wdiff rfc8628.original rfc8628.txt

OAuth
Internet Engineering Task Force (IETF) W. Denniss
Internet-Draft
Request for Comments: 8628 Google
Intended status:
Category: Standards Track J. Bradley
Expires: September 12, 2019
ISSN: 2070-1721 Ping Identity
M. Jones
Microsoft
H. Tschofenig
ARM Limited
March 11,
August 2019

OAuth 2.0 Device Authorization Grant
draft-ietf-oauth-device-flow-15

Abstract

The OAuth 2.0 Device Authorization Grant device authorization grant is designed for internet- Internet-
connected devices that either lack a browser to perform a user-agent user-agent-
based authorization, authorization or are input-constrained input constrained to the extent that
requiring the user to input text in order to authenticate during the
authorization flow is impractical. It enables OAuth clients on such
devices (like smart TVs, media consoles, digital picture frames, and
printers) to obtain user authorization to access protected resources
without
by using an on-device user-agent. a user agent on a separate device.

Status of This Memo

This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force
(IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list It represents the consensus of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.

Internet-Drafts are draft documents valid the IETF community. It has
received public review and has been approved for a maximum publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.

Information about the current status of six months this document, any errata,
and how to provide feedback on it may be updated, replaced, or obsoleted by other documents obtained at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."

This Internet-Draft will expire on September 12, 2019.
https://www.rfc-editor.org/info/rfc8628.

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://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
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described in the Simplified BSD License.

Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Protocol . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.1. Device Authorization Request . . . . . . . . . . . . . . 5
3.2. Device Authorization Response . . . . . . . . . . . . . . 7
3.3. User Interaction . . . . . . . . . . . . . . . . . . . . 8
3.3.1. Non-textual Non-Textual Verification URI Optimization . . . . . . 9
3.4. Device Access Token Request . . . . . . . . . . . . . . . 10
3.5. Device Access Token Response . . . . . . . . . . . . . . 11
4. Discovery Metadata . . . . . . . . . . . . . . . . . . . . . 12
5. Security Considerations . . . . . . . . . . . . . . . . . . . 12
5.1. User Code Brute Forcing . . . . . . . . . . . . . . . . . 13
5.2. Device Code Brute Forcing . . . . . . . . . . . . . . . . 13
5.3. Device Trustworthiness . . . . . . . . . . . . . . . . . 14 13
5.4. Remote Phishing . . . . . . . . . . . . . . . . . . . . . 14
5.5. Session Spying . . . . . . . . . . . . . . . . . . . . . 15
5.6. Non-confidential Non-Confidential Clients . . . . . . . . . . . . . . . . 15
5.7. Non-Visual Code Transmission . . . . . . . . . . . . . . 15
6. Usability Considerations . . . . . . . . . . . . . . . . . . 15 16
6.1. User Code Recommendations . . . . . . . . . . . . . . . . 16
6.2. Non-Browser User Interaction . . . . . . . . . . . . . . 17
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17
7.1. OAuth Parameters Parameter Registration . . . . . . . . . . . . . . 17
7.1.1. Registry Contents . . . . . . . . . . . . . . . . . . 17
7.2. OAuth URI Registration . . . . . . . . . . . . . . . . . 17
7.2.1. Registry Contents . . . . . . . . . . . . . . . . . . 17
7.3. OAuth Extensions Error Registration . . . . . . . . . . . 17
7.3.1. Registry Contents . . . . . . . . . . . . . . . . . . 18
7.4. OAuth 2.0 Authorization Server Metadata . . . . . . . . . 18
7.4.1. Registry Contents . . . . . . . . . . . . . . . . . . 18
8. Normative References . . . . . . . . . . . . . . . . . . . . 18
Appendix A. 19
Acknowledgements . . . . . . . . . . . . . . . . . . 19
Appendix B. Document History . . . . . . . . . . . . . . . . . . 20
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 22 21

1. Introduction

This OAuth 2.0 [RFC6749] protocol extension, sometimes referred to as
"device flow", extension enables OAuth clients to
request user authorization from applications on devices that have
limited input capabilities or lack a suitable browser. Such devices
include those smart TVs, media
console, consoles, picture frames frames, and printers printers,
which lack an easy input method or a suitable browser required for
traditional OAuth interactions. The authorization flow defined by
this specification specification, sometimes referred to as the "device flow",
instructs the user to review the authorization request on a secondary
device, such as a
smartphone smartphone, which does have the requisite input and
browser capabilities to complete the user interaction.

The Device Authorization Grant is not device authorization grant is not intended to replace browser-
based OAuth in native apps on capable devices like smartphones.
Those apps should follow the practices specified in OAuth "OAuth 2.0 for
Native Apps Apps" [RFC8252].

The operating requirements to be able to use for using this authorization grant type
are:

(1) The device is already connected to the Internet.

(2) The device is able to make outbound HTTPS requests.

(3) The device is able to display or otherwise communicate a URI and
code sequence to the user.

(4) The user has a secondary device (e.g., personal computer or
smartphone) from which they can process the request.

As the device authorization grant does not require two-way
communication between the OAuth client on the device and the user-agent user
agent (unlike other OAuth 2 grant types types, such as the Authorization Code authorization
code and Implicit implicit grant types), it supports several use cases that
cannot be served by those other approaches.

Instead of interacting directly with the end user's user agent, agent (i.e.,
browser), the device client instructs the end user to use another
computer or device and connect to the authorization server to approve
the access request. Since the protocol supports clients that can't
receive incoming requests, clients poll the authorization server
repeatedly until the end user completes the approval process.

The device client typically chooses the set of authorization servers
to support (i.e., its own authorization server, server or those by of providers
with which it has relationships with). relationships). It is not uncommon common for the device
application client
to support only a single one authorization server, such as
with in the case of a TV
application for a specific media provider that supports only that
media provider's authorization server. The user may not yet have an
established relationship yet with that authorization provider, though one
can potentially be set up during the authorization flow.

Figure 1: Device Authorization Flow

The device authorization flow illustrated in Figure 1 includes the
following steps:

(A) The client requests access from the authorization server and
includes its client identifier in the request.

(B) The authorization server issues a device code, code and an end-user
code,
code and provides the end-user verification URI.

(C) The client instructs the end user to use its a user agent (on on another device)
device and visit the provided end-user verification URI. The
client provides the user with the end-user code to enter in
order to review the authorization request.

(D) The authorization server authenticates the end user (via the
user agent) agent), and prompts the user to grant the client's access
request. If the user agrees to the client's access request, the
user enters input the user code
provided by the device client. The authorization server
validates the user code provided by the user. user, and prompts the
user to accept or decline the request.

(E) While the end user reviews the client's request (step D), the
client repeatedly polls the authorization server to find out if
the user completed the user authorization step. The client
includes the verification device code and its client identifier.

(F) The authorization server validates the verification device code provided by
the client and responds back with the access token if the user client is
granted access, an error if they are denied access, or
indicates an
indication that the client should continue to poll.

2. Terminology

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.

Device Authorization Endpoint:
The authorization server's endpoint capable of issuing device
verification codes, user codes, and verification URLs.

Device Verification Code:
A short-lived token representing an authorization session.

End-User Verification Code:
A short-lived token which the device displays to the end user, is
entered by the user on the authorization server, and is thus used
to bind the device to the user.

3. Protocol

3.1. Device Authorization Request

This specification defines a new OAuth endpoint, endpoint: the device
authorization endpoint. This is separate from the OAuth
authorization endpoint defined in [RFC6749] with which the user
interacts with via a user-agent user agent (i.e., a browser). By comparison, when
using the device authorization endpoint, the OAuth client on the
device interacts with the authorization server directly without
presenting the request in a user-agent, user agent, and the end user authorizes
the request on a separate device. This interaction is defined as
follows.

The client initiates the authorization flow by requesting a set of
verification codes from the authorization server by making an HTTP
"POST" request to the device authorization endpoint.

The client constructs the makes a device authorization request with to the device
authorization endpoint by including the following parameters, sent
as parameters using
the body "application/x-www-form-urlencoded" format, per Appendix B of the request, encoded
[RFC6749], with the "application/x-www-form-
urlencoded" a character encoding algorithm defined by Section 4.10.22.6 of
[HTML5]: UTF-8 in the HTTP request
entity-body:

client_id
REQUIRED,
REQUIRED if the client is not authenticating with the
authorization server as described in Section 3.2.1. of [RFC6749].
The client identifier as described in Section 2.2 of [RFC6749].

scope
OPTIONAL. The scope of the access request as described defined by
Section 3.3 of [RFC6749].

For example, the client makes the following HTTPS request:

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

client_id=459691054427

client_id=1406020730&scope=example_scope

All requests from the device MUST use the Transport Layer Security
(TLS) [RFC8446] protocol [RFC8446] and implement the best practices of BCP 195
[RFC7525].

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.

The client authentication requirements of Section 3.2.1 of [RFC6749]
apply to requests on this endpoint, which means that confidential
clients (those that have established client credentials) authenticate
in the same manner as when making requests to the token endpoint, and
public clients provide the "client_id" parameter to identify
themselves.

Due to the polling nature of this protocol (as specified in
Section 3.4), care is needed to avoid overloading the capacity of the
token endpoint. To avoid unneeded requests on the token endpoint,
the client SHOULD only commence a device authorization request when
prompted by the user, user and not automatically, such as when the app
starts or when the previous authorization session expires or fails.

3.2. Device Authorization Response

In response, the authorization server generates a unique device
verification code and an end-user code that are valid for a limited
time and includes them in the HTTP response body using the
"application/json" format [RFC8259] with a 200 (OK) status code. The
response contains the following parameters:

device_code
REQUIRED. The device verification code.

user_code
REQUIRED. The end-user verification code.

verification_uri
REQUIRED. The end-user verification URI on the authorization
server. The URI should be short and easy to remember as end users
will be asked to manually type it into their user-agent. user agent.

verification_uri_complete
OPTIONAL. A verification URI that includes the "user_code" (or
other information with the same function as the "user_code"),
which is designed for non-textual transmission.

expires_in
REQUIRED. The lifetime in seconds of the "device_code" and
"user_code".

interval
OPTIONAL. The minimum amount of time in seconds that the client
SHOULD wait between polling requests to the token endpoint. If no
value is provided, clients MUST use 5 as the default.

For example:

HTTP/1.1 200 OK
Content-Type: application/json
Cache-Control: no-store

{
"device_code": "GmRhmhcxhwAzkoEqiMEg_DnyEysNkuNhszIySk9eS",
"user_code": "WDJB-MJHT",
"verification_uri": "https://example.com/device",
"verification_uri_complete":
"https://example.com/device?user_code=WDJB-MJHT",
"expires_in": 1800,
"interval": 5
}
In the event of an error (such as an invalidly configured client),
the authorization server responds in the same way as the token
endpoint specified in Section 5.2 of [RFC6749].

3.3. User Interaction

After receiving a successful Authorization Response, authorization response, the client
displays or otherwise communicates the "user_code" and the
"verification_uri" to the end user and instructs them to visit the
URI in a user agent on a secondary device (for example, in a browser
on their mobile phone), phone) and enter the user code.

Figure 2: Example User Instruction

The authorizing user navigates to the "verification_uri" and
authenticates with the authorization server in a secure TLS-protected
([RFC8446])
[RFC8446] session. The authorization server prompts the end user to
identify the device authorization session by entering the "user_code"
provided by the client. The authorization server should then inform
the user about the action they are undertaking and ask them to
approve or deny the request. Once the user interaction is complete,
the server MAY inform instructs the user to return to their device.

During the user interaction, the device continuously polls the token
endpoint with the "device_code", as detailed in Section 3.4, until
the user completes the interaction, the code expires, or another
error occurs. The "device_code" is not intended for the end user
directly, and thus
directly; thus, it should not be displayed during the interaction to
avoid confusing the end user.

Authorization servers supporting this specification MUST implement a
user interaction
user-interaction sequence that starts with the user navigating to
"verification_uri" and continues with them supplying the "user_code"
at some stage during the interaction. Other than that, the exact
sequence and implementation of the user interaction is up to the
authorization server, server; for example, the authorization server may
enable new users to sign up for an account during the authorization
flow,
flow or add additional security verification steps.

It is NOT RECOMMENDED for authorization servers to include the user
code ("user_code") in the verification URI ("verification_uri"), as
this increases the length and complexity of the URI that the user
must type. While the user must still type the same a similar number of
characters with the "user_code" separated, once they successfully
navigate to the "verification_uri", any errors in entering the code
can be highlighted by the authorization server to improve the user
experience. The next section documents the user interaction with
"verification_uri_complete", which is designed to carry both pieces
of information.

3.3.1. Non-textual Non-Textual Verification URI Optimization

When "verification_uri_complete" is included in the Authorization
Response authorization
response (Section 3.2), clients MAY present this URI in a non-textual
manner using any method that results in the browser being opened with
the URI, such as with QR (Quick Response) codes or NFC (Near Field
Communication), to save the user from typing the URI.

For usability reasons, it is RECOMMENDED for clients to still display
the textual verification URI ("verification_uri") for users who are
not able to use such a shortcut. Clients MUST still display the
"user_code", as the authorization server will require the user to
confirm it to disambiguate devices, devices or as a remote phishing mitigation (See
(see Section 5.4).

If the user starts the user interaction by browsing navigating to
"verification_uri_complete", then the user interaction described in
Section 3.3 is still followed, but with the optimization that the user
does not need to type in the "user_code". The server SHOULD display
the "user_code" to the user and ask them to verify that it matches
the "user_code" being displayed on the device, device to confirm they are
authorizing the correct device. As before, in addition to taking
steps to confirm the identity of the device, the user should also be
afforded the choice to approve or deny the authorization request.

+-------------------------------------------------+
| |
| Scan the QR code, or code or, using +------------+ |
| a browser on another device, |[_].. . [_]| |
| visit: | . .. . .| |
| https://example.com/device | . . . ....| |
| |. . . . | |
| And enter the code: |[_]. ... . | |
| WDJB-MJHT +------------+ |
| |
+-------------------------------------------------+

Figure 3: Example User Instruction with QR Code Representation
of the Complete Verification URI

3.4. Device Access Token Request

After displaying instructions to the user, the client makes creates an Access
Token Request
access token request and sends it to the token endpoint (as defined
by Section 3.2 of [RFC6749]) with a "grant_type" of
"urn:ietf:params:oauth:grant-type:device_code". This is an extension
grant type (as defined by Section 4.5 of [RFC6749]) created by this
specification, with the following parameters:

grant_type
REQUIRED. Value MUST be set to
"urn:ietf:params:oauth:grant-type:device_code".

device_code
REQUIRED. The device verification code, "device_code" from the
Device Authorization Response,
device authorization response, defined in Section 3.2.

For example, the client makes the following HTTPS request (line
breaks are for display purposes only):

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

grant_type=urn%3Aietf%3Aparams%3Aoauth%3Agrant-type%3Adevice_code
&device_code=GmRhmhcxhwAzkoEqiMEg_DnyEysNkuNhszIySk9eS
&client_id=459691054427
&client_id=1406020730
If 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 of [RFC6749].
Note that there are security implications of statically distributed
client credentials, credentials; see Section 5.6.

The response to this request is defined in Section 3.5. Unlike other
OAuth grant types, it is expected for the client to try the Access
Token Request repeatedly in access
token request repeatedly in a polling fashion, fashion based on the error code
in the response.

3.5. Device Access Token Response

If the user has approved the grant, the token endpoint responds with
a success response defined in Section 5.1 of [RFC6749]; otherwise otherwise, it
responds with an error, as defined in Section 5.2 of [RFC6749].

In addition to the error codes defined in Section 5.2 of [RFC6749],
the following error codes are specified for use with the device
authorization grant in token endpoint responses:

authorization_pending
The authorization request is still pending as the end user hasn't
yet completed the user interaction user-interaction steps (Section 3.3). The
client SHOULD repeat the Access Token Request access token request to the token
endpoint (a process known as polling). Before each new request request,
the client MUST wait at least the number of seconds specified by
the "interval" parameter of the Device Authorization Response device authorization response (see
Section 3.2), or 5 seconds if none was provided, and respect any
increase in the polling interval required by the "slow_down"
error.

slow_down
A variant of "authorization_pending", the authorization request is
still pending and polling should continue, but the interval MUST
be increased by 5 seconds for this and all subsequent requests.

access_denied
The end user denied the authorization request. request was denied.

expired_token
The "device_code" has expired expired, and the device authorization
session has concluded. The client MAY commence a new Device Authorization
Request device
authorization request but SHOULD wait for user interaction before
restarting to avoid unnecessary polling.

The "authorization_pending" and "slow_down" error codes define
particularly unique behavior, as they indicate that the OAuth client
should continue to poll the token endpoint by repeating the token
request (implementing the precise behavior defined above). If the
client receives an error response with any other error code, it MUST
stop polling and SHOULD react accordingly, for example, by displaying
an error to the user.

On encountering a connection timeout, clients MUST unilaterally
reduce their polling frequency before retrying. The use of an
exponential backoff algorithm to achieve this, such as by doubling the
polling interval on each such connection timeout, is RECOMMENDED.

The assumption of this specification is that the separate device on
which the user is authorizing the request on does not have a way to
communicate back to the device with the OAuth client. This protocol
only requires a one-way channel in order to maximise maximize the viability of
the protocol in restricted environments, like an application running
on a TV that is only capable of outbound requests. If a return
channel were to exist for the chosen user interaction user-interaction interface, then
the device MAY wait until notified on that channel that the user has
completed the action before initiating the token request (as an
alternative to polling). Such behavior is, however, outside the
scope of this specification.

4. Discovery Metadata

Support for this specification MAY be protocol is declared in the OAuth 2.0 Authorization
Server Metadata [RFC8414] by including the as follows. The value
"urn:ietf:params:oauth:grant-type:device_code" is included in values
of the "grant_types_supported" parameter, key, and by adding the following new
parameter: key value
pair is added:

device_authorization_endpoint
OPTIONAL. URL of the authorization server's device authorization
endpoint
endpoint, as defined in Section 3.1.

5. Security Considerations

5.1. User Code Brute Forcing

Since the user code is typed by the user, shorter codes are more
desirable for usability reasons. This means the entropy is typically
less than would be used for the device code or other OAuth bearer
token types where the code length does not impact usability. It
Therefore, it is
therefore recommended that the server rate-limit user code
attempts.

The user code SHOULD have enough entropy that that, when combined with rate
limiting
rate-limiting and other mitigations makes mitigations, a brute-force attack becomes
infeasible. For example, it's generally held that 128-bit symmetric
keys for encryption are seen as good enough today because an attacker
has to put in 2^96 work to have a 2^-32 chance of guessing correctly
via brute force. The rate limiting rate-limiting and finite lifetime on the user
code
places place an artificial limit on the amount of work an attacker can
"do", so if,
"do". If, for instance, one uses a an 8-character base-20 base 20 user code
(with roughly 34.5 bits of entropy), the rate-limiting interval and
validity period would need to only allow 5 attempts in order to get
the same 2^-32 probability of success by random guessing.

A successful brute forcing of the user code would enable the attacker
to authenticate approve the authorization grant with their own credentials and make an credentials, after
which the device would receive a device authorization grant linked to
the device. attacker's account. This is the opposite scenario to an OAuth
bearer token being brute forced, whereby the attacker gains control
of the victim's authorization grant. Such attacks may not always
make economic sense, for example sense. For example, for a video app app, the device owner
may then be able to purchase movies using the attacker's account, though account
(though even in this case a privacy risk would still remain and thus
is important to protect
against. against). Furthermore, some uses of the
device flow give the granting account the ability to perform actions
that need to be protected, such as controlling the
device, which needs to be protected. device.

The precise length of the user code and the entropy contained within
is at the discretion of the authorization server, which needs to
consider the sensitivity of their specific protected resources, the
practicality of the code length from a usability standpoint, and any
mitigations that are in place place, such as rate-limiting, when
determining the user code format.

5.2. Device Code Brute Forcing

An attacker who guesses the device code would be able to potentially
obtain the authorization code once the user completes the flow. As
the device code is not displayed to the user and thus there are no
usability considerations on the length, a very high entropy code
SHOULD be used.

5.3. Device Trustworthiness

Unlike other native application OAuth 2.0 flows, the device
requesting the authorization is not the same as the device that from which
the user grants access from. access. Thus, signals from the approving user's
session and device are not always relevant to the trustworthiness of
the client device.

Note that if an authorization server used with this flow is
malicious, then it could perform a man-in-the-middle attack on the
backchannel flow to another authorization server. In this scenario,
the man-in-the-
middle man-in-the-middle is not completely hidden from sight, as the end
user would end up on the authorization page of the wrong service,
giving them an opportunity to notice that the URL in the browser's
address bar is wrong. For this to be possible, the device
manufacturer must either
directly be the attacker, attacker and shipping a device
intended to perform the man-in-the-middle attack, or be using an
authorization server that is controlled by an attacker, possibly
because the attacker compromised the authorization server used by the
device. In part, the person purchasing the device is counting on it the
manufacturer and its business partners to be trustworthy.

5.4. Remote Phishing

It is possible for the device flow to be initiated on a device in an
attacker's possession. For example, an attacker might send an email
instructing the target user to visit the verification URL and enter
the user code. To mitigate such an attack, it is RECOMMENDED to
inform the user that they are authorizing a device during the user user-
interaction step (see Section 3.3), 3.3) and to confirm that the device is
in their possession. The authorization server SHOULD display
information about the device so that the person can user could notice if a
software client was attempting to impersonating impersonate a hardware device.

For authorization servers that support the option specified
"verification_uri_complete" optimization discussed in Section 3.3.1 for the client to append the user code to the
authorization URI, 3.3.1,
it is particularly important to confirm that the device is in the
user's possession, as the user no longer has to type in the code
being displayed on the device manually. One possibility suggestion is to display
the code during the authorization flow and asking ask the user to verify
that the same code is currently being displayed on the device they
are setting up.

The user code needs to have a long enough lifetime to be useable
(allowing the user to retrieve their secondary device, navigate to
the verification URI, login, etc.), log in, etc.) but should be sufficiently short
to limit the usability of a code obtained for phishing. This doesn't
prevent a phisher from presenting a fresh token, particularly in the case if they
are interacting with the user in real time, but it does limit the
viability of codes sent over email or SMS. text message.

5.5. Session Spying

While the device is pending authorization, it may be possible for a
malicious user to physically spy on the device user interface (by
viewing the screen on which it's displayed, for example) and hijack
the session by completing the authorization faster than the user that
initiated it. Devices SHOULD take into account the operating
environment when considering how to communicate the code to the user
to reduce the chances it will be observed by a malicious user.

5.6. Non-confidential Non-Confidential Clients

Device clients are generally incapable of maintaining the
confidentiality of their credentials, as users in possession of the
device can reverse engineer reverse-engineer it and extract the credentials.
Therefore, unless additional measures are taken, they should be
treated as public clients (as defined by Section 2.1 of OAuth 2.0) [RFC6749]),
which are susceptible to impersonation. The security considerations
of Section 5.3.1 of [RFC6819] and Sections 8.5 and 8.6 of [RFC8252]
apply to such clients.

The user may also be able to obtain the device_code "device_code" and/or other
OAuth bearer tokens issued to their client, which would allow them to
use their own authorization grant directly by impersonating the
client. Given that the user in possession of the client credentials
can already impersonate the client and create a new authorization
grant (with a new device_code), "device_code"), this doesn't represent a separate
impersonation vector.

5.7. Non-Visual Code Transmission

There is no requirement that the user code be displayed by the device
visually. Other methods of one-way communication can potentially be
used, such as text-to-speech audio, audio or Bluetooth Low Energy. To
mitigate an attack in which a malicious user can bootstrap their
credentials on a device not in their control, it is RECOMMENDED that
any chosen communication channel only be accessible by people in
close proximity. E.g., proximity, for example, users who can see, see or hear the device.

6. Usability Considerations

This section is a non-normative discussion of usability
considerations.

6.1. User Code Recommendations

For many users, their nearest Internet-connected device will be their
mobile phone, and typically phone; typically, these devices offer input methods that are
more time consuming time-consuming than a computer keyboard to change the case or
input numbers. To improve usability (improving entry speed, speed and
reducing retries), these the limitations of such devices should be taken
into account when selecting the user-code user code character set.

One way to improve input speed is to restrict the character set to
case-insensitive A-Z characters, with no digits. These characters
can typically be entered on a mobile keyboard without using modifier
keys. Further removing vowels to avoid randomly creating words
results in the base-20 base 20 character set: set "BCDFGHJKLMNPQRSTVWXZ". Dashes
or other punctuation may be included for readability.

An example user code following this guideline containing guideline, "WDJB-MJHT", contains
8 significant characters and has dashes added for end-user readability,
with a
readability. The resulting entropy of 20^8: "WDJB-MJHT". is 20^8.

Pure numeric codes are also a good choice for usability, especially
for clients targeting locales where A-Z character keyboards are not
used, though their the length of such a code needs to be longer to maintain a
high entropy.

An example numeric user code containing that contains 9 significant digits and
dashes added for end-user readability, readability with an entropy of 10^9: 10^9 is
"019-450-730".

When processing the inputted user code, the server should strip
dashes and other punctuation that it added for readability (making
the inclusion of that such punctuation by the user optional). For codes
using only characters in the A-Z range range, as with the base-20 base 20 charset
defined above, the user's input should be upper-cased uppercased before a
comparison to account for the fact that the user may input the
equivalent lower-
case lowercase characters. Further stripping of all characters
outside the
user_code charset chosen character set is recommended to reduce instances
where an errantly typed character (like a space character)
invalidates otherwise valid input.

It is RECOMMENDED to avoid character sets that contain two or more
characters that can easily be confused with each other other, like "0" and
"O",
"O" or "1", "l" and "I". Furthermore, to the extent practical, where when
a character set contains one a character that may be confused with
characters outside the character set the set, a character outside the set MAY
be substituted with the one in the character set that with which it is
commonly confused with (for confused; for example, "O" may be substituted for "0" when
using a the numerical 0-9 character set). set.

6.2. Non-Browser User Interaction

Devices and authorization servers MAY negotiate an alternative code
transmission and user interaction user-interaction method in addition to the one
described in Section 3.3. Such an alternative user interaction user-interaction flow
could obviate the need for a browser and manual input of the code,
for example, by using Bluetooth to transmit the code to the
authorization server's companion app. Such interaction methods can
utilize this protocol, as protocol as, ultimately, the user just needs to identify
the authorization session to the authorization server; however, user
interaction other than via through the verification URI is outside the
scope of this specification.

7. IANA Considerations

7.1. OAuth Parameters Parameter Registration

This specification registers the following values in the IANA "OAuth
Parameters" registry [IANA.OAuth.Parameters] established by
[RFC6749].

7.1.1. Registry Contents

o Parameter name:

Name: device_code
o
Parameter usage location: Usage Location: token request
o
Change controller: Controller: IESG
o Specification Document:
Reference: Section 3.1 3.4 of [[ this specification ]] RFC 8628

7.2. OAuth URI Registration

This specification registers the following values in the IANA "OAuth
URI" registry [IANA.OAuth.Parameters] established by [RFC6755].

7.2.1. Registry Contents

URN: urn:ietf:params:oauth:grant-type:device_code
o
Common Name: Device flow grant type Authorization Grant Type for OAuth 2.0
o
Change controller: Controller: IESG
o
Specification Document: Section 3.1 3.4 of [[ this specification ]] RFC 8628

7.3. OAuth Extensions Error Registration

This specification registers the following values in the IANA "OAuth
Extensions Error Registry" registry [IANA.OAuth.Parameters]
established by [RFC6749].

7.3.1. Registry Contents

o Error name:

Name: authorization_pending
o Error usage location:
Usage Location: Token endpoint response
o Related protocol extension: [[ this specification ]]
o
Protocol Extension: RFC 8628
Change controller: Controller: IETF
o Specification Document:
Reference: Section 3.5 of [[ this specification ]]

o Error name: RFC 8628

Name: access_denied
o Error usage location:
Usage Location: Token endpoint response
o Related protocol extension: [[ this specification ]]
o
Protocol Extension: RFC 8628
Change controller: Controller: IETF
o Specification Document:
Reference: Section 3.5 of [[ this specification ]]

o Error name: RFC 8628

Name: slow_down
o Error usage location:
Usage Location: Token endpoint response
o Related protocol extension: [[ this specification ]]
o
Protocol Extension: RFC 8628
Change controller: Controller: IETF
o Specification Document:
Reference: Section 3.5 of [[ this specification ]]

o Error name: RFC 8628

Name: expired_token
o Error usage location:
Usage Location: Token endpoint response
o Related protocol extension: [[ this specification ]]
o
Protocol Extension: RFC 8628
Change controller: Controller: IETF
o Specification Document:
Reference: Section 3.5 of [[ this specification ]] RFC 8628

7.4. OAuth 2.0 Authorization Server Metadata

This specification registers the following values in the IANA "OAuth
2.0
Authorization Server Metadata" registry [IANA.OAuth.Parameters]
established by [RFC8414].

7.4.1. Registry Contents

Metadata name: device_authorization_endpoint
o
Metadata Description: The Device Authorization Endpoint.
o URL of the authorization server's device
authorization endpoint
Change controller: Controller: IESG
o Specification Document:
Reference: Section 4 of [[ this specification ]] RFC 8628

8. Normative References

[HTML5] IANA, "HTML5",
<https://www.w3.org/TR/2014/REC-html5-20141028/>.

[IANA.OAuth.Parameters]
IANA, "OAuth Parameters",
<http://www.iana.org/assignments/oauth-parameters>.

[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/info/rfc2119>.

[RFC6749] Hardt, D., Ed., "The OAuth 2.0 Authorization Framework",
RFC 6749, DOI 10.17487/RFC6749, October 2012,
<https://www.rfc-editor.org/info/rfc6749>.

[RFC6755] Campbell, B. and H. Tschofenig, "An IETF URN Sub-Namespace
for OAuth", RFC 6755, DOI 10.17487/RFC6755, October 2012,
<https://www.rfc-editor.org/info/rfc6755>.

[RFC6819] Lodderstedt, T., Ed., McGloin, M., and P. Hunt, "OAuth 2.0
Threat Model and Security Considerations", RFC 6819,
DOI 10.17487/RFC6819, January 2013,
<https://www.rfc-editor.org/info/rfc6819>.

[RFC7525] Sheffer, Y., Holz, R., and P. Saint-Andre,
"Recommendations for Secure Use of Transport Layer
Security (TLS) and Datagram Transport Layer Security
(DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May
2015, <https://www.rfc-editor.org/info/rfc7525>.

[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/info/rfc8174>.

[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/info/rfc8252>.

[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/info/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/info/rfc8414>.

[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>.

Appendix A.

Acknowledgements

The starting point for this document was the Internet-Draft draft-
recordon-oauth-v2-device,
draft-recordon-oauth-v2-device, authored by David Recordon and Brent
Goldman, which itself was based on content in draft versions of the
OAuth 2.0 protocol specification removed prior to publication due to
a then lack then-lack of sufficient deployment expertise. Thank you to the
OAuth working group Working Group members who contributed to those earlier drafts.

This document was produced in the OAuth working group Working Group under the
chairpersonship of Rifaat Shekh-Yusef and Hannes Tschofenig Tschofenig, with
Benjamin Kaduk, Kathleen Moriarty, and Eric Rescorla serving as
Security Area Directors.

The following individuals contributed ideas, feedback, and wording
that shaped and formed the final specification:

Alissa Cooper,

Ben Campbell, Brian Campbell, Roshni Chandrashekhar, Alissa Cooper,
Eric Fazendin, Benjamin Kaduk, Jamshid Khosravian, Mirja Kuehlewind,
Torsten Lodderstedt, James Manger, Dan McNulty, Breno de Medeiros,
Alexey Melnikov, Simon Moffatt, Stein Myrseth, Emond Papegaaij,
Justin Richer, Adam Roach, Nat Sakimura, Andrew Sciberras, Marius
Scurtescu, Filip Skokan, Robert Sparks, Ken Wang, and Christopher Wood,
Steven E. Wright.

Appendix B. Document History

[[ to be removed by the RFC Editor before publication as an RFC ]]

-15

o Renamed and dropped most usage of the term "flow"
o Documented error responses on the authorization endpoint
o Documented client authentication for the authorization endpoint

-14

o Added more normative text on polling behavior.
o Added discussion on risk of user retrieving their own device_code.
o Editorial improvements.

-13

o Added a longer discussion about entropy, proposed by Benjamin
Kaduk.
o Added device_code to OAuth IANA registry.
o Expanded explanation of "case insensitive".
o Added security section on Device Code Brute Forcing.
o application/x-www-form-urlencoded normativly referenced.
o Editorial improvements.

-12

o Set a default polling interval to 5s explicitly.

o Defined the slow_down behavior that it should increase the current
interval by 5s.
o expires_in now REQUIRED
o Other changes in response to review feedback.

-11

o Updated reference to OAuth 2.0 Authorization Server Metadata.

-10

o Added a missing definition of access_denied for use on the token
endpoint.
o Corrected text documenting which error code should be returned for
expired tokens (it's "expired_token", not "invalid_grant").
o Corrected section reference to RFC 8252 (the section numbers had
changed after the initial reference was made).
o Fixed line length of one diagram (was causing xml2rfc warnings).
o Added line breaks so the URN grant_type is presented on an
unbroken line.
o Typos fixed Wright, and other stylistic improvements.

-09

o Addressed review comments by Security Area Director Eric Rescorla
about the potential of a confused deputy attack.

-08

o Expanded the User Code Brute Forcing section to include more
detail on this attack.

-07

o Replaced the "user_code" URI parameter optimization with
verification_uri_complete following the IETF99 working group
discussion.
o Added security consideration about spying.
o Required that device_code not be shown.
o Added text regarding a minimum polling interval.

-06

o Clarified usage of the "user_code" URI parameter optimization
following the IETF98 working group discussion.

-05
o response_type parameter removed from authorization request.
o Added option for clients to include the user_code on the
verification URI.
o Clarified token expiry, and other nits.

-04

o Security & Usability sections. OAuth Discovery Metadata.

-03

o device_code is now a URN. Added IANA Considerations

-02

o Added token request & response specification.

-01

o Applied spelling and grammar corrections and added the Document
History appendix.

-00

o Initial working group draft based on draft-recordon-oauth-
v2-device. Qin Wu.

Authors' Addresses

William Denniss
Google
1600 Amphitheatre Pkwy
Mountain View, CA 94043
USA
United States of America

Email: wdenniss@google.com
URI: http://wdenniss.com/device-flow https://wdenniss.com/deviceflow

John Bradley
Ping Identity

Email: ve7jtb@ve7jtb.com
URI: http://www.thread-safe.com/

Michael B. Jones
Microsoft

Email: mbj@microsoft.com
URI: http://self-issued.info/

Hannes Tschofenig
ARM Limited
Austria

Email: Hannes.Tschofenig@gmx.net
URI: http://www.tschofenig.priv.at