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RFC 8628

OAuth 2.0 Device Authorization Grant

Pages: 21
Proposed STD
Errata

Top   ToC   RFC8628 - Page 1
Internet Engineering Task Force (IETF)                        W. Denniss
Request for Comments: 8628                                        Google
Category: Standards Track                                     J. Bradley
ISSN: 2070-1721                                            Ping Identity
                                                                M. Jones
                                                               Microsoft
                                                           H. Tschofenig
                                                             ARM Limited
                                                             August 2019


                  OAuth 2.0 Device Authorization Grant

Abstract

   The OAuth 2.0 device authorization grant is designed for Internet-
   connected devices that either lack a browser to perform a user-agent-
   based authorization or are 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
   by using a user agent on a separate device.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8628.
Top   ToC   RFC8628 - Page 2
Copyright Notice

   Copyright (c) 2019 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/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

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 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 . . . . . . . . . . . . . . . . .  12
     5.2.  Device Code Brute Forcing . . . . . . . . . . . . . . . .  13
     5.3.  Device Trustworthiness  . . . . . . . . . . . . . . . . .  13
     5.4.  Remote Phishing . . . . . . . . . . . . . . . . . . . . .  14
     5.5.  Session Spying  . . . . . . . . . . . . . . . . . . . . .  15
     5.6.  Non-Confidential Clients  . . . . . . . . . . . . . . . .  15
     5.7.  Non-Visual Code Transmission  . . . . . . . . . . . . . .  15
   6.  Usability Considerations  . . . . . . . . . . . . . . . . . .  16
     6.1.  User Code Recommendations . . . . . . . . . . . . . . . .  16
     6.2.  Non-Browser User Interaction  . . . . . . . . . . . . . .  17
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  17
     7.1.  OAuth Parameter Registration  . . . . . . . . . . . . . .  17
     7.2.  OAuth URI Registration  . . . . . . . . . . . . . . . . .  17
     7.3.  OAuth Extensions Error Registration . . . . . . . . . . .  18
     7.4.  OAuth Authorization Server Metadata . . . . . . . . . . .  18
   8.  Normative References  . . . . . . . . . . . . . . . . . . . .  19
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  20
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  21
Top   ToC   RFC8628 - Page 3
1.  Introduction

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

   The 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 2.0 for
   Native Apps" [RFC8252].

   The operating requirements 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 (unlike other OAuth 2 grant types, such as the authorization
   code and 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 (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.
Top   ToC   RFC8628 - Page 4
   The device client typically chooses the set of authorization servers
   to support (i.e., its own authorization server or those of providers
   with which it has relationships).  It is common for the device client
   to support only one authorization server, such as 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 with that authorization provider, though one
   can potentially be set up during the authorization flow.

      +----------+                                +----------------+
      |          |>---(A)-- Client Identifier --->|                |
      |          |                                |                |
      |          |<---(B)-- Device Code,      ---<|                |
      |          |          User Code,            |                |
      |  Device  |          & Verification URI    |                |
      |  Client  |                                |                |
      |          |  [polling]                     |                |
      |          |>---(E)-- Device Code       --->|                |
      |          |          & Client Identifier   |                |
      |          |                                |  Authorization |
      |          |<---(F)-- Access Token      ---<|     Server     |
      +----------+   (& Optional Refresh Token)   |                |
            v                                     |                |
            :                                     |                |
           (C) User Code & Verification URI       |                |
            :                                     |                |
            v                                     |                |
      +----------+                                |                |
      | End User |                                |                |
      |    at    |<---(D)-- End user reviews  --->|                |
      |  Browser |          authorization request |                |
      +----------+                                +----------------+

                    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 and an end-user
        code and provides the end-user verification URI.

   (C)  The client instructs the end user to use a user agent on another
        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.
Top   ToC   RFC8628 - Page 5
   (D)  The authorization server authenticates the end user (via the
        user agent), and prompts the user to input the user code
        provided by the device client.  The authorization server
        validates the user code provided by the 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 device code and its client identifier.

   (F)  The authorization server validates the device code provided by
        the client and responds with the access token if the client is
        granted access, an error if they are denied access, or 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.

3.  Protocol

3.1.  Device Authorization Request

   This specification defines a new OAuth endpoint: the device
   authorization endpoint.  This is separate from the OAuth
   authorization endpoint defined in [RFC6749] with which the user
   interacts via a 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, 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.
Top   ToC   RFC8628 - Page 6
   The client makes a device authorization request to the device
   authorization endpoint by including the following parameters using
   the "application/x-www-form-urlencoded" format, per Appendix B of
   [RFC6749], with a character encoding of UTF-8 in the HTTP request
   entity-body:

   client_id
      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 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=1406020730&scope=example_scope

   All requests from the device MUST use the Transport Layer Security
   (TLS) 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 and not automatically, such as when the app
   starts or when the previous authorization session expires or fails.
Top   ToC   RFC8628 - Page 7
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.

   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
      }
Top   ToC   RFC8628 - Page 8
   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, 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) and enter the user code.

            +-----------------------------------------------+
            |                                               |
            |  Using a browser on another device, visit:    |
            |  https://example.com/device                   |
            |                                               |
            |  And enter the code:                          |
            |  WDJB-MJHT                                    |
            |                                               |
            +-----------------------------------------------+

                    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] 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 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; 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 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; for example, the authorization server may
   enable new users to sign up for an account during the authorization
   flow or add additional security verification steps.
Top   ToC   RFC8628 - Page 9
   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 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 Verification URI Optimization

   When "verification_uri_complete" is included in the 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 or as remote phishing mitigation
   (see Section 5.4).

   If the user starts the user interaction by navigating to
   "verification_uri_complete", then the user interaction described in
   Section 3.3 is still followed, 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 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.
Top   ToC   RFC8628 - Page 10
            +-------------------------------------------------+
            |                                                 |
            |  Scan the QR 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 creates an
   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, defined in Section 3.2.

   client_id
      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].

   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=1406020730
Top   ToC   RFC8628 - Page 11
   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; 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 a polling 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, 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 steps (Section 3.3).  The
      client SHOULD repeat the access token request to the token
      endpoint (a process known as polling).  Before each new request,
      the client MUST wait at least the number of seconds specified by
      the "interval" parameter of the 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 authorization request was denied.

   expired_token
      The "device_code" has expired, and the device authorization
      session has concluded.  The client MAY commence a new device
      authorization request but SHOULD wait for user interaction before
      restarting to avoid unnecessary polling.
Top   ToC   RFC8628 - Page 12
   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 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 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 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 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 protocol is declared in OAuth 2.0 Authorization
   Server Metadata [RFC8414] as follows.  The value
   "urn:ietf:params:oauth:grant-type:device_code" is included in values
   of the "grant_types_supported" key, and the following new key value
   pair is added:

   device_authorization_endpoint
      OPTIONAL.  URL of the authorization server's device authorization
      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.
   Therefore, it is recommended that the server rate-limit user code
   attempts.
Top   ToC   RFC8628 - Page 13
   The user code SHOULD have enough entropy that, when combined with
   rate-limiting and other 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 and finite lifetime on the user
   code place an artificial limit on the amount of work an attacker can
   "do".  If, for instance, one uses an 8-character 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 approve the authorization grant with their own credentials, after
   which the device would receive a device authorization grant linked to
   the 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, for a video app, the device owner
   may then be able to purchase movies using the attacker's account
   (though even in this case a privacy risk would still remain and thus
   is important to protect 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.

   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, 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 from which
   the user grants access.  Thus, signals from the approving user's
   session and device are not always relevant to the trustworthiness of
   the client device.
Top   ToC   RFC8628 - Page 14
   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 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 be the 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 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-
   interaction step (see Section 3.3) and to confirm that the device is
   in their possession.  The authorization server SHOULD display
   information about the device so that the user could notice if a
   software client was attempting to impersonate a hardware device.

   For authorization servers that support the
   "verification_uri_complete" optimization discussed in Section 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 suggestion is to display
   the code during the authorization flow and 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, 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 if they
   are interacting with the user in real time, but it does limit the
   viability of codes sent over email or text message.
Top   ToC   RFC8628 - Page 15
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 Clients

   Device clients are generally incapable of maintaining the
   confidentiality of their credentials, as users in possession of the
   device can 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 [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" 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"), 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 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, for example, users who can see or hear the device.
Top   ToC   RFC8628 - Page 16
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; typically, these devices offer input methods that are
   more time-consuming than a computer keyboard to change the case or
   input numbers.  To improve usability (improving entry speed and
   reducing retries), the limitations of such devices should be taken
   into account when selecting the 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 character set "BCDFGHJKLMNPQRSTVWXZ".  Dashes
   or other punctuation may be included for readability.

   An example user code following this guideline, "WDJB-MJHT", contains
   8 significant characters and has dashes added for end-user
   readability.  The resulting entropy 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 the length of such a code needs to be longer to maintain
   high entropy.

   An example numeric user code that contains 9 significant digits and
   dashes added for end-user readability with an entropy of 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 such punctuation by the user optional).  For codes
   using only characters in the A-Z range, as with the base 20 charset
   defined above, the user's input should be uppercased before a
   comparison to account for the fact that the user may input the
   equivalent lowercase characters.  Further stripping of all characters
   outside the chosen character set is recommended to reduce instances
   where an errantly typed character (like a space character)
   invalidates otherwise valid input.
Top   ToC   RFC8628 - Page 17
   It is RECOMMENDED to avoid character sets that contain two or more
   characters that can easily be confused with each other, like "0" and
   "O" or "1", "l" and "I".  Furthermore, to the extent practical, when
   a character set contains a character that may be confused with
   characters outside the character set, a character outside the set MAY
   be substituted with the one in the character set with which it is
   commonly confused; for example, "O" may be substituted for "0" when
   using the numerical 0-9 character set.

6.2.  Non-Browser User Interaction

   Devices and authorization servers MAY negotiate an alternative code
   transmission and user-interaction method in addition to the one
   described in Section 3.3.  Such an alternative 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, ultimately, the user just needs to identify
   the authorization session to the authorization server; however, user
   interaction other than through the verification URI is outside the
   scope of this specification.

7.  IANA Considerations

7.1.  OAuth Parameter Registration

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

      Name: device_code
      Parameter Usage Location: token request
      Change Controller: IESG
      Reference: Section 3.4 of 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].

      URN: urn:ietf:params:oauth:grant-type:device_code
      Common Name: Device Authorization Grant Type for OAuth 2.0
      Change Controller: IESG
      Specification Document: Section 3.4 of RFC 8628
Top   ToC   RFC8628 - Page 18
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].

      Name: authorization_pending
      Usage Location: Token endpoint response
      Protocol Extension: RFC 8628
      Change Controller: IETF
      Reference: Section 3.5 of RFC 8628

      Name: access_denied
      Usage Location: Token endpoint response
      Protocol Extension: RFC 8628
      Change Controller: IETF
      Reference: Section 3.5 of RFC 8628

      Name: slow_down
      Usage Location: Token endpoint response
      Protocol Extension: RFC 8628
      Change Controller: IETF
      Reference: Section 3.5 of RFC 8628

      Name: expired_token
      Usage Location: Token endpoint response
      Protocol Extension: RFC 8628
      Change Controller: IETF
      Reference: Section 3.5 of RFC 8628

7.4.  OAuth Authorization Server Metadata

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

      Metadata name: device_authorization_endpoint
      Metadata Description: URL of the authorization server's device
      authorization endpoint
      Change Controller: IESG
      Reference: Section 4 of RFC 8628
Top   ToC   RFC8628 - Page 19
8.  Normative References

   [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>.
Top   ToC   RFC8628 - Page 20
   [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>.

Acknowledgements

   The starting point for this document was the Internet-Draft
   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 of sufficient deployment expertise.  Thank you to the
   OAuth Working Group members who contributed to those earlier drafts.

   This document was produced in the OAuth Working Group under the
   chairpersonship of Rifaat Shekh-Yusef and Hannes 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:

   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, Christopher Wood,
   Steven E. Wright, and Qin Wu.
Top   ToC   RFC8628 - Page 21
Authors' Addresses

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

   Email: wdenniss@google.com
   URI:   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