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Internet Engineering Task Force (IETF) D. Hardt, Ed.
Request for Comments: 6749 Microsoft
Obsoletes: 5849 October 2012
Category: Standards Track
ISSN: 2070-1721
The OAuth 2.0 Authorization Framework
Abstract
The OAuth 2.0 authorization framework enables a third-party
application to obtain limited access to an HTTP service, either on
behalf of a resource owner by orchestrating an approval interaction
between the resource owner and the HTTP service, or by allowing the
third-party application to obtain access on its own behalf. This
specification replaces and obsoletes the OAuth 1.0 protocol described
in RFC 5849.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6749.
Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust’s Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Hardt Standards Track [Page 1]

RFC 6749 OAuth 2.0 October 2012
Table of Contents
1. Introduction ....................................................4
1.1. Roles ......................................................6
1.2. Protocol Flow ..............................................7
1.3. Authorization Grant ........................................8
1.3.1. Authorization Code ..................................8
1.3.2. Implicit ............................................8
1.3.3. Resource Owner Password Credentials .................9
1.3.4. Client Credentials ..................................9
1.4. Access Token ..............................................10
1.5. Refresh Token .............................................10
1.6. TLS Version ...............................................12
1.7. HTTP Redirections .........................................12
1.8. Interoperability ..........................................12
1.9. Notational Conventions ....................................13
2. Client Registration ............................................13
2.1. Client Types ..............................................14
2.2. Client Identifier .........................................15
2.3. Client Authentication .....................................16
2.3.1. Client Password ....................................16
2.3.2. Other Authentication Methods .......................17
2.4. Unregistered Clients ......................................17
3. Protocol Endpoints .............................................18
3.1. Authorization Endpoint ....................................18
3.1.1. Response Type ......................................19
3.1.2. Redirection Endpoint ...............................19
3.2. Token Endpoint ............................................21
3.2.1. Client Authentication ..............................22
3.3. Access Token Scope ........................................23
4. Obtaining Authorization ........................................23
4.1. Authorization Code Grant ..................................24
4.1.1. Authorization Request ..............................25
4.1.2. Authorization Response .............................26
4.1.3. Access Token Request ...............................29
4.1.4. Access Token Response ..............................30
4.2. Implicit Grant ............................................31
4.2.1. Authorization Request ..............................33
4.3. Resource Owner Password Credentials Grant .................37
4.3.1. Authorization Request and Response .................39
4.4. Client Credentials Grant ..................................40
4.4.1. Authorization Request and Response .................41
4.5. Extension Grants ..........................................42

5. Issuing an Access Token ........................................43
5.1. Successful Response .......................................43
5.2. Error Response ............................................45
6. Refreshing an Access Token .....................................47
7. Accessing Protected Resources ..................................48
7.1. Access Token Types ........................................49
7.2. Error Response ............................................49
8. Extensibility ..................................................50
8.1. Defining Access Token Types ...............................50
8.2. Defining New Endpoint Parameters ..........................50
8.3. Defining New Authorization Grant Types ....................51
8.4. Defining New Authorization Endpoint Response Types ........51
8.5. Defining Additional Error Codes ...........................51
9. Native Applications ............................................52
10. Security Considerations .......................................53
10.1. Client Authentication ....................................53
10.2. Client Impersonation .....................................54
10.3. Access Tokens ............................................55
10.4. Refresh Tokens ...........................................55
10.5. Authorization Codes ......................................56
10.6. Authorization Code Redirection URI Manipulation ..........56
10.7. Resource Owner Password Credentials ......................57
10.8. Request Confidentiality ..................................58
10.9. Ensuring Endpoint Authenticity ...........................58
10.10. Credentials-Guessing Attacks ............................58
10.11. Phishing Attacks ........................................58
10.12. Cross-Site Request Forgery ..............................59
10.13. Clickjacking ............................................60
10.14. Code Injection and Input Validation .....................60
10.15. Open Redirectors ........................................60
10.16. Misuse of Access Token to Impersonate Resource
Owner in Implicit Flow ..................................61
11. IANA Considerations ...........................................62
11.1. OAuth Access Token Types Registry ........................62
11.1.1. Registration Template .............................62
11.2. OAuth Parameters Registry ................................63
11.2.2. Initial Registry Contents .........................64
11.3. OAuth Authorization Endpoint Response Types Registry .....66
11.3.2. Initial Registry Contents .........................67
11.4. OAuth Extensions Error Registry ..........................67
12. References ....................................................68
12.1. Normative References .....................................68
12.2. Informative References ...................................70

Appendix A. Augmented Backus-Naur Form (ABNF) Syntax ..............71
A.1. "client_id" Syntax ........................................71
A.2. "client_secret" Syntax ....................................71
A.3. "response_type" Syntax ....................................71
A.4. "scope" Syntax ............................................72
A.5. "state" Syntax ............................................72
A.6. "redirect_uri" Syntax .....................................72
A.7. "error" Syntax ............................................72
A.8. "error_description" Syntax ................................72
A.9. "error_uri" Syntax ........................................72
A.10. "grant_type" Syntax .......................................73
A.11. "code" Syntax .............................................73
A.12. "access_token" Syntax .....................................73
A.13. "token_type" Syntax .......................................73
A.14. "expires_in" Syntax .......................................73
A.15. "username" Syntax .........................................73
A.16. "password" Syntax .........................................73
A.17. "refresh_token" Syntax ....................................74
A.18. Endpoint Parameter Syntax .................................74
Appendix B. Use of application/x-www-form-urlencoded Media Type ...74
Appendix C. Acknowledgements ......................................75
1. Introduction
In the traditional client-server authentication model, the client
requests an access-restricted resource (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
restricted 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
owner’s credentials for future use, typically a password in
clear-text.
o Servers are required to support password authentication, despite
the security weaknesses inherent in passwords.
o Third-party applications gain overly broad access to the 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 the third party’s 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, lifetime, 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, 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 over any protocol other than HTTP is out of scope.
The OAuth 1.0 protocol ([RFC5849]), published as an informational
document, was the result of a small ad hoc community effort. This
Standards Track specification builds on the OAuth 1.0 deployment
experience, as well as additional use cases and extensibility
requirements gathered from the wider IETF community. The OAuth 2.0
protocol is not backward compatible with OAuth 1.0. The two versions
may co-exist on the network, and implementations may choose to
support both. However, it is the intention of this specification
that new implementations support OAuth 2.0 as specified in this
document and that OAuth 1.0 is used only to support existing
deployments. The OAuth 2.0 protocol shares very few implementation
details with the OAuth 1.0 protocol. Implementers familiar with
OAuth 1.0 should approach this document without any assumptions as to
its structure and details.

1.1. Roles
OAuth defines four roles:
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.
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. The term "client" does
not imply any particular implementation characteristics (e.g.,
whether the application executes on a server, a desktop, or other
devices).
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
+--------+ +---------------+
| |--(A)- Authorization Request ->| Resource |
| | | Owner |
| |<-(B)-- Authorization Grant ---| |
| | +---------------+
| |
| | +---------------+
| |--(C)-- Authorization Grant -->| Authorization |
| Client | | Server |
| |<-(D)----- Access Token -------| |
| | +---------------+
| |
| | +---------------+
| |--(E)----- Access Token ------>| Resource |
| | | Server |
| |<-(F)--- Protected Resource ---| |
+--------+ +---------------+
Figure 1: Abstract Protocol Flow
The abstract OAuth 2.0 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 the authorization
server as an intermediary.
(B) The client receives an authorization grant, which is a
credential representing the resource owner’s authorization,
expressed using one of four grant types defined in this
specification or using an extension grant type. The
authorization grant type depends on the method used by the
client to request authorization and the types supported by the
authorization server.
(C) The client requests an access token by authenticating with the
authorization server and presenting the authorization grant.
(D) The authorization server authenticates the client 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.
The preferred method for the client to obtain an authorization grant
from the resource owner (depicted in steps (A) and (B)) is to use the
authorization server as an intermediary, which is illustrated in
Figure 3 in Section 4.1.
1.3. Authorization Grant
An authorization grant is a credential representing the resource
owner’s authorization (to access its protected resources) 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.
1.3.1. 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, as well as the
transmission of the access token directly to the client without
passing it through the resource owner’s user-agent and potentially
exposing it to others, including the resource owner.
1.3.2. Implicit
The implicit grant is a simplified authorization code flow optimized
for clients implemented in a browser using a scripting language such
as JavaScript. In the implicit flow, instead of issuing the client
an authorization code, the client is issued an access token directly

(as the result of the resource owner authorization). The grant type
is implicit, as no intermediate credentials (such as an authorization
code) are issued (and later used to obtain an access token).
When issuing an access token during the implicit grant flow, the
authorization server does not authenticate the client. In some
cases, the client identity can be verified via the 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 weighed against
the security implications of using implicit grants, such as those
described in Sections 10.3 and 10.16, especially when the
authorization code grant type is available.
1.3.3. Resource Owner Password Credentials
The resource owner password credentials (i.e., 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., the
client is part of the device 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. This
grant type can eliminate the need for the client to store the
resource owner credentials for future use, by exchanging the
credentials with a long-lived access token or refresh token.
1.3.4. 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) or is requesting access to protected
resources based on an authorization previously arranged with the

1.4. 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 may self-contain the authorization information in a
verifiable manner (i.e., a token string consisting of some data and a
signature). Additional authentication 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 such
as [RFC6750].
1.5. Refresh Token
Refresh tokens are credentials used to obtain access tokens. Refresh
tokens are issued to the client by the authorization server and are
used to obtain a new access token when the current access token
becomes invalid or expires, or to obtain additional access tokens
with identical or narrower scope (access tokens may have a shorter
lifetime and fewer permissions than authorized by the resource
owner). Issuing a refresh token is optional at the discretion of the
authorization server. If the authorization server issues a refresh
token, it is included when issuing an access token (i.e., step (D) in
Figure 1).
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 denotes an identifier used to retrieve the

authorization information. Unlike access tokens, refresh tokens are
intended for use only with authorization servers and are never sent
to resource servers.
+--------+ +---------------+
| |--(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 ----------->| |
| | | |
| |<-(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 and presenting an authorization grant.
(B) The authorization server authenticates the client and validates
the authorization grant, and if valid, issues an access token
and a refresh token.
(C) The client makes a protected resource request 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 and presenting the refresh token. The
client authentication requirements are based on the client type
and on the authorization server policies.
(H) The authorization server authenticates the client and validates
the refresh token, and if valid, issues a new access token (and,
optionally, a new refresh token).
Steps (C), (D), (E), and (F) are outside the scope of this
specification, as described in Section 7.
1.6. TLS Version
Whenever Transport Layer Security (TLS) is used by this
specification, the appropriate version (or versions) of TLS will vary
over time, based on the widespread deployment and known security
vulnerabilities. At the time of this writing, TLS version 1.2
[RFC5246] is the most recent version, but has a very limited
deployment base and might not be readily available for
implementation. TLS version 1.0 [RFC2246] is the most widely
deployed version and will provide the broadest interoperability.
Implementations MAY also support additional transport-layer security
mechanisms that meet their security requirements.
1.7. HTTP Redirections
This specification makes extensive use of HTTP redirections, in which
the client or the authorization server directs the resource owner’s
user-agent to another destination. While the examples in this
specification show the use of the HTTP 302 status code, any other
method available via the user-agent to accomplish this redirection is
allowed and is considered to be an implementation detail.
1.8. Interoperability
OAuth 2.0 provides a rich authorization framework with well-defined
security properties. However, as a rich and highly extensible
framework with many optional components, on its own, this
specification is likely to produce a wide range of non-interoperable
implementations.
In addition, this specification leaves a few required components
partially or fully undefined (e.g., client registration,
authorization server capabilities, endpoint discovery). Without

these components, clients must be manually and specifically
configured against a specific authorization server and resource
server in order to interoperate.
This framework was designed with the clear expectation that future
work will define prescriptive profiles and extensions necessary to
achieve full web-scale interoperability.
1.9. 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]. Additionally, the rule URI-reference is
included from "Uniform Resource Identifier (URI): Generic Syntax"
[RFC3986].
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
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 end-user 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.

When registering a client, the client developer SHALL:
o specify the client type as described in Section 2.1,
o provide its client redirection URIs as described in Section 3.1.2,
and
o include any other information required by the authorization server
(e.g., application name, website, description, logo image, the
acceptance of legal terms).
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):
confidential
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 device used by the
resource owner, such as an installed native application or a web
browser-based application), and incapable of secure client
authentication via any other means.
The client type designation is based on the authorization server’s
definition of secure authentication and its acceptable exposure
levels of client credentials. The authorization server SHOULD NOT
make assumptions about the client type.
A client may be implemented as a distributed set of components, each
with a different client type and security context (e.g., a
distributed client with both a confidential server-based component
and a public browser-based component). If the authorization server
does not provide support for such clients or does not provide
guidance with regard to their registration, the client SHOULD
register each component as a separate client.

This specification has been designed around the following client
profiles:
web application
A web application is a confidential client running on a web
server. Resource owners access the client via an HTML user
interface rendered in a user-agent on the device used by the
resource owner. 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 resource owner.
user-agent-based application
A user-agent-based application is a public client in which the
client code is downloaded from a web server and executes within a
user-agent (e.g., web browser) on the device used by the resource
owner. Protocol data and credentials are easily accessible (and
often visible) to the resource owner. Since such applications
reside within the user-agent, they can make seamless use of the
user-agent capabilities when requesting authorization.
native application
A native application is a public client installed and executed on
the device used by the resource owner. Protocol data and
credentials are accessible to the resource owner. It is assumed
that any client authentication credentials included in the
application can be extracted. On the other hand, dynamically
issued credentials such as access tokens or refresh tokens can
receive an acceptable level of protection. At a minimum, these
credentials are protected from hostile servers with which the
application may interact. On some platforms, these credentials
might be protected from other applications residing on the same
device.
2.2. 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 MUST NOT be used
alone for client authentication. The client identifier is unique to
the authorization server.
The client identifier string size is left undefined by this
specification. The client should avoid making assumptions about the
identifier size. The authorization server SHOULD document the size
of any identifier it issues.

2.3. Client Authentication
If the client type is confidential, the client and authorization
server establish a client authentication method suitable for the
security requirements of the authorization server. The authorization
server MAY accept any form of client authentication meeting its
security requirements.
Confidential 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 MAY establish a client authentication method
with public clients. However, the authorization server MUST NOT rely
on public client authentication for the purpose of identifying the
client.
The client MUST NOT use more than one authentication method in each
request.
2.3.1. 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 encoded using the
"application/x-www-form-urlencoded" encoding algorithm per
Appendix B, and the encoded value is used as the username; the client
password is encoded using the same algorithm and used as the
password. The authorization server MUST support the HTTP Basic
authentication scheme for authenticating clients that were issued a
client password.
For example (with extra line breaks for display purposes only):
Authorization: Basic czZCaGRSa3F0Mzo3RmpmcDBaQnIxS3REUmJuZlZkbUl3
Alternatively, the authorization server MAY support 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 2.2.
client_secret
REQUIRED. The client secret. The client MAY omit the
parameter if the client secret is an empty string.

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). The parameters can only
be transmitted in the request-body and MUST NOT be included in the
request URI.
For example, a request to refresh an access token (Section 6) using
the body parameters (with extra line breaks for display purposes
only):
POST /token HTTP/1.1
Host: server.example.com
Content-Type: application/x-www-form-urlencoded
grant_type=refresh_token&refresh_token=tGzv3JOkF0XG5Qx2TlKWIA
&client_id=s6BhdRkqt3&client_secret=7Fjfp0ZBr1KtDRbnfVdmIw
The authorization server MUST require the use of TLS as described in
Section 1.6 when sending requests using password authentication.
Since this client authentication method involves a password, the
authorization server MUST protect any endpoint utilizing it against
brute force attacks.
2.3.2. 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.
2.4. Unregistered Clients
This specification does not exclude the use of unregistered clients.
However, the use of 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 authorization server endpoints
(HTTP resources):
o Authorization endpoint - used by the client to obtain
authorization from the resource owner via user-agent redirection.
o Token endpoint - used by the client to exchange an authorization
grant for an access token, typically with client authentication.
As well as one client endpoint:
o Redirection endpoint - used by the authorization server to return
responses containing authorization credentials to the client via
the resource owner user-agent.
Not every authorization grant type utilizes both endpoints.
Extension grant types MAY define additional endpoints as needed.
3.1. Authorization Endpoint
The authorization endpoint is used to interact with the resource
owner and obtain an authorization grant. 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 an "application/x-www-form-urlencoded"
formatted (per Appendix B) query component ([RFC3986] Section 3.4),
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 TLS
as described in Section 1.6 when sending requests to the
authorization endpoint.
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.

Parameters sent without a value MUST be treated as if they were
omitted from the request. The authorization server MUST ignore
unrecognized request parameters. Request and response parameters
MUST NOT be included more than once.
3.1.1. Response Type
The authorization endpoint is used by the authorization code grant
type and implicit grant type flows. The client informs the
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 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 registered extension value as described by
Section 8.4.
Extension response types MAY contain a space-delimited (%x20) list of
values, where the order of values does not matter (e.g., response
type "a b" is the same as "b a"). The meaning of such composite
response types is defined by their respective specifications.
If an authorization request is missing the "response_type" parameter,
or if the response type is not understood, the authorization server
MUST return an error response as described in Section 4.1.2.1.
3.1.2. Redirection Endpoint
After completing its interaction with the resource owner, the
authorization server directs the resource owner’s user-agent back to
the client. The authorization server redirects the user-agent to the
client’s redirection endpoint previously established with the
authorization server during the client registration process or when
making the authorization request.
The redirection endpoint URI MUST be an absolute URI as defined by
[RFC3986] Section 4.3. The endpoint URI MAY include an
"application/x-www-form-urlencoded" formatted (per Appendix B) query
component ([RFC3986] Section 3.4), which MUST be retained when adding
additional query parameters. The endpoint URI MUST NOT include a
fragment component.

3.1.2.1. Endpoint Request Confidentiality
The redirection endpoint SHOULD require the use of TLS as described
in Section 1.6 when the requested response type is "code" or "token",
or when the redirection request will result in the transmission of
sensitive credentials over an open network. This specification does
not mandate the use of TLS because at the time of this writing,
requiring clients to deploy TLS is a significant hurdle for many
client developers. If TLS is not available, the authorization server
SHOULD warn the resource owner about the insecure endpoint prior to
redirection (e.g., display a message during the authorization
request).
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 the following clients to
register their redirection endpoint:
o Public clients.
o Confidential clients utilizing the implicit grant type.
The authorization server SHOULD require all clients to register their
redirection endpoint prior to utilizing the authorization endpoint.
The authorization server SHOULD require the client to provide the
complete redirection URI (the client MAY use the "state" request
parameter to achieve per-request customization). If requiring the
registration of the complete redirection URI is not possible, the
authorization server SHOULD require the registration of the URI
scheme, authority, and path (allowing the client to dynamically vary
only the query component of the redirection URI when requesting
authorization).
The authorization server MAY allow the client to register multiple
redirection endpoints.
Lack of a redirection URI registration requirement can enable an
attacker to use the authorization endpoint as an open redirector as
described in Section 10.15.

3.1.2.3. Dynamic Configuration
If multiple redirection URIs have been registered, if only part of
the redirection URI 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 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 client registration included the full
redirection URI, the authorization server MUST compare the two URIs
using simple string comparison as defined in [RFC3986] Section 6.2.1.
3.1.2.4. Invalid Endpoint
If an authorization request fails validation due to a missing,
invalid, or mismatching redirection URI, the authorization server
SHOULD inform the resource owner of the error and MUST NOT
automatically redirect the user-agent to the invalid redirection URI.
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 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 and the credentials it contains.
The client SHOULD NOT include any third-party scripts (e.g., third-
party analytics, social plug-ins, ad networks) in the redirection
endpoint response. Instead, it SHOULD extract the credentials from
the URI and redirect the user-agent again to another endpoint without
exposing the credentials (in the URI or elsewhere). If third-party
scripts are included, the client MUST ensure 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
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 the location
is typically provided in the service documentation.
The endpoint URI MAY include an "application/x-www-form-urlencoded"
formatted (per Appendix B) query component ([RFC3986] Section 3.4),
which MUST be retained when adding additional query parameters. The
endpoint URI MUST NOT include a fragment component.
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 TLS as described in
Section 1.6 when sending requests to the token endpoint.
The client MUST use the HTTP "POST" method when making access token
requests.
Parameters sent without a value MUST be treated as if they were
omitted from the request. The authorization server MUST ignore
unrecognized request parameters. Request and response parameters
MUST NOT be included more than once.
3.2.1. Client Authentication
Confidential clients or other clients issued client credentials MUST
authenticate with the authorization server as described in
Section 2.3 when making requests to the token endpoint. Client
authentication is used for:
o Enforcing the binding of refresh tokens and authorization codes to
the client they were issued to. Client authentication is critical
when an authorization code is transmitted to the redirection
endpoint over an insecure channel or when the redirection URI has
not been registered in full.
o Recovering from a compromised client by disabling the client or
changing its credentials, thus preventing an attacker from abusing
stolen refresh tokens. Changing a single set of client
credentials is significantly faster than revoking an entire set of
refresh tokens.
o Implementing authentication management best practices, which
require periodic credential rotation. Rotation of an entire set
of refresh tokens can be challenging, while rotation of a single
set of client credentials is significantly easier.

A client MAY use the "client_id" request parameter to identify itself
when sending requests to the token endpoint. In the
"authorization_code" "grant_type" request to the token endpoint, an
unauthenticated client MUST send its "client_id" to prevent itself
from inadvertently accepting a code intended for a client with a
different "client_id". This protects the client from substitution of
the authentication code. (It provides no additional security for the
protected resource.)
3.3. Access Token Scope
The authorization and token endpoints allow the client to specify the
scope of the access request using the "scope" request parameter. In
turn, the authorization server uses the "scope" response parameter to
inform the client of the scope of the access token issued.
The value of the scope parameter is expressed as a list of space-
delimited, case-sensitive strings. The strings are 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.
scope = scope-token *( SP scope-token )
scope-token = 1*( %x21 / %x23-5B / %x5D-7E )
The authorization server MAY fully or partially ignore the scope
requested by the client, based on the authorization server policy or
the resource owner’s instructions. If the issued access token scope
is different from the one requested by the client, the authorization
server MUST include the "scope" response parameter to inform the
client of the actual scope granted.
If the client omits the scope parameter when requesting
authorization, the authorization server MUST either process the
request using a pre-defined default value or fail the request
indicating an invalid scope. The authorization server SHOULD
document its scope requirements and default value (if defined).
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 Grant
The authorization code grant type is used to obtain both access
tokens and refresh tokens and is optimized for confidential clients.
Since this is 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)-- & Redirection URI ---->| |
| User- | | Authorization |
| Agent -+----(B)-- User authenticates --->| Server |
| | | |
| -+----(C)-- Authorization Code ---<| |
+-|----|---+ +---------------+
| | ^ v
(A) (C) | |
| | | |
^ v | |
+---------+ | |
| |>---(D)-- Authorization Code ---------’ |
| Client | & Redirection URI |
| | |
| |<---(E)----- Access Token -------------------’
+---------+ (w/ Optional Refresh Token)
Note: The lines illustrating steps (A), (B), and (C) are broken into
two parts as they pass through the user-agent.
Figure 3: Authorization Code Flow

(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 (in the request or during
client registration). 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 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 authenticates the client, validates the
authorization code, and ensures that the redirection URI
received matches the URI used to redirect the client in
step (C). If valid, the authorization server responds back with
an access token and, optionally, a refresh 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, per Appendix B:
response_type
REQUIRED. Value MUST be set to "code".
client_id
REQUIRED. The client identifier as described in Section 2.2.
redirect_uri
OPTIONAL. As described in Section 3.1.2.

scope
OPTIONAL. The scope of the access request as described by
Section 3.3.
state
RECOMMENDED. 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 parameter SHOULD be used for preventing
cross-site request forgery as described in Section 10.12.
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 TLS (with extra line breaks for display purposes
only):
GET /authorize?response_type=code&client_id=s6BhdRkqt3&state=xyz
&redirect_uri=https%3A%2F%2Fclient%2Eexample%2Ecom%2Fcb HTTP/1.1
The authorization server validates the request to ensure that 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,
per Appendix B:
code
REQUIRED. The authorization code generated by the
authorization server. The authorization code MUST expire
shortly after it is issued to mitigate the risk of leaks. A
maximum authorization code lifetime of 10 minutes is
RECOMMENDED. The client MUST NOT use the authorization code

more than once. If an authorization code is used more than
once, the authorization server MUST deny the request and SHOULD
revoke (when possible) 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. 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=SplxlOBeZQQYbYS6WxSbIA
&state=xyz
The client MUST 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 is missing or 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, per Appendix B:
error
REQUIRED. A single ASCII [USASCII] error code from the
following:
invalid_request
The request is missing a required parameter, includes an
invalid parameter value, includes a parameter more than
once, 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.
server_error
The authorization server encountered an unexpected
condition that prevented it from fulfilling the request.
(This error code is needed because a 500 Internal Server
Error HTTP status code cannot be returned to the client
via an HTTP redirect.)
temporarily_unavailable
The authorization server is currently unable to handle
the request due to a temporary overloading or maintenance
of the server. (This error code is needed because a 503
Service Unavailable HTTP status code cannot be returned
to the client via an HTTP redirect.)
Values for the "error" parameter MUST NOT include characters
outside the set %x20-21 / %x23-5B / %x5D-7E.
error_description
OPTIONAL. Human-readable ASCII [USASCII] text providing
additional information, used to assist the client developer in
understanding the error that occurred.
Values for the "error_description" parameter MUST NOT include
characters outside the set %x20-21 / %x23-5B / %x5D-7E.
error_uri
OPTIONAL. A URI identifying a human-readable web page with
information about the error, used to provide the client
developer with additional information about the error.
Values for the "error_uri" parameter MUST conform to the
URI-reference syntax and thus MUST NOT include characters
outside the set %x21 / %x23-5B / %x5D-7E.

state
REQUIRED if a "state" parameter was present in the client
client.
HTTP/1.1 302 Found
Location: 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 sending the
following parameters using the "application/x-www-form-urlencoded"
format per Appendix B with a character encoding of UTF-8 in the HTTP
request entity-body:
grant_type
REQUIRED. Value MUST be set to "authorization_code".
code
REQUIRED. The authorization code received from the
redirect_uri
REQUIRED, if the "redirect_uri" parameter was included in the
authorization request as described in Section 4.1.1, and their
values MUST be identical.
client_id
REQUIRED, if the client is not authenticating with the
authorization server as described in Section 3.2.1.
If the client type is confidential or the client was issued client
credentials (or assigned other authentication requirements), the
client MUST authenticate with the authorization server as described
in Section 3.2.1.

For example, the client makes the following HTTP request using TLS
(with extra line breaks for display purposes only):
Authorization: Basic czZCaGRSa3F0MzpnWDFmQmF0M2JW
grant_type=authorization_code&code=SplxlOBeZQQYbYS6WxSbIA
&redirect_uri=https%3A%2F%2Fclient%2Eexample%2Ecom%2Fcb
The authorization server MUST:
o require client authentication for confidential clients or for any
client that was issued client credentials (or with other
authentication requirements),
o authenticate the client if client authentication is included,
o ensure that the authorization code was issued to the authenticated
confidential client, or if the client is public, ensure that the
code was issued to "client_id" in the request,
o verify that the authorization code is valid, and
o ensure that the "redirect_uri" parameter is present if the
"redirect_uri" parameter was included in the initial authorization
request as described in Section 4.1.1, and if included ensure 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;charset=UTF-8
Cache-Control: no-store
Pragma: no-cache
{
"access_token":"2YotnFZFEjr1zCsicMWpAA",
"token_type":"example",
"expires_in":3600,
"refresh_token":"tGzv3JOkF0XG5Qx2TlKWIA",
"example_parameter":"example_value"
}
4.2. Implicit Grant
The implicit grant type is used to obtain access tokens (it does not
support the issuance of refresh tokens) and is optimized for public
clients known to operate a particular redirection URI. These clients
are typically implemented in a browser using a scripting language
such as JavaScript.
Since this is 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 for an access token, the
client receives the access token as the result of the authorization
request.
The implicit grant type does not include client authentication, and
relies on the presence of the resource owner and 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 the same device.

+----------+
| Resource |
| Owner |
| |
+----------+
^
|
(B)
+----|-----+ Client Identifier +---------------+
| -+----(A)-- & Redirection URI --->| |
| User- | | Authorization |
| Agent -|----(B)-- User authenticates -->| Server |
| | | |
| |<---(C)--- Redirection URI ----<| |
| | with Access Token +---------------+
| | in Fragment
| | +---------------+
| |----(D)--- Redirection URI ---->| Web-Hosted |
| | without Fragment | Client |
| | | Resource |
| (F) |<---(E)------- Script ---------<| |
| | +---------------+
+-|--------+
| |
(A) (G) Access Token
| |
^ v
+---------+
| |
| Client |
| |
+---------+
Note: The lines illustrating steps (A) and (B) are broken into two
parts as they pass through the user-agent.
Figure 4: Implicit Grant Flow

(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
the access token in the URI fragment.
(D) The user-agent follows the redirection instructions by making a
request to the web-hosted client resource (which does not
include the fragment per [RFC2616]). The user-agent retains the
fragment information locally.
(E) The 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, and extracting the access token (and other
parameters) contained in the fragment.
(F) The user-agent executes the script provided by the web-hosted
client resource locally, which extracts the access token.
(G) The user-agent passes the access token to the client.
See Sections 1.3.2 and 9 for background on using the implicit grant.
See Sections 10.3 and 10.16 for important security considerations
when using the implicit grant.
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, per Appendix B:
response_type
REQUIRED. Value MUST be set to "token".
client_id
REQUIRED. The client identifier as described in Section 2.2.

redirect_uri
OPTIONAL. As described in Section 3.1.2.
scope
Section 3.3.
state
RECOMMENDED. 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 parameter SHOULD be used for preventing
cross-site request forgery as described in Section 10.12.
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 TLS (with extra line breaks for display purposes
only):
GET /authorize?response_type=token&client_id=s6BhdRkqt3&state=xyz
&redirect_uri=https%3A%2F%2Fclient%2Eexample%2Ecom%2Fcb HTTP/1.1
The authorization server validates the request to ensure that 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).
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.

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, per
Appendix B:
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
RECOMMENDED. The lifetime in seconds of the access token. For
example, the value "3600" denotes that the access token will
expire in one hour from the time the response was generated.
If omitted, the authorization server SHOULD provide the
expiration time via other means or document the default value.
scope
OPTIONAL, if identical to the scope requested by the client;
otherwise, REQUIRED. The scope of the access token as
described by Section 3.3.
state
REQUIRED if the "state" parameter was present in the client
client.
The authorization server MUST NOT issue a refresh token.
sending the following HTTP response (with extra line breaks for
display purposes only):
HTTP/1.1 302 Found
Location: http://example.com/cb#access_token=2YotnFZFEjr1zCsicMWpAA
&state=xyz&token_type=example&expires_in=3600
Developers should note that some user-agents do not support the
inclusion of a fragment component in the HTTP "Location" response
header field. Such clients will require using other methods for
redirecting the client than a 3xx redirection response -- for
example, returning an HTML page that includes a ’continue’ button
with an action linked to the redirection URI.

The client MUST 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 request fails due to a missing, invalid, or mismatching
redirection URI, or if the client identifier is missing or 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 fragment component of the redirection URI using the
"application/x-www-form-urlencoded" format, per Appendix B:
error
following:
invalid_request
invalid parameter value, includes a parameter more than
once, 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.

server_error
The authorization server encountered an unexpected
condition that prevented it from fulfilling the request.
(This error code is needed because a 500 Internal Server
Error HTTP status code cannot be returned to the client
via an HTTP redirect.)
temporarily_unavailable
The authorization server is currently unable to handle
the request due to a temporary overloading or maintenance
of the server. (This error code is needed because a 503
Service Unavailable HTTP status code cannot be returned
to the client via an HTTP redirect.)
error_description
error_uri
state
REQUIRED if a "state" parameter was present in the client
client.
HTTP/1.1 302 Found
Location: https://client.example.com/cb#error=access_denied&state=xyz
4.3. Resource Owner Password Credentials Grant
The resource owner password credentials grant type is suitable in
cases where the resource owner has a trust relationship with the
client, such as the device operating system or a highly privileged

application. The authorization server should take special care when
enabling this grant type and only allow it when other flows are not
viable.
This grant type is suitable for clients capable of obtaining the
resource owner’s 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 to an
access token.
+----------+
| Resource |
| Owner |
| |
+----------+
v
| Resource Owner
(A) Password Credentials
|
v
+---------+ +---------------+
| |>--(B)---- Resource Owner ------->| |
| | Password Credentials | Authorization |
| |<--(C)---- Access Token ---------<| |
| | (w/ Optional Refresh Token) | |
+---------+ +---------------+
Figure 5: Resource Owner Password Credentials Flow
(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 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 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.
The client makes a request to the token endpoint by adding the
grant_type
REQUIRED. Value MUST be set to "password".
username
REQUIRED. The resource owner username.
password
REQUIRED. The resource owner password.
scope
Section 3.3.
If the client type is confidential or the client was issued client
credentials (or assigned other authentication requirements), the
client MUST authenticate with the authorization server as described
in Section 3.2.1.
For example, the client makes the following HTTP request using
transport-layer security (with extra line breaks for display purposes
only):
grant_type=password&username=johndoe&password=A3ddj3w

o authenticate the client if client authentication is included, and
o validate the resource owner password credentials using its
existing password validation algorithm.
Since this access token request utilizes the resource owner’s
password, the authorization server MUST protect the endpoint against
brute force attacks (e.g., using rate-limitation or generating
alerts).
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.
HTTP/1.1 200 OK
Pragma: no-cache
{
"expires_in":3600,
}
4.4. Client Credentials Grant
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 that have 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 confidential
clients.
+---------+ +---------------+
| | | |
| |>--(A)- Client Authentication --->| Authorization |
| |<--(B)---- Access Token ---------<| |
| | | |
+---------+ +---------------+
Figure 6: Client Credentials Flow
(A) The client authenticates with the authorization server and
requests an access token from the token endpoint.
(B) The authorization server authenticates the client, and if valid,
issues an access token.
4.4.1. Authorization Request and Response
Since the client authentication is used as the authorization grant,
no additional authorization request is needed.
The client makes a request to the token endpoint by adding the
grant_type
REQUIRED. Value MUST be set to "client_credentials".
scope
Section 3.3.
The client MUST authenticate with the authorization server as
described in Section 3.2.1.

only):
grant_type=client_credentials
The authorization server MUST authenticate the client.
authorization server issues an access 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.
HTTP/1.1 200 OK
Pragma: no-cache
{
"expires_in":3600,
}
4.5. Extension Grants
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 Security Assertion
Markup Language (SAML) 2.0 assertion grant type as defined by
[OAuth-SAML2], the client could make the following HTTP request using
TLS (with extra line breaks for display purposes only):
grant_type=urn%3Aietf%3Aparams%3Aoauth%3Agrant-type%3Asaml2-
bearer&assertion=PEFzc2VydGlvbiBJc3N1ZUluc3RhbnQ9IjIwMTEtMDU
[...omitted for brevity...]aG5TdGF0ZW1lbnQ-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:
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
RECOMMENDED. The lifetime in seconds of the access token. For
example, the value "3600" denotes that the access token will
expire in one hour from the time the response was generated.
If omitted, the authorization server SHOULD provide the
expiration time via other means or document the default value.

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, if identical to the scope requested by the client;
otherwise, REQUIRED. The scope of the access token as
described by Section 3.3.
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 JavaScript Object Notation (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 order of
parameters does not matter and can vary.
The authorization server MUST include the HTTP "Cache-Control"
response header field [RFC2616] with a value of "no-store" in any
response containing tokens, credentials, or other sensitive
information, as well as the "Pragma" response header field [RFC2616]
with a value of "no-cache".
For example:
HTTP/1.1 200 OK
Pragma: no-cache
{
"expires_in":3600,
}
The client MUST ignore unrecognized value names in the response. 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 (unless specified otherwise) and includes the following
parameters with the response:
error
following:
invalid_request
unsupported parameter value (other than grant type),
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 authentication included, or unsupported
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 (e.g., authorization
code, resource owner credentials) or refresh token 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

invalid_scope
The requested scope is invalid, unknown, malformed, or
exceeds the scope granted by the resource owner.
error_description
error_uri
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 order of parameters does not matter and can
vary.
For example:
HTTP/1.1 400 Bad Request
Pragma: no-cache
{
"error":"invalid_request"
}

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
grant_type
REQUIRED. Value MUST be set to "refresh_token".
refresh_token
REQUIRED. The refresh token issued to the client.
scope
Section 3.3. The requested scope MUST NOT include any scope
not originally granted by the resource owner, and if omitted is
treated as equal to the scope originally granted by the
resource owner.
Because refresh tokens are typically long-lasting credentials used to
request additional access tokens, the refresh token is bound to the
client to which it was issued. If the client type is confidential or
the client was issued client credentials (or assigned other
authentication requirements), the client MUST authenticate with the
authorization server as described in Section 3.2.1.
only):
grant_type=refresh_token&refresh_token=tGzv3JOkF0XG5Qx2TlKWIA

o authenticate the client if client authentication is included and
ensure that the refresh token was issued to the authenticated
client, and
o validate the refresh token.
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
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, the refresh token scope MUST be
identical to that of the refresh token included by the client 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 that 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
The method in which the client utilizes 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 specification of the access
token type used, such as [RFC6750].

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 the token
type.
For example, the "bearer" token type defined in [RFC6750] is utilized
by simply including the access token string in the request:
GET /resource/1 HTTP/1.1
Host: example.com
Authorization: Bearer mF_9.B5f-4.1JqM
while the "mac" token type defined in [OAuth-HTTP-MAC] is utilized by
issuing a Message Authentication Code (MAC) key together with the
access token that 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 [RFC6750] and [OAuth-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.
7.2. Error Response
If a resource access request fails, the resource server SHOULD inform
the client of the error. While the specifics of such error responses
are beyond the scope of this specification, this document establishes
a common registry in Section 11.4 for error values to be shared among
OAuth token authentication schemes.
New authentication schemes designed primarily for OAuth token
authentication SHOULD define a mechanism for providing an error
status code to the client, in which the error values allowed are
registered in the error registry established by this specification.

Such schemes MAY limit the set of valid error codes to a subset of
the registered values. If the error code is returned using a named
parameter, the parameter name SHOULD be "error".
Other schemes capable of being used for OAuth token authentication,
but not primarily designed for that purpose, MAY bind their error
values to the registry in the same manner.
New authentication schemes MAY choose to also specify the use of the
"error_description" and "error_uri" parameters to return error
information in a manner parallel to their usage in this
specification.
8. Extensibility
8.1. Defining Access Token Types
Access token types can be defined in one of two ways: registered in
the Access Token Types registry (following the procedures in
Section 11.1), or by using 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]). The token type
"example" is reserved for use in examples.
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
OAuth 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

Unregistered vendor-specific parameter extensions that are not
commonly applicable and that 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
New response types for use with the authorization endpoint are
defined and registered in the Authorization Endpoint Response Types
registry following the procedure in Section 11.3. Response type
names MUST conform to the response-type ABNF.
response-type = response-name *( SP response-name )
response-name = 1*response-char
response-char = "_" / DIGIT / ALPHA
If a response type contains one or more space characters (%x20), it
is compared as a space-delimited list of values in which the order of
values does not matter. Only one order of values can be registered,
which covers all other arrangements of the same set of values.
For example, the response type "token code" is left undefined by this
specification. However, an extension can define and register the
"token code" response type. Once registered, the same combination
cannot be registered as "code token", but both values can be used to
denote the same response type.
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), the token error response (Section 5.2), or the
resource access error response (Section 7.2), such error codes MAY be
defined.

Extension error codes MUST be registered (following the procedures in
Section 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 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 = 1*error-char
error-char = %x20-21 / %x23-5B / %x5D-7E
9. Native Applications
Native applications are clients installed and executed on the device
used by the resource owner (i.e., desktop application, native mobile
application). Native applications require special consideration
related to security, platform capabilities, and overall end-user
experience.
The authorization endpoint requires interaction between the client
and the resource owner’s user-agent. Native applications can invoke
an external user-agent or embed a user-agent within the application.
For example:
o External user-agent - the native application can capture the
response from the authorization server using a redirection URI
with a scheme registered with the operating system to invoke the
client as the handler, manual copy-and-paste of the credentials,
running a local web server, installing a user-agent extension, or
by providing a redirection URI identifying a server-hosted
resource under the client’s control, which in turn makes the
response available to the native application.
o Embedded user-agent - the native application obtains the response
by directly communicating with the embedded user-agent by
monitoring state changes emitted during the resource load, or
accessing the user-agent’s cookies storage.
When choosing between an external or embedded user-agent, developers
should consider the following:
o An external user-agent may improve completion rate, as the
resource owner may already have an active session with the
authorization server, removing the need to re-authenticate. It
provides a familiar end-user experience and functionality. The

resource owner may also rely on user-agent features or extensions
to assist with authentication (e.g., password manager, 2-factor
device reader).
o An embedded user-agent may offer improved usability, as it removes
the need to switch context and open new windows.
o An embedded user-agent poses a security challenge because resource
owners are authenticating in an unidentified window without access
to the visual protections found in most external user-agents. An
embedded user-agent educates end-users to trust unidentified
requests for authentication (making phishing attacks easier to
execute).
When choosing between the implicit grant type and the authorization
code grant type, the following should be considered:
o Native applications that use the authorization code grant type
SHOULD do so without using client credentials, due to the native
application’s inability to keep client credentials confidential.
o When using the implicit grant type flow, a refresh token is not
returned, which requires repeating the authorization process once
the access token expires.
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 the three
client profiles described in Section 2.1: web application,
user-agent-based application, and native application.
A comprehensive OAuth security model and analysis, as well as
background for the protocol design, is provided by
[OAuth-THREATMODEL].
10.1. Client Authentication
The authorization server establishes client credentials with web
application clients for the purpose of client authentication. The
authorization server is encouraged to consider stronger client
authentication means than a client password. Web application clients
MUST ensure confidentiality of client passwords and other client
credentials.

The authorization server MUST NOT issue client passwords or other
client credentials to native application or user-agent-based
application clients 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 client on a
specific device.
When client authentication is not possible, the authorization server
SHOULD employ other means to validate the client’s identity -- for
example, by requiring the registration of the client redirection URI
or enlisting the resource owner to confirm identity. A valid
redirection URI is not sufficient to verify the client’s identity
when asking for resource owner authorization but can be used to
prevent delivering credentials to a counterfeit client after
obtaining resource owner authorization.
The authorization server must consider the security implications of
interacting with unauthenticated clients and take measures to limit
the potential exposure of other credentials (e.g., refresh tokens)
issued to such clients.
10.2. Client Impersonation
A malicious client can impersonate another client and obtain access
to protected resources if the impersonated client fails to, or is
unable to, keep its client credentials confidential.
The authorization server MUST authenticate the client whenever
possible. If the authorization server cannot authenticate the client
due to the client’s nature, the authorization server MUST require the
registration of any redirection URI used for receiving authorization
responses and SHOULD utilize other means to protect resource owners
from such potentially malicious clients. For example, the
authorization server can engage the resource owner to assist in
identifying the client and its origin.
The authorization server SHOULD enforce explicit resource owner
authentication and provide the resource owner with information about
the client and the requested authorization scope and lifetime. It is
up to the resource owner to review the information in the context of
the current client and to authorize or deny the request.
The authorization server SHOULD NOT process repeated authorization
requests automatically (without active resource owner interaction)
without authenticating the client or relying on other measures to
ensure that the repeated request comes from the original client and
not an impersonator.

10.3. Access Tokens
Access token credentials (as well as any confidential access token
attributes) MUST be kept confidential in transit and storage, and
only shared among the authorization server, the resource servers the
access token is valid for, and the client to whom the access token is
issued. Access token credentials MUST only be transmitted using TLS
as described in Section 1.6 with server authentication as defined by
[RFC2818].
When using the implicit grant type, the access token is transmitted
in the URI fragment, which can expose it to unauthorized parties.
The authorization server MUST ensure that access tokens cannot be
generated, modified, or guessed to produce valid access tokens by
unauthorized parties.
The client SHOULD request access tokens with the minimal scope
necessary. The authorization server SHOULD take the client identity
into account when choosing how to honor the requested scope and MAY
issue an access token with less rights than requested.
This specification does not provide any methods for the resource
server to ensure that an access token presented to it by a given
client was issued to that client by the authorization server.
10.4. Refresh Tokens
Authorization servers MAY issue refresh tokens to web application
clients and native application clients.
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 binding between a refresh token and the client to whom it was
issued. Refresh tokens MUST only be transmitted using TLS as
described in Section 1.6 with server authentication as defined by
[RFC2818].
The authorization server MUST verify the binding between the refresh
token and client identity whenever the client identity can be
authenticated. When client authentication is not possible, the
authorization server SHOULD deploy other means to detect refresh
token abuse.
For example, the authorization server could employ refresh token
rotation in which a new refresh token is issued with every access
token refresh response. The previous refresh token is invalidated

but retained by the authorization server. If a refresh token is
compromised and subsequently used by both the attacker and the
legitimate client, one of them will present an invalidated refresh
token, which will inform the authorization server of the breach.
The authorization server MUST ensure that refresh tokens cannot be
generated, modified, or guessed to produce valid refresh tokens by
unauthorized parties.
10.5. Authorization Codes
The transmission of authorization codes SHOULD be made over a secure
channel, and the client SHOULD require the use of TLS with its
redirection URI if the URI identifies a network resource. 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 resource owner who granted authorization at the
authorization server is the same resource owner returning to the
client to complete the process. Therefore, if the client relies on
the authorization code for its own resource owner authentication, the
client redirection endpoint MUST require the use of TLS.
Authorization codes MUST be short lived and 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.
10.6. Authorization Code Redirection URI Manipulation
When requesting authorization using the authorization code grant
type, the client can specify a redirection URI via the "redirect_uri"
parameter. If an attacker can manipulate the value of the
redirection URI, it can cause the authorization server to redirect
the resource owner user-agent to a URI under the control of the
attacker with the authorization code.
An attacker can create an account at a legitimate client and initiate
the authorization flow. When the attacker’s user-agent is sent to
the authorization server to grant access, the attacker grabs the
authorization URI provided by the legitimate client and replaces the

client’s redirection URI with a URI under the control of the
attacker. The attacker then tricks the victim into following the
manipulated link to authorize access to the legitimate client.
Once at the authorization server, the victim is prompted with a
normal, valid request on behalf of a legitimate and trusted client,
and authorizes the request. The victim is then redirected to an
endpoint under the control of the attacker with the authorization
code. The attacker completes the authorization flow by sending the
authorization code to the client using the original redirection URI
provided by the client. The client exchanges the authorization code
with an access token and links it to the attacker’s client account,
which can now gain access to the protected resources authorized by
the victim (via the client).
In order to prevent such an attack, the authorization server MUST
ensure that the redirection URI used to obtain the authorization code
is identical to the redirection URI provided when exchanging the
authorization code for an access token. The authorization server
MUST require public clients and SHOULD require confidential clients
to register their redirection URIs. If a redirection URI is provided
in the request, the authorization server MUST validate it against the
registered value.
10.7. 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
usernames and passwords by the client but does not eliminate the need
to expose highly privileged credentials to the client.
This grant type carries a higher risk than other grant types because
it maintains the password anti-pattern this protocol 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’s involvement ends when
it hands over its credentials to the client), the client can obtain
access tokens with a broader scope than desired by the resource
owner. The authorization server should consider the scope and
lifetime 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.

10.8. Request Confidentiality
Access tokens, refresh tokens, resource owner passwords, and client
credentials MUST NOT be transmitted in the clear. Authorization
codes SHOULD NOT be transmitted in the clear.
The "state" and "scope" parameters SHOULD NOT include sensitive
client or resource owner information in plain text, as they can be
transmitted over insecure channels or stored insecurely.
10.9. Ensuring Endpoint Authenticity
In order to prevent man-in-the-middle attacks, the authorization
server MUST require the use of TLS with server authentication as
defined by [RFC2818] for any request sent to the authorization and
token endpoints. The client MUST validate the authorization server’s
TLS certificate as defined by [RFC6125] and in accordance with its
requirements for server identity authentication.
10.10. Credentials-Guessing Attacks
The authorization server MUST prevent attackers from guessing access
tokens, authorization codes, refresh tokens, resource owner
passwords, and client credentials.
The probability of an attacker guessing generated tokens (and other
credentials not intended for handling by end-users) MUST be less than
or equal to 2^(-128) and SHOULD be less than or equal to 2^(-160).
The authorization server MUST utilize other means to protect
credentials intended for end-user usage.
10.11. Phishing Attacks
Wide deployment of this and similar protocols may cause end-users to
become inured to the practice of being redirected to websites where
they are asked to enter their passwords. If end-users are not
careful to verify the authenticity of these websites before entering
their credentials, it will be possible for attackers to exploit this
practice to steal resource owners’ passwords.
Service providers should attempt to educate end-users about the risks
phishing attacks pose and should provide mechanisms that make it easy
for end-users to confirm the authenticity of their sites. Client
developers should consider the security implications of how they
interact with the user-agent (e.g., external, embedded), and the
ability of the end-user to verify the authenticity of the

To reduce the risk of phishing attacks, the authorization servers
MUST require the use of TLS on every endpoint used for end-user
interaction.
10.12. Cross-Site Request Forgery
Cross-site request forgery (CSRF) is an exploit in which an attacker
causes the user-agent of a victim end-user to follow a malicious URI
(e.g., provided to the user-agent as a misleading link, image, or
redirection) to a trusting server (usually established via the
presence of a valid session cookie).
A CSRF attack against the client’s redirection URI allows an attacker
to inject its own authorization code or access token, which can
result in the client using an access token associated with the
attacker’s protected resources rather than the victim’s (e.g., save
the victim’s bank account information to a protected resource
controlled by the attacker).
The client MUST implement CSRF protection for its redirection URI.
This is typically accomplished by requiring any request sent to the
redirection URI endpoint to include a value that binds the request to
the user-agent’s authenticated state (e.g., a hash of the session
cookie used to authenticate the user-agent). The client SHOULD
utilize the "state" request parameter to deliver this value to the
authorization server when making an authorization request.
Once authorization has been obtained from the end-user, the
authorization server redirects the end-user’s user-agent back to the
client with the required binding value contained in the "state"
parameter. The binding value enables the client to verify the
validity of the request by matching the binding value to the
user-agent’s authenticated state. The binding value used for CSRF
protection MUST contain a non-guessable value (as described in
Section 10.10), and the user-agent’s authenticated state (e.g.,
session cookie, HTML5 local storage) MUST be kept in a location
accessible only to the client and the user-agent (i.e., protected by
same-origin policy).
A CSRF attack against the authorization server’s authorization
endpoint can result in an attacker obtaining end-user authorization
for a malicious client without involving or alerting the end-user.
The authorization server MUST implement CSRF protection for its
authorization endpoint and ensure that a malicious client cannot
obtain authorization without the awareness and explicit consent of
the resource owner.

10.13. Clickjacking
In a clickjacking attack, an attacker registers a legitimate client
and then constructs a malicious site in which it loads the
authorization server’s authorization endpoint web page in a
transparent iframe overlaid on top of a set of dummy buttons, which
are carefully constructed to be placed directly under important
buttons on the authorization page. When an end-user clicks a
misleading visible button, the end-user is actually clicking an
invisible button on the authorization page (such as an "Authorize"
button). This allows an attacker to trick a resource owner into
granting its client access without the end-user’s knowledge.
To prevent this form of attack, native applications SHOULD use
external browsers instead of embedding browsers within the
application when requesting end-user authorization. For most newer
browsers, avoidance of iframes can be enforced by the authorization
server using the (non-standard) "x-frame-options" header. This
header can have two values, "deny" and "sameorigin", which will block
any framing, or framing by sites with a different origin,
respectively. For older browsers, JavaScript frame-busting
techniques can be used but may not be effective in all browsers.
10.14. Code Injection and Input Validation
A code injection attack occurs when an input or otherwise external
variable is used by an application unsanitized and causes
modification to the application logic. This may allow an attacker to
gain access to the application device or its data, cause denial of
service, or introduce a wide range of malicious side-effects.
The authorization server and client MUST sanitize (and validate when
possible) any value received -- in particular, the value of the
"state" and "redirect_uri" parameters.
10.15. Open Redirectors
The authorization server, authorization endpoint, and client
redirection endpoint can be improperly configured and operate as open
redirectors. An open redirector is an endpoint using a parameter to
automatically redirect a user-agent to the location specified by the
parameter value without any validation.
Open redirectors can be used in phishing attacks, or by an attacker
to get end-users to visit malicious sites by using the URI authority
component of a familiar and trusted destination. In addition, if the
authorization server allows the client to register only part of the
redirection URI, an attacker can use an open redirector operated by

the client to construct a redirection URI that will pass the
authorization server validation but will send the authorization code
or access token to an endpoint under the control of the attacker.
10.16. Misuse of Access Token to Impersonate Resource Owner in Implicit
Flow
For public clients using implicit flows, this specification does not
provide any method for the client to determine what client an access
token was issued to.
A resource owner may willingly delegate access to a resource by
granting an access token to an attacker’s malicious client. This may
be due to phishing or some other pretext. An attacker may also steal
a token via some other mechanism. An attacker may then attempt to
impersonate the resource owner by providing the access token to a
legitimate public client.
In the implicit flow (response_type=token), the attacker can easily
switch the token in the response from the authorization server,
replacing the real access token with the one previously issued to the
attacker.
Servers communicating with native applications that rely on being
passed an access token in the back channel to identify the user of
the client may be similarly compromised by an attacker creating a
compromised application that can inject arbitrary stolen access
tokens.
Any public client that makes the assumption that only the resource
owner can present it with a valid access token for the resource is
vulnerable to this type of attack.
This type of attack may expose information about the resource owner
at the legitimate client to the attacker (malicious client). This
will also allow the attacker to perform operations at the legitimate
client with the same permissions as the resource owner who originally
granted the access token or authorization code.
Authenticating resource owners to clients is out of scope for this
specification. Any specification that uses the authorization process
as a form of delegated end-user authentication to the client (e.g.,
third-party sign-in service) MUST NOT use the implicit flow without
additional security mechanisms that would enable the client to
determine if the access token was issued for its use (e.g., audience-
restricting the access token).

11. IANA Considerations
11.1. OAuth Access Token Types Registry
This specification establishes the OAuth Access Token Types registry.
Access token types are registered with a Specification Required
([RFC5226]) after a two-week review period on the
oauth-ext-review@ietf.org mailing list, on the advice of one or more
Designated Experts. 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 must be sent to the oauth-ext-review@ietf.org
mailing list for review and comment, with an appropriate subject
(e.g., "Request for access token type: example").
Within the review period, 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.
IANA must 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 resource requests using access tokens 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,
email address, home page URI) may also be included.

Specification document(s):
Reference to the document(s) that specify the parameter,
preferably including a URI that can be used to retrieve a copy of
the document(s). An indication of the relevant sections may also
be included but is not required.
11.2. 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 with a
Specification Required ([RFC5226]) after a two-week review period on
the oauth-ext-review@ietf.org mailing list, on the advice of one or
more Designated Experts. 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.
(e.g., "Request for parameter: example").
successful.
list.
Parameter name:
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:

Reference to the document(s) that specify the parameter,
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): RFC 6749
o Parameter name: client_secret
o Parameter usage location: token request
o Parameter name: response_type
o Parameter usage location: authorization request
o Parameter name: redirect_uri
o Parameter usage location: authorization request, token request
o Parameter name: scope
o Parameter usage location: authorization request, authorization
response, token request, token response
o Parameter name: state
o Parameter usage location: authorization request, authorization
response
o Parameter name: code
o Parameter usage location: authorization response, token request

o Parameter name: error_description
o Parameter usage location: authorization response, token response
o Parameter name: error_uri
o Parameter name: grant_type
o Parameter name: access_token
o Parameter name: token_type
o Parameter name: expires_in
o Parameter name: username
o Parameter name: password
o Parameter name: refresh_token
o Parameter usage location: token request, token response

11.3. OAuth Authorization Endpoint Response Types Registry
This specification establishes the OAuth Authorization Endpoint
Response Types registry.
Additional response types for use with the authorization endpoint are
registered with a Specification Required ([RFC5226]) after a two-week
review period on the oauth-ext-review@ietf.org mailing list, on the
advice of one or more Designated Experts. 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.
(e.g., "Request for response type: example").
successful.
list.
Response type name:
Change controller:
Reference to the document(s) that specify the type, preferably
including a URI that can be used to retrieve a copy of the
document(s). 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 Types registry’s initial
contents are:
o Response type name: code
o Response type name: token
11.4. 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 with a Specification Required ([RFC5226])
after a two-week review period on the oauth-ext-review@ietf.org
mailing list, on the advice of one or more Designated Experts.
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.
(e.g., "Request for error code: example").
successful.
list.

Error name:
The name requested (e.g., "example"). Values for the error name
MUST NOT include characters outside the set %x20-21 / %x23-5B /
%x5D-7E.
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), token error response (Section 5.2), or resource
access error response (Section 7.2).
Related protocol extension:
The name of the extension grant type, access token type, or
extension parameter that the error code is used in conjunction
with.
Change controller:
Reference to the document(s) that specify the error code,
12. References
12.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2246] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
RFC 2246, January 1999.
[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.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of
ISO 10646", STD 63, RFC 3629, November 2003.
[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.
[RFC6125] Saint-Andre, P. and J. Hodges, "Representation and
Verification of Domain-Based Application Service Identity
within Internet Public Key Infrastructure Using X.509
(PKIX) Certificates in the Context of Transport Layer
Security (TLS)", RFC 6125, March 2011.
[USASCII] American National Standards Institute, "Coded Character
Set -- 7-bit American Standard Code for Information
Interchange", ANSI X3.4, 1986.
[W3C.REC-html401-19991224]
Raggett, D., Le Hors, A., and I. Jacobs, "HTML 4.01
Specification", World Wide Web Consortium
Recommendation REC-html401-19991224, December 1999,
<http://www.w3.org/TR/1999/REC-html401-19991224>.
[W3C.REC-xml-20081126]
Bray, T., Paoli, J., Sperberg-McQueen, C., Maler, E.,
and F. Yergeau, "Extensible Markup Language (XML) 1.0
(Fifth Edition)", World Wide Web Consortium
Recommendation REC-xml-20081126, November 2008,
<http://www.w3.org/TR/2008/REC-xml-20081126>.

12.2. Informative References
[OAuth-HTTP-MAC]
Hammer-Lahav, E., Ed., "HTTP Authentication: MAC Access
Authentication", Work in Progress, February 2012.
[OAuth-SAML2]
Campbell, B. and C. Mortimore, "SAML 2.0 Bearer Assertion
Profiles for OAuth 2.0", Work in Progress, September 2012.
[OAuth-THREATMODEL]
Lodderstedt, T., Ed., McGloin, M., and P. Hunt, "OAuth 2.0
Threat Model and Security Considerations", Work
in Progress, October 2012.
[OAuth-WRAP]
Hardt, D., Ed., Tom, A., Eaton, B., and Y. Goland, "OAuth
Web Resource Authorization Profiles", Work in Progress,
January 2010.
[RFC5849] Hammer-Lahav, E., "The OAuth 1.0 Protocol", RFC 5849,
April 2010.
[RFC6750] Jones, M. and D. Hardt, "The OAuth 2.0 Authorization
Framework: Bearer Token Usage", RFC 6750, October 2012.

Appendix A. Augmented Backus-Naur Form (ABNF) Syntax
This section provides Augmented Backus-Naur Form (ABNF) syntax
descriptions for the elements defined in this specification using the
notation of [RFC5234]. The ABNF below is defined in terms of Unicode
code points [W3C.REC-xml-20081126]; these characters are typically
encoded in UTF-8. Elements are presented in the order first defined.
Some of the definitions that follow use the "URI-reference"
definition from [RFC3986].
Some of the definitions that follow use these common definitions:
VSCHAR = %x20-7E
NQCHAR = %x21 / %x23-5B / %x5D-7E
NQSCHAR = %x20-21 / %x23-5B / %x5D-7E
UNICODECHARNOCRLF = %x09 /%x20-7E / %x80-D7FF /
%xE000-FFFD / %x10000-10FFFF
(The UNICODECHARNOCRLF definition is based upon the Char definition
in Section 2.2 of [W3C.REC-xml-20081126], but omitting the Carriage
Return and Linefeed characters.)
A.1. "client_id" Syntax
The "client_id" element is defined in Section 2.3.1:
client-id = *VSCHAR
A.2. "client_secret" Syntax
The "client_secret" element is defined in Section 2.3.1:
client-secret = *VSCHAR
A.3. "response_type" Syntax
The "response_type" element is defined in Sections 3.1.1 and 8.4:
response-type = response-name *( SP response-name )
response-name = 1*response-char
response-char = "_" / DIGIT / ALPHA

A.4. "scope" Syntax
The "scope" element is defined in Section 3.3:
scope = scope-token *( SP scope-token )
scope-token = 1*NQCHAR
A.5. "state" Syntax
The "state" element is defined in Sections 4.1.1, 4.1.2, 4.1.2.1,
4.2.1, 4.2.2, and 4.2.2.1:
state = 1*VSCHAR
A.6. "redirect_uri" Syntax
The "redirect_uri" element is defined in Sections 4.1.1, 4.1.3,
and 4.2.1:
redirect-uri = URI-reference
A.7. "error" Syntax
The "error" element is defined in Sections 4.1.2.1, 4.2.2.1, 5.2,
7.2, and 8.5:
error = 1*NQSCHAR
A.8. "error_description" Syntax
The "error_description" element is defined in Sections 4.1.2.1,
4.2.2.1, 5.2, and 7.2:
error-description = 1*NQSCHAR
A.9. "error_uri" Syntax
The "error_uri" element is defined in Sections 4.1.2.1, 4.2.2.1, 5.2,
and 7.2:
error-uri = URI-reference

A.10. "grant_type" Syntax
The "grant_type" element is defined in Sections 4.1.3, 4.3.2, 4.4.2,
4.5, and 6:
grant-type = grant-name / URI-reference
grant-name = 1*name-char
A.11. "code" Syntax
The "code" element is defined in Section 4.1.3:
code = 1*VSCHAR
A.12. "access_token" Syntax
The "access_token" element is defined in Sections 4.2.2 and 5.1:
access-token = 1*VSCHAR
A.13. "token_type" Syntax
The "token_type" element is defined in Sections 4.2.2, 5.1, and 8.1:
token-type = type-name / URI-reference
type-name = 1*name-char
A.14. "expires_in" Syntax
The "expires_in" element is defined in Sections 4.2.2 and 5.1:
expires-in = 1*DIGIT
A.15. "username" Syntax
The "username" element is defined in Section 4.3.2:
username = *UNICODECHARNOCRLF
A.16. "password" Syntax
The "password" element is defined in Section 4.3.2:
password = *UNICODECHARNOCRLF

A.17. "refresh_token" Syntax
The "refresh_token" element is defined in Sections 5.1 and 6:
refresh-token = 1*VSCHAR
A.18. Endpoint Parameter Syntax
The syntax for new endpoint parameters is defined in Section 8.2:
param-name = 1*name-char
Appendix B. Use of application/x-www-form-urlencoded Media Type
At the time of publication of this specification, the
"application/x-www-form-urlencoded" media type was defined in
Section 17.13.4 of [W3C.REC-html401-19991224] but not registered in
the IANA MIME Media Types registry
(<http://www.iana.org/assignments/media-types>). Furthermore, that
definition is incomplete, as it does not consider non-US-ASCII
characters.
To address this shortcoming when generating payloads using this media
type, names and values MUST be encoded using the UTF-8 character
encoding scheme [RFC3629] first; the resulting octet sequence then
needs to be further encoded using the escaping rules defined in
[W3C.REC-html401-19991224].
When parsing data from a payload using this media type, the names and
values resulting from reversing the name/value encoding consequently
need to be treated as octet sequences, to be decoded using the UTF-8
character encoding scheme.
For example, the value consisting of the six Unicode code points
(1) U+0020 (SPACE), (2) U+0025 (PERCENT SIGN),
(3) U+0026 (AMPERSAND), (4) U+002B (PLUS SIGN),
(5) U+00A3 (POUND SIGN), and (6) U+20AC (EURO SIGN) would be encoded
into the octet sequence below (using hexadecimal notation):
20 25 26 2B C2 A3 E2 82 AC
and then represented in the payload as:
+%25%26%2B%C2%A3%E2%82%AC

Appendix C. 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) [OAuth-WRAP]. Eran Hammer then edited many
of the intermediate drafts that evolved into this RFC. The Security
Considerations section was drafted by Torsten Lodderstedt, Mark
McGloin, Phil Hunt, Anthony Nadalin, and John Bradley. The section
on use of the "application/x-www-form-urlencoded" media type was
drafted by Julian Reschke. The ABNF section was drafted by Michael
B. Jones.
The OAuth 1.0 community specification was edited by Eran Hammer 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, 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 Y. 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
that shaped and formed the final specification:
Michael Adams, Amanda Anganes, Andrew Arnott, Dirk Balfanz, Aiden
Bell, John Bradley, Marcos Caceres, Brian Campbell, Scott Cantor,
Blaine Cook, Roger Crew, Leah Culver, Bill de hOra, Andre DeMarre,
Brian Eaton, Wesley Eddy, Wolter Eldering, Brian Ellin, Igor
Faynberg, George Fletcher, Tim Freeman, Luca Frosini, Evan Gilbert,
Yaron Y. Goland, Brent Goldman, Kristoffer Gronowski, Eran Hammer,
Dick Hardt, Justin Hart, Craig Heath, Phil Hunt, Michael B. Jones,
Terry Jones, John Kemp, Mark Kent, Raffi Krikorian, Chasen Le Hara,
Rasmus Lerdorf, Torsten Lodderstedt, Hui-Lan Lu, Casey Lucas, Paul
Madsen, Alastair Mair, Eve Maler, James Manger, Mark McGloin,
Laurence Miao, William Mills, Chuck Mortimore, Anthony Nadalin,
Julian Reschke, Justin Richer, Peter Saint-Andre, Nat Sakimura, Rob
Sayre, Marius Scurtescu, Naitik Shah, Luke Shepard, Vlad Skvortsov,
Justin Smith, Haibin Song, Niv Steingarten, Christian Stuebner,
Jeremy Suriel, Paul Tarjan, Christopher Thomas, Henry S. Thompson,
Allen Tom, Franklin Tse, Nick Walker, Shane Weeden, and Skylar
Woodward.

This document was produced under the chairmanship of Blaine Cook,
Peter Saint-Andre, Hannes Tschofenig, Barry Leiba, and Derek Atkins.
The area directors included Lisa Dusseault, Peter Saint-Andre, and
Stephen Farrell.
Author’s Address
Dick Hardt (editor)
Microsoft
EMail: dick.hardt@gmail.com
URI: http://dickhardt.org/

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