OAuth 2.0 for First-Party Applications
draft-parecki-oauth-first-party-apps-01
| Document | Type | Active Internet-Draft (individual) | |
|---|---|---|---|
| Authors | Aaron Parecki , George Fletcher , Pieter Kasselman | ||
| Last updated | 2024-03-01 | ||
| Replaces | draft-parecki-oauth-first-party-native-apps | ||
| RFC stream | (None) | ||
| Intended RFC status | (None) | ||
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| Stream | Stream state | (No stream defined) | |
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| IESG | IESG state | I-D Exists | |
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| Send notices to | (None) |
draft-parecki-oauth-first-party-apps-01
Web Authorization Protocol A. Parecki
Internet-Draft Okta
Intended status: Standards Track G. Fletcher
Expires: 2 September 2024 Capital One Financial
P. Kasselman
Microsoft
1 March 2024
OAuth 2.0 for First-Party Applications
draft-parecki-oauth-first-party-apps-01
Abstract
This document defines the Authorization Challenge Endpoint, which
supports a first-party client that wants to control the process of
obtaining authorization from the user using a native experience.
In many cases, this can provide an entirely browserless OAuth 2.0
experience suited for native applications, only delegating to the
browser in unexpected, high risk, or error conditions.
About This Document
This note is to be removed before publishing as an RFC.
The latest revision of this draft can be found at
https://aaronpk.github.io/oauth-first-party-apps/draft-parecki-oauth-
first-party-apps.html. Status information for this document may be
found at https://datatracker.ietf.org/doc/draft-parecki-oauth-first-
party-apps/.
Discussion of this document takes place on the Web Authorization
Protocol Working Group mailing list (mailto:oauth@ietf.org), which is
archived at https://mailarchive.ietf.org/arch/browse/oauth/.
Subscribe at https://www.ietf.org/mailman/listinfo/oauth/.
Source for this draft and an issue tracker can be found at
https://github.com/aaronpk/oauth-first-party-apps.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on 2 September 2024.
Copyright Notice
Copyright (c) 2024 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 (https://trustee.ietf.org/
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Please review these documents carefully, as they describe your rights
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Usage and Applicability . . . . . . . . . . . . . . . . . 4
1.2. Limitations of this specification . . . . . . . . . . . . 5
1.3. User Experience Considerations . . . . . . . . . . . . . 5
2. Conventions and Definitions . . . . . . . . . . . . . . . . . 6
2.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 6
3. Protocol Overview . . . . . . . . . . . . . . . . . . . . . . 6
3.1. Initial Authorization Request . . . . . . . . . . . . . . 6
3.2. Refresh Token Request . . . . . . . . . . . . . . . . . . 8
3.3. Resource Request . . . . . . . . . . . . . . . . . . . . 8
4. Protocol Endpoints . . . . . . . . . . . . . . . . . . . . . 8
4.1. Authorization Challenge Endpoint . . . . . . . . . . . . 8
4.2. Token endpoint . . . . . . . . . . . . . . . . . . . . . 9
5. Authorization Initiation . . . . . . . . . . . . . . . . . . 10
5.1. Authorization Challenge Request . . . . . . . . . . . . . 10
5.2. Authorization Challenge Response . . . . . . . . . . . . 11
5.2.1. Authorization Code Response . . . . . . . . . . . . . 11
5.2.2. Error Response . . . . . . . . . . . . . . . . . . . 11
5.3. Intermediate Requests . . . . . . . . . . . . . . . . . . 14
5.3.1. Auth Session . . . . . . . . . . . . . . . . . . . . 14
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6. Token Request . . . . . . . . . . . . . . . . . . . . . . . . 15
6.1. Token Endpoint Successful Response . . . . . . . . . . . 15
6.2. Token Endpoint Error Response . . . . . . . . . . . . . . 16
7. Resource Server Error Response . . . . . . . . . . . . . . . 17
8. Authorization Server Metadata . . . . . . . . . . . . . . . . 17
9. Security Considerations . . . . . . . . . . . . . . . . . . . 17
9.1. First-Party Applications . . . . . . . . . . . . . . . . 17
9.2. Phishing . . . . . . . . . . . . . . . . . . . . . . . . 18
9.3. Credential Stuffing Attacks . . . . . . . . . . . . . . . 18
9.4. Client Authentication . . . . . . . . . . . . . . . . . . 18
9.5. Sender Constrained Tokens . . . . . . . . . . . . . . . . 19
9.5.1. DPoP: Demonstrating Proof-of-Possession . . . . . . . 19
9.5.2. Other Proof of Possession Mechanisms . . . . . . . . 20
9.6. Auth Session . . . . . . . . . . . . . . . . . . . . . . 20
9.6.1. Auth Session DPoP Binding . . . . . . . . . . . . . . 20
9.6.2. Auth Session Lifetime . . . . . . . . . . . . . . . . 20
9.7. Multiple Applications . . . . . . . . . . . . . . . . . . 21
9.7.1. User Experience Risk . . . . . . . . . . . . . . . . 21
9.7.2. Technical Risk . . . . . . . . . . . . . . . . . . . 21
9.7.3. Mitigation . . . . . . . . . . . . . . . . . . . . . 21
9.8. Single Page Applications . . . . . . . . . . . . . . . . 21
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 22
10.1. OAuth Parameters Registration . . . . . . . . . . . . . 22
10.2. OAuth Server Metadata Registration . . . . . . . . . . . 22
10.3. JSON Web Token Claims Registration . . . . . . . . . . . 23
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 23
11.1. Normative References . . . . . . . . . . . . . . . . . . 23
11.2. Informative References . . . . . . . . . . . . . . . . . 25
Appendix A. Example User Experiences . . . . . . . . . . . . . . 25
A.1. Passkey . . . . . . . . . . . . . . . . . . . . . . . . . 25
A.2. Redirect to Authorization Server . . . . . . . . . . . . 26
A.3. Passwordless One-Time Password (OTP) . . . . . . . . . . 26
A.4. E-Mail Confirmation Code . . . . . . . . . . . . . . . . 27
A.5. SMS Confirmation Code . . . . . . . . . . . . . . . . . . 28
A.6. Re-authenticating to an app a week later using OTP . . . 28
A.7. Step-up Authentication using Confirmation SMS . . . . . . 29
A.8. Registration . . . . . . . . . . . . . . . . . . . . . . 31
Appendix B. Example Implementations . . . . . . . . . . . . . . 32
B.1. Authorization Challenge Request Parameters . . . . . . . 32
B.2. Authorization Challenge Response Parameters . . . . . . . 33
B.3. Example Sequence . . . . . . . . . . . . . . . . . . . . 33
Appendix C. Design Goals . . . . . . . . . . . . . . . . . . . . 35
Appendix D. Document History . . . . . . . . . . . . . . . . . . 35
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 36
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 36
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1. Introduction
This document extends the OAuth 2.0 Authorization Framework [RFC6749]
with a new endpoint, authorization_challenge_endpoint, to support
first-party applications that want to control the process of
obtaining authorization from the user using a native experience.
The client collects any initial information from the user and POSTs
that information as well as information about the client's request to
the Authorization Challenge Endpoint, and receives either an
authorization code or an error code in response. The error code may
indicate that the client can continue to prompt the user for more
information, or can indicate that the client needs to launch a
browser to have the user complete the flow in a browser.
The Authorization Challenge Endpoint is used to initiate the OAuth
flow in place of redirecting or launching a browser to the
authorization endpoint.
While a fully-delegated approach using the redirect-based
Authorization Code grant is generally preferred, this draft provides
a mechanism for the client to directly interact with the user. This
requires a high degree of trust between the authorization server and
the client, as there typically is for first-party applications. It
should only be considered when there are usability concerns with a
redirect-based approach, such as for native mobile or desktop
applications.
This draft also extends the token response (typically for use in
response to a refresh token request) and resource server response to
allow the authorization server or resource server to indicate that
the client should re-request authorization from the user. This can
include requesting step-up authentication by including parameters
defined in [RFC9470] as well.
1.1. Usage and Applicability
This specification MUST only be used by first-party applications,
which is when the authorization server and application are operated
by the same entity and the user understands them both as the same
entity.
This specification MUST NOT be used by third party applications, and
the authorization server SHOULD take measures to prevent use by third
party applications. (e.g. only enable this grant for certain client
IDs, and take measures to authenticate first-party apps when
possible.)
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Using this specification in scenarios other than those described will
lead to unintended security and privacy problems for users and
service providers.
This specification is designed to be used by first-party native
applications, which includes both mobile and desktop applications.
If you provide multiple apps and expect users to use multiple apps on
the same device, there may be better ways of sharing a user's login
between the apps other than each app implementing this specification
or using an SDK that implements this specification. For example,
[OpenID.Native-SSO] provides a mechanism for one app to obtain new
tokens by exchanging tokens from another app, without any user
interaction. See Section 9.7 for more details.
1.2. Limitations of this specification
The scope of this specification is limited to first-party
applications. Please review the entirety of Section 9, and when more
than one first-party application is supported, Section 9.7.
While this draft provides the framework for a native OAuth
experience, each implementation will need to define the specific
behavior that it expects from OAuth clients interacting with the
authorization server. While this lack of clearly defining the
details would typically lead to less interoperability, it is
acceptable in this case since we intend this specification to be
deployed in a tightly coupled environment since it is only applicable
to first-party applications.
1.3. User Experience Considerations
It is important to consider the user experience implications of
different authentication challenges as well as the device with which
the user is attempting to authorize.
For example, requesting a user to enter a password on a limited-input
device (e.g. TV) creates a lot of user friction while also exposing
the user's password to anyone else in the room. On the other hand,
using a challenge method that involves, for example, a fingerprint
reader on the TV remote allowing for a FIDO2 passkey authentication
would be a good experience.
The Authorization Server SHOULD consider the user's device when
presenting authentication challenges and developers SHOULD consider
whether the device implementing this specification can provide a good
experience for the user. If the combination of user device and
authentication challenge methods creates a lot of friction or
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security risk, consider using a specification like OAuth 2.0 Device
Authorization Grant [RFC8628]. If selecting OAuth 2.0 Device
Authorization Grant [RFC8628] which uses a cross-device authorization
mechanism, please incorporate the security best practices identified
in Cross-Device Flows: Security Best Current Practice
[I-D.ietf-oauth-cross-device-security].
2. Conventions and Definitions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
2.1. Terminology
This specification uses the terms "Access Token", "Authorization
Code", "Authorization Endpoint", "Authorization Server" (AS),
"Client", "Client Authentication", "Client Identifier", "Client
Secret", "Grant Type", "Protected Resource", "Redirection URI",
"Refresh Token", "Resource Owner", "Resource Server" (RS) and "Token
Endpoint" defined by [RFC6749].
TODO: Replace RFC6749 references with OAuth 2.1
3. Protocol Overview
There are three primary ways this specification extends various parts
of an OAuth system.
3.1. Initial Authorization Request
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+-------------------+
| Authorization |
(B)Authorization | Server |
+----------+ Challenge Request |+-----------------+|
(A)Client+---| First- |---------------------->|| Authorization ||
Starts| | Party | || Challenge ||
Flow +-->| Client |<----------------------|| Endpoint ||
| | (C)Authorization || ||
| | Error Response || ||
| | : || ||
| | : || ||
| | (D)Authorization || ||
| | Challenge Request || ||
| |---------------------->|| ||
| | || ||
| |<----------------------|| ||
| | (E) Authorization |+-----------------+|
| | Code Response | |
| | | |
| | | |
| | | |
| | (F) Token | |
| | Request |+-----------------+|
| |---------------------->|| Token ||
| | || Endpoint ||
| |<----------------------|| ||
| | (G) Access Token |+-----------------+|
| | | |
+----------+ +-------------------+
Figure: First-Party Client Authorization Code Request
* (A) The first-party client starts the flow, by presenting the user
with a "sign in" button, or collecting information from the user,
such as their email address or username.
* (B) The client initiates the authorization request by making a
POST request to the Authorization Challenge Endpoint, optionally
with information collected from the user (e.g. email or username)
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* (C) The authorization server determines whether the information
provided to the Authorization Challenge Endpoint is sufficient to
grant authorization, and either responds with an authorization
code or responds with an error. In this example, it determines
that additional information is needed and responds with an error.
The error may contain additional information to guide the Client
on what information to collect next. This pattern of collecting
information, submitting it to the Authorization Challenge Endpoint
and then receing an error or authorization code may repeat several
times.
* (D) The client gathers additional information (e.g. signed passkey
challenge, or one-time code from email) and makes a POST request
to the Authorization Challenge Endpoint.
* (E) The Authorization Challenge Endpoint returns an authorization
code.
* (F) The client sends the authorization code received in step (E)
to obtain a token from the Token Endpoint.
* (G) The Authorization Server returns an Access Token from the
Token Endpoint.
3.2. Refresh Token Request
When the client uses a refresh token to obtain a new access token,
the authorization server MAY respond with an error to indicate that
re-authorization of the user is required.
3.3. Resource Request
When making a resource request to a resource server, the resource
server MAY respond with an error according to OAuth 2.0 Step-Up
Authentication Challenge Protocol [RFC9470], indicating that re-
authorization of the user is required.
4. Protocol Endpoints
4.1. Authorization Challenge Endpoint
The authorization challenge endpoint is a new endpoint defined by
this specification which the first-party application uses to obtain
an authorization code.
The authorization challenge endpoint is an HTTP API at the
authorization server that accepts HTTP POST requests with parameters
in the HTTP request message body using the application/x-www-form-
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urlencoded format. This format has a character encoding of UTF-8, as
described in Appendix B of [RFC6749]. The authorization challenge
endpoint URL MUST use the "https" scheme.
If the authorization server requires client authentication for this
client on the Token Endpoint, then the authorization server MUST also
require client authentication for this client on the Authorization
Challenge Endpoint. See Section 9.4 for more details.
Authorization servers supporting this specification SHOULD include
the URL of their authorization challenge endpoint in their
authorization server metadata document [RFC8414] using the
authorization_challenge_endpoint parameter as defined in Section 8.
The endpoint accepts the authorization request parameters defined in
[RFC6749] for the authorization endpoint as well as all applicable
extensions defined for the authorization endpoint. Some examples of
such extensions include Proof Key for Code Exchange (PKCE) [RFC7636],
Resource Indicators [RFC8707], and OpenID Connect [OpenID]. It is
important to note that some extension parameters have meaning in a
web context but don't have meaning in a native mechanism (e.g.
response_mode=query). It is out of scope as to what the AS does in
the case that an extension defines a parameter that is has no meaning
in this use case.
The client initiates the authorization flow with or without
information collected from the user (e.g. a signed passkey challenge
or MFA code).
The authorization challenge endpoint response is either an
authorization code or an error code, and may also contain an
auth_session which the client uses on subsequent requests to the
authorization challenge endpoint.
4.2. Token endpoint
The token endpoint is used by the client to obtain an access token by
presenting its authorization grant or refresh token, as described in
Section 3.2 of OAuth 2.0 [RFC6749].
This specification extends the token endpoint response to allow the
authorization server to indicate that further authentication of the
user is required.
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5. Authorization Initiation
A client may wish to initiate an authorization flow by first
prompting the user for their user identifier or other account
information. The authorization challenge endpoint is a new endpoint
to collect this login hint and direct the client with the next steps,
whether that is to do an MFA flow, or perform an OAuth redirect-based
flow.
In order to preserve the security of this specification, the
Authorization Server MUST verify the "first-partyness" of the client
before continuing with the authentication flow. Please see
Section 9.1 for additional considerations.
5.1. Authorization Challenge Request
The client makes a request to the authorization challenge endpoint by
adding the following parameters, as well as parameters from any
extensions, using the application/x-www-form-urlencoded format with a
character encoding of UTF-8 in the HTTP request body:
"client_id": REQUIRED if the client is not authenticating with the
authorization server and if no auth_session is included.
"scope": OPTIONAL. The OAuth scope defined in [RFC6749].
"acr_values": OPTIONAL. The acr_values requested by the client.
"auth_session": OPTIONAL. If the client has previously obtained an
auth session, described in Section 5.3.1.
"code_challenge": OPTIONAL. The code challenge as defined by
[RFC7636]. See Section 5.2.2.1 for details.
"code_challenge_method": OPTIONAL. The code challenge method as
defined by [RFC7636]. See Section 5.2.2.1 for details.
Specific implementations as well as extensions to this specification
MAY define additional parameters to be used at this endpoint.
For example, the client makes the following request to initiate a
flow given the user's phone number, line breaks shown for
illustration purposes only:
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POST /authorize HTTP/1.1
Host: server.example.com
Content-Type: application/x-www-form-urlencoded
login_hint=%2B1-310-123-4567&scope=profile
&client_id=bb16c14c73415
5.2. Authorization Challenge Response
The authorization server determines whether the information provided
up to this point is sufficient to issue an authorization code, and if
so responds with an authorization code. If the information is not
sufficient for issuing an authorization code, then the authorization
server MUST respond with an error response.
5.2.1. Authorization Code Response
The authorization server issues an authorization code by creating an
HTTP response content using the application/json media type as
defined by [RFC8259] with the following parameters and an HTTP 200
(OK) status code:
"authorization_code": REQUIRED. The authorization code issued by
the authorization server.
For example,
HTTP/1.1 200 OK
Content-Type: application/json
Cache-Control: no-store
{
"authorization_code": "uY29tL2F1dGhlbnRpY"
}
5.2.2. Error Response
If the request contains invalid parameters or incorrect data, or if
the authorization server wishes to interact with the user directly,
the authorization server responds with an HTTP 400 (Bad Request)
status code (unless specified otherwise below) and includes the
following parameters with the response:
"error": REQUIRED. A single ASCII [USASCII] error code from the
following:
"invalid_request": The request is missing a required parameter,
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includes an unsupported parameter value, repeats a parameter,
includes multiple credentials, utilizes more than one mechanism
for authenticating the client, or is otherwise malformed.
"invalid_client": Client authentication failed (e.g., unknown
client, no client 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 or auth_session
is invalid, expired, revoked, or is otherwise invalid.
"unauthorized_client": The authenticated client is not authorized
to use this authorization grant type.
"invalid_scope": The requested scope is invalid, unknown,
malformed, or exceeds the scope granted by the resource owner.
"insufficient_authorization": The presented authorization is
insufficient, and the authorization server is requesting the
client take additional steps to complete the authorization.
"redirect_to_web": The request is not able to be fulfilled with
any further direct interaction with the user. Instead, the
client should initiate a new authorization code flow so that
the user interacts with the authorization server in a web
browser.
Values for the error parameter MUST NOT include characters outside
the set %x20-21 / %x23-5B / %x5D-7E.
The authorization server MAY extend these error codes with custom
messages based on the requirements of the authorization server.
"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
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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.
"auth_session": OPTIONAL. The auth session allows the authorization
server to associate subsequent requests by this client with an
ongoing authorization request sequence. The client MUST include
the auth_session in follow-up requests to the challenge endpoint
if it receives one along with the error response.
"request_uri": OPTIONAL. A request URI as described by [RFC9126]
Section 2.2.
"expires_in": OPTIONAL. The lifetime of the request_uri in seconds,
as described by [RFC9126] Section 2.2.
This specification requires the authorization server to define new
error codes that relate to the actions the client must take in order
to properly authenticate the user. These new error codes are
specific to the authorization server's implementation of this
specification and are intentionally left out of scope.
The parameters are included in the content of the HTTP response using
the application/json media type as defined by [RFC7159]. 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.
The authorization server MAY define additional parameters in the
response depending on the implmentation. The authorization server
MAY also define more specific content types for the error responses
as long as the response is JSON and conforms to application/<AS-
defined>+json.
5.2.2.1. Redirect to Web Error Response
The authorization server may choose to interact directly with the
user based on a risk assesment, the introduction of a new
authentication method not supported in the application, or to handle
an exception flow like account recovery. To indicate this error to
the client, the authorization server returns an error response as
defined above with the redirect_to_web error code.
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In this case, the client is expected to initiate a new OAuth
Authorization Code flow with PKCE according to [RFC6749] and
[RFC7636].
If the client expects the frequency of this error response to be
high, the client MAY include a PKCE ([RFC7636]) code_challenge in the
initial authorization challenge request. This enables the
authorization server to essentially treat the authorization challenge
request as a PAR [RFC9126] request, and return the request_uri and
expires_in as defined by [RFC9126] in the error response. The client
then uses the request_uri value to build an authorization request as
defined in [RFC9126] Section 4.
5.3. Intermediate Requests
If the authorization server returns an insufficient_authorization
error as described above, this is an indication that there is further
information the client should request from the user, and continue to
make requests to the authorization server until the authorization
request is fulfilled and an authorization code returned.
These intermediate requests are out of scope of this specification,
and are expected to be defined by the authorization server. The
format of these requests are not required to conform to the format of
the initial authorization challenge requests (e.g. the request format
may be application/json rather than application/x-www-form-
urlencoded).
5.3.1. Auth Session
The auth_session is a value that the authorization server issues in
order to be able to associate subsequent requests from the same
client. It is intended to be analagous to how a browser cookie
associates multiple requests by the same browser to the authorization
server.
The auth_session value is completely opaque to the client, and as
such the authorization server MUST adequately protect the value from
inspection by the client, for example by using a random string or
using a JWE if the authorization server is not maintaining state on
the backend.
If the client has an auth_session, the client MUST include it in
future requests to the authorization challenge endpoint. The client
MUST store the auth_session beyond the issuance of the authorization
code to be able to use it in future requests.
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Every response defined by this specification may include a new
auth_session value. Clients MUST NOT assume that auth_session values
are static, and MUST be prepared to update the stored auth_session
value if one is received in a response.
To mitigate the risk of session hijacking, the 'auth_session' MUST be
bound to the device, and the authorization server MUST reject an
'auth_session' if it is presented from a different device than the
one it was bound to.
See Section 9.6 for additional security considerations.
6. Token Request
The client makes a request to the token endpoint using the
authorization code it obtained from the authorization challenge
endpoint.
This specification does not define any additional parameters beyond
the token request parameters defined in Section 4.1.3 of [RFC6749].
However, notably, the redirect_uri parameter will not be included in
this request, because no redirect_uri parameter was included in the
authorization request.
6.1. Token Endpoint Successful Response
This specification extends the OAuth 2.0 [RFC6749] token response
defined in Section 5.1 with the additional parameter auth_session,
defined in Section 5.3.1.
The response MAY include an auth_session parameter which the client
is expected to include on a subsequent request to the authorization
challenge endpoint. The auth_session parameter MAY also be included
even if the authorization code was obtained through a traditional
OAuth authorization code flow rather than the flow defined by this
specification.
An example successful token response is below:
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HTTP/1.1 200 OK
Content-Type: application/json
Cache-Control: no-store
{
"access_token": "2YotnFZFEjr1zCsicMWpAA",
"token_type": "Bearer",
"expires_in": 3600,
"refresh_token": "tGzv3JOkF0XG5Qx2TlKWIA",
"auth_session": "uY29tL2F1dGhlbnRpY"
}
6.2. Token Endpoint Error Response
Upon any request to the token endpoint, including a request with a
valid refresh token, the authorization server can respond with an
authorization challenge instead of a successful access token
response.
An authorization challenge error response is a particular type of
error response as defined in Section 5.2 of OAuth 2.0 [RFC6749] where
the error code is set to the following value:
"error": "insufficient_authorization": The presented authorization
is insufficient, and the authorization server is requesting the
client take additional steps to complete the authorization.
Additionally, the response MAY contain an auth_session parameter
which the client is expected to include on a subsequent request to
the authorization challenge endpoint.
"auth_session": OPTIONAL. The optional auth session value allows
the authorization server to associate subsequent requests by this
client with an ongoing authorization request sequence. The client
MUST include the auth_session in follow-up requests to the
challenge endpoint if it receives one along with the error
response.
For example:
HTTP/1.1 403 Forbidden
Content-Type: application/json
Cache-Control: no-store
{
"error": "insufficient_authorization",
"auth_session": "uY29tL2F1dGhlbnRpY"
}
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7. Resource Server Error Response
Step-Up Authentication [RFC9470] defines a mechanism for resource
servers to tell the client to start a new authorization request,
including acr_values and max_age, and scope from RFC6750. Upon
receiving this request, the client starts a new authorization request
according to this specification, and includes the acr_values, max_age
and scope returned in the error response.
This specification does not define any new parameters for the
resource server error response beyond those defined in [RFC9470].
8. Authorization Server Metadata
The following authorization server metadata parameters [RFC8414] are
introduced to signal the server's capability and policy with respect
to first-party applications.
"authorization_challenge_endpoint": The URL of the authorization
challenge endpoint at which a client can initiate an authorization
request and eventually obtain an authorization code.
9. Security Considerations
9.1. First-Party Applications
First-party applications are applications that the user recognizes as
belonging to the same brand as the authorization server. For
example, a bank publishing their own mobile application.
Because this specification enables a client application to interact
directly with the end user, and the application handles sending any
information collected from the user to the authorization server, it
is expected to be used only for first-party applications when the
authorization server also has a high degree of trust of the client.
This specification is not prescriptive on how the Authorization
Server establishes it's trust in the first-partyness of the
application. For mobile platforms, most support some mechanism for
application attestation that can be used to identify the entity that
created/signed/uploaded the app to the app store. App attestation
can be combined with other mechanisms like Dynamic Client
Registration [RFC7591] to enable strong client authentication in
addition to client verification (first-partyness). The exact steps
required are out of scope for this specification. Note that
applications running inside a browser (e.g. Single Page Apps)
context it is much more difficult to verify the first-partyness of
the client. Please see Section 9.8 for additional details.
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9.2. Phishing
There are two ways using this specification increases the risk of
phishing.
With this specification, the client interacts directly with the end
user, collecting information provided by the user and sending it to
the authorization server. If an attacker impersonates the client and
successfully tricks a user into using it, they may not realize they
are giving their credentials to the malicious application.
In a traditional OAuth deployment using the redirect-based
authorization code flow, the user will only ever enter their
credentials at the authorization server, and it is straightforward to
explain to avoid entering credentials in other "fake" websites. By
introducing a new place the user is expected to enter their
credentials using this specification, it is more complicated to teach
users how to recognize other fake login prompts that might be
attempting to steal their credentials.
Because of these risks, the authorization server MAY decide to
require that the user go through a redirect-based flow at any stage
of the process based on its own risk assessment.
9.3. Credential Stuffing Attacks
The authorization challenge endpoint is capable of directly receiving
user credentials and returning authorization codes. This exposes a
new vector to perform credential stuffing attacks, if additional
measures are not taken to ensure the authenticity of the application.
An authorization server may already have a combination of built-in or
3rd party security tools in place to monitor and reduce this risk in
browser-based authentication flows. Implementors SHOULD consider
similar security measures to reduce this risk in the authorization
challenge endpoint. Additionally, the attestation APIs SHOULD be
used when possible to assert a level of confidence to the
authorization server that the request is originating from an
application owned by the same party.
9.4. Client Authentication
Typically, mobile and desktop applications are considered "public
clients" in OAuth, since they cannot be shipped with a statically
configured set of client credentials [RFC8252]. Because of this,
client impersonation should be a concern of anyone deploying this
pattern. Without client authentication, a malicious user or attacker
can mimick the requests the application makes to the authorization
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server, pretending to be the legitimate client.
Because this specification is intended for first-party applications,
it is likely that the intent is to also avoid prompting the user with
a consent screen as recommended by [RFC6749].
Implementers SHOULD consider additional measures to limit the risk of
client impersonation, such as using attestation APIs available from
the operating system.
9.5. Sender Constrained Tokens
Tokens issued in response to an authorization challenge request
SHOULD be sender constrained to mitigate the risk of token theft and
replay.
Proof-of-Possession techniques constrain tokens by binding them to a
cryptographic key. Whenever the token is presented, it MUST be
accompanied by a proof that the client presenting the token also
controls the cryptographic key bound to the token. If a proof-of-
possession sender constrained token is presented without valid proof
of possession of the cryptographic key, it MUST be rejected.
9.5.1. DPoP: Demonstrating Proof-of-Possession
DPoP ([RFC9449]) is an application-level mechanism for sender-
constraining OAuth [RFC6749] access and refresh tokens. If DPoP is
used to sender constrain tokens, the client SHOULD use DPoP for every
token request to the Authorization Server and interaction with the
Resource Server.
DPoP includes an optional capability to bind the authorization code
to the DPoP key to enable end-to-end binding of the entire
authorization flow. Given the back-channel nature of this
specification, there are far fewer opportunities for an attacker to
access the authorization code and PKCE code verifier compared to the
redirect-based Authorization Code Flow. In this specification, the
Authorization Code is obtained via a back-channel request. Despite
this, omitting Authorization Code binding leaves a gap in the end-to-
end protection that DPoP provides, so DPoP Authorization Code binding
SHOULD be used.
The mechanism for Authorization Code binding with DPoP is similar as
that defined for Pushed Authorization Requests (PARs) in Section 10.1
of [RFC9449]. In order to bind the Authorization Code with DPoP, the
client MUST add the DPoP header to the Authorization Challenge
Request. The authorization server MUST check the DPoP proof JWT that
was included in the DPoP header as defined in Section 4.3 of
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[RFC9449]. The authorization server MUST ensure that the same key is
used in all subsequent Authorization Challenge Requests, or in the
eventual token request. The authorization server MUST reject
subsequent Authorization Challenge Requests, or the eventual token
request, unless a DPoP proof for the same key presented in the
original Authorization Challenge Request is provided.
The above mechanism simplifies the implementation of the client, as
it can attach the DPoP header to all requests to the authorization
server regardless of the type of request. This mechanism provides a
stronger binding than using the dpop_jkt parameter, as the DPoP
header contains a proof of possession of the private key.
9.5.2. Other Proof of Possession Mechanisms
It may be possible to use other proof of possession mechanisms to
sender constrain access and refresh tokens. Defining these
mechanisms are out of scope for this specification.
9.6. Auth Session
9.6.1. Auth Session DPoP Binding
If the client and authorization server are using DPoP binding of
access tokens and/or authorization codes, then the auth_session value
SHOULD be protected by the DPoP binding as well. The authorization
server SHOULD bind the auth_session value to the DPoP public key. If
the authorization server is binding the auth_session value to the
DPoP public key, it MUST check that the same DPoP public key is being
used and MUST verify the DPoP proof to ensure the client controls the
corresponding private key whenever the client includes the
auth_session in an Authorization Challenge Request as described in
Section 5.1.
DPoP binding of the auth_session value ensures that the context
referenced by the auth_session cannot be stolen and reused by another
device.
9.6.2. Auth Session Lifetime
This specification makes no requirements or assumptions on the
lifetime of the auth_session value. The lifetime and expiration is
at the discretion of the authorization server, and the authorization
server may choose to invalidate the value for any reason such as
scheduled expiration, security events, or revocation events.
Clients MUST NOT make any assumptions or depend on any particular
lifetime of the auth_session value.
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9.7. Multiple Applications
When multiple first-party applications are supported by the AS, then
it is important to consider a number of additional risks. These
risks fall into two main categories: Experience Risk and Technical
Risk which are described below.
9.7.1. User Experience Risk
Any time a user is asked to provide the authentication credentials in
user experiences that differ, it has the effect of increasing the
likelihood that the user will fall prey to a phishing attack because
they are used to entering credentials in different looking
experiences. When multiple first-party applications are supported,
the implementation MUST ensure the native experience is identical
across all the first-party applications.
Another experience risk is user confusion caused by different looking
experiences and behaviors. This can increase the likelihood the user
will not complete the authentication experience for the first-party
application.
9.7.2. Technical Risk
In addition to the experience risks, multiple implementations in
first-party applications increases the risk of an incorrect
implementation as well as increasing the attack surface as each
implementation may expose its own weaknesses.
9.7.3. Mitigation
To address these risks, when multiple first-party applications must
be supported, and other methods such as [OpenID.Native-SSO] are not
applicable, it is RECOMMENDED that a client-side SDK be used to
ensure the implementation is consistent across the different
applications and to ensure the user experience is identical for all
first-party apps.
9.8. Single Page Applications
Single Page Applications (SPA) run in a scripting language inside the
context of a browser instance. This environment poses several unique
challenges compared to native applications, in particular:
* Significant attack vectors due to the possibility of Cross-Site
Scripting (XSS) attacks
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* Fewer options to securely attest to the first-partyness of a
browser based application
See [I-D.ietf-oauth-browser-based-apps] for a detailed discussion of
the risks of XSS attacks in browsers.
Additionally, the nature of a Single-Page App means the user is
already in a browser context, so the user experience cost of doing a
full page redirect or a popup window for the traditional OAuth
Authorization Code Flow is much less than the cost of doing so in a
native application. The complexity and risk of implementing this
specification in a browser likely does not outweigh the user
experience benefits that would be gained in that context.
For these reasons, it is NOT RECOMMENDED to use this specification in
browser-based applications.
10. IANA Considerations
10.1. OAuth Parameters Registration
IANA has (TBD) registered the following values in the IANA "OAuth
Parameters" registry of [IANA.oauth-parameters] established by
[RFC6749].
*Parameter name*: auth_session
*Parameter usage location*: token response
*Change Controller*: IETF
*Specification Document*: Section 5.4 of this specification
10.2. OAuth Server Metadata Registration
IANA has (TBD) registered the following values in the IANA "OAuth
Authorization Server Metadata" registry of [IANA.oauth-parameters]
established by [RFC8414].
*Metadata Name*: authorization_challenge_endpoint
*Metadata Description*: URL of the authorization server's
authorization challenge endpoint.
*Change Controller*: IESG
*Specification Document*: Section 4.1 of [[ this specification ]]
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10.3. JSON Web Token Claims Registration
IANA has (TBD) registered the following Claims in the "JSON Web Token
Claims" registry [IANA.JWT] established by [RFC7519].
11. References
11.1. Normative References
[I-D.ietf-oauth-cross-device-security]
Kasselman, P., Fett, D., and F. Skokan, "Cross-Device
Flows: Security Best Current Practice", Work in Progress,
Internet-Draft, draft-ietf-oauth-cross-device-security-05,
1 March 2024, <https://datatracker.ietf.org/doc/html/
draft-ietf-oauth-cross-device-security-05>.
[IANA.JWT] "*** BROKEN REFERENCE ***".
[IANA.oauth-parameters]
IANA, "OAuth Parameters",
<http://www.iana.org/assignments/oauth-parameters>.
[OpenID] Sakimura, N., Bradley, J., Jones, M., de Medeiros, B., and
C. Mortimore, "OpenID Connect Core 1.0", November 2014,
<https://openid.net/specs/openid-connect-core-1_0.html>.
[OpenID.Native-SSO]
Fletcher, G., "OpenID Connect Native SSO for Mobile Apps",
November 2022, <https://openid.net/specs/openid-connect-
native-sso-1_0.html>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/rfc/rfc2119>.
[RFC6749] Hardt, D., Ed., "The OAuth 2.0 Authorization Framework",
RFC 6749, DOI 10.17487/RFC6749, October 2012,
<https://www.rfc-editor.org/rfc/rfc6749>.
[RFC7159] Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
Interchange Format", RFC 7159, DOI 10.17487/RFC7159, March
2014, <https://www.rfc-editor.org/rfc/rfc7159>.
[RFC7515] Jones, M., Bradley, J., and N. Sakimura, "JSON Web
Signature (JWS)", RFC 7515, DOI 10.17487/RFC7515, May
2015, <https://www.rfc-editor.org/rfc/rfc7515>.
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[RFC7519] Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
(JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015,
<https://www.rfc-editor.org/rfc/rfc7519>.
[RFC7591] Richer, J., Ed., Jones, M., Bradley, J., Machulak, M., and
P. Hunt, "OAuth 2.0 Dynamic Client Registration Protocol",
RFC 7591, DOI 10.17487/RFC7591, July 2015,
<https://www.rfc-editor.org/rfc/rfc7591>.
[RFC7636] Sakimura, N., Ed., Bradley, J., and N. Agarwal, "Proof Key
for Code Exchange by OAuth Public Clients", RFC 7636,
DOI 10.17487/RFC7636, September 2015,
<https://www.rfc-editor.org/rfc/rfc7636>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.
[RFC8259] Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
Interchange Format", STD 90, RFC 8259,
DOI 10.17487/RFC8259, December 2017,
<https://www.rfc-editor.org/rfc/rfc8259>.
[RFC8414] Jones, M., Sakimura, N., and J. Bradley, "OAuth 2.0
Authorization Server Metadata", RFC 8414,
DOI 10.17487/RFC8414, June 2018,
<https://www.rfc-editor.org/rfc/rfc8414>.
[RFC8628] Denniss, W., Bradley, J., Jones, M., and H. Tschofenig,
"OAuth 2.0 Device Authorization Grant", RFC 8628,
DOI 10.17487/RFC8628, August 2019,
<https://www.rfc-editor.org/rfc/rfc8628>.
[RFC8707] Campbell, B., Bradley, J., and H. Tschofenig, "Resource
Indicators for OAuth 2.0", RFC 8707, DOI 10.17487/RFC8707,
February 2020, <https://www.rfc-editor.org/rfc/rfc8707>.
[RFC9126] Lodderstedt, T., Campbell, B., Sakimura, N., Tonge, D.,
and F. Skokan, "OAuth 2.0 Pushed Authorization Requests",
RFC 9126, DOI 10.17487/RFC9126, September 2021,
<https://www.rfc-editor.org/rfc/rfc9126>.
[RFC9449] Fett, D., Campbell, B., Bradley, J., Lodderstedt, T.,
Jones, M., and D. Waite, "OAuth 2.0 Demonstrating Proof of
Possession (DPoP)", RFC 9449, DOI 10.17487/RFC9449,
September 2023, <https://www.rfc-editor.org/rfc/rfc9449>.
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[RFC9470] Bertocci, V. and B. Campbell, "OAuth 2.0 Step Up
Authentication Challenge Protocol", RFC 9470,
DOI 10.17487/RFC9470, September 2023,
<https://www.rfc-editor.org/rfc/rfc9470>.
[SHS] Technology, N. I. of S. and., ""Secure Hash Standard
(SHS)", FIPS PUB 180-4, DOI 10.6028/NIST.FIPS.180-4",
August 2015, <http://dx.doi.org/10.6028/NIST.FIPS.180-4>.
[USASCII] Institute, A. N. S., "Coded Character Set -- 7-bit
American Standard Code for Information Interchange, ANSI
X3.4", 1986.
11.2. Informative References
[I-D.ietf-oauth-browser-based-apps]
Parecki, A., Waite, D., and P. De Ryck, "OAuth 2.0 for
Browser-Based Apps", Work in Progress, Internet-Draft,
draft-ietf-oauth-browser-based-apps-17, 28 February 2024,
<https://datatracker.ietf.org/doc/html/draft-ietf-oauth-
browser-based-apps-17>.
[RFC8252] Denniss, W. and J. Bradley, "OAuth 2.0 for Native Apps",
BCP 212, RFC 8252, DOI 10.17487/RFC8252, October 2017,
<https://www.rfc-editor.org/rfc/rfc8252>.
Appendix A. Example User Experiences
This section provides non-normative examples of how this
specification may be used to support specific use cases.
A.1. Passkey
A user may log in with a passkey (without a password).
* The Client collects the username from the user.
* The Client sends an Authorization Challenge Request (Section 5.1)
to the Authorization Challenge Endpoint (Section 4.1) including
the username.
* The Authorization Server verifies the username and returns a
challenge
* The Client signs the challenge using the platform authenticator,
which results in the user being prompted for verification with
biometrics or a PIN.
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* The Client sends the signed challenge, username, and credential ID
to the Authorization Challenge Endpoint (Section 4.1).
* The Authorization Server verifies the signed challenge and returns
an Authorization Code.
* The Client requests an Access Token and Refresh Token by issuing a
Token Request (Section 6) to the Token Endpoint.
* The Authorization Server verifies the Authorization Code and
issues the requested tokens.
A.2. Redirect to Authorization Server
A user may be redirected to the Authorization Server to perfrom an
account reset.
* The Client collects username from the user.
* The Client sends an Authorization Challenge Request (Section 5.1)
to the Authorization Challenge Endpoint (Section 4.1) including
the username.
* The Authorization Server verifies the username and determines that
the account is locked and returns a Redirect error response.
* The Client parses the redirect message, opens a browser and
redirects the user to the Authorization Server performing an OAuth
2.0 flow with PKCE.
* The user resets their account by performing a multi-step
authentication flow with the Authorization Server.
* The Authorization Server issues an Authorization Code in a
redirect back to the client, which then exchanges it for an access
and refresh token.
A.3. Passwordless One-Time Password (OTP)
In a passwordless One-Time Password (OTP) scheme, the user is in
possession of a one-time password generator. This generator may be a
hardware device, or implemented as an app on a mobile phone. The
user provides a user identifier and one-time password, which is
verified by the Authorization Server before it issues an
Authorization Code, which can be exchanged for an Access and Refresh
Token.
* The Client collects username and OTP from user.
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* The Client sends an Authorization Challenge Request (Section 5.1)
to the Authorization Challenge Endpoint (Section 4.1) including
the username and OTP.
* The Authorization Server verifies the username and OTP and returns
an Authorization Code.
* The Client requests an Access Token and Refresh Token by issuing a
Token Request (Section 6) to the Token Endpoint.
* The Authorization Server verifies the Authorization Code and
issues the requested tokens.
A.4. E-Mail Confirmation Code
A user may be required to provide an e-mail confirmation code as part
of an authentication ceremony to prove they control an e-mail
address. The user provides an e-mail address and is then required to
enter a verification code sent to the e-mail address. If the correct
verification code is returned to the Authorization Server, it issues
Access and Refresh Tokens.
* The Client collects an e-mail address from the user.
* The Client sends the e-mail address in an Authorization Challenge
Request (Section 5.1) to the Authorization Challenge Endpoint
(Section 4.1).
* The Authorization Server sends a verification code to the e-mail
address and returns an Error Response (Section 5.2.2) including
"error": "insufficient_authorization", "auth_session" and a custom
property indicating that an e-mail verification code must be
entered.
* The Client presents a user experience guiding the user to copy the
e-mail verification code to the Client. Once the e-mail
verification code is entered, the Client sends an Authorization
Challenge Request to the Authorization Challenge Endpoint,
including the e-mail verification code as well as the auth_session
parameter returned in the previous Error Response.
* The Authorization Server uses the auth_session to maintain the
session and verifies the e-mail verification code before issuing
an Authorization Code to the Client.
* The Client sends the Authorization Code in a Token Request
(Section 6) to the Token Endpoint.
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* The Authorization Server verifies the Authorization Code and
issues the Access Token and Refresh Token.
A.5. SMS Confirmation Code
A user may be required to provide an SMS confirmation code as part of
an authentication ceremony to prove they control a mobile phone
number. The user provides a phone number and is then required to
enter an SMS confirmation code sent to the phone. If the correct
confirmation code is returned to the Authorization Server, it issues
Access and Refresh Tokens.
* The Client collects a mobile phone number from the user.
* The Client sends the phone number in an Authorization Challenge
Request (Section 5.1) to the Authorization Challenge Endpoint
(Section 4.1).
* The Authorization Server sends a confirmation code to the phone
number and returns an Error Response (Section 5.2.2) including
"error": "insufficient_authorization", "auth_session" and a custom
property indicating that a SMS confirmation code must be entered.
* The Client presents a user experience guiding the user to enter
the SMS confirmation code. Once the SMS verification code is
entered, the Client sends an Authorization Challenge Request to
the Authorization Challenge Endpoint, including the confirmation
code as well as the auth_session parameter returned in the
previous Error Response.
* The Authorization Server uses the auth_session to maintain the
session context and verifies the SMS code before issuing an
Authorization Code to the Client.
* The Client sends the Authorization Code in a Token Request
(Section 6) to the Token Endpoint.
* The Authorization Server verifies the Authorization Code and
issues the Access Token and Refresh Token.
A.6. Re-authenticating to an app a week later using OTP
A client may be in possession of an Access and Refresh Token as the
result of a previous succesful user authentication. The user returns
to the app a week later and accesses the app. The Client presents
the Access Token, but receives an error indicating the Access Token
is no longer valid. The Client presents a Refresh Token to the
Authorization Server to obtain a new Access Token. If the
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Authorization Server requires user interaction for reasons based on
its own policies, it rejects the Refresh Token and the Client re-
starts the user authentication flow to obtain new Access and Refresh
Tokens.
* The Client has a short-lived access token and long-lived refresh
token following a previous completion of an Authorization Grant
Flow which included user authentication.
* A week later, the user launches the app and tries to access a
protected resource at the Resource Server.
* The Resource Server responds with an error code indicating an
invalid access token since it has expired.
* The Client presents the refresh token to the Authorization Server
to obtain a new access token (section 6 [RFC6749])
* The Authorization Server responds with an error code indicating
that an OTP from the user is required, as well as an auth_session.
* The Client prompts the user to enter an OTP.
* The Client sends the OTP and auth_session in an Authorization
Challenge Request (Section 5.1) to the Authorization Challenge
Endpoint (Section 4.1).
* The Authorization Server verifies the auth_session and OTP, and
returns an Authorization Code.
* The Client sends the Authorization Code in a Token Request
(Section 6) to the Token Endpoint.
* The Authorization Server verifies the Authorization Code and
issues the requested tokens.
* The Client presents the new Access Token to the Resource Server in
order to access the protected resource.
A.7. Step-up Authentication using Confirmation SMS
A Client previously obtained an Access and Refresh Token after the
user authenticated with an OTP. When the user attempts to access a
protected resource, the Resource Server determines that it needs an
additional level of authentication and triggers a step-up
authentication, indicating the desired level of authentication using
acr_values and max_age as defined in the Step-up Authentication
specification. The Client initiates an authorization request with
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the Authorization Server indicating the acr_values and max_age
parameters. The Authorization Server responds with error messages
promptng for additional authentication until the acr_values and
max_age values are satisfied before issuing fresh Access and Refresh
Tokens.
* The Client has a short-lived access token and long-lived refresh
token following the completion of an Authorization Code Grant Flow
which included user authentication.
* When the Client presents the Access token to the Resource Server,
the Resource Server determines that the acr claim in the Access
Token is insufficient given the resource the user wants to access
and responds with an insufficient_user_authentication error code,
along with the desired acr_values and desired max_age.
* The Client sends an Authorization Challenge Request (Section 5.1)
to the Authorization Challenge Endpoint (Section 4.1) including
the auth_session, acr_values and max_age parameters.
* The Authorization Server verifies the auth_session and determines
which authentication methods must be satisfied based on the
acr_values, and responds with an Error Response (Section 5.2.2)
including "error": "insufficient_authorization" and a custom
property indicating that an OTP must be entered.
* The Client prompts the user for an OTP, which the user obtains and
enters.
* The Client sends an Authorization Challenge Request to the
Authorization Challenge Endpoint including the auth_session and
OTP.
* The Authorization Server verifies the OTP and returns an
Authorization Code.
* The Client sends the Authorization Code in a Token Request
(Section 6) to the Token Endpoint.
* The Authorization Server verifies the Authorization Code and
issues an Access Token with the updated acr value along with the
Refresh Token.
* The Client presents the Access Token to the Resources Server,
which verifies that the acr value meets its requirements before
granting access to the prtoected resource.
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A.8. Registration
This example describes how to use the mechanisms defined in this
draft to create a complete user registration flow starting with an
email address. In this example, it is the Authorization Server's
policy to allow these challenges to be sent to email and phone number
that were previously unrecognized, and creating the user account on
the fly.
* The Client collects a username from the user.
* The Client sends an Authorization Challenge Request (Section 5.1)
to the Authorization Challenge Endpoint (Section 4.1) including
the username.
* The Authorization Server returns an Error Response (Section 5.2.2)
including "error": "insufficient_authorization", "auth_session",
and a custom property indicating that an e-mail address must be
collected.
* The Client collects an e-mail address from the user.
* The Client sends the e-mail address as part of a second
Authorization Challenge Request to the Authorization Challenge
Endpoint, along with the auth_session parameter.
* The Authorization Server sends a verification code to the e-mail
address and returns an Error Response including "error":
"insufficient_authorization", "auth_session" and a custom property
indicating that an e-mail verification code must be entered.
* The Client presents a user experience guiding the user to copy the
e-mail verification code to the Client. Once the e-mail
verification code is entered, the Client sends an Authorization
Challenge Request to the Authorization Challenge Endpoint,
including the e-mail verification code as well as the auth_session
parameter returned in the previous Error Response.
* The Authorization Server uses the auth_session to maintain the
session context, and verifies the e-mail verification code. It
determines that it also needs a phone number for account recovery
purposes and returns an Error Response including "error":
"insufficient_authorization", "auth_session" and a custom property
indicating that a phone number must be collected.
* The Client collects a mobile phone number from the user.
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* The Client sends the phone number in an Authorization Challenge
Request to the Authorization Challenge Endpoint, along with the
auth_session.
* The Authorization Server uses the auth_session parameter to link
the previous requests. It sends a confirmation code to the phone
number and returns an Error Response including "error":
"insufficient_authorization", "auth_session" and a custom property
indicating that a SMS confirmation code must be entered.
* The Client presents a user experience guiding the user to enter
the SMS confirmation code. Once the SMS verification code is
entered, the Client sends an Authorization Challenge Request to
the Authorization Challenge Endpoint, including the confirmation
code as well as the auth_session parameter returned in the
previous Error Response.
* The Authorization Server uses the auth_session to maintain the
session context, and verifies the SMS verification code before
issuing an Authorization Code to the Client.
* The Client sends the Authorization Code in a Token Request
(Section 6) to the Token Endpoint.
* The Authorization Server verifies the Authorization Code and
issues the requested tokens.
Appendix B. Example Implementations
In order to successfully implement this specification, the
Authorization Server will need to define its own specific
requirements for what values clients are expected to send in the
Authorization Challenge Request (Section 5.1), as well as its own
specific error codes in the Authorization Challenge Response
(Section 5.2).
Below is an example of parameters required for a complete
implementation that enables the user to log in with a username and
OTP.
B.1. Authorization Challenge Request Parameters
In addition to the request parameters defined in Section 5.1, the
authorization server defines the additional parameters below.
"username": REQUIRED for the initial Authorization Challenge
Request.
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"otp": The OTP collected from the user. REQUIRED when re-trying an
Authorization Challenge Request in response to the otp_required
error defined below.
B.2. Authorization Challenge Response Parameters
In addition to the response parameters defined in Section 5.2, the
authorization server defines the additional value for the error
response below.
"otp_required": The client should collect an OTP from the user and
send the OTP in a second request to the Authorization Challenge
Endpoint. The HTTP response code to use with this error value is
401 Unauthorized.
B.3. Example Sequence
The client prompts the user to enter their username, and sends the
username in an initial Authorization Challenge Request.
POST /authorize HTTP/1.1
Host: server.example.com
Content-Type: application/x-www-form-urlencoded
username=alice
&scope=photos
&client_id=bb16c14c73415
The Authorization Server sends an error response indicating that an
OTP is required.
HTTP/1.1 401 Unauthorized
Content-Type: application/json
Cache-Control: no-store
{
"error": "otp_required",
"auth_session": "ce6772f5e07bc8361572f"
}
The client prompts the user for an OTP, and sends a new Authorization
Challenge Request.
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POST /authorize HTTP/1.1
Host: server.example.com
Content-Type: application/x-www-form-urlencoded
auth_session=ce6772f5e07bc8361572f
&otp=555121
The Authorization Server validates the auth_session to find the
expected user, then validates the OTP for that user, and responds
with an authorization code.
HTTP/1.1 200 OK
Content-Type: application/json
Cache-Control: no-store
{
"authorization_code": "uY29tL2F1dGhlbnRpY"
}
The client sends the authorization code to the token endpoint.
POST /token HTTP/1.1
Host: server.example.com
Content-Type: application/x-www-form-urlencoded
grant_type=authorization_code
&client_id=bb16c14c73415
&code=uY29tL2F1dGhlbnRpY
The Authorization Server responds with an access token and refresh
token.
HTTP/1.1 200 OK
Content-Type: application/json
Cache-Control: no-store
{
"token_type": "Bearer",
"expires_in": 3600,
"access_token": "d41c0692f1187fd9b326c63d",
"refresh_token": "e090366ac1c448b8aed84cbc07"
}
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Appendix C. Design Goals
Rather than extend the OAuth token endpoint with additional grant
types, this specification defines a new authorization flow the client
can use to obtain an authorization flow. There are two primary
reasons for designing the specification this way.
This enables existing OAuth implementations to make fewer
modifications to existing code by not needing to extend the token
endpoint with new logic. Instead, the new logic can be encapsulated
in an entirely new endpoint, the output of which is an authorization
code which can be redeemed for an access token at the existing token
endpoint.
This also mirrors more closely the existing architecture of the
redirect-based authorization code flow. In the authorization code
flow, the client first initiates a request by redirecting a browser
to the authorization endpoint, at which point the authorization
server takes over with its own custom logic to authenticate the user
in whatever way appropriate. Afterwards, the authorization server
redirects the user back to the client application with an
authorization code in the query string. This specification mirrors
the existing approach by having the client first make a POST request
to the "authorization challenge endpoint", at which point the
authorization server provides its own custom logic to authenticate
the user, eventually returning an authorization code.
These design decisions should enable authorization server
implementations to isolate and encapsulate the changes needed to
support this specification.
Appendix D. Document History
-01
* Added clarification on use of authorization code binding when
using DPoP with the authorization challenge endpoint.
* Removed ash claim to simplify DPoP binding with auth_session
value.
* Fixed how "redirect to web" mechanism works with PKCE.
* Added "intermediate requests" section to clarify these requests
are out of scope, moved "auth session" description to that
section.
-00
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* Renamed authorization_required to insufficient_authorization
* Defined insufficient_authorization on the Authorization Challenge
Endpoint
* Renamed device_session to auth_session
* Added explicit method to indicate the client should restart the
flow in a browser
* Described how to use DPoP in conjunction with this spec
Acknowledgments
The authors would like to thank the attendees of the OAuth Security
Workshop 2023 session in which this was discussed, as well as the
following individuals who contributed ideas, feedback, and wording
that shaped and formed the final specification:
Brian Campbell, Dick Hardt, Dmitry Telegin, John Bradley, Justin
Richer, Mike Jones, Orie Steele, Tobias Looker.
Authors' Addresses
Aaron Parecki
Okta
Email: aaron@parecki.com
George Fletcher
Capital One Financial
Email: george.fletcher@capitalone.com
Pieter Kasselman
Microsoft
Email: pieter.kasselman@microsoft.com
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