OAuth Working Group T. Lodderstedt, Ed.
Internet-Draft Deutsche Telekom AG
Intended status: Standards Track S. Dronia
Expires: August 16, 2013
M. Scurtescu
Google
February 12, 2013
Token Revocation
draft-ietf-oauth-revocation-05
Abstract
This document proposes an additional endpoint for OAuth authorization
servers, which allows clients to notify the authorization server that
a previously obtained refresh or access token is no longer needed.
This allows the authorization server to cleanup security credentials.
A revocation request will invalidate the actual token and, if
applicable, other tokens based on the same authorization grant.
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
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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Copyright Notice
Copyright (c) 2013 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Token Revocation . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Revocation Request . . . . . . . . . . . . . . . . . . . . 4
2.2. Revocation Response . . . . . . . . . . . . . . . . . . . . 5
2.2.1. Error Response . . . . . . . . . . . . . . . . . . . . 5
2.3. Cross-Origin Support . . . . . . . . . . . . . . . . . . . 5
3. Implementation Note . . . . . . . . . . . . . . . . . . . . . . 6
4. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 7
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7
6. Security Considerations . . . . . . . . . . . . . . . . . . . . 7
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8
7.1. Normative References . . . . . . . . . . . . . . . . . . . 8
7.2. Informative References . . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 9
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1. Introduction
The OAuth 2.0 core specification [RFC6749] defines several ways for a
client to obtain refresh and access tokens. This specification
supplements the core specification with a mechanism to revoke both
types of tokens. A token is a string representing an authorization
grant issued by the resource owner to the client. A revocation
request will invalidate the actual token and, if applicable, other
tokens based on the same authorization grant and the authorization
grant itself.
From an end-user's perspective, OAuth is often used to log into a
certain site or app. This revocation mechanism allows a client to
invalidate its tokens if the end-user logs out, changes identity, or
uninstalls the respective app. Notifying the authorization server
that the token is no longer needed allows the authorization server to
clean up data associated with that token (e.g. session data) and the
underlying authorization grant. This behavior prevents a situation
where there is still a valid authorization grant for a particular
client which the end user is not aware of. This way, token
revocation prevents abuse of abandoned tokens and facilitates a
better end-user experience since invalidated authorization grants
will no longer turn up in a list of authorization grants the
authorization server might present to the end-user.
2. Token Revocation
Implementations MUST support the revocation of refresh tokens and
SHOULD support the revocation of access tokens (see Implementation
Note).
The client requests the revocation of a particular token by making an
HTTP POST request to the token revocation endpoint URL. The token
endpoint URL MAY include a query component. The means to obtain the
location of the revocation endpoint is out of scope of this
specification. For example, the client developler may consult the
server's documentation or automatic discovery may be used. As this
endpoint is handling security credentials, the endpoint location MUST
be obtained from a trustworthy source.
Since requests to the token revocation endpoint result in the
transmission of plain text credentials in the HTTP request, the
authorization server MUST require the use of a transport-layer
security mechanism when sending requests to the token revocation
endpoints. The authorization server MUST support TLS 1.0
([RFC2246]), SHOULD support TLS 1.2 ([RFC5246]) and its future
replacements, and MAY support additional transport-layer mechanisms
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meeting its security requirements.
2.1. Revocation Request
The client constructs the request by including the following
parameters using the "application/x-www-form-urlencoded" format in
the HTTP request entity-body:
token REQUIRED. The token that the client wants to get revoked.
token_type_hint OPTIONAL. A hint about the type of the token
submitted for revocation. Clients MAY pass this parameter in
order to help the authorization server to optimize the token
lookup. If the server is unable to locate the token using
the given hint, it MUST extend its search accross all of its
supported token types. An authorization server MAY ignore
this parameter, particularly if it is able to detect the
token type automatically. This specification defines two
such values:
* access_token An Access Token as defined in [RFC6749]
section 1.4
* refresh_token A Refresh Token as defined in [RFC6749]
section 1.5
Specific implementations, profiles, and extensions of this
specification MAY define other values for this parameter.
The client also includes its authentication credentials as described
in Section 2.3. of [RFC6749].
For example, a client may request the revocation of a refresh token
with the following request (line breaks are for display purposes
only):
POST /revoke HTTP/1.1
Host: server.example.com
Content-Type: application/x-www-form-urlencoded
Authorization: Basic czZCaGRSa3F0MzpnWDFmQmF0M2JW
token=45ghiukldjahdnhzdauz&token_type_hint=refresh_token
The authorization server first validates the client credentials (in
case of a confidential client) and then verifies whether the token
was issued to the client making the revocation request. If this
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validation fails, the request is refused and the client is informed
of the error by the authorization server as described below.
In the next step, the authorization server invalidates the token.
The client MUST assume the revocation is immediate upon the return of
the request. The client MUST NOT use the token again after the
revocation.
Depending on the authorization server's revocation policy, the
revocation of a particular token may cause the revocation of related
tokens and the underlying authorization grant. If the particular
token is a refresh token and the authorization server supports the
revocation of access tokens, then the authorization server SHOULD
also invalidate all access tokens based on the same authorization
grant (see Implementation Note). If the token passed to the request
is an access token, the server MAY decide to revoke the respective
refresh token as well.
Note: A client compliant with [RFC6749] must be prepared to handle
unexpected token invalidation at any time. Independent of the
revocation mechanism specified in this document, resource owners may
decide to revoke authorization grants or the authorization server may
invalidate tokens in order to mitigate security threats. Thus having
different server policies with respect to cascading the revocation of
tokens should not pose interoperability problems.
2.2. Revocation Response
The authorization server indicates a successful processing of the
request by a HTTP status code 200. This also applies if the client
submits an invalid token to the revocation endpoint.
2.2.1. Error Response
The error presentation conforms to the defintion in section 5.2 of
[RFC6749]. The following additional error code is defined for the
token revocation endpoint:
unsupported_token_type The authorization server does not support the
revocation of the presented token type. I.e. the client
tried to revoke an access token on a server not supporting
this feature.
2.3. Cross-Origin Support
The revocation end-point MAY support CORS [W3C.WD-cors-20120403] if
it is aimed at use in combination with user-agent-based applications.
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In addition, for interoperability with legacy user-agents, it MAY
also offer JSONP [jsonp] by allowing GET requests with an additional
parameter:
callback OPTIONAL. The qualified name of a JavaScript function.
For example, a client may request the revocation of an access token
with the following request (line breaks are for display purposes
only):
https://example.com/revoke?token=agabcdefddddafdd&
callback=package.myCallback
Successful response:
package.myCallback();
Error response:
package.myCallback({"error":"unsupported_token_type"});
Clients should be aware that when relying on JSONP, a malicious
revocation end-point may attempt to inject malicious code into the
client.
3. Implementation Note
OAuth 2.0 allows deployment flexibility with respect to the style of
access tokens. The access tokens may be self-contained so that an
resource server needs no further interaction with an authorization
server issuing these tokens to perform an authorization decision of
the client requesting access to a protected resource. A system
design may, however, instead use access tokens that are handles
referring to authorization data stored at the authorization server.
This consequently requires a resource server to issue a request to
the respective authorization server to retrieve the content of the
access token every time a client presents an access token.
While these are not the only options they illustrate the implications
for revocation. In the latter case the authorization server is able
to revoke an access token previously issued to a client when the
resource server relays a received access token. In the former case
some (currently non-standardized) backend interaction between the
authorization server and the resource server may be used when
immediate access token revocation is desired. Another design
alternative is to issue short-lived access tokens, which can be
refreshed at any time using the corresponding refresh tokens. This
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allows the authorization server to impose a limit on the time revoked
access tokens are in use.
Which approach of token revocation is chosen will depend on the
overall system design and on the application service provider's risk
analysis. The cost of revocation in terms of required state and
communication overhead is ultimately the result of the desired
security properties.
4. Acknowledgements
We would like to thank Peter, Mauritius, Amanda Anganes, Mark Wubben,
Hannes Tschofenig, Michiel de Jong, Doug Foiles, Paul Madsen, George
Fletcher, Sebastian Ebling, Christian Stuebner, Brian Campbell, Igor
Faynberg, Lukas Rosenstock, and Justin Richer for their valuable
feedback.
5. IANA Considerations
This draft includes no request to IANA.
6. Security Considerations
If the authorization server does not support access token revocation,
access tokens will not be immediately invalidated when the
corresponding refresh token is revoked. Deployments MUST take this
into account when conducting their security risk analysis.
Cleaning up tokens using revocation contributes to overall security
and privacy since it reduces the likelihood for abuse of abandoned
tokens. This specification in general does not intend to provide
countermeasures against token theft and abuse. For a discussion of
respective threats and countermeasures, consult the security
considerations given in section 10 of the OAuth core specification
[RFC6749] and the OAuth threat model document
[I-D.ietf-oauth-v2-threatmodel].
Malicious clients could attempt to use the new endpoint to launch
denial of service attacks on the authorization server. Appropriate
countermeasures, which should be in place for the token endpoint as
well, MUST be applied to the revocation endpoint (see
[I-D.ietf-oauth-v2-threatmodel], section 4.4.1.11).
A malicious client may attempt to guess valid tokens on this endpoint
by making revocation requests against potential token strings.
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According to this specification, a client's request must contain a
valid client_id, in the case of a public client, or valid client
credentials, in the case of a confidential client. The token being
revoked must also belong to the requesting client. If an attacker is
able to successfully guess a public client's client_id and one of
their tokens, or a private client's credentials and one of their
tokens, they could do much worse damage by using the token elsewhere
than by revoking it. If they chose to revoke the token, the
legitimate client will lose its authorization grant and will need to
prompt the user again. No further damage is done and the guessed
token is now worthless.
Since the revocation endpoint is handling security credentials,
clients shall obtain its location from a trustworthy source only.
Otherwise, an attacker could capture valid security tokens by
utilizing a counterfeit revocation endpoint.
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.
[RFC2246] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
RFC 2246, January 1999.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
[RFC6749] Hardt, D., "The OAuth 2.0 Authorization Framework",
RFC 6749, October 2012.
7.2. Informative References
[I-D.ietf-oauth-v2-threatmodel]
Lodderstedt, T., McGloin, M., and P. Hunt, "OAuth 2.0
Threat Model and Security Considerations",
draft-ietf-oauth-v2-threatmodel-08 (work in progress),
October 2012.
[W3C.WD-cors-20120403]
Kesteren, A., "Cross-Origin Resource Sharing", World Wide
Web Consortium LastCall WD-cors-20120403, April 2012,
<http://www.w3.org/TR/2012/WD-cors-20120403>.
[jsonp] Ippolito, B., "Remote JSON - JSONP", December 2005.
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Authors' Addresses
Torsten Lodderstedt (editor)
Deutsche Telekom AG
Email: torsten@lodderstedt.net
Stefanie Dronia
Email: sdronia@gmx.de
Marius Scurtescu
Google
Email: mscurtescu@google.com
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