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RFC 6750
Network Working Group M. Jones
Internet-Draft Microsoft
Intended status: Standards Track D. Hardt
Expires: August 1, 2011 independent
D. Recordon
Facebook
January 28, 2011
The OAuth 2.0 Protocol: Bearer Tokens
draft-ietf-oauth-v2-bearer-02
Abstract
This specification describes how to use bearer tokens when accessing
OAuth 2.0 protected resources.
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 August 1, 2011.
Copyright Notice
Copyright (c) 2011 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
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Notational Conventions . . . . . . . . . . . . . . . . . . 3
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
1.3. Overview . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Authenticated Requests . . . . . . . . . . . . . . . . . . . . 4
2.1. The Authorization Request Header Field . . . . . . . . . . 5
2.2. Form-Encoded Body Parameter . . . . . . . . . . . . . . . 5
2.3. URI Query Parameter . . . . . . . . . . . . . . . . . . . 6
3. Security Considerations . . . . . . . . . . . . . . . . . . . 7
3.1. Security Threats . . . . . . . . . . . . . . . . . . . . . 7
3.2. Threat Mitigation . . . . . . . . . . . . . . . . . . . . 7
3.3. Summary of Recommendations . . . . . . . . . . . . . . . . 8
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
4.1. OAuth Access Token Type Registration . . . . . . . . . . . 9
4.1.1. The "OAuth2" OAuth Access Token Type . . . . . . . . . 9
4.2. OAuth Parameters Registration . . . . . . . . . . . . . . 10
4.2.1. The "oauth_token" OAuth Parameter . . . . . . . . . . 10
5. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
5.1. Normative References . . . . . . . . . . . . . . . . . . . 10
5.2. Informative References . . . . . . . . . . . . . . . . . . 11
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 11
Appendix B. Document History . . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12
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1. Introduction
OAuth enables clients to access protected resources by obtaining an
access token, which is defined in [OAuth2] as "a string representing
an access authorization issued to the client", rather than using the
resource owner's credentials.
Tokens are issued to clients by an authorization server with the
approval of the resource owner. The client uses the access token to
access the protected resources hosted by the resource server. This
specification describes how to make protected resource requests by
treating an OAuth access token as a bearer token.
This specification defines the use of bearer tokens with OAuth over
HTTP [RFC2616] using TLS [RFC2818]. Other specifications may extend
it for use with other transport protocols.
1.1. Notational Conventions
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 [RFC2119].
This document uses the Augmented Backus-Naur Form (ABNF) notation of
[I-D.ietf-httpbis-p1-messaging]. Additionally, the following rules
are included from [RFC2617]: auth-param; and from
[I-D.ietf-httpbis-p1-messaging]: RWS.
Unless otherwise noted, all the protocol parameter names and values
are case sensitive.
1.2. Terminology
All terms are as defined in [OAuth2].
1.3. Overview
OAuth provides a method for clients to access a protected resource on
behalf of a resource owner. Before a client can access a protected
resource, it must first obtain authorization (access grant) from the
resource owner and then exchange the access grant for an access token
(representing the grant's scope, duration, and other attributes).
The client accesses the protected resource by presenting the access
token to the resource server.
The access token provides an abstraction layer, replacing different
authorization constructs (e.g. username and password, assertion) for
a single token understood by the resource server. This abstraction
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enables issuing access tokens valid for a short time period, as well
as removing the resource server's need to understand a wide range of
authentication schemes.
+--------+ +---------------+
| |--(A)- Authorization Request ->| Resource |
| | | Owner |
| |<-(B)----- Access Grant -------| |
| | +---------------+
| |
| | Access Grant & +---------------+
| |--(C)--- Client Credentials -->| Authorization |
| Client | | Server |
| |<-(D)----- Access Token -------| |
| | +---------------+
| |
| | +---------------+
| |--(E)----- Access Token ------>| Resource |
| | | Server |
| |<-(F)--- Protected Resource ---| |
+--------+ +---------------+
Figure 1: Abstract Protocol Flow
The abstract flow illustrated in Figure 1 describes the overall OAuth
2.0 protocol architecture. The following steps are specified within
this document:
E) The client makes a protected resource request to the resource
server by presenting the access token.
F) The resource server validates the access token, and if valid,
serves the request.
2. Authenticated Requests
Clients make authenticated token requests using the "Authorization"
request header field. Resource servers MUST accept authenticated
requests using the "OAuth2" HTTP authentication scheme as described
in Section 2.1, and MAY support additional methods.
Alternatively, clients MAY attempt to include the access token in the
HTTP body when using the "application/x-www-form-urlencoded" content
type as described in Section 2.2 or using the HTTP request URI in the
query component as described in Section 2.3. Resource servers MAY
support these alternative methods.
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Clients SHOULD only use the request body or URI when the
"Authorization" request header field is not available, and MUST NOT
use more than one method to transport the token in each request.
Because of the Security Considerations (Section 3) associated with
the URI method, it SHOULD only be used if no other method is
feasible.
2.1. The Authorization Request Header Field
The "Authorization" request header field is used by clients to make
authenticated token requests. The client uses the "OAuth2"
authentication scheme to include the access token in the request.
For example:
GET /resource HTTP/1.1
Host: server.example.com
Authorization: OAuth2 vF9dft4qmT
The "Authorization" header field uses the framework defined by
[RFC2617] as follows:
credentials = "OAuth2" RWS access-token [ RWS 1#auth-param ]
access-token = 1*( quoted-char / <"> )
quoted-char = "!" / "#" / "$" / "%" / "&" / "'" / "("
/ ")" / "*" / "+" / "-" / "." / "/" / DIGIT
/ ":" / "<" / "=" / ">" / "?" / "@" / ALPHA
/ "[" / "]" / "^" / "_" / "`" / "{" / "|"
/ "}" / "~" / "\" / "," / ";"
2.2. Form-Encoded Body Parameter
When including the access token in the HTTP request entity-body, the
client adds the access token to the request body using the
"oauth_token" parameter. The client can use this method only if the
following REQUIRED conditions are met:
o The HTTP request entity-body is single-part.
o The entity-body follows the encoding requirements of the
"application/x-www-form-urlencoded" content-type as defined by
[W3C.REC-html401-19991224].
o The HTTP request entity-header includes the "Content-Type" header
field set to "application/x-www-form-urlencoded".
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o The HTTP request method is one for which a body is permitted to be
present in the request. In particular, this means that the "GET"
method MAY NOT be used.
The entity-body can include other request-specific parameters, in
which case, the "oauth_token" parameters SHOULD be appended following
the request-specific parameters, properly separated by an "&"
character (ASCII code 38).
For example, the client makes the following HTTP request using
transport-layer security:
POST /resource HTTP/1.1
Host: server.example.com
Content-Type: application/x-www-form-urlencoded
oauth_token=vF9dft4qmT
The "application/x-www-form-urlencoded" method should typically only
be used in application contexts where participating browsers do not
have access to the "Authorization" request header field.
2.3. URI Query Parameter
When including the access token in the HTTP request URI, the client
adds the access token to the request URI query component as defined
by [RFC3986] using the "oauth_token" parameter.
For example, the client makes the following HTTP request using
transport-layer security:
GET /resource?oauth_token=vF9dft4qmT HTTP/1.1
Host: server.example.com
The HTTP request URI query can include other request-specific
parameters, in which case, the "oauth_token" parameters SHOULD be
appended following the request-specific parameters, properly
separated by an "&" character (ASCII code 38).
For example:
http://example.com/resource?x=y&oauth_token=vF9dft4qmT
Because of the Security Considerations (Section 3) associated with
the URI method, it SHOULD only be used if no other method is
feasible.
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3. Security Considerations
This section describes the relevant security threats regarding token
handling when using bearer tokens and describes how to mitigate these
threats.
3.1. Security Threats
The following list presents several common threats against protocols
utilizing some form of tokens. This list of threats is based on NIST
Special Publication 800-63 [NIST800-63]. Since this document builds
on the OAuth 2.0 specification, we exclude a discussion of threats
that are described there or in related documents.
Token manufacture/modification: An attacker may generate a bogus
token or modify the token contents (such as the authentication or
attribute statements) of an existing token, causing the resource
server to grant inappropriate access to the client. For example,
an attacker may modify the token to extend the validity period; a
malicious client may modify the assertion to gain access to
information that they should not be able to view.
Token disclosure: Tokens may contain authentication and attribute
statements that include sensitive information.
Token redirect: An attacker uses the token generated for consumption
by resource server to obtain access to another resource server.
Token replay: An attacker attempts to use a token that has already
been used with that resource server in the past.
3.2. Threat Mitigation
A large range of threats can be mitigated by protecting the contents
of the token by using a digital signature or a keyed message digest.
Alternatively, the contents of the token could be passed by reference
rather than by value (requiring a separate message exchange to
resolve the reference to the token contents).
This document does not specify the encoding or the contents of the
token; hence detailed recommendations for token integrity protection
are outside the scope of this document. We assume that the token
integrity protection is sufficient to prevent the token from being
modified.
To deal with token redirect, it is important for the authorization
server to include the identity of the intended recipients, namely a
single resource server (or a list of resource servers). Restricting
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the use of the token to a specific scope is also recommended.
To provide protection against token disclosure, confidentiality
protection is applied via TLS with a ciphersuite that offers
confidentiality protection. This requires that the communication
interaction between the client and the authorization server, as well
as the interaction between the client and the resource server,
utilize confidentiality protection. Since TLS is mandatory to
implement and to use with this specification, it is the preferred
approach for preventing token disclosure via the communication
channel. For those cases where the client is prevented from
observing the contents of the token, token encryption has to be
applied in addition to the usage of TLS protection.
To deal with token capture and replay, the following recommendations
are made: First, the lifetime of the token has to be limited by
putting a validity time field inside the protected part of the token.
Note that using short-lived (one hour or less) tokens significantly
reduces the impact of one of them being leaked. Second,
confidentiality protection of the exchanges between the client and
the authorization server and between the client and the resource
server MUST be applied. As a consequence, no eavesdropper along the
communication path is able to observe the token exchange.
Consequently, such an on-path adversary cannot replay the token.
Furthermore, when presenting the token to a resource server, the
client MUST verify the identity of that resource server. Note that
the client MUST validate the TLS certificate chain when making these
requests to protected resources. Presenting the token to an
unauthenticated and unauthorized resource server or failing to
validate the certificate chain will allow adversaries to steal the
token and gain unauthorized access to protected resources.
3.3. Summary of Recommendations
Safeguard bearer tokens Client implementations MUST ensure that
bearer tokens are not leaked to unintended parties, as they will
be able to use them to gain access to protected resources. This
is the primary security consideration when using bearer tokens
with OAuth and underlies all the more specific recommendations
that follow.
Validate SSL certificate chains The client must validate the TLS
certificate chain when making requests to protected resources.
Failing to do so may enable DNS hijacking attacks to steal the
token and gain unintended access.
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Always use TLS (https) Clients MUST always use TLS (https) when
making requests with bearer tokens. Failing to do so exposes the
token to numerous attacks that could give attackers unintended
access.
Don't store bearer tokens in cookies As cookies are generally sent
in the clear, implementations MUST NOT store bearer tokens within
them.
Issue short-lived bearer tokens Using short-lived (one hour or less)
bearer tokens can reduce the impact of one of them being leaked.
The User-Agent flow should only issue short lived access tokens.
Don't pass bearer tokens in page URLs Browsers, web servers, and
other software may not adequately secure URLs in the browser
history, web server logs, and other data structures. If bearer
tokens are passed in page URLs (typically as query string
parameters), attackers might be able to steal them from the
history data, logs, or other unsecured locations. Instead, pass
browser tokens in message bodies for which confidentiality
measures are taken.
4. IANA Considerations
4.1. OAuth Access Token Type Registration
This specification registers the following access token type in the
OAuth Access Token Type Registry.
4.1.1. The "OAuth2" OAuth Access Token Type
Type name:
OAuth2
Additional Token Request Parameters:
oauth_token
HTTP Authentication Scheme(s):
OAuth2
Change controller:
IETF
Specification document(s):
[[ this document ]]
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4.2. OAuth Parameters Registration
This specification registers the following parameters in the OAuth
Parameters Registry established by [OAuth2].
4.2.1. The "oauth_token" OAuth Parameter
Parameter name: oauth_token
Parameter usage location: Token requests
Change controller: IETF
Specification document(s): [[ this document ]]
Related information: None
5. References
5.1. Normative References
[I-D.ietf-httpbis-p1-messaging]
Fielding, R., Gettys, J., Mogul, J., Nielsen, H.,
Masinter, L., Leach, P., Berners-Lee, T., and J. Reschke,
"HTTP/1.1, part 1: URIs, Connections, and Message
Parsing", draft-ietf-httpbis-p1-messaging-09 (work in
progress), March 2010.
[OAuth2] Hammer-Lahav, E., Ed., Recordon, D., and D. Hardt, "The
OAuth 2.0 Protocol Framework", 2011.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
[RFC2617] Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S.,
Leach, P., Luotonen, A., and L. Stewart, "HTTP
Authentication: Basic and Digest Access Authentication",
RFC 2617, June 1999.
[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
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RFC 3986, January 2005.
[W3C.REC-html401-19991224]
Hors, A., Raggett, D., and I. Jacobs, "HTML 4.01
Specification", World Wide Web Consortium
Recommendation REC-html401-19991224, December 1999,
<http://www.w3.org/TR/1999/REC-html401-19991224>.
5.2. Informative References
[NIST800-63]
Burr, W., Dodson, D., Perlner, R., Polk, T., Gupta, S.,
and E. Nabbus, "NIST Special Publication 800-63-1,
INFORMATION SECURITY", December 2008.
Appendix A. Acknowledgements
The following people contributed to preliminary versions of this
document: Blaine Cook (BT), Brian Eaton (Google), Yaron Goland
(Microsoft), Brent Goldman (Facebook), Raffi Krikorian (Twitter),
Luke Shepard (Facebook), and Allen Tom (Yahoo!). The content and
concepts within are a product of the OAuth community, the WRAP
community, and the OAuth Working Group.
The OAuth Working Group has dozens of very active contributors who
proposed ideas and wording for this document, including: Michael
Adams, Andrew Arnott, Dirk Balfanz, Brian Campbell, Leah Culver, Bill
de hOra, Brian Ellin, Igor Faynberg, George Fletcher, Tim Freeman,
Evan Gilbert, Justin Hart, John Kemp, Eran Hammer-Lahav, Chasen Le
Hara, Michael B. Jones, Torsten Lodderstedt, Eve Maler, James Manger,
Laurence Miao, Chuck Mortimore, Anthony Nadalin, Justin Richer, Peter
Saint-Andre, Nat Sakimura, Rob Sayre, Marius Scurtescu, Naitik Shah,
Justin Smith, Jeremy Suriel, Christian Stuebner, Paul Tarjan, and
Franklin Tse.
Appendix B. Document History
[[ to be removed by RFC editor before publication as an RFC ]]
-02
o Incorporated feedback received on draft 01. Most changes were to
the security considerations section. No normative changes were
made. Specific changes included:
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o Changed terminology from "token reuse" to "token capture and
replay".
o Removed sentence "Encrypting the token contents is another
alternative" from the security considerations since it was
redundant and potentially confusing.
o Corrected some references to "resource server" to be
"authorization server" in the security considerations.
o Generalized security considerations language about obtaining
consent of the resource owner.
o Broadened scope of security considerations description for
recommendation "Don't pass bearer tokens in page URLs".
o Removed unused reference to OAuth 1.0.
o Updated reference to framework specification and updated David
Recordon's e-mail address.
o Removed security considerations text on authenticating clients.
o Registered the "OAuth2" OAuth access token type and "oauth_token"
parameter.
-01
o First public draft, which incorporates feedback received on -00
including enhanced Security Considerations content. This version
is intended to accompany OAuth 2.0 draft 11.
-00
o Initial draft based on preliminary version of OAuth 2.0 draft 11.
Authors' Addresses
Michael B. Jones
Microsoft
Email: mbj@microsoft.com
URI: http://self-issued.info/
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Dick Hardt
independent
Email: dick.hardt@gmail.com
URI: http://dickhardt.org/
David Recordon
Facebook
Email: dr@fb.com
URI: http://www.davidrecordon.com/
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