wdiff draft-ietf-oauth-v2-16.txt draft-ietf-oauth-v2-17.txt

Network Working Group E. Hammer-Lahav, Ed.
Internet-Draft Yahoo!
Obsoletes: 5849 (if approved) D. Recordon
Intended status: Standards Track Facebook
Expires: November 20, 2011 January 9, 2012 D. Hardt
Microsoft
May 19,
July 8, 2011

The OAuth 2.0 Authorization Protocol
draft-ietf-oauth-v2-16
draft-ietf-oauth-v2-17

Abstract

The OAuth 2.0 authorization protocol enables a third-party
application to obtain limited access to an HTTP service, either on
behalf of an end-user a resource owner by orchestrating an approval interaction
between the end-user resource owner and the HTTP service, or by allowing the third-
party
third-party application to obtain access on its own behalf.

Status of this Memo

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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 5
1.1. Roles . . . . . . . . . . . . . . . . . . . . . . . . . . 5 6
1.2. Protocol Flow . . . . . . . . . . . . . . . . . . . . . . 5 6
1.3. Access Token . . . . . . . . . . . . . . . . . . . . . . 6 7
1.4. Authorization Grant . . . . . . . . . . . . . . . . . . . 7
1.5. Refresh Token 8
1.4.1. Authorization Code . . . . . . . . . . . . . . . . . . 8
1.4.2. Implicit . . . . . . . . . . . . . . . . . . . . . . . 8
1.4.3. Resource Owner Password Credentials . . . . . . . . . 9
1.6. Document Structure
1.4.4. Client Credentials . . . . . . . . . . . . . . . . . . 9
1.4.5. Extensions . 11
1.7. . . . . . . . . . . . . . . . . . . . . . 9
1.5. Refresh Token . . . . . . . . . . . . . . . . . . . . . . 9
1.6. Notational Conventions . . . . . . . . . . . . . . . . . 11
2. Protocol Endpoints . Client Registration . . . . . . . . . . . . . . . . . . . . . 11
2.1. Authorization Endpoint Client Types . . . . . . . . . . . . . . . . . . . . . . 12
2.2. Token Endpoint Registration Requirements . . . . . . . . . . . . . . . . 12
2.3. Client Identifier . . . . . 13
3. . . . . . . . . . . . . . . . 12
2.4. Client Authentication . . . . . . . . . . . . . . . . . . 12
2.4.1. Client Password . . . . . . . . . . . . . . . . . . . 13
2.4.2. Other Authentication Methods . . . . . . . . . . . . . 14
2.5. Unregistered Clients . . . . . . . . . . . . . . . . . . 14
3. Protocol Endpoints . . . . . . . . . . . . . . . . . . . . . . 14
3.1. Client Password Authentication Authorization Endpoint . . . . . . . . . . . . . . . . . 14
3.1.1. Response Type . . . . . . . . . . . . . . . . . . . . 15
3.1.2. Redirection URI . . . . . . . . . . . . . . . . . . . 16
3.2. Other Token Endpoint . . . . . . . . . . . . . . . . . . . . . 18
3.2.1. Client Authentication Methods . . . . . . . . . . . 16 . . . . . 18
4. Obtaining Authorization . . . . . . . . . . . . . . . . . . . 16 19
4.1. Authorization Code . . . . . . . . . . . . . . . . . . . 16 19
4.1.1. Authorization Request . . . . . . . . . . . . . . . . 21
4.1.2. Authorization Response . . . . . . . . . . . . . . . . 22
4.1.3. Access Token Request . . . . . . . . . . . . . . . . . 24
4.1.4. Access Token Response . . . . . . . . . . . . . . . . 25
4.2. Implicit Grant . . . . . . . . . . . . . . . . . . . . . 22 25
4.2.1. Authorization Request . . . . . . . . . . . . . . . . 28
4.2.2. Access Token Response . . . . . . . . . . . . . . . . 29
4.3. Resource Owner Password Credentials . . . . . . . . . . . 28 32
4.3.1. Authorization Request and Response . . . . . . . . . . 33
4.3.2. Access Token Request . . . . . . . . . . . . . . . . . 33
4.3.3. Access Token Response . . . . . . . . . . . . . . . . 34
4.4. Client Credentials . . . . . . . . . . . . . . . . . . . 30 34
4.4.1. Authorization Request and Response . . . . . . . . . . 35
4.4.2. Access Token Request . . . . . . . . . . . . . . . . . 35
4.4.3. Access Token Response . . . . . . . . . . . . . . . . 36
4.5. Extensions . . . . . . . . . . . . . . . . . . . . . . . 32 36
5. Issuing an Access Token . . . . . . . . . . . . . . . . . . . 33 37
5.1. Successful Response . . . . . . . . . . . . . . . . . . . 33 37
5.2. Error Response . . . . . . . . . . . . . . . . . . . . . 34 39
6. Refreshing an Access Token . . . . . . . . . . . . . . . . . . 36 40
7. Accessing Protected Resources . . . . . . . . . . . . . . . . 37 41
7.1. Access Token Types . . . . . . . . . . . . . . . . . . . 38 42
8. Extensibility . . . . . . . . . . . . . . . . . . . . . . . . 38 43
8.1. Defining Access Token Types . . . . . . . . . . . . . . . 38 43
8.2. Defining New Endpoint Parameters . . . . . . . . . . . . 39 43
8.3. Defining New Authorization Grant Types . . . . . . . . . 39 44
8.4. Defining New Authorization Endpoint Response Types . . . 44
8.5. Defining Additional Error Codes . . . . . . . . . . . . . 39 44
9. Native Applications . . . . . . . . . . . . . . . . . . . . . 40 45
10. Security Considerations . . . . . . . . . . . . . . . . . . . 41 46
10.1. Client Authentication . . . . . . . . . . . . . . . . . . 42 47
10.2. Client Impersonation . . . . . . . . . . . . . . . . . . 42 47
10.3. Access Token Credentials . . . . . . . . . . . . . . . . 43 48
10.4. Refresh Tokens . . . . . . . . . . . . . . . . . . . . . 43 48
10.5. Request Confidentiality . . . . . . . . . . . . . . . . . 44 49
10.6. Endpoints Authenticity . . . . . . . . . . . . . . . . . 44 49
10.7. Credentials Guessing Attacks . . . . . . . . . . . . . . 44 49
10.8. Phishing Attacks . . . . . . . . . . . . . . . . . . . . 44 49
10.9. Authorization Codes . . . . . . . . . . . . . . . . . . . 45 50
10.10. Session Fixation . . . . . Authorization Code Leakage . . . . . . . . . . . . . . . 45 50
10.11. Redirection URI Validation . . . . . . . . . . . . . . . 46 51
10.12. Resource Owner Password Credentials . . . . . . . . . . . 46 51
10.13. XSRF/CSRF Prevention Cross-Site Request Forgery . . . . . . . . . . . . . . . 51
10.14. Clickjacking . . . 46 . . . . . . . . . . . . . . . . . . . 52
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 46 52
11.1. The OAuth Access Token Type Registry . . . . . . . . . . 46 52
11.1.1. Registration Template . . . . . . . . . . . . . . . . 53
11.2. The OAuth Parameters Registry . . . . . . . . . . . . . . 48 53
11.2.1. Registration Template . . . . . . . . . . . . . . . . 54
11.2.2. Initial Registry Contents . . . . . . . . . . . . . . 54
11.3. The OAuth Authorization Endpoint Response Type
Registry . . . . . . . . . . . . . . . . . . . . . . . . 56
11.3.1. Registration Template . . . . . . . . . . . . . . . . 57
11.3.2. Initial Registry Contents . . . . . . . . . . . . . . 57
11.4. The OAuth Extensions Error Registry . . . . . . . . . . . 51 57
11.4.1. Registration Template . . . . . . . . . . . . . . . . 58
12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 52 59
Appendix A. Editor's Notes . . . . . . . . . . . . . . . . . . . 53 59
13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 53 60
13.1. Normative References . . . . . . . . . . . . . . . . . . 53 60
13.2. Informative References . . . . . . . . . . . . . . . . . 54 61

Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 55 62

1. Introduction

In the traditional client-server authentication model, the client
accesses a protected resource on the server by authenticating with
the server using the resource owner's credentials. In order to
provide third-party applications access to protected resources, the
resource owner shares its credentials with the third-party. This
creates several problems and limitations:

o Third-party applications are required to store the resource- resource
owner's credentials for future use, typically a password in clear-
text.
o Servers are required to support password authentication, despite
the security weaknesses created by passwords.
o Third-party applications gain overly broad access to the resource- resource
owner's protected resources, leaving resource owners without any
ability to restrict duration or access to a limited subset of
resources.
o Resource owners cannot revoke access to an individual third-party
without revoking access to all third-parties, and must do so by
changing their password.
o Compromise of any third-party application results in compromise of
the end-user's password and all of the data protected by that
password.

OAuth addresses these issues by introducing an authorization layer
and separating the role of the client from that of the resource
owner. In OAuth, the client requests access to resources controlled
by the resource owner and hosted by the resource server, and is
issued a different set of credentials than those of the resource
owner.

Instead of using the resource owner's credentials to access protected
resources, the client obtains an access token - a string denoting a
specific scope, duration, and other access attributes. Access tokens
are issued to third-party 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.

For example, a web an end-user (resource owner) can grant a printing
service (client) access to her protected photos stored at a photo
sharing service (resource server), without sharing her username and
password with the printing service. Instead, she authenticates
directly with a server trusted by the photo sharing service
(authorization server) which issues the printing service delegation-
specific credentials (access token).

This specification is designed for use with HTTP [RFC2616]. The use
of OAuth with any transport protocol other than HTTP is undefined.

1.1. Roles

OAuth includes four roles working together to grant and provide
access to protected resources - access restricted resources which
require authentication to access: requiring
authentication:

resource owner
An entity capable of granting access to a protected resource.
When the resource owner is a person, it is referred to as an end-
user. (e.g.
end-user).
resource server
The server hosting the protected resources, capable of accepting
and responding to protected resource requests using access tokens.
client
An application making protected resource requests on behalf of the
resource owner and with its authorization.
authorization server
The server issuing access tokens to the client after successfully
authenticating the resource owner and obtaining authorization.

The interaction between the authorization server and resource server
is beyond the scope of this specification. The authorization server
may be the same server as the resource server or a separate entity.
A single authorization server may issue access tokens accepted by
multiple resource servers.

1.2. Protocol Flow

When interacting with the authorization server, the client identifies
itself using a set of client credentials which include a client
identifier and other authentication attributes. The means through
which the client obtains its credentials are beyond the scope of this
specification, but typically involve registration with the
authorization server.

+--------+ +---------------+
| |--(A)- Authorization Request ->| Resource |
| | | Owner |
| |<-(B)-- Authorization Grant ---| |
| | +---------------+
| |
| | Authorization Grant & +---------------+
| |--(C)--- Client Credentials |--(C)-- Authorization Grant -->| 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 interaction
between the four roles and includes the following steps:

(A) The client requests authorization from the resource owner. The
authorization request can be made directly to the resource owner
(as shown), or preferably indirectly via an intermediary such as
an authorization server.
(B) The client receives an authorization grant which represents the
authorization provided by the resource owner. The authorization
grant type depends on the method used by the client and
supported by the authorization server to obtain it.
(C) The client requests an access token by authenticating with the
authorization server using its client credentials (prearranged
between the client and authorization server) and presenting the authorization grant.
(D) The authorization server validates authenticates the client credentials and validates
the authorization grant, and if valid issues an access token.
(E) The client requests the protected resource from the resource
server and authenticates by presenting the access token.
(F) The resource server validates the access token, and if valid,
serves the request.

1.3. Access Token

Access tokens are credentials used to access protected resources. An
access token is a string representing an authorization issued to the
client. The string is usually opaque to the client. Tokens
represent specific scopes and durations of access, granted by the
resource owner, and enforced by the resource server and authorization
server.

The token may denote an identifier used to retrieve the authorization
information, or self-contain the authorization information in a
verifiable manner (i.e. a token string consisting of some data and a
signature). Additional authentication credentials credentials, which are beyond
the scope of this specification, may be required in order for the
client to use a token.

The access token provides an abstraction layer, replacing different
authorization constructs (e.g. username and password) with a single
token understood by the resource server. This abstraction enables
issuing access tokens more restrictive than the authorization grant
used to obtain them, as well as removing the resource server's need
to understand a wide range of authentication methods.

Access tokens can have different formats, structures, and methods of
utilization (e.g. cryptographic properties) based on the resource
server security requirements. Access token attributes and the
methods used to access protected resources are beyond the scope of
this specification and are defined by companion specifications.

1.4. Authorization Grant

An authorization grant is a general term used to describe the
intermediate credentials representing the resource owner
authorization (to access its protected resources), and serves as an
abstraction layer. An authorization grant is used by the client to
obtain an access token.

This specification defines four grant types: authorization code,
implicit, resource owner password credentials, and client
credentials, as well as an extensibility mechanism for defining
additional types.

1.4.1. Authorization Code

The authorization code is obtained by using an authorization server
as an intermediary between the client and resource owner. Instead of
requesting authorization directly from the resource owner, the client
directs the resource owner to an authorization server (via its user-
agent as defined in [RFC2616]), which in turn directs the resource
owner back to the client with the authorization code.

Before directing the resource owner back to the client with the
authorization code, the authorization server authenticates the
resource owner and obtains authorization. Because the resource owner
only authenticates with the authorization server, the resource
owner's credentials are never shared with the client.

The authorization code provides a few important security benefits
such as the ability to authenticate the client and issuing the access
token directly to the client without potentially exposing it to
others, including the resource owner.

1.4.2. Implicit

When

The authorization grant is implicit when an access token is issued to
the client directly as the result of the resource owner
authorization, without an intermediary
authorization grant using intermediate credentials (such as an
authorization code), the grant is
considered implicit. code).

When issuing an implicit grant, the authorization server cannot
verify does not
authenticate the client and the client identity of is verified via the client, and
redirection URI used to deliver the access token to the client. The
access token may be exposed to the resource owner or other
applications with access to the resource owner's user-agent.

Implicit grants improve the responsiveness and efficiency of some
clients (such as a client implemented as an in-browser application)
since it reduces the number of round trips required to obtain an
access token. However, this convenience should be weighted against
the security implications of using implicit grants, especially when
the authorization code grant type is available.

1.4.3. Resource Owner Password Credentials

The resource owner password credentials (e.g. a username and
password) can be used directly as an authorization grant to obtain an
access token. The credentials should only be used when there is a
high degree of trust between the resource owner and the client (e.g.
its computer operating system or a highly privileged application),
and when other authorization grant types are not available (such as
an authorization code).

Even though this grant type requires direct client access to the
resource owner credentials, the resource owner credentials are used
for a single request and are exchanged for an access token. Unlike
the HTTP Basic authentication scheme defined in [RFC2617], this grant
type (when combined with a refresh token) eliminates the need for the
client to store the resource-owner resource owner credentials for future use.

1.4.4. Client Credentials

The client credentials (or other forms of client authentication) can
be used as an authorization grant when the authorization scope is
limited to the protected resources under the control of the client,
or to protected resources previously arranged with the authorization
server. Client credentials are used as an authorization grant
typically when the client is acting on its own behalf (the client is
also the resource owner).

1.4.5. Extensions

Additional grant types may be defined to provide a bridge between
OAuth and other protocols. For example,
[I-D.ietf-oauth-saml2-bearer] defines a SAML 2.0
[OASIS.saml-core-2.0-os] bearer assertion grant type, which can be

1.5. Refresh Token

Refresh tokens are credentials used to obtain an access token.

1.5. tokens. Refresh Token

A refresh token is optionally
tokens are issued to the client by the authorization server and are
used to
the client together with an obtain a new access token. The client can use the
refresh token to request another when the current access token based on the same
authorization, without having to involve the resource owner again,
becomes invalid or expires, or
having to retain the original authorization grant used to obtain additional access tokens
with identical or narrower scope (access tokens may have a shorter
lifetime and fewer permissions than authorized by the
initial resource
owner). Issuing a refresh token is optional and is included when
issuing an access token.

A refresh token is a string representing the authorization granted to
the client by the resource owner. The string is usually opaque to
the client. The token may denote denotes an identifier used to retrieve the
authorization information, or self-contain the authorization
information in a verifiable manner. The information. Unlike access tokens, refresh token is bound to
the client it was issued to, tokens are
intended for use only with authorization servers and its usage requires client
authentication.

The refresh token can be used are never sent
to obtain a new access token when the
current access token expires (access tokens may have a shorter
lifetime than authorized by the resource owner), no longer valid, or
to obtain additional access tokens with identical or narrower scope. servers.

+--------+ Authorization Grant & +---------------+
| |--(A)-------- Client Credentials |--(A)------- Authorization Grant --------->| |
| | | |
| |<-(B)----------- Access Token -------------| |
| | & Refresh Token | |
| | | |
| | +----------+ | |
| |--(C)---- Access Token ---->| | | |
| | | | | |
| |<-(D)- Protected Resource --| Resource | | Authorization |
| Client | | Server | | Server |
| |--(E)---- Access Token ---->| | | |
| | | | | |
| |<-(F)- Invalid Token Error -| | | |
| | +----------+ | |
| | | |
| | |--(G)----------- Refresh Token & | |
| |--(G)-------- Client Credentials --------->| ----------->| |
| | | |
| |<-(H)----------- Access Token -------------| |
+--------+ & Optional Refresh Token +---------------+

Figure 2: Refreshing an Expired Access Token

The flow illustrated in Figure 2 includes the following steps:

(A) The client requests an access token by authenticating with the
authorization server using its client credentials, server, and presenting an authorization grant.
(B) The authorization server validates authenticates the client credentials and validates
the authorization grant, and if valid issues an access token and
a refresh token.
(C) The client makes a protected resource requests to the resource
server by presenting the access token.
(D) The resource server validates the access token, and if valid,
serves the request.

(E) Steps (C) and (D) repeat until the access token expires. If the
client knows the access token expired, it skips to step (G),
otherwise it makes another protected resource request.
(F) Since the access token is invalid, the resource server returns
an invalid token error.
(G) The client requests a new access token by authenticating with
the authorization server using its client credentials, and presenting the refresh token.
(H) The authorization server validates authenticates the client credentials and validates
the refresh token, and if valid issues a new access token (and
optionally, a new refresh token).

1.6. Document Structure

This specification is organized into the following sections:

o Section 2 - describes the two endpoints used to obtain and utilize
the various authorization grant types.
o Section 3 - describes client identification and authentication in
general, and provides one such method for client authentication
using password credentials.
o Section 4 - describes the complete flow for each authorization
grant type, including requesting authorization, authorization
response, and requesting an access token.
o Section 5 - describes the common access token response used for
all non-implicit authorization grant types.
o Section 6 - describes the use of a refresh token to obtain
additional access tokens using the same resource owner
authorization.
o Section 7 - describes how access tokens are used to access
protected resources.
o Section 8 - describes how to extend certain elements of the
protocol.
o Section 9 - provides a security analysis of the protocol.

1.7. Notational Conventions

The key words 'MUST', 'MUST NOT', 'REQUIRED', 'SHALL', 'SHALL NOT',
'SHOULD', 'SHOULD NOT', 'RECOMMENDED', 'MAY', and 'OPTIONAL' in this
specification are to be interpreted as described in [RFC2119].

This specification uses the Augmented Backus-Naur Form (ABNF)
notation of [RFC5234].

Certain security-related terms are to be understood in the sense
defined in [RFC4949]. These terms include, but are not limited to,
'attack', 'authentication', 'authorization', 'certificate',
'confidentiality', 'credential', 'encryption', 'identity', 'sign',
'signature', 'trust', 'validate', and 'verify'.

Unless otherwise noted, all the protocol parameter names and values
are case sensitive.

2. Client Registration

[[ Pending Consensus ]]

Before initiating the protocol, the client registers with the
authorization server. The means through which the client registers
with the authorization server are beyond the scope of this
specification, but typically involve human interaction with an HTML
registration form.

Client registration does not require a direct interaction between the
client and the authorization server. When supported by the
authorization server, registration can rely on other means for
establishing trust and obtaining the required client properties (e.g.
redirection URI, client type). For example, registration can be
accomplished using a self-issued or third-party-issued assertion, or
by the authorization server performing client discovery using a
trusted channel.

2.1. Client Types

OAuth defines two client types, based on their ability to
authenticate securely with the authorization server (i.e. ability to
maintain the confidentiality of their client credentials):

private
Clients capable of maintaining the confidentiality of their
credentials (e.g. client implemented on a secure server with
restricted access to the client credentials), or capable of secure
client authentication using other means.
public
Clients incapable of maintaining the confidentiality of their
credentials (e.g. clients executing on the resource owner's device
such as an installed native application or a user-agent-based
application), and incapable of secure client authentication via
any other mean.

The client type designation is based on the authorization server's
definition of secure authentication and its acceptable exposure
levels of client credentials.

2.2. Registration Requirements

When registering a client, the client developer MUST specify:

o the client type as described in Section 2.1,
o the client redirection URIs as described in Section 3.1.2, and
o any other information required by the authorization server (e.g.
application name, website, description, logo image, the acceptance
of legal terms).

2.3. Client Identifier

The authorization server issues the registered client a client
identifier - a unique string representing the registration
information provided by the client. The client identifier is not a
secret, it is exposed to the resource owner, and cannot not be used
alone for client authentication.

2.4. Client Authentication

In addition, the client and authorization server establish a client
authentication method suitable for the client type and security
requirements of the authorization server. The authorization server
MAY accept any form of client authentication meeting its security
requirements.

Private clients are typically issued (or establish) a set of client
credentials used for authenticating with the authorization server
(e.g. password, public/private key pair).

The authorization server SHOULD NOT make assumptions about the client
type or accept the type information provided without establishing
trust with the client or its developer. The authorization server
MUST NOT rely on client authentication performed by public clients.

The client MUST NOT use more than one authentication method in each
request.

2.4.1. Client Password

Clients in possession of a client password MAY use the HTTP Basic
authentication scheme as defined in [RFC2617] to authenticate with
the authorization server. The client identifier is used as the
username, and the client password is used as the password.

For example (extra line breaks are for display purposes only):

Authorization: Basic czZCaGRSa3F0MzpnWDFmQmF0M2JW

Alternatively, the authorization server MAY allow including the
client credentials in the request body using the following
parameters:

client_id
REQUIRED. The client identifier issued to the client during
the registration process described by Section 2.3.
client_secret
REQUIRED. The client secret.

Including the client credentials in the request body using the two
parameters is NOT RECOMMENDED, and should be limited to clients
unable to directly utilize the HTTP Basic authentication scheme (or
other password-based HTTP authentication schemes).

For example, requesting to refresh an access token (Section 6) using
the body parameters (extra line breaks are for display purposes
only):

POST /token HTTP/1.1
Host: server.example.com
Content-Type: application/x-www-form-urlencoded;charset=UTF-8

grant_type=refresh_token&refresh_token=tGzv3JOkF0XG5Qx2TlKWIA
&client_id=s6BhdRkqt3&client_secret=7Fjfp0ZBr1KtDRbnfVdmIw

The authorization server MUST require the use of a transport-layer
security mechanism when sending requests to the token endpoint, as
requests using this authentication method result in the transmission
of clear-text credentials.

2.4.2. Other Authentication Methods

The authorization server MAY support any suitable HTTP authentication
scheme matching its security requirements. When using other
authentication methods, the authorization server MUST define a
mapping between the client identifier (registration record) and
authentication scheme.

2.5. Unregistered Clients

This specification does not exclude the use of unregistered clients.
However, the use with such clients is beyond the scope of this
specification, and requires additional security analysis and review
of its interoperability impact.

3. Protocol Endpoints

The authorization process utilizes two endpoints (HTTP resources):

o Authorization endpoint - used to obtain authorization from the
resource owner via user-agent redirection.
o Token endpoint - used to exchange an authorization grant for an
access token, typically with client authentication.

Not every authorization grant type utilizes both endpoints.
Extension grant types MAY define additional endpoints as needed.

2.1.

3.1. Authorization Endpoint

The authorization endpoint is used to interact with the resource
owner and obtain authorization which is expressed explicitly as an
authorization code (exchanged (later exchanged for an access token), or
implicitly by direct issuance of an access token.

The authorization server MUST first verify the identity of the
resource owner. The way in which the authorization server
authenticates the resource owner (e.g. username and password login,
session cookies) is beyond the scope of this specification.

The means through which the client obtains the location of the
authorization endpoint are beyond the scope of this specification but
the location is typically provided in the service documentation. The
endpoint URI MAY include a query component as defined by [RFC3986]
section 3, which MUST be retained when adding additional query
parameters. The endpoint URI MUST NOT include a fragment component.

Since requests to the authorization endpoint result in user
authentication and the transmission of clear-text credentials (in the
HTTP response), the authorization server MUST require the use of a
transport-layer security mechanism when sending requests to the
authorization endpoint. The authorization server MUST support TLS
1.2 as defined in [RFC5246], and MAY support additional transport-
layer mechanisms meeting its security requirements.

The authorization server MUST support the use of the HTTP "GET"
method [RFC2616] for the authorization endpoint, and MAY support the
use of the "POST" method as well.

The REQUIRED "response_type" request parameter is used to identify
which grant type the client is requesting: authorization code or
implicit, described in Section 4.1.1 and Section 4.2.1 respectively.
If the request is missing the "response_type" parameter, the
authorization server SHOULD return an error response as described in
Section 4.1.2.1.

Parameters sent without a value MUST be treated as if they were
omitted from the request. The authorization server SHOULD ignore
unrecognized request parameters. Request and response parameters
MUST NOT repeat be included more than once,
unless noted otherwise.

2.1.1. Redirection URI once.

3.1.1. Response Type

The authorization endpoint is used by the authorization code grant
type and implicit grant type flows. The client directs informs the resource owner's user-agent to
authorization server of the desired grant type using the following
parameter:

response_type
REQUIRED. The value MUST be one of "code" for requesting an
authorization endpoint and includes code as described by Section 4.1.1, "token" for
requesting an access token (implicit grant) as described by
Section 4.2.1, or a redirection registered extension value as described by
Section 8.4.

If an authorization request is missing the "response_type" parameter,
the authorization server SHOULD return an error response as described
in Section 4.1.2.1.

3.1.2. Redirection URI to which

[[ Pending Consensus ]]

After completing its interaction with the resource owner, the
authorization server will redirect directs the resource owner's user-agent back once
authorization has been obtained (or denied). to
the client. The client MAY omit authorization server redirects the user-agent to the
client's redirection URI if one has been previously established between with the client and
authorization server via other means, such as during the client registration process.

The redirection URI MUST be an absolute URI and as defined by [RFC3986]
section 4.3, MAY include a query
component, component which MUST be retained by
the authorization server when adding additional query parameters. parameters, and
MUST NOT include a fragment component.

3.1.2.1. Endpoint Confidentiality

If a redirection request will result in the transmission of an
authorization code or access token over an open network (between the
resource owner's user-agent and the client), the client SHOULD
require the use of a transport-layer security mechanism.

Lack of transport-layer security can have a severe impact on the
security of the client and the protected resources it is authorized
to access. The use of transport-layer security is particularly
critical when the authorization process is used as a form of
delegated end-user authentication by the client (e.g. third-party
sign-in service).

3.1.2.2. Registration Requirements

The authorization server MUST require public clients to register
their redirection URI, MUST require all clients to register their
redirection URI prior to utilizing the implicit grant type, and
SHOULD require all clients to register their redirection URI prior to
utilizing the authorization code grant type.

The authorization server SHOULD require the client to pre-register
their provide the
complete redirection URI or at least certain components such as (the client MAY use the "state" request
parameter to achieve per-request customization). The authorization
server MAY allow the client to register multiple redirection URIs.
If requiring the registration of the complete redirection URI is not
possible, the authorization server SHOULD require the registration of
the URI scheme, host, port authority, and path.

3.1.2.3. Dynamic Configuration

If a multiple redirection URIs have been registered, if only part of
the redirection URI was has been registered, or if no redirection URI has
been registered, the client MUST include a redirection URI with the
authorization request using the "redirect_uri" request parameter.

When a redirection URI is included in an authorization request, the
authorization server MUST compare any redirection URI and match the value received
against at least one of the registered redirection URIs (or URI
components) as defined in [RFC3986] section 6, if any redirection
URIs were registered.

If the authorization server allows the client to dynamically change
the query component of the redirection URI, the client MUST ensure
that manipulation of the query component by an attacker cannot lead
to an abuse of the redirection endpoint with as an open redirector.

3.1.2.4. Invalid Endpoint

If an authorization request fails validation due to a missing,
invalid, or mismatching redirection URI, the registered authorization server
SHOULD inform the resource owner of the error, and MUST NOT
automatically redirect the user-agent to the invalid redirection URI.

The authorization server SHOULD NOT redirect the user-agent to
unregistered or untrusted URIs to prevent the authorization endpoint
from being used as an open redirector.

3.1.2.5. Endpoint Content

The redirection request to the client's endpoint typically results in
an HTML document response, processed by the user-agent. If no valid the HTML
response is served directly as the result of the redirection request,
any script included in the HTML document will execute with full
access to the redirection URI is
available, and the authorization server credentials it contains.

The client SHOULD inform NOT include any third-party scripts in the resource owner
directly of
redirection endpoint response. Instead, it should extract the error.

2.2.
credentials from the URI and redirect the user-agent again to another
endpoint without the credentials in the URI.

The client MUST NOT include any untrusted third-party scripts in the
redirection endpoint response (e.g. third-party analytics, social
plug-ins, ad networks) without first ensuring that its own scripts
used to extract and remove the credentials from the URI will execute
first.

3.2. Token Endpoint

The token endpoint is used by the client to obtain an access token by
authenticating with the authorization server and
presenting its authorization grant or refresh token. The token
endpoint is used with every authorization grant except for the
implicit grant type (since an access token is issued directly).

The means through which the client obtains the location of the token
endpoint are beyond the scope of this specification but is typically
provided in the service documentation. The endpoint URI MAY include
a query component, which MUST be retained when adding additional
query parameters.

Since requests to the token endpoint result in the transmission of
clear-text credentials (in the HTTP request and response), the
authorization server MUST require the use of a transport-layer
security mechanism when sending requests to the token endpoint. The
authorization server MUST support TLS 1.2 as defined in [RFC5246],
and MAY support additional transport-layer mechanisms meeting its
security requirements.

The token endpoint requires client authentication as described in
Section 3. The authorization server MAY accept any form of client
authentication meeting its security requirements. The client MUST
NOT use more than one authentication method in each request.

The client MUST use the HTTP "POST" method when making access token
requests.

3. once.

3.2.1. Client Authentication

Client credentials are used to identify and authenticate the client.
The client credentials include a client identifier - a unique string

[[ Pending Consensus ]]

Private clients, clients issued to the client to identify itself to credentials, or clients
assigned other authentication requirements, MUST authenticate with
the authorization server.
The client identifier is not a secret, it is exposed server as described in Section 2.4 when making
requests to the resource
owner, and MUST NOT be used alone for client authentication. token endpoint. Client authentication is accomplished via additional means such as a
matching client password.

The methods through which the client obtains its client credentials
are beyond used for:

o Enforcing the scope binding of this specification. However, the client
registration process typically includes gathering relevant
information which is used refresh tokens and authorization codes to educate the resource owner about
the client they are issued. Client authentication is critical
when requesting authorization.

Due an authorization code is transmitted to the nature of some clients, redirection URI
endpoint over an insecure channel, or when the authorization server should redirection URI has
not make assumptions about the confidentiality of been registered in full.
o Recovery from a compromised client credentials
without establishing trust with by disabling the client. The authorization server
SHOULD NOT issue client or
changing its credentials, by preventing an attacker from abusing
stolen refresh tokens. Changing a single set of client
credentials to clients incapable is significantly faster than revoking an entire set of keeping
their
refresh tokens.

o Implementing authentication management best practices which
require periodic credentials confidential (typically determined during the rotation. Rotation of an entire set
of refresh tokens can be challenging, while rotation of a single
set of client registration process). credentials is significantly easier. In addition, the authorization server MAY allow unauthenticated
access token requests when the client identity does not matter (e.g.
anonymous client) or when the client identity is established via
other means. For readability purposes only,
this specification is
written under the assumption that the authorization server requires
some form of client authentication. However, such language does not
affect the authorization server's discretion in allowing
unauthenticated client requests.

3.1. Client Password Authentication

[[ Pending Consensus ]]

Clients in possession of client password credentials (the client
identifier together with provide a shared symmetric secret) MAY use the HTTP
Basic authentication scheme as defined in [RFC2617] to authenticate
with the authorization server. The client identifier is used as the
username, and the secret is used as the password.

When using the HTTP Basic authentication scheme, the client
identifier is included twice in the request (in the "Authorization"
header and in the "client_id" parameter). The authorization server
MUST ensure the two identifiers belong to the same client.

For example (extra line breaks are mechanism for display purposes only):

POST /token HTTP/1.1
Host: server.example.com
Authorization: Basic czZCaGRSa3F0MzpnWDFmQmF0M2JW
Content-Type: application/x-www-form-urlencoded

grant_type=authorization_code&client_id=s6BhdRkqt3&
code=i1WsRn1uB1&
redirect_uri=https%3A%2F%2Fclient%2Eexample%2Ecom%2Fcb

Alternatively, the authorization server MAY allow including the
client secret in the request body using the following parameter:

client_secret
REQUIRED. The client secret. refresh token
rotation.

The use security ramifications of the "client_secret" parameter is NOT RECOMMENDED, and
should be limited to allowing unauthenticated access by
public clients unable to directly utilize the HTTP
Basic authentication scheme.

For example (extra line breaks are for display purposes only):

POST /token HTTP/1.1
Host: server.example.com
Content-Type: application/x-www-form-urlencoded

grant_type=authorization_code&client_id=s6BhdRkqt3&
client_secret=gX1fBat3bV&code=i1WsRn1uB1&
redirect_uri=https%3A%2F%2Fclient%2Eexample%2Ecom%2Fcb

Since requests using this authentication method result in the
transmission of clear-text credentials, the authorization server token endpoint MUST
require be considered, as well as
the use issuance of a transport-layer security mechanism when sending
requests refresh tokens to the token endpoint.

3.2. Other Client Authentication Methods

4. Obtaining Authorization

To request an access token, the client obtains authorization from the
resource owner. The authorization is expressed in the form of an
authorization grant which the client uses to request the access
token. OAuth defines four grant types: authorization code, implicit,
resource owner password credentials, and client credentials. It also
provides an extension mechanism for defining additional grant types.

4.1. Authorization Code

The authorization code grant type is suitable used to obtain both access
tokens and refresh tokens and is optimized for clients capable of
maintaining their client credentials confidential (for authenticating
with the authorization server) such as a client implemented on a
secure server. private clients. As a
redirection-based flow, the client must be capable of interacting
with the resource owner's user-agent (typically a web browser) and
capable of receiving incoming requests (via redirection) from the
authorization server.

+----------+
| resource |
| owner |
| |
+----------+
^
|
(B)
+----|-----+ Client Identifier +---------------+
| -+----(A)--- -+----(A)-- & Redirect Redirection URI ------>| ---->| |
| User- | | Authorization |
| Agent -+----(B)-- User authenticates --->| Server |
| | | |
| -+----(C)-- Authorization Code ---<| |
+-|----|---+ +---------------+
| | ^ v
(A) (C) | |
| | | |
^ v | |
+---------+ | |
| |>---(D)-- Client Credentials, --------' |
| | Authorization Code, Code ---------' |
| Client | & Redirect Redirection URI |
| | |
| |<---(E)----- Access Token -------------------'
+---------+ (w/ Optional Refresh Token)

Figure 3: Authorization Code Flow

The flow illustrated in Figure 3 includes the following steps:

(A) The client initiates the flow by directing the resource owner's
user-agent to the authorization endpoint. The client includes
its client identifier, requested scope, local state, and a
redirection URI to which the authorization server will send the
user-agent back once access is granted (or denied).
(B) The authorization server authenticates the resource owner (via
the user-agent) and establishes whether the resource owner
grants or denies the client's access request.
(C) Assuming the resource owner grants access, the authorization
server redirects the user-agent back to the client using the
redirection URI provided earlier. The redirection URI includes
an authorization code and any local state provided by the client
earlier.

(D) The client requests an access token from the authorization
server's token endpoint by authenticating using its client
credentials, and includes including the authorization code
received in the previous step. When making the request, the
client authenticates with the authorization server. The client
includes the redirection URI used to obtain the authorization
code for verification.
(E) The authorization server validates authenticates the client credentials, client, validates the
authorization code, and ensures the redirection URI received
matches the URI used to redirect the client in step (C). If
valid, responds back with an access token.

4.1.1. Authorization Request

The client constructs the request URI by adding the following
parameters to the query component of the authorization endpoint URI
using the "application/x-www-form-urlencoded" format as defined by
[W3C.REC-html401-19991224]:

response_type
REQUIRED. Value MUST be set to "code".
client_id
REQUIRED. The client identifier as described in Section 3. 2.3.
redirect_uri
REQUIRED, unless a redirection URI has been established between
the client and authorization server via other means. Described
OPTIONAL, as described in Section 2.1.1. 3.1.2.
scope
OPTIONAL. The scope of the access request expressed as a list
of space-delimited, case sensitive strings. The value is
defined by the authorization server. If the value contains
multiple space-delimited strings, their order does not matter,
and each string adds an additional access range to the
requested scope.
state
OPTIONAL. An opaque value used by the client to maintain state
between the request and callback. The authorization server
includes this value when redirecting the user-agent back to the
client.

The client directs the resource owner to the constructed URI using an
HTTP redirection response, or by other means available to it via the
user-agent.

For example, the client directs the user-agent to make the following
HTTP request using transport-layer security (extra line breaks are
for display purposes only):

GET /authorize?response_type=code&client_id=s6BhdRkqt3&
redirect_uri=https%3A%2F%2Fclient%2Eexample%2Ecom%2Fcb /authorize?response_type=code&client_id=s6BhdRkqt3&state=xyz
&redirect_uri=https%3A%2F%2Fclient%2Eexample%2Ecom%2Fcb HTTP/1.1

Host: server.example.com

The authorization server validates the request to ensure all required
parameters are present and valid. If the request is valid, the
authorization server authenticates the resource owner and obtains an
authorization decision (by asking the resource owner or by
establishing approval via other means).

When a decision is established, the authorization server directs the
user-agent to the provided client redirection URI using an HTTP
redirection response, or by other means available to it via the user-
agent.

4.1.2. Authorization Response

If the resource owner grants the access request, the authorization
server issues an authorization code and delivers it to the client by
adding the following parameters to the query component of the
redirection URI using the "application/x-www-form-urlencoded" format:

code
REQUIRED. The authorization code generated by the
authorization server. The authorization code SHOULD MUST expire
shortly after it is issued to minimize mitigate the risk of leaks. A
maximum authorization code lifetime of 10 minutes is
RECOMMENDED. The client MUST NOT reuse the authorization code.
If an authorization code is used more than once, the
authorization server MAY SHOULD attempt to revoke all tokens
previously issued based on that authorization code. The
authorization code is bound to the client identifier and
redirection URI.
state
REQUIRED if the "state" parameter was present in the client
authorization request. Set to the exact value received from
the client.

For example, the authorization server redirects the user-agent by
sending the following HTTP response:

HTTP/1.1 302 Found
Location: https://client.example.com/cb?code=i1WsRn1uB1 https://client.example.com/cb?code=SplxlOBeZQQYbYS6WxSbIA
&state=xyz

The client SHOULD ignore unrecognized response parameters. The
authorization code string size is left undefined by this
specification. The client should avoid making assumptions about code
value sizes. The authorization server should document the size of
any value it issues.

4.1.2.1. Error Response

If the request fails due to a missing, invalid, or mismatching
redirection URI, or if the client identifier provided is invalid, the
authorization server SHOULD inform the resource owner of the error,
and MUST NOT automatically redirect the user-agent to the invalid
redirection URI.

If the resource owner denies the access request or if the request
fails for reasons other than a missing or invalid redirection URI,
the authorization server informs the client by adding the following
parameters to the query component of the redirection URI using the
"application/x-www-form-urlencoded" format:

error
REQUIRED. A single error code from the following:
invalid_request
The request is missing a required parameter, includes an
unsupported parameter or parameter value, or is otherwise
malformed.
unauthorized_client
The client is not authorized to request an authorization
code using this method.
access_denied
The resource owner or authorization server denied the
request.
unsupported_response_type
The authorization server does not support obtaining an
authorization code using this method.
invalid_scope
The requested scope is invalid, unknown, or malformed.
a 4xx or 5xx HTTP status code (except for 400 and 401)
server_error
The authorization server MAY set encountered an unexpected
condition which prevented it from fulfilling the "error" parameter
value request.
temporarily_unavailable
The authorization server is currently unable to a numerical HTTP status code from handle
the 4xx request due to a temporary overloading or 5xx
range, with the exception maintenance
of the 400 (Bad Request) and
401 (Unauthorized) status codes. For example, if the
service is temporarily unavailable, the authorization
server MAY return an error response with "error" set to
"503". server.
error_description
OPTIONAL. A human-readable UTF-8 encoded text providing
additional information, used to assist in the client developer in
understanding and resolution
of the error that occurred. [[ add language and encoding information
]]

error_uri
OPTIONAL. A URI identifying a human-readable web page with
information about the error, used to provide the resource owner client
developer with additional information about the error.
state
REQUIRED if a valid "state" parameter was present in the client
authorization request. Set to the exact value received from
the client.

For example, the authorization server redirects the user-agent by
sending the following HTTP response:

HTTP/1.1 302 Found
Location: https://client.example.com/cb?error=access_denied https://client.example.com/cb?error=access_denied&state=xyz

4.1.3. Access Token Request

The client makes a request to the token endpoint by adding the
following parameters using the "application/x-www-form-urlencoded"
format in the HTTP request entity-body:

grant_type
REQUIRED. Value MUST be set to "authorization_code".
client_id
REQUIRED. The client identifier as described in Section 3.
code
REQUIRED. The authorization code received from the
authorization server.
redirect_uri
REQUIRED. The redirection URI used by
REQUIRED, if the authorization server
to return "redirect_uri" parameter was included in the
authorization response request described in Section 4.1.1, and their
values MUST be identical.

If the previous step.

The client includes its authentication type is private or was issued client credentials (or
assigned other authentication requirements), the client MUST
authenticate with the authorization server as described in
Section 3 3.2.1.

For example, the client makes the following HTTP using transport-
layer security (extra line breaks are for display purposes only):

POST /token HTTP/1.1
Host: server.example.com
Authorization: Basic czZCaGRSa3F0MzpnWDFmQmF0M2JW
Content-Type: application/x-www-form-urlencoded

grant_type=authorization_code&client_id=s6BhdRkqt3&
code=i1WsRn1uB1&
redirect_uri=https%3A%2F%2Fclient%2Eexample%2Ecom%2Fcb application/x-www-form-urlencoded;charset=UTF-8

grant_type=authorization_code&code=SplxlOBeZQQYbYS6WxSbIA
&redirect_uri=https%3A%2F%2Fclient%2Eexample%2Ecom%2Fcb

The authorization server MUST:

o Validate the require client authentication for private clients or for any
client issued client credentials (or with other authentication
requirements),
o authenticate the client if client authentication is included and
ensure that the authorization code was issued to that client. the authenticated
client,
o Verify verify that the authorization code is valid, and
o ensure that the
redirection URI matches the redirection URI used by "redirect_uri" parameter is present if the
authorization server to deliver
"redirect_uri" parameter was included in the initial authorization code.
request described in Section 4.1.1, and that their values are
identical.

4.1.4. Access Token Response

If the access token request is valid and authorized, the
authorization server issues an access token and optional refresh
token as described in Section 5.1. If the request client
authentication failed or is invalid, the authorization server returns
an error response as described in Section 5.2.

An example successful response:

HTTP/1.1 200 OK
Content-Type: application/json application/json;charset=UTF-8
Cache-Control: no-store
Pragma: no-cache

{
"access_token":"SlAV32hkKG",
"access_token":"2YotnFZFEjr1zCsicMWpAA",
"token_type":"example",
"expires_in":3600,
"refresh_token":"8xLOxBtZp8",
"refresh_token":"tGzv3JOkF0XG5Qx2TlKWIA",
"example_parameter":"example_value"
}

4.2. Implicit Grant

The implicit grant type is suitable used to obtain access tokens (it does not
support the issuance of refresh tokens) and is optimized for public
clients incapable of
maintaining their client credentials confidential (for authenticating
with the authorization server) such as client applications residing
in known to operate a user-agent, particular redirection URI. These clients
are typically implemented in a browser using a scripting language
such as JavaScript.

As a redirection-based flow, the client must be capable of
interacting with the resource owner's user-agent (typically a web
browser) and capable of receiving incoming requests (via redirection)
from the authorization server.

Unlike the authorization code grant type in which the client makes
separate requests for authorization and access token, the client
receives the access token as the result of the authorization request.

Using the

The implicit grant type does not include client authentication
since the client is unable to maintain their credential
confidentiality (the client resides authentication, and
relies on the resource owner's computer
or device which makes presence of the client credentials accessible resource owner and
exploitable). the registration of
the redirection URI. Because the access token is encoded into the
redirection URI, it may be exposed to the resource owner and other
applications residing on its computer or device.

+----------+
| Resource |
| Owner |
| |
+----------+
^
|
(B)
+----|-----+ Client Identifier +---------------+
| -+----(A)--- -+----(A)-- & Redirect Redirection URI ----->| --->| |
| User- | | Authorization |
| Agent -|----(B)-- User authenticates -->| Server |
| | | |
| |<---(C)---- Redirect |<---(C)--- Redirection URI ------<| ----<| |
| | with Access Token +---------------+
| | in Fragment
| | +---------------+
| |----(D)---- Redirect |----(D)--- Redirection URI ------>| Web Server ---->| Web-Hosted |
| | without Fragment | with Client |
| | | Resource |
| (F) |<---(E)------- Script ---------<| |
| | +---------------+
+-|--------+
| |
(A) (G) Access Token
| |
^ v
+---------+
| |
| Client |
| |
+---------+

Figure 4: Implicit Grant Flow

The flow illustrated in Figure 4 includes the following steps:

(C) Assuming the resource owner grants access, the authorization
server redirects the user-agent back to the client using the
redirection URI provided earlier. The redirection URI includes
the access token in the URI fragment.
(D) The user-agent follows the redirection instructions by making a
request to the web server (does web-hosted client resource (which does not
include the fragment). The user-agent retains the fragment
information locally.
(E) The web server web-hosted client resource returns a web page (typically an
HTML document with an embedded script) capable of accessing the
full redirection URI including the fragment retained by the user-
agent,
user-agent, and extracting the access token (and other
parameters) contained in the fragment.
(F) The user-agent executes the script provided by the web server web-hosted
client resource locally, which extracts the access token and
passes it to the client.

4.2.1. Authorization Request

The client constructs the request URI by adding the following
parameters to the query component of the authorization endpoint URI
using the "application/x-www-form-urlencoded" format:

response_type
REQUIRED. Value MUST be set to "token".
client_id
REQUIRED. The client identifier as described in Section 3. 2.3.
redirect_uri
REQUIRED, unless a redirection URI has been established between
the client and authorization server via other means. Described
OPTIONAL, as described in Section 2.1.1. 3.1.2.
scope
OPTIONAL. The scope of the access request expressed as a list
of space-delimited, case sensitive strings. The value is
defined by the authorization server. If the value contains
multiple space-delimited strings, their order does not matter,
and each string adds an additional access range to the
requested scope.
state
OPTIONAL. An opaque value used by the client to maintain state
between the request and callback. The authorization server
includes this value when redirecting the user-agent back to the
client.

The client directs the resource owner to the constructed URI using an
HTTP redirection response, or by other means available to it via the
user-agent.

For example, the client directs the user-agent to make the following
HTTP request using transport-layer security (extra line breaks are
for display purposes only):

GET /authorize?response_type=token&client_id=s6BhdRkqt3&
redirect_uri=https%3A%2F%2Fclient%2Eexample%2Ecom%2Fcb /authorize?response_type=token&client_id=s6BhdRkqt3&state=xyz
&redirect_uri=https%3A%2F%2Fclient%2Eexample%2Ecom%2Fcb HTTP/1.1
Host: server.example.com

The authorization server validates the request to ensure all required
parameters are present and valid. The authorization server MUST
verify that the redirection URI to which it will redirect the access
token matches a redirection URI registered by the client as described
in Section 3.1.2.

If the request is valid, the authorization server authenticates the
resource owner and obtains an authorization decision (by asking the
resource owner or by establishing approval via other means).

4.2.2. Access Token Response

If the resource owner grants the access request, the authorization
server issues an access token and delivers it to the client by adding
the following parameters to the fragment component of the redirection
URI using the "application/x-www-form-urlencoded" format:

access_token
REQUIRED. The access token issued by the authorization server.
token_type
REQUIRED. The type of the token issued as described in
Section 7.1. Value is case insensitive.
expires_in
OPTIONAL. The duration in seconds of the access token
lifetime. For example, the value "3600" denotes that the
access token will expire in one hour from the time the response
was generated.
scope
OPTIONAL. The scope of the access request token expressed as a list of
space-delimited, case sensitive strings. The value is defined
by the authorization server. If the value contains multiple
space-delimited strings, their order does not matter, and each
string adds an additional access range to the requested scope.

The authorization server SHOULD include the parameter if the requested
access token scope is different from the one requested by the
client.
state
REQUIRED if the "state" parameter was present in the client
authorization request. Set to the exact value received from
the client.

For example, the authorization server redirects the user-agent by
sending the following HTTP response (URI extra line breaks are for
display purposes only):

HTTP/1.1 302 Found
Location: http://example.com/rd#access_token=FJQbwq9&
token_type=example&expires_in=3600 http://example.com/rd#access_token=2YotnFZFEjr1zCsicMWpAA
&state=xyz&token_type=example&expires_in=3600

Developers should note that some HTTP client implementations do not
support the inclusion of a fragment component in the HTTP "Location"
response header field. Such client will require using other methods
for redirecting the client than a 3xx redirection response. For
example, returning an HTML page which includes a 'continue' button
with an action linked to the redirection URI.

The client SHOULD ignore unrecognized response parameters. The
access token string size is left undefined by this specification.
The client should avoid making assumptions about value sizes. The
authorization server should document the size of any value it issues.

4.2.2.1. Error Response

If the resource owner denies the access request or if the request
fails for reasons other than a missing or invalid redirection URI,
the authorization server informs the client by adding the following
parameters to the fragment component of the redirection URI using the
"application/x-www-form-urlencoded" format:

error
REQUIRED. A single error code from the following:
invalid_request
The request is missing a required parameter, includes an
unsupported parameter or parameter value, or is otherwise
malformed.
unauthorized_client
The client is not authorized to request an access token
using this method.
access_denied
The resource owner or authorization server denied the
request.
unsupported_response_type
The authorization server does not support obtaining an
access token using this method.
invalid_scope
The requested scope is invalid, unknown, or malformed.
a 4xx or 5xx HTTP status code (except for 400 and 401)
server_error
The authorization server MAY set encountered an unexpected
condition which prevented it from fulfilling the "error" parameter
value request.
temporarily_unavailable
The authorization server is currently unable to a numerical HTTP status code from handle
the 4xx request due to a temporary overloading or 5xx
range, with the exception maintenance
of the 400 (Bad Request) and
401 (Unauthorized) status codes. For example, if the
service is temporarily unavailable, the authorization
server MAY return an error response with "error" set to
"503". server.
error_description
OPTIONAL. A human-readable UTF-8 encoded text providing
additional information, used to assist in the client developer in
understanding and resolution
of the error that occurred. [[ add language and encoding information
]]
error_uri
OPTIONAL. A URI identifying a human-readable web page with
information about the error, used to provide the resource owner client
developer with additional information about the error.
state
REQUIRED if a valid "state" parameter was present in the client
authorization request. Set to the exact value received from
the client.

For example, the authorization server redirects the user-agent by
sending the following HTTP response:

HTTP/1.1 302 Found
Location: https://client.example.com/cb#error=access_denied https://client.example.com/cb#error=access_denied&state=xyz

4.3. Resource Owner Password Credentials

The resource owner password credentials grant type is suitable in
cases where the resource owner has a trust relationship with the
client, such as its computer operating system or a highly privileged
application. The authorization server should take special care when
enabling the grant type, and only when other flows are not viable.

The grant type is suitable for clients capable of obtaining the
resource owner credentials (username and password, typically using an
interactive form). It is also used to migrate existing clients using
direct authentication schemes such as HTTP Basic or Digest
authentication to OAuth by converting the stored credentials with to an
access token.

+----------+
| Resource |
| Owner |
| |
+----------+
v
| Resource Owner
(A) Password Credentials
|
v
+---------+ +---------------+
| | Client Credentials | |
| |>--(B)---- & Resource Owner ----->| ------->| |
| Client | Password Credentials | Authorization |
| Client | | Server |
| |<--(C)---- Access Token ---------<| |
| | (w/ Optional Refresh Token) | |
+---------+ +---------------+

Figure 5: Resource Owner Password Credentials Flow

The flow illustrated in Figure 5 includes the following steps:

(A) The resource owner provides the client with its username and
password.
(B) The client requests an access token from the authorization
server's token endpoint by authenticating using its client
credentials, and includes including the credentials received
from the resource owner. When making the request, the client
authenticates with the authorization server.

(C) The authorization server authenticates the client and validates
the resource owner
credentials and the client credentials credentials, and if valid issues an access
token.

4.3.1. Authorization Request and Response

The method through which the client obtains the resource owner
credentials is beyond the scope of this specification. The client
MUST discard the credentials once an access token has been obtained.

4.3.2. Access Token Request

The client makes a request to the token endpoint by adding the
following parameters using the "application/x-www-form-urlencoded"
format in the HTTP request entity-body:

grant_type
REQUIRED. Value MUST be set to "password".
client_id
REQUIRED. The client identifier as described in Section 3.
username
REQUIRED. The resource owner username, encoded as UTF-8.
password
REQUIRED. The resource owner password, encoded as UTF-8.
scope
OPTIONAL. The scope of the access request expressed as a list
of space-delimited, case sensitive strings. The value is
defined by the authorization server. If the value contains
multiple space-delimited strings, their order does not matter,
and each string adds an additional access range to the
requested scope.

The

If the client type is private or was issued client includes its authentication credentials (or
assigned other authentication requirements), the client MUST
authenticate with the authorization server as described in
Section 3 3.2.1.

For example, the client makes the following HTTP request using
transport-layer security (extra line breaks are for display purposes
only):

POST /token HTTP/1.1
Host: server.example.com
Authorization: Basic czZCaGRSa3F0MzpnWDFmQmF0M2JW
Content-Type: application/x-www-form-urlencoded

grant_type=password&client_id=s6BhdRkqt3&
username=johndoe&password=A3ddj3w application/x-www-form-urlencoded;charset=UTF-8

grant_type=password&username=johndoe&password=A3ddj3w

The authorization server MUST:

o Validate require client authentication for private clients or for any
client issued client credentials (or with other authentication
requirements),
o authenticate the client credentials. if client authentication is included, and
o Validate validate the resource owner password credentials.

Since this access token request utilizes the resource owner's
password, the authorization server MUST protect the endpoint against
brute force attacks.

4.3.3. Access Token Response

If the access token request is valid and authorized, the
authorization server issues an access token and optional refresh
token as described in Section 5.1. If the request failed client
authentication or is invalid, the authorization server returns an
error response as described in Section 5.2.

An example successful response:

HTTP/1.1 200 OK
Content-Type: application/json application/json;charset=UTF-8
Cache-Control: no-store
Pragma: no-cache

4.4. Client Credentials

The client can request an access token using only its client
credentials (or other supported means of authentication) when the
client is requesting access to the protected resources under its
control, or those of another resource owner which has been previously
arranged with the authorization server (the method of which is beyond
the scope of this specification).

The client credentials grant type MUST only be used by private
clients.

Figure 6: Client Credentials Flow

The flow illustrated in Figure 6 includes the following steps:

(A) The client authenticates with the authorization server and
requests an access token from the token endpoint by
authenticating using its client credentials. endpoint.
(B) The authorization server validates authenticates the client credentials client, and if valid
issues an access token.

4.4.1. Authorization Request and Response

Since the client credentials are authentication is used as the authorization grant,
no additional authorization request is needed as the client is already
in the possession of its client credentials. needed.

4.4.2. Access Token Request

The client makes a request to the token endpoint by adding the
following parameters using the "application/x-www-form-urlencoded"
format in the HTTP request entity-body:

grant_type
REQUIRED. Value MUST be set to "client_credentials".
client_id
REQUIRED. The client identifier as described in Section 3.
scope
OPTIONAL. The scope of the access request expressed as a list
of space-delimited, case sensitive strings. The value is
defined by the authorization server. If the value contains
multiple space-delimited strings, their order does not matter,
and each string adds an additional access range to the
requested scope.

The client includes its authentication credentials MUST authenticate with the authorization server as
described in Section 3 3.2.1.

For example, the client makes the following HTTP request using
transport-layer security (extra line breaks are for display purposes
only):

POST /token HTTP/1.1
Host: server.example.com
Authorization: Basic czZCaGRSa3F0MzpnWDFmQmF0M2JW
Content-Type: application/x-www-form-urlencoded

grant_type=client_credentials&client_id=s6BhdRkqt3 application/x-www-form-urlencoded;charset=UTF-8

grant_type=client_credentials

The authorization server MUST validate authenticate the client credentials. client.

4.4.3. Access Token Response

If the access token request is valid and authorized, the
authorization server issues an access token and optional refresh
token as described in
Section 5.1. A refresh token SHOULD NOT be included. If the request
failed client authentication or is invalid, the authorization server
returns an error response as described in Section 5.2.

An example successful response:

HTTP/1.1 200 OK
Content-Type: application/json application/json;charset=UTF-8
Cache-Control: no-store
Pragma: no-cache

{
"access_token":"SlAV32hkKG",
"access_token":"2YotnFZFEjr1zCsicMWpAA",
"token_type":"example",
"expires_in":3600,
"refresh_token":"8xLOxBtZp8",
"example_parameter":"example_value"
}

4.5. Extensions

The client uses an extension grant type by specifying the grant type
using an absolute URI (defined by the authorization server) as the
value of the "grant_type" parameter of the token endpoint, and by
adding any additional parameters necessary.

For example, to request an access token using a SAML 2.0 assertion
grant type as defined by [I-D.ietf-oauth-saml2-bearer], the client
makes the following HTTP request using transport-layer security (line
breaks are for display purposes only):

POST /token HTTP/1.1
Host: server.example.com
Content-Type: application/x-www-form-urlencoded application/x-www-form-urlencoded;charset=UTF-8

grant_type=http%3A%2F%2Foauth.net%2Fgrant_type%2Fassertion%2F
saml%2F2.0%2Fbearer&assertion=PEFzc2VydGlvbiBJc3N1ZUluc3RhbnQ
[...omitted for brevity...]V0aG5TdGF0ZW1lbnQ-PC9Bc3NlcnRpb24-

5. Issuing an Access Token

5.1. Successful Response

The authorization server issues an access token and optional refresh
token, and constructs the response by adding the following parameters
to the entity body of the HTTP response with a 200 (OK) status code:

refresh_token
OPTIONAL. The refresh token which can be used to obtain new
access tokens using the same authorization grant as described
in Section 6.
scope
OPTIONAL. The scope of the access request token expressed as a list of
space-delimited, case sensitive strings. The value is defined
by the authorization server. If the value contains multiple
space-delimited strings, their order does not matter, and each
string adds an additional access range to the requested scope.
The authorization server SHOULD include the parameter if the requested
access token scope is different from the one requested by the
client.

The parameters are included in the entity body of the HTTP response
using the "application/json" media type as defined by [RFC4627]. The
parameters are serialized into a JSON structure by adding each
parameter at the highest structure level. Parameter names and string
values are included as JSON strings. Numerical values are included
as JSON numbers.

The authorization server MUST include the HTTP "Cache-Control"
response header field [RFC2616] with a value of "no-store" in any
response containing tokens, secrets, or other sensitive information. information,
as well as the "Pragma" response header field [RFC2616] with a value
of "no-cache".

For example:

HTTP/1.1 200 OK
Content-Type: application/json application/json;charset=UTF-8
Cache-Control: no-store
Pragma: no-cache

The client SHOULD ignore unrecognized response parameters. The sizes
of tokens and other values received from the authorization server are
left undefined. The client should avoid making assumptions about
value sizes. The authorization server should document the size of
any value it issues.

5.2. Error Response

The authorization server responds with an HTTP 400 (Bad Request)
status code and includes the following parameters with the response:

error
REQUIRED. A single error code from the following:
invalid_request
The request is missing a required parameter, includes an
unsupported parameter or parameter value, repeats a
parameter, includes multiple credentials, utilizes more
than one mechanism for authenticating the client, or is
otherwise malformed.
invalid_client
Client authentication failed (e.g. unknown client, no
client credentials authentication included, multiple client credentials
authentications included, or unsupported credentials type). authentication
method). The authorization server MAY return an HTTP 401
(Unauthorized) status code to indicate which HTTP
authentication schemes are supported. If the client
attempted to authenticate via the "Authorization" request
header field, the authorization server MUST respond with
an HTTP 401 (Unauthorized) status code, and include the
"WWW-Authenticate" response header field matching the
authentication scheme used by the client.
invalid_grant
The provided authorization grant is invalid, expired,
revoked, does not match the redirection URI used in the
authorization request, or was issued to another client.
unauthorized_client
The authenticated client is not authorized to use this
authorization grant type.
unsupported_grant_type
The authorization grant type is not supported by the
authorization server.
invalid_scope
The requested scope is invalid, unknown, malformed, or
exceeds the scope granted by the resource owner.
error_description
OPTIONAL. A human-readable UTF-8 encoded text providing
additional information, used to assist in the client developer in
understanding and resolution
of the error that occurred. [[ add language and encoding information
]]

error_uri
OPTIONAL. A URI identifying a human-readable web page with
information about the error, used to provide the resource owner client
developer with additional information about the error.

For example:

HTTP/1.1 400 Bad Request
Content-Type: application/json application/json;charset=UTF-8
Cache-Control: no-store
Pragma: no-cache

{
"error":"invalid_request"
}

If the authorization server encounters an error condition other than
the 400 (Bad Request) and 401 (Unauthorized) responses described
above (e.g. the service is temporarily unavailable), the
authorization server SHOULD include an error response in the entity
body, and set the "error" parameter value to the numerical HTTP
status code returned.

For example:

HTTP/1.1 503 Service Unavailable
Content-Type: application/json

{
"error":"503"
}

6. Refreshing an Access Token

If the authorization server issued a refresh token to the client, the
client makes a refresh request to the token endpoint by adding the
following parameters using the "application/x-www-form-urlencoded"
format in the HTTP request entity-body:

grant_type
REQUIRED. Value MUST be set to "refresh_token".
client_id
REQUIRED. The client identifier as described in Section 3.
refresh_token
REQUIRED. The refresh token issued to the client.
scope
OPTIONAL. The scope of the access request expressed as a list
of space-delimited, case sensitive strings. The value is
defined by the authorization server. If the value contains
multiple space-delimited strings, their order does not matter,
and each string adds an additional access range to the
requested scope. The requested scope MUST be equal or lesser
than the scope originally granted by the resource owner, and if
omitted is treated as equal to the scope originally granted by
the resource owner.

The

Because refresh tokens are typically long-lasting credentials used to
request additional access tokens, the refresh token is bound to the
client it was issued. If the client type is private or was issued
client includes its authentication credentials (or assigned other authentication requirements),
the client MUST authenticate with the authorization server as
described in Section 3. 3.2.1.

For example, the client makes the following HTTP request using
transport-layer security (extra line breaks are for display purposes
only):

POST /token HTTP/1.1
Host: server.example.com
Authorization: Basic czZCaGRSa3F0MzpnWDFmQmF0M2JW
Content-Type: application/x-www-form-urlencoded

grant_type=refresh_token&client_id=s6BhdRkqt3&
refresh_token=n4E9O119d application/x-www-form-urlencoded;charset=UTF-8

grant_type=refresh_token&refresh_token=tGzv3JOkF0XG5Qx2TlKWIA

The authorization server MUST validate MUST:

o require client authentication for private clients or for any
client issued client credentials (or with other authentication
requirements),
o authenticate the client credentials, if client authentication is included and
ensure
that the refresh token was issued to the authenticated client,
o validate the refresh token, and
o verify that the resource owner's authorization is still valid.

If valid and authorized, the authorization server issues an access
token as described in Section 5.1. If the request failed
verification or is invalid, the authorization server returns an error
response as described in Section 5.2.

The authorization server MAY issue a new refresh token, in which
case, case
the client MUST discard the old refresh token and replace it with the
new refresh token. The authorization server MAY revoke the old
refresh token after issuing a new refresh token to the client. If a
new refresh token is issued, its scope MUST be identical to that of
the refresh token included in the request.

7. Accessing Protected Resources

The client accesses protected resources by presenting the access
token to the resource server. The resource server MUST validate the
access token and ensure it has not expired and that its scope covers
the requested resource. The methods used by the resource server to
validate the access token (as well as any error responses) are beyond
the scope of this specification, but generally involve an interaction
or coordination between the resource server and the authorization
server.

The method in which the client utilized the access token to
authenticate with the resource server depends on the type of access
token issued by the authorization server. Typically, it involves
using the HTTP "Authorization" request header field [RFC2617] with an
authentication scheme defined by the access token type specification.

7.1. Access Token Types

The access token type provides the client with the information
required to successfully utilize the access token to make a protected
resource request (along with type-specific attributes). The client
MUST NOT use an access token if it does not understand or does not
trust the token type.

For example, the "bearer" token type defined in
[I-D.ietf-oauth-v2-bearer] is utilized by simply including the access
token string in the request:

GET /resource/1 HTTP/1.1
Host: example.com
Authorization: Bearer 7Fjfp0ZBr1KtDRbnfVdmIw

while the "mac" token type defined in [I-D.ietf-oauth-v2-http-mac] is
utilized by issuing a MAC key together with the access token which is
used to sign certain components of the HTTP requests:

GET /resource/1 HTTP/1.1
Host: example.com
Authorization: MAC id="h480djs93hd8",
nonce="274312:dj83hs9s",
mac="kDZvddkndxvhGRXZhvuDjEWhGeE="

The above examples are provided for illustration purposes only.
Developers are advised to consult the [I-D.ietf-oauth-v2-bearer] and
[I-D.ietf-oauth-v2-http-mac] specifications before use.

Each access token type definition specifies the additional attributes
(if any) sent to the client together with the "access_token" response
parameter. It also defines the HTTP authentication method used to
include the access token when making a protected resource request.

8. Extensibility

8.1. Defining Access Token Types

Access token types can be defined in one of two ways: registered in
the access token type registry (following the procedures in
Section 11.1), or use a unique absolute URI as its name.

Types utilizing a URI name SHOULD be limited to vendor-specific
implementations that are not commonly applicable, and are specific to
the implementation details of the resource server where they are
used.

All other types MUST be registered. Type names MUST conform to the
type-name ABNF. If the type definition includes a new HTTP
authentication scheme, the type name SHOULD be identical to the HTTP
authentication scheme name (as defined by [RFC2617]).

type-name = 1*name-char
name-char = "-" / "." / "_" / DIGIT / ALPHA

8.2. Defining New Endpoint Parameters

New request or response parameters for use with the authorization
endpoint or the token endpoint are defined and registered in the
parameters registry following the procedure in Section 11.2.

Parameter names MUST conform to the param-name ABNF and parameter
values syntax MUST be well-defined (e.g., using ABNF, or a reference
to the syntax of an existing parameter).

param-name = 1*name-char
name-char = "-" / "." / "_" / DIGIT / ALPHA

Unregistered vendor-specific parameter extensions that are not
commonly applicable, and are specific to the implementation details
of the authorization server where they are used SHOULD utilize a
vendor-specific prefix that is not likely to conflict with other
registered values (e.g. begin with 'companyname_').

8.3. Defining New Authorization Grant Types

New authorization grant types can be defined by assigning them a
unique absolute URI for use with the "grant_type" parameter. If the
extension grant type requires additional token endpoint parameters,
they MUST be registered in the OAuth parameters registry as described
by Section 11.2.

8.4. Defining New Authorization Endpoint Response Types

[[ Pending consensus ]]

New response types for use with the authorization endpoint are
defined and registered in the authorization endpoint response type
registry following the procedure in Section 11.3. Response type
names MUST conform to the response-type ABNF.

response-type = response-name *( "+" response-name )
response-name = 1*response-char
response-char = "_" / DIGIT / ALPHA

The "+" character is reserved for defining composite response types
made up of two or more existing registered response types. Only one
response type of each combination may be registered and used for
making requests. Composite response types are treated and compared
in the same as manner as non-composite response types. The "+"
notation is meant only to improve human readability and is not used
for machine parsing.

For example, an extension can define and register the "token+code"
response type. However, once registered, the same combination cannot
be registered as "code+token", or used to make an authorization
request.

8.5. Defining Additional Error Codes

In cases where protocol extensions (i.e. access token types,
extension parameters, or extension grant types) require additional
error codes to be used with the authorization code grant error
response (Section 4.1.2.1), the implicit grant error response
(Section 4.2.2.1), or the token error response (Section 5.2), such
error codes MAY be defined.

Extension error codes MUST be registered (following the procedures in
Section 11.3) 11.4) if the extension they are used in conjunction with is a
registered access token type, a registered endpoint parameter, or an
extension grant type. Error codes used with unregistered extensions
MAY be registered.

Error codes MUST conform to the error-code ABNF, and SHOULD be
prefixed by an identifying name when possible. For example, an error
identifying an invalid value set to the extension parameter "example"
should be named "example_invalid".

error-code = ALPHA *error-char
error-char = "-" / "." / "_" / DIGIT / ALPHA

9. Native Applications

[[ Pending consensus ]]

A native application is a client which is

Native applications are clients installed and executes executed on the end-user's
resource owner's device (i.e. desktop application, native mobile
application). Native applications are often capable of interacting
with (or embedding) a user-agent but are limited in how such
interactions affects their may require special consideration
related to security, platform capabilities, and overall end-user
experience. In many
cases, native applications are incapable of receiving redirection
requests from the

The authorization server (e.g. due to firewall rules,
operating system restrictions). endpoint requires interaction between the client
and the resource owner's user-agent. Native applications can utilize OAuth in different ways, based on
their requirements and desired end-user experience:

o Use the authorization code grant type flow described in
Section 4.1 by launching invoke
an external user-agent. The user-agent or embed a user-agent within the application.
For example:

o External user-agent - the native application can capture the
response by providing from the authorization server using a redirection URI identifying a local (non-network) resource (registered
with an scheme registered with the operating system to invoke the native application
client as handler), the handler, manual copy-and-paste of the credentials,
running a local web server, installing a user-agent plug-in, or by
providing a redirection URI identifying a server-hosted resource
under the native application's client's control, which in turn makes the response
available to the native application (e.g. using
the user-agent window title or other locations accessible from
outside the user-agent). application.
o Use Embedded user-agent - the authorization code grant type flow described in
Section 4.1 by embedding a user-agent. The native application obtains the response
by directly communicating with the embedded
user-agent. Embedded user-agents are discouraged as they
typically provide a less consistent user experience and do not
enable the end-user to verify the authorization server's
authenticity.

Native applications SHOULD use user-agent by
monitoring state changes emitted during the authorization code grant type flow
without client password credentials (due to their inability to keep resource load,
monitoring HTTP headers, or accessing the credentials confidential) to obtain short-lived access tokens,
and use refresh tokens to maintain access. user-agent's cookies
storage.

When choosing between launching an external user-agent and an
embedding a or embedded user-agent, native application developers
should consider
the following: consider:

o External user-agents may improve completion rate as the end-user resource
owner may already have an active session with the authorization
server removing the need to re-authenticate, and provide a
familiar user-
agent user-agent user experience. experience and functionality. The end-user
resource owner may also rely on extensions or add-ons to assist
with authentication (e.g. password managers or 2-factor device
reader).
o Embedded user-agents often may offer a better end-user flow, an improved usability, as they
remove the need to switch context and open new windows but also
may provide less familiar features than the external user-agent. windows.
o Embedded user-agents pose a security challenge because end-users resource
owners are authenticating in an unidentified window without access
to the visual protections offered found on by many user-agents. Embedded of the external user-
agents
agents. Embedded user-agents educate end-user to trust
unidentified requests for authentication (making phishing attacks
easier to execute).

When choosing between implicit and authorization code grant types,
the following should be considered:

o Native applications that use the authorization code grant type
flow SHOULD do so without using client password credentials, due
to the native application's inability to keep those credentials
secure.
o When using the implicit grant type flow a refresh token is not
returned.

10. Security Considerations

As a flexible and extensible framework, OAuth's security
considerations depend on many factors. The following sections
provide implementers with security guidelines focused on three common
client types:

Web Application
A web application is a client running on a web server. End-users Resource
owners access the client via an HTML user interface rendered in a user-
agent
user-agent on the end-user's resource-owner's device. The client credentials
as well as any access token issued to the client are stored on the
web server and are not exposed to or accessible by the end-user. resource
owner.
User-Agent-based Application
A user-agent-based application is a client in which the client
code is downloaded from a web server and executes within a user-
agent on the end-user's resource owner's device. The OAuth protocol data and
credentials are accessible to the end-user. resource owner. Since such
applications directly reside within the user-agent, they can make
seamless use of the user-agent capabilities in the end-user resource owner
authorization process.

Native Application
A native application is a client which is installed and executes
on the end-user's resource owner's device. The OAuth protocol data and
credentials are accessible to the end-user. resource owner. It is assumed
that such an any client authentication credentials included in the
application can protect dynamically be extracted, and furthermore that rotation of the
client authentication credentials is not practical. Dynamically
issued credentials, credentials such as access tokens or refresh tokens, on the
other hand, can receive an acceptable level of protection. At a
minimum these credentials are protected from hostile servers which
the application may contact. On some platform those credentials
might be protected from eavesdropping by other applications residing on the same
device.

A comprehensive OAuth security model and analysis, as well as
background for the protocol design is provided in
[I-D.lodderstedt-oauth-security].

10.1. Client Authentication

The authorization server issues client credentials to web
applications for the purpose of authenticating them. The
authorization server is encouraged to consider using stronger client
authentication means than a client password. Application developers
MUST ensure confidentiality of client passwords and other
credentials.

The authorization server MUST NOT issue client passwords or other
credentials to native or user-agent-based applications for the
purpose of client authentication. The authorization server MAY issue
a client password or other credentials for a specific installation of
a native application on a specific device.

10.2. Client Impersonation

Given the inability of some clients to keep their client credentials
confidential, a malicious client can impersonate another client and
obtain access to protected resources. The authorization server MUST
authenticate the client whenever possible. If the authorization
server cannot authenticate the a client due to the client's
limitations, the authorization server should utilize other means to
protect resource owners from such malicious clients, including but
not limited to engaging the end-user resource owner to assist in identifying
the client and its source.

The authorization server SHOULD enforce explicit end-user resource owner
authentication, or prompt the end-user resource owner to authorize access
again, providing the end-user resource owner with information about the
client, scope, and duration of the authorization. It is up to the end-user
resource owner to review the information in the context of the
current client, and authorize the request.

The authorization server SHOULD NOT automatically, without active
end-user
resource owner interaction, process repeated authorization requests
without authenticating the client or relying on other measures to
ensure the repeated request comes from a valid client and not an
impersonator.

The authorization server SHOULD require the client to pre-register register its
redirection URI and validate the value of the "redirect_uri" against
the pre-registered registered value. The client MUST NOT serve an open redirector
resource which can be used by an attacker to construct an URI that
will pass the authorization server's redirection URI matching rules,
and will redirect the end-user's resource owner's user-agent to the attacker's
server.

The authorization server SHOULD issue access tokens with limited
scope and duration to clients incapable of authenticating.

10.3. Access Token Credentials

Access token credentials MUST be kept confidential in transit and
storage, and shared only among the authorization server, the resource
servers the credentials are valid for, and the client to whom the
credentials were issued.

When using the implicit grant type, the access token credentials are
transmitted in the URI fragment, which can expose the credentials to
unauthorized parties.

The authorization server MUST ensure that access token credentials
cannot be generated, modified, or guessed to produce valid access
token credentials.

The client SHOULD request access token credentials with the minimal
scope and duration necessary. The authorization server SHOULD take
the client identity into account when choosing to honor the requested
scope, and MAY issue credentials with a lesser scope than requested.

10.4. Refresh Tokens

Authorization servers MAY issue refresh tokens to web and native
applications.

Refresh tokens MUST be kept confidential in transit and storage, and
shared only among the authorization server and the client to whom the
refresh tokens were issued. The authorization server MUST maintain
the link between a refresh token and the client to whom it was
issued.

The authorization server MUST verify the link between the refresh
token and client identity whenever the client's identity can be
authenticated. When client authentication is not possible, the
authorization server SHOULD deploy other means to detect refresh
token abuse.

The authorization server MUST ensure that refresh tokens cannot be
generated, modified, or guessed to produce valid refresh tokens.

10.5. Request Confidentiality

Access token credentials, refresh tokens, resource owner passwords,
and client secrets MUST NOT be transmitted in the clear.
Authorization codes SHOULD NOT be transmitted in the clear.

10.6. Endpoints Authenticity

In order to prevent man-in-the-middle and phishing attacks, the
authorization server MUST implement and require TLS with server-side server
authentication in all exchanges. exchanges as described by [RFC2818]. The
client MUST verify validate the authorization server's TLS certificate, as well as the respective certificate chain. in
accordance with its requirements for authentication of the server's
identity.

10.7. Credentials Guessing Attacks

The authorization server MUST prevent attackers from guessing access
tokens, authorization codes, refresh tokens, resource owner
passwords, and client secrets.

When generating tokens and other secrets not intended for direct
human utilization, the authorization server MUST use a reasonable
level of entropy in order to mitigate the risk of guessing attacks.
When creating secrets intended for human usage, the authorization
server MUST utilize other means to protect those secrets.

10.8. Phishing Attacks

Native applications SHOULD use external browsers instead of embedding
browsers within the application when requesting end-user resource owner
authorization. External browsers offer a familiar user experience
and a trusted environment in which end-users resource owners can confirm the
authenticity of the authorization server.

To reduce the risk of phishing attacks, the authorization servers
MUST utilize TLS to allow user-agents to validate the authorization
server's identity. Service providers should educate their end-users
about the risks of phishing attacks and how they can verify the
authorization server's identity.

10.9. Authorization Codes

The transmission of authorization codes SHOULD be made over a secure
channel, and the client SHOULD implement TLS for use with its
redirection URI if the URI identifies a network resource.
Authorization codes MUST be kept confidential. Since authorization
codes are transmitted via user-agent redirections, they could
potentially be disclosed through user-agent history and HTTP referrer
headers.

Authorization codes operate as plaintext bearer credentials, used to
verify that the end-user resource owner who granted authorization at the
authorization server, is the same end-user resource owner returning to the
client to complete the process. Therefore, if the client relies on
the authorization code for its own end-user resource owner authentication, the
client redirection endpoint MUST require TLS.

Authorization codes SHOULD MUST be short lived and MUST be single use. If the
authorization server observes multiple attempts to exchange an
authorization code for an access token, the authorization server
SHOULD attempt to revoke all access tokens already granted based on
the compromised authorization code.

If the client can be authenticated, the authorization servers MUST
authenticate the client and ensure that the authorization code was
issued to the same client.

10.10. Session Fixation Authorization Code Leakage

Session fixation attacks leverage the authorization code grant type,
by tricking an end-user a resource owner to authorize access to a legitimate
client, but to a client account under the control of the attacker.
The only difference between a valid flow and the attack flow is in
how the victim reached the authorization server to grant access.
Once at the authorization server, the victim is prompted with a
normal, valid request on behalf of a legitimate and familiar client.
The attacker then uses the victim's authorization to gain access to
the information authorized by the victim.

In order to prevent such an attack, authorization servers MUST ensure
that the redirection URI used to obtain the authorization code, is
the same as the redirection URI provided when exchanging the
authorization code for an access token. The authorization server
SHOULD require the client to pre-register register their redirection URI and if
provided, MUST validate the redirection URI received in the
authorization request against the pre-registered registered value.

10.11. Redirection URI Validation

[[ Add specific recommendations about redirection validation and
matching ]]

10.12. Resource Owner Password Credentials

The resource owner password credentials grant type is often used for
legacy or migration reasons. It reduces the overall risk of storing
username and password in the client, but does not eliminate the need
to expose highly privileged credentials to the client.

This grant type carries a higher risk than the other grant types
because it maintains the password anti-pattern OAuth seeks to avoid.
The client could abuse the password or the password could
unintentionally be disclosed to an attacker (e.g. via log files or
other records kept by the client).

Additionally, because the resource owner does not have control over
the authorization process (the resource owner involvement ends when
it hands over its credentials to the client), the client can obtain
access tokens with a broader scope and longer duration than desired
by the resource owner. The authorization server SHOULD restrict the
scope and duration of access tokens issued via this grant type.

The authorization server and client SHOULD minimize use of this grant
type and utilize other grant types whenever possible.

10.13. XSRF/CSRF Prevention

[[ Add text Cross-Site Request Forgery

Cross-site request forgery (CSRF) is a web-based attack whereby HTTP
requests are transmitted from the user-agent of an end-user the
server trusts or has authenticated. CSRF attacks on the
authorization endpoint can allow an attacker to obtain authorization
without the consent of the resource owner.

The "state" request parameter SHOULD be used to mitigate against CSRF
attacks, particularly for login CSRF attacks. CSRF attacks against
the client's redirection URI allow an attacker to inject their own
authorization code or access token, which can result in the client
using an access token associated with reference the attacker's account rather
than the victim's. Depending on the nature of the client and the
protected resources, this can have undesirable and damaging effects.

It is strongly RECOMMENDED that the client includes the "state"
request parameter with authorization requests to the 'state' authorization
server. The "state" request parameter ]] MUST contain a non-guessable
value, and the client MUST keep it in a location accessible only by
the client or the user-agent (i.e., protected by same-origin policy).

For example, using a DOM variable (protected by JavaScript or other
DOM-binding language's enforcement of SOP), HTTP cookie, or HTML5
client-side storage. The authorization server includes the value of
the "state" parameter when redirecting the user-agent back to the
client which MUST then ensure the received value matches the stored
value.

10.14. Clickjacking

11. IANA Considerations

11.1. The OAuth Access Token Type Registry

This specification establishes the OAuth access token type registry.

Access token types are registered on the advice of one or more
Designated Experts (appointed by the IESG or their delegate), with a
Specification Required (using terminology from [RFC5226]). However,
to allow for the allocation of values prior to publication, the
Designated Expert(s) may approve registration once they are satisfied
that such a specification will be published.

Registration requests should be sent to the [TBD]@ietf.org mailing
list for review and comment, with an appropriate subject (e.g.,
"Request for access toke type: example"). [[ Note to RFC-EDITOR: The
name of the mailing list should be determined in consultation with
the IESG and IANA. Suggested name: oauth-ext-review. ]]

Within at most 14 days of the request, the Designated Expert(s) will
either approve or deny the registration request, communicating this
decision to the review list and IANA. Denials should include an
explanation and, if applicable, suggestions as to how to make the
request successful.

Decisions (or lack thereof) made by the Designated Expert can be
first appealed to Application Area Directors (contactable using
app-ads@tools.ietf.org email address or directly by looking up their
email addresses on http://www.iesg.org/ website) and, if the
appellant is not satisfied with the response, to the full IESG (using
the iesg@iesg.org mailing list).

IANA should only accept registry updates from the Designated
Expert(s), and should direct all requests for registration to the
review mailing list.

11.1.1. Registration Template

Type name:
The name requested (e.g., "example").
Additional Token Endpoint Response Parameters:
Additional response parameters returned together with the
"access_token" parameter. New parameters MUST be separately
registered in the OAuth parameters registry as described by
Section 11.2.
HTTP Authentication Scheme(s):
The HTTP authentication scheme name(s), if any, used to
authenticate protected resources requests using access token of
this type.
Change controller:
For standards-track RFCs, state "IETF". For others, give the name
of the responsible party. Other details (e.g., postal address,
e-mail address, home page URI) may also be included.
Specification document(s):
Reference to document that specifies the parameter, preferably
including a URI that can be used to retrieve a copy of the
document. An indication of the relevant sections may also be
included, but is not required.

11.2. The OAuth Parameters Registry

This specification establishes the OAuth parameters registry.

Additional parameters for inclusion in the authorization endpoint
request, the authorization endpoint response, the token endpoint
request, or the token endpoint response, are registered on the advice
of one or more Designated Experts (appointed by the IESG or their
delegate), with a Specification Required (using terminology from
[RFC5226]). However, to allow for the allocation of values prior to
publication, the Designated Expert(s) may approve registration once
they are satisfied that such a specification will be published.

Registration requests should be sent to the [TBD]@ietf.org mailing
list for review and comment, with an appropriate subject (e.g.,
"Request for parameter: example"). [[ Note to RFC-EDITOR: The name of
the mailing list should be determined in consultation with the IESG
and IANA. Suggested name: oauth-ext-review. ]]

IANA should only accept registry updates from the Designated
Expert(s), and should direct all requests for registration to the
review mailing list.

11.2.1. Registration Template

Parameter name:
The name requested (e.g., "example").
Parameter usage location:
The location(s) where parameter can be used. The possible
locations are: authorization request, authorization response,
token request, or token response.
Change controller:
For standards-track RFCs, state "IETF". For others, give the name
of the responsible party. Other details (e.g., postal address,
e-mail address, home page URI) may also be included.
Specification document(s):
Reference to document that specifies the parameter, preferably
including a URI that can be used to retrieve a copy of the
document. An indication of the relevant sections may also be
included, but is not required.

11.2.2. Initial Registry Contents

The OAuth Parameters Registry's initial contents are:

o Parameter name: client_id
o Parameter usage location: authorization request, token request
o Change controller: IETF
o Specification document(s): [[ this document ]]

o Parameter name: client_secret
o Parameter usage location: token request
o Change controller: IETF
o Specification document(s): [[ this document ]]

o Parameter name: response_type
o Parameter usage location: authorization request
o Change controller: IETF
o Specification document(s): [[ this document ]]

o Parameter name: redirect_uri
o Parameter usage location: authorization request, token request
o Change controller: IETF
o Specification document(s): [[ this document ]]

o Parameter name: scope
o Parameter usage location: authorization request, authorization
response, token request, token response
o Change controller: IETF
o Specification document(s): [[ this document ]]

o Parameter name: state
o Parameter usage location: authorization request, authorization
response
o Change controller: IETF
o Specification document(s): [[ this document ]]

o Parameter name: code
o Parameter usage location: authorization response, token request
o Change controller: IETF
o Specification document(s): [[ this document ]]

o Parameter name: error_description
o Parameter usage location: authorization response, token response
o Change controller: IETF
o Specification document(s): [[ this document ]]

o Parameter name: error_uri
o Parameter usage location: authorization response, token response
o Change controller: IETF
o Specification document(s): [[ this document ]]

o Parameter name: grant_type
o Parameter usage location: token request
o Change controller: IETF
o Specification document(s): [[ this document ]]

o Parameter name: access_token
o Parameter usage location: authorization response, token response
o Change controller: IETF
o Specification document(s): [[ this document ]]

o Parameter name: token_type
o Parameter usage location: authorization response, token response
o Change controller: IETF
o Specification document(s): [[ this document ]]

o Parameter name: expires_in
o Parameter usage location: authorization response, token response
o Change controller: IETF
o Specification document(s): [[ this document ]]

o Parameter name: username
o Parameter usage location: token request
o Change controller: IETF
o Specification document(s): [[ this document ]]

o Parameter name: password
o Parameter usage location: token request
o Change controller: IETF
o Specification document(s): [[ this document ]]

o Parameter name: refresh_token
o Parameter usage location: token request, token response
o Change controller: IETF
o Specification document(s): [[ this document ]]

11.3. The OAuth Authorization Endpoint Response Type Registry

This specification establishes the OAuth authorization endpoint
response type registry.

Additional response type for use with the authorization endpoint are
registered on the advice of one or more Designated Experts (appointed
by the IESG or their delegate), with a Specification Required (using
terminology from [RFC5226]). However, to allow for the allocation of
values prior to publication, the Designated Expert(s) may approve
registration once they are satisfied that such a specification will
be published.

Registration requests should be sent to the [TBD]@ietf.org mailing
list for review and comment, with an appropriate subject (e.g.,
"Request for response type: example"). [[ Note to RFC-EDITOR: The
name of the mailing list should be determined in consultation with
the IESG and IANA. Suggested name: oauth-ext-review. ]]

IANA should only accept registry updates from the Designated
Expert(s), and should direct all requests for registration to the
review mailing list.

11.3.1. Registration Template

Response type name:
The name requested (e.g., "example").
Change controller:
For standards-track RFCs, state "IETF". For others, give the name
of the responsible party. Other details (e.g., postal address,
e-mail address, home page URI) may also be included.
Specification document(s):
Reference to document that specifies the type, preferably
including a URI that can be used to retrieve a copy of the
document. An indication of the relevant sections may also be
included, but is not required.

11.3.2. Initial Registry Contents

The OAuth Authorization Endpoint Response Type Registry's initial
contents are:

o Response type name: code
o Change controller: IETF
o Specification document(s): [[ this document ]]

o Response type name: token
o Change controller: IETF
o Specification document(s): [[ this document ]]

11.4. The OAuth Extensions Error Registry

This specification establishes the OAuth extensions error registry.

Additional error codes used together with other protocol extensions
(i.e. extension grant types, access token types, or extension
parameters) are registered on the advice of one or more Designated
Experts (appointed by the IESG or their delegate), with a
Specification Required (using terminology from [RFC5226]). However,
to allow for the allocation of values prior to publication, the
Designated Expert(s) may approve registration once they are satisfied
that such a specification will be published.

Registration requests should be sent to the [TBD]@ietf.org mailing
list for review and comment, with an appropriate subject (e.g.,
"Request for error code: example"). [[ Note to RFC-EDITOR: The name
of the mailing list should be determined in consultation with the
IESG and IANA. Suggested name: oauth-ext-review. ]]

IANA should only accept registry updates from the Designated
Expert(s), and should direct all requests for registration to the
review mailing list.

11.3.1.

11.4.1. Registration Template

Error name:
The name requested (e.g., "example").
Error usage location:
The location(s) where the error can be used. The possible
locations are: authorization code grant error response
(Section 4.1.2.1), implicit grant error response
(Section 4.2.2.1), or token error response (Section 5.2).
Related protocol extension:
The name of the extension grant type, access token type, or
extension parameter, the error code is used in conjunction with.
Change controller:
For standards-track RFCs, state "IETF". For others, give the name
of the responsible party. Other details (e.g., postal address,
e-mail address, home page URI) may also be included.
Specification document(s):
Reference to document that specifies the error code, preferably
including a URI that can be used to retrieve a copy of the
document. An indication of the relevant sections may also be
included, but is not required.

12. Acknowledgements

The initial OAuth 2.0 protocol specification was edited by David
Recordon, based on two previous publications: the OAuth 1.0 community
specification [RFC5849], and OAuth WRAP (OAuth Web Resource
Authorization Profiles) [I-D.draft-hardt-oauth-01]. The Security
Considerations section was drafted by Torsten Lodderstedt, Mark
McGloin, Phil Hunt, and Anthony Nadalin.

The OAuth 1.0 community specification was edited by Eran Hammer-Lahav
and authored by Mark Atwood, Dirk Balfanz, Darren Bounds, Richard M.
Conlan, Blaine Cook, Leah Culver, Breno de Medeiros, Brian Eaton,
Kellan Elliott-McCrea, Larry Halff, Eran Hammer-Lahav, Ben Laurie,
Chris Messina, John Panzer, Sam Quigley, David Recordon, Eran
Sandler, Jonathan Sergent, Todd Sieling, Brian Slesinsky, and Andy
Smith.

The OAuth WRAP specification was edited by Dick Hardt and authored by
Brian Eaton, Yaron Goland, Dick Hardt, and Allen Tom.

This specification is the work of the OAuth Working Group which
includes dozens of active and dedicated participants. In particular,
the following individuals contributed ideas, feedback, and wording
which shaped and formed the final specification:

Michael Adams, Andrew Arnott, Dirk Balfanz, Scott Cantor, Blaine
Cook, Brian Campbell, Brian Eaton, Leah Culver, Bill de hOra, Brian
Eaton, Brian Ellin, Igor Faynberg, George Fletcher, Tim Freeman, Evan
Gilbert, Yaron Goland, Brent Goldman, Kristoffer Gronowski, Justin
Hart, Dick Hardt, Craig Heath, Phil Hunt, Michael B. Jones, John
Kemp, Mark Kent, Raffi Krikorian, Chasen Le Hara, Rasmus Lerdorf,
Torsten Lodderstedt, Hui-
Lan Hui-Lan Lu, Paul Madsen, Alastair Mair, Eve
Maler, James Manger, Mark McGloin, Laurence Miao, Chuck Mortimore,
Anthony Nadalin, Justin Richer, Peter Saint-
Andre, Saint-Andre, Nat Sakimura, Rob
Sayre, Marius Scurtescu, Naitik Shah, Luke Shepard, Vlad Skvortsov,
Justin Smith, Jeremy Suriel, Christian Stuebner, Paul Tarjan, Allen
Tom, Franklin Tse, Nick Walker, Shane Weeden, and Skylar Woodward.

Appendix A. Editor's Notes

While many people contributed to this specification throughout its
long journey, the editor would like to acknowledge and thank a few
individuals for their outstanding and invaluable efforts leading up
to the publication of this specification. It is these individuals
without whom this work would not have existed, existed or reached its
successful conclusion.

David Recordon for continuously being one of OAuth's most valuable
assets, bringing pragmatism and urgency to the work, and helping
shape it from its very beginning, as well as being one of the best
collaborators I had the pleasure of working with.

Mark Nottingham for introducing OAuth to the IETF and setting the
community on this course. Lisa Dusseault for her support and
guidance as the Application area director. Blaine Cook, Peter Saint-
Andre, and Hannes Tschofenig for their work as working group chairs.

James Manger for his creative ideas and always insightful feedback.
Brian Campbell, Torsten Lodderstedt, Chuck Mortimore, Justin Richer,
Marius Scurtescu, and Luke Shepard for their continued participation
and valuable feedback.

Special thanks goes to Mike Curtis and Yahoo! for their unconditional
support of this work for over three years.

13. References

13.1. Normative References

[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,
RFC 3986, January 2005.

[RFC4627] Crockford, D., "The application/json Media Type for
JavaScript Object Notation (JSON)", RFC 4627, July 2006.

[RFC4949] Shirey, R., "Internet Security Glossary, Version 2",
RFC 4949, August 2007.

[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.

[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.

[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.

[W3C.REC-html401-19991224]
Hors, A.,
Jacobs, I., Hors, A., and D. Raggett, "HTML 4.01
Specification", World Wide Web Consortium
Recommendation REC-html401-19991224, December 1999,
<http://www.w3.org/TR/1999/REC-html401-19991224>.

13.2. Informative References

[I-D.draft-hardt-oauth-01]
Hardt, D., Ed., Tom, A., Eaton, B., and Y. Goland, "OAuth
Web Resource Authorization Profiles", January 2010.

[I-D.ietf-oauth-saml2-bearer]
Campbell, B. and C. Mortimore, "SAML 2.0 Bearer Assertion
Grant Type Profile for OAuth 2.0",
draft-ietf-oauth-saml2-bearer-03 (work in progress),
February 2011.

[I-D.ietf-oauth-v2-bearer]
Jones, M., Hardt, D., and D. Recordon, "The OAuth 2.0
Protocol: Bearer Tokens", draft-ietf-oauth-v2-bearer-04
(work in progress), March 2011.

[I-D.ietf-oauth-v2-http-mac]
Hammer-Lahav, E., Barth, A., and B. Adida, "HTTP
Authentication: MAC Access Authentication",
draft-ietf-oauth-v2-http-mac-00 (work in progress),
May 2011.

[I-D.lodderstedt-oauth-security]
Lodderstedt, T., McGloin, M., and P. Hunt, "OAuth 2.0
Threat Model and Security Considerations",
draft-lodderstedt-oauth-security-01 (work in progress),
March 2011.

[OASIS.saml-core-2.0-os]
Cantor, S., Kemp, J., Philpott, R., and E. Maler,
"Assertions and Protocol for the OASIS Security Assertion
Markup Language (SAML) V2.0", OASIS Standard saml-core-
2.0-os, March 2005.

[RFC5849] Hammer-Lahav, E., "The OAuth 1.0 Protocol", RFC 5849,
April 2010.

Authors' Addresses

Eran Hammer-Lahav (editor)
Yahoo!

Email: eran@hueniverse.com
URI: http://hueniverse.com

David Recordon
Facebook

Email: dr@fb.com
URI: http://www.davidrecordon.com/

Dick Hardt
Microsoft

Email: dick.hardt@gmail.com
URI: http://dickhardt.org/