Network Working Group M. Jones, Ed.
Internet-Draft Microsoft
Intended status: Standards Track D. Hardt
Expires: June 4, 2011 independent
D. Recordon
Facebook
December 1, 2010
The OAuth 2.0 Protocol: Bearer Tokens
draft-ietf-oauth-v2-bearer-01
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
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This Internet-Draft will expire on June 4, 2011.
Copyright Notice
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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
5. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9
5.1. Normative References . . . . . . . . . . . . . . . . . . . 9
5.2. Informative References . . . . . . . . . . . . . . . . . . 10
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 10
Appendix B. Document History . . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 11
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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 The OAuth 2.0 Protocol [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, 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 reuse: An attacker attempts to use a token that has already
been used once 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. Encrypting the token contents is
another alternative. 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 rare 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 reuse, 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, the resource server MUST ensure that it only hands out
tokens to clients it has authenticated first and authorized. For
this purpose, the client MUST be authenticated and authorized by the
resource server. The authorization server MUST also receive a
confirmation (the consent of the resource owner) prior to providing a
token to the client. 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.
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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.
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 may not adequately
secure URLs in the browser history. If bearer tokens are passed
in page URLs (typically as query string parameters), attackers
might be able to steal them from the history data. Instead, pass
browser tokens in message bodies for which confidentiality
measures are taken.
4. IANA Considerations
This document neither establishes new IANA registries nor adds new
values to existing registries.
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", 2010.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
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[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,
RFC 3986, January 2005.
[RFC5849] Hammer-Lahav, E., "The OAuth 1.0 Protocol", RFC 5849,
April 2010.
[W3C.REC-html401-19991224]
Raggett, D., Hors, A., 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, 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: [[ If your
name is missing or you think someone should be added here, please
send Mike Jones a note - don't be shy! ]]
Michael Adams, Andrew Arnott, Dirk Balfanz, Brian Campbell, Leah
Culver, Bill de hOra, Brian Ellin, Igor Faynberg, George Fletcher,
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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, 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 ]]
-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 (editor)
Microsoft
Email: mbj@microsoft.com
URI: http://self-issued.info/
Dick Hardt
independent
Email: dick.hardt@gmail.com
URI: http://dickhardt.org/
David Recordon
Facebook
Email: davidrecordon@facebook.com
URI: http://www.davidrecordon.com/
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