Token", "Authorization
Code", "Authorization Endpoint", "Authorization Grant",
"Authorization Server", "Client", "Client Authentication", "Client
Identifier", "Client Secret", "Grant Type", "Protected Resource",
"Redirection URI", "Refresh Token", "Resource Owner", "Resource
Server", "Response Type", and "Token Endpoint" defined by OAuth 2.0
[RFC6749], the terms "Claim", "Claim Name", "Claim Value", and "JSON
Web Token (JWT)" defined by JSON Web Token (JWT) [JWT], the term
"User Agent" defined by RFC 7230 [RFC7230], and the terms "Provided",
"Referred", "Token Binding" and "Token Binding ID" defined by Token
Binding over HTTP [I-D.ietf-tokbind-https].
2. Token Binding for Refresh Tokens
Token Binding of refresh tokens is a straightforward first-party
scenario, applying term "first-party" as used in Token Binding over
HTTP [I-D.ietf-tokbind-https]. It cryptographically binds the
refresh token to the TLS connection between the client and the token
endpoint. This case is straightforward because the refresh token is
both retrieved by the client from the token endpoint and sent by the
client to the token endpoint. Unlike the federated scenarios
described in Section 3 (Federation Use Cases) of Token Binding over
HTTP [I-D.ietf-tokbind-https] and the access token case described in
the next section, only a single TLS connection is involved in the
refresh token case.
Token Binding a refresh token requires that the authorization server
do two things. First, when refresh token is sent to the client, the
authorization server needs to remember the Provided Token Binding ID
and remember its association with the issued refresh token. Second,
when a token request containing a refresh token is received at the
token endpoint, the authorization server needs to verify that the
Provided Token Binding ID for the request matches the remembered
Token Binding ID associated with the refresh token. If the Token
Binding IDs do not match, the authorization server should return an
error in response to the request.
The means by which the authorization server remembers the association
between the refresh token and the Token Binding ID is an
Jones, et al. Expires March 11, 2017 [Page 3]
Internet-Draft OAuth 2.0 Token Binding September 2016
implementation detail that beyond the scope of this specification.
Some authorization servers will choose to store the Token Binding ID
(or a cryptographic hash of it, such a SHA-256 hash [SHS]) in the
refresh token itself, thus reducing the amount of state to be kept by
the server. Other authorization servers will add the Token Binding
ID value (or a hash of it) to an internal data structure also
containing other information about the refresh token, such as grant
type information. These choices make no difference to the client,
since the refresh token is opaque to it.
3. Token Binding for Access Tokens
Token Binding for access tokens cryptographically binds the access
token to the TLS connection between the client and the resource
server. Token Binding is applied to access tokens in a similar
manner to that described in Section 3 (Federation Use Cases) of Token
Binding over HTTP [I-D.ietf-tokbind-https]. It is also builds upon
the mechanisms for Token Binding of ID Tokens defined in OpenID
Connect Token Bound Authentication 1.0 [OpenID.TokenBinding].
In the OpenID Connect [OpenID.Core] use case, HTTP redirects are used
to pass information between the identity provider and the relying
party; this HTTP redirect makes the Token Binding ID of the relying
party available to the identity provider as the Referred Token
Binding ID, information about which is then added to the ID Token.
No such redirect occurs between the authorization server and the
resource server in the access token case; therefore, information
about the Token Binding ID for the TLS connection between the client
and the resource server needs to be explicitly communicated by the
client to the authorization server to achieve Token Binding of the
access token. This information is passed to the authorization server
using this request parameter:
resource_tbh
Base64url encoding of the SHA-256 hash [SHS] of the Token Binding
ID for the TLS connection between the client and the resource
server.
Note that to obtain this Token Binding ID, the client needs to
establish a TLS connection between itself and the resource server
prior to making the authorization request so that the Provided Token
Binding ID for the TLS connection to the resource server can be
obtained. The means by which the client retrieves this Token Binding
ID from the underlying Token Binding API is implementation and
operating system specific. An alternative, if supported, is for the
client to generate a Token Binding key to use for the resource
server, use the Token Binding ID for that key, and then later use
Jones, et al. Expires March 11, 2017 [Page 4]
Internet-Draft OAuth 2.0 Token Binding September 2016
that key when the TLS connection to the resource server is
established.
The authorization server MUST ignore the "resource_tbh" parameter if
it does not support Token Binding for the access token.
3.1. Initial Access Tokens
Upon receiving the hash of the Token Binding ID in an authorization
request containing the "resource_tbh" (resource token binding hash)
authorization request parameter, the authorization server then
records it in the issued access token. Alternatively, in some
implementations, the resource's Token Binding ID hash might be
communicated to the resource server by other means, such as by
introspecting [RFC7662] the access token.
3.2. Refreshed Access Tokens
Access tokens obtained from refresh requests can also be token bound.
In this case, the hash of the Token Binding ID of the TLS connection
between the client and the resource server is sent to the
authorization server at the token endpoint using the "resource_tbh"
(resource token binding hash) token request parameter; its syntax is
exactly the same as the corresponding authorization request
parameter. The authorization server then records it in the issued
access token or communicates it to the resource server by other
means, just as in the previous case.
3.3. Resource Server Token Binding Validation
Upon receiving a token bound access token, the resource server
validates the binding by computing a SHA-256 hash of the Provided
Token Binding ID and comparing it to the token binding hash value for
the access token. If these values do not match, the resource access
attempt MUST be rejected with an error.
3.4. Representing Token Binding in JWT Access Tokens
If the access token is represented as a JWT, the token binding
information SHOULD be represented in the same way that it is in token
bound OpenID Connect ID Tokens [OpenID.TokenBinding]. That
specification defines the new JWT Confirmation Method RFC 7800
[RFC7800] member "tbh" (token binding hash) to represent the SHA-256
hash of a Token Binding ID in an ID Token. The value of the "tbh"
member is the base64url encoding of the SHA-256 hash of the Token
Binding ID.
Jones, et al. Expires March 11, 2017 [Page 5]
Internet-Draft OAuth 2.0 Token Binding September 2016
The following example demonstrates the JWT Claims Set of an access
token containing the base64url encoding of the SHA-256 hash of a
Token Binding ID as the value of the "tbh" (token binding hash)
element in the "cnf" (confirmation) claim:
{
"iss": "https://server.example.com",
"aud": "https://resource.example.com",
"iat": 1467324320,
"exp": 1467324920,
"cnf":{
"tbh": "n0jI3trBK6_Gp2qiLOf48ZEZTjpBnhm-QOyzJxhBeAk"
}
}
4. Phasing in Token Binding and Preventing Downgrade Attacks
Many OAuth implementations will be deployed in situations in which
not all participants support Token Binding. Any of combination of
the client, the authorization server, the resource server, and the
User Agent may not yet support Token Binding, in which case it will
not work end-to-end.
It is a context-dependent deployment choice whether to allow
interactions to proceed in which Token Binding is not supported or
whether to treat Token Binding failures at any step as fatal errors.
Particularly in dynamic deployment environments in which End Users
have choices of clients, authorization servers, resource servers,
and/or User Agents, it is RECOMMENDED that authorizations using one
or more components that do not implement Token Binding be allowed to
successfully proceed. This enables different components to be
upgraded to supporting Token Binding at different times, providing a
smooth transition path for phasing in Token Binding. However, when
Token Binding has been performed, any Token Binding key mismatches
MUST be treated as fatal errors.
If all the participants in an authorization interaction support Token
Binding and yet one or more of them does not use it, this is likely
evidence of a downgrade attack. In this case, the authorization
SHOULD be aborted with an error. For instance, if the resource
server knows that the authorization server and the User Agent both
support Token Binding and yet the access token received does not
contain Token Binding information, this is almost certainly a sign of
an attack.
The authorization server and client can determine whether the other
supports Token Binding using the metadata values defined in the next
section. They can determine whether the User Agent supports Token
Jones, et al. Expires March 11, 2017 [Page 6]
Internet-Draft OAuth 2.0 Token Binding September 2016
Binding by whether it negotiated Token Binding for the TLS
connection. At present, there is no defined mechanism for
determining whether the resource server supports Token Binding or
not. However, it always safe to proceed as if it does; at worst, the
resource server simply won't verify the Token Binding.
5. Token Binding Metadata
5.1. Token Binding Client Metadata
Clients supporting Token Binding that also support the OAuth 2.0
Dynamic Client Registration Protocol [RFC7591] use these metadata
values to register their support for Token Binding of Access Tokens
and Refresh Tokens:
client_access_token_token_binding_supported
OPTIONAL. Boolean value specifying whether the Client supports
Token Binding of Access Tokens. If omitted, the default value is
"false".
client_refresh_token_token_binding_supported
OPTIONAL. Boolean value specifying whether the Client supports
Token Binding of Refresh Tokens. If omitted, the default value is
"false".
5.2. Token Binding Authorization Server Metadata
Authorization Servers supporting Token Binding that also support
OAuth 2.0 Authorization Server Metadata [OAuth.AuthorizationMetadata]
use these metadata values to register their support for Token Binding
of Access Tokens and Refresh Tokens:
as_access_token_token_binding_supported
OPTIONAL. Boolean value specifying whether the Authorization
Server supports Token Binding of Access Tokens. If omitted, the
default value is "false".
as_refresh_token_token_binding_supported
OPTIONAL. Boolean value specifying whether the Authorization
Server supports Token Binding of Refresh Tokens. If omitted, the
default value is "false".
6. Security Considerations
If a refresh request is received by the authorization server
containing a "resource_tbh" (resource token binding hash) value
requesting a token bound access token and the refresh token in the
request is not itself token bound, then it is not clear that token
Jones, et al. Expires March 11, 2017 [Page 7]
Internet-Draft OAuth 2.0 Token Binding September 2016
binding the access token adds significant value. This situation
should be considered an open issue for discussion by the working
group.
7. IANA Considerations
7.1. OAuth Parameters Registration
This specification registers the following parameter in the IANA
"OAuth Parameters" registry [IANA.OAuth.Parameters] established by
RFC 6749 [RFC6749]:
7.1.1. Registry Contents
o Parameter name: "resource_tbh"
o Parameter usage location: Authorization Request, Token Request
o Change controller: IESG
o Specification document(s): Section 3 of this document
o Related information: None
7.2. OAuth Dynamic Client Registration Metadata Registration
This specification registers the following client metadata
definitions in the IANA "OAuth Dynamic Client Registration Metadata"
registry [IANA.OAuth.Parameters] established by [RFC7591]:
7.2.1. Registry Contents
o Client Metadata Name:
"client_access_token_token_binding_supported"
o Client Metadata Description: Boolean value specifying whether the
Client supports Token Binding of Access Tokens
o Change Controller: IESG
o Specification Document(s): Section 5.1 of [[ this specification ]]
o Client Metadata Name:
"client_refresh_token_token_binding_supported"
o Client Metadata Description: Boolean value specifying whether the
Client supports Token Binding of Refresh Tokens
o Change Controller: IESG
o Specification Document(s): Section 5.1 of [[ this specification ]]
7.3. OAuth Authorization Server Discovery Metadata Registration
This specification registers the following discovery metadata
definitions in the IANA "OAuth Authorization Server Discovery
Metadata" registry established by [OAuth.AuthorizationMetadata]:
Jones, et al. Expires March 11, 2017 [Page 8]
Internet-Draft OAuth 2.0 Token Binding September 2016
7.3.1. Registry Contents
o Discovery Metadata Name: "as_access_token_token_binding_supported"
o Discovery Metadata Description: Boolean value specifying whether
the Authorization Server supports Token Binding of Access Tokens
o Change Controller: IESG
o Specification Document(s): Section 5.2 of [[ this specification ]]
o Discovery Metadata Name:
"as_refresh_token_token_binding_supported"
o Discovery Metadata Description: Boolean value specifying whether
the Authorization Server supports Token Binding of Refresh Tokens
o Change Controller: IESG
o Specification Document(s): Section 5.2 of [[ this specification ]]
8. References
8.1. Normative References
[I-D.ietf-tokbind-https]
Popov, A., Nystrom, M., Balfanz, D., Langley, A., and J.
Hodges, "Token Binding over HTTP", draft-ietf-tokbind-
https-06 (work in progress), August 2016.
[I-D.ietf-tokbind-protocol]
Popov, A., Nystrom, M., Balfanz, D., Langley, A., and J.
Hodges, "The Token Binding Protocol Version 1.0", draft-
ietf-tokbind-protocol-10 (work in progress), September
2016.
[IANA.OAuth.Parameters]
IANA, "OAuth Parameters",
<http://www.iana.org/assignments/oauth-parameters>.
[JWT] Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
(JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015,
<http://tools.ietf.org/html/rfc7519>.
[OpenID.TokenBinding]
Jones, M., Bradley, J., and B. Campbell, "OpenID Connect
Token Bound Authentication 1.0", July 2016,
<http://openid.net/specs/
openid-connect-token-bound-authentication-1_0.html>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
Jones, et al. Expires March 11, 2017 [Page 9]
Internet-Draft OAuth 2.0 Token Binding September 2016
[RFC6749] Hardt, D., Ed., "The OAuth 2.0 Authorization Framework",
RFC 6749, DOI 10.17487/RFC6749, October 2012,
<http://www.rfc-editor.org/info/rfc6749>.
[RFC7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Message Syntax and Routing",
RFC 7230, DOI 10.17487/RFC7230, June 2014,
<http://www.rfc-editor.org/info/rfc7230>.
[RFC7662] Richer, J., Ed., "OAuth 2.0 Token Introspection",
RFC 7662, DOI 10.17487/RFC7662, October 2015,
<http://www.rfc-editor.org/info/rfc7662>.
[RFC7800] Jones, M., Bradley, J., and H. Tschofenig, "Proof-of-
Possession Key Semantics for JSON Web Tokens (JWTs)",
RFC 7800, DOI 10.17487/RFC7800, April 2016,
<http://www.rfc-editor.org/info/rfc7800>.
[SHS] National Institute of Standards and Technology, "Secure
Hash Standard (SHS)", FIPS PUB 180-4, March 2012,
<http://csrc.nist.gov/publications/fips/fips180-4/
fips-180-4.pdf>.
8.2. Informative References
[OAuth.AuthorizationMetadata]
Jones, M., Sakimura, N., and J. Bradley, "OAuth 2.0
Authorization Server Metadata", draft-ietf-oauth-
discovery-02 (work in progress), August 2016,
<http://tools.ietf.org/html/
draft-ietf-oauth-discovery-04>.
[OpenID.Core]
Sakimura, N., Bradley, J., Jones, M., de Medeiros, B., and
C. Mortimore, "OpenID Connect Core 1.0", November 2014,
<http://openid.net/specs/openid-connect-core-1_0.html>.
[RFC7591] Richer, J., Ed., Jones, M., Bradley, J., Machulak, M., and
P. Hunt, "OAuth 2.0 Dynamic Client Registration Protocol",
RFC 7591, DOI 10.17487/RFC7591, July 2015,
<http://www.rfc-editor.org/info/rfc7591>.
Appendix A. Acknowledgements
The authors would like to thank the following people for their
contributions to the specification: Dirk Balfanz, William Denniss,
Andrei Popov, and Nat Sakimura.
Jones, et al. Expires March 11, 2017 [Page 10]
Internet-Draft OAuth 2.0 Token Binding September 2016
Appendix B. Open Issues
o Some token binding implementations apparently provide APIs that
enable native applications to provide Referred Token Bindings,
just as the federation support in the HTTPS Token Binding spec
does. Can we count on these APIs being supported on all
platforms, and if so, does this enable us to somehow do without
the "resource_tbh" parameter by mandating that the client send
both a Provided and a Referred Token Binding to the authorization
server? If this isn't the case, is "resource_tbh" actually secure
or does this open a cross-channel validation hole? This area
probably needs more attention from both the Token Binding and
OAuth working groups.
o How should we support crypto agility for the hash function?
Appendix C. Document History
[[ to be removed by the RFC Editor before publication as an RFC ]]
-00
o Created the initial working group version from draft-jones-oauth-
token-binding-00.
Authors' Addresses
Michael B. Jones
Microsoft
Email: mbj@microsoft.com
URI: http://self-issued.info/
John Bradley
Ping Identity
Email: ve7jtb@ve7jtb.com
URI: http://www.thread-safe.com/
Brian Campbell
Ping Identity
Email: brian.d.campbell@gmail.com
URI: https://twitter.com/__b_c
Jones, et al. Expires March 11, 2017 [Page 11]
