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Cross-Device Flows: Security Best Current Practice
draft-ietf-oauth-cross-device-security-04

Document Type Active Internet-Draft (oauth WG)
Authors Pieter Kasselman , Daniel Fett , Filip Skokan
Last updated 2023-10-22
Replaces draft-kasselman-cross-device-security
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draft-ietf-oauth-cross-device-security-04
Web Authorization Protocol                                  P. Kasselman
Internet-Draft                                                 Microsoft
Intended status: Best Current Practice                           D. Fett
Expires: 24 April 2024                                          Authlete
                                                               F. Skokan
                                                                    Okta
                                                         22 October 2023

           Cross-Device Flows: Security Best Current Practice
               draft-ietf-oauth-cross-device-security-04

Abstract

   This document describes threats against cross-device flows along with
   near term mitigations, protocol selection guidance, and the
   analytical tools needed to evaluate the effectiveness of these
   mitigations.  It serves as a security guide to system designers,
   architects, product managers, security specialists, fraud analysts
   and engineers implementing cross-device flows.

Discussion Venues

   This note is to be removed before publishing as an RFC.

   Discussion of this document takes place on the Web Authorization
   Protocol Working Group mailing list (oauth@ietf.org), which is
   archived at https://mailarchive.ietf.org/arch/browse/oauth/.

   Source for this draft and an issue tracker can be found at
   https://github.com/oauth-wg/oauth-cross-device-security.

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

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents 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 24 April 2024.

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Copyright Notice

   Copyright (c) 2023 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 Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
     1.1.  Cross-Device Authorization  . . . . . . . . . . . . . . .   4
     1.2.  Cross-Device Session Transfer . . . . . . . . . . . . . .   5
     1.3.  Defending Against Cross-Device Attacks  . . . . . . . . .   6
     1.4.  Conventions and Terminology . . . . . . . . . . . . . . .   6
   2.  Cross-Device Flow Patterns  . . . . . . . . . . . . . . . . .   7
     2.1.  Cross-Device Authorization  . . . . . . . . . . . . . . .   7
       2.1.1.  User-Transferred Session Data Pattern . . . . . . . .   8
       2.1.2.  Backchannel-Transferred Session Pattern . . . . . . .   9
       2.1.3.  User-Transferred Authorization Data Pattern . . . . .  11
     2.2.  Cross-Device Session Transfer . . . . . . . . . . . . . .  12
       2.2.1.  Cross-Device Session Transfer Pattern . . . . . . . .  12
     2.3.  Examples of Cross-Device Flows  . . . . . . . . . . . . .  14
       2.3.1.  Example A1: Authorize Access to a Video Streaming
               Service (User-Transferred Session Data Pattern) . . .  14
       2.3.2.  Example A2: Authorize Access to Productivity Services
               (User-Transferred Session Data Pattern) . . . . . . .  14
       2.3.3.  Example A3: Authorize Use of a Bike Sharing Scheme
               (User-Transferred Session Data Pattern) . . . . . . .  14
       2.3.4.  Example A4: Authorize a Financial Transaction
               (Backchannel-Transferred Session Pattern) . . . . . .  14
       2.3.5.  Example A5: Add a Device to a Network (Session Transfer
               Pattern)  . . . . . . . . . . . . . . . . . . . . . .  15
       2.3.6.  Example A6: Remote Onboarding (User-Transferred Session
               Data Pattern) . . . . . . . . . . . . . . . . . . . .  15
       2.3.7.  Example A7: Application Bootstrap (Session Transfer
               Pattern)  . . . . . . . . . . . . . . . . . . . . . .  15
       2.3.8.  Example A8: Access a Productivity Application
               (User-Transferred Authorization Data Pattern) . . . .  15
       2.3.9.  Example A9: Administer a System
               (Backchannel-Transferred Session Pattern) . . . . . .  16
   3.  Cross-Device Flow Exploits  . . . . . . . . . . . . . . . . .  16

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     3.1.  Cross-Device Authorization Flow Exploits  . . . . . . . .  16
       3.1.1.  User-Transferred Session Data Pattern Exploits  . . .  17
       3.1.2.  Backchannel-Transferred Session Pattern Exploits  . .  19
       3.1.3.  User-Transferred Authorization Data Pattern
               Exploits  . . . . . . . . . . . . . . . . . . . . . .  21
     3.2.  Cross-Device Session Transfer Exploits  . . . . . . . . .  23
     3.3.  Examples of Cross-Device Flow Exploits  . . . . . . . . .  25
       3.3.1.  Example B1: Illicit Access to a Video Streaming Service
               (User-Transferred Session Data Pattern) . . . . . . .  25
       3.3.2.  Example B2: Illicit Access to Productivity Services
               (User-Transferred Session Data Pattern) . . . . . . .  26
       3.3.3.  Example B3: Illicit Access to Physical Assets
               (User-Transferred Session Data Pattern) . . . . . . .  26
       3.3.4.  Example B4.1: Illicit Transaction Authorization
               (Backchannel-Transferred Session Pattern) . . . . . .  27
       3.3.5.  Example B4.2: Fake Helpdesk (Backchannel-Transferred
               Session Pattern)  . . . . . . . . . . . . . . . . . .  27
       3.3.6.  Example B5: Illicit Network Join (Session Transfer
               Pattern Exploit)  . . . . . . . . . . . . . . . . . .  27
       3.3.7.  Example B6: Illicit Onboarding (User-Transferred
               Session Data Pattern) . . . . . . . . . . . . . . . .  28
       3.3.8.  Example B7: Illicit Application Bootstrap (Session
               Transfer Pattern Exploit) . . . . . . . . . . . . . .  28
       3.3.9.  Example B8: Account Takeover (User-Transferred Session
               Data Pattern) . . . . . . . . . . . . . . . . . . . .  28
       3.3.10. Example B9: Illicit Access to Administration
               Capabilities Through Consent Request Overload
               (Backchannel-Transferred Session Pattern) . . . . . .  29
       3.3.11. Out of Scope  . . . . . . . . . . . . . . . . . . . .  29
   4.  Cross-Device Protocols and Standards  . . . . . . . . . . . .  29
   5.  Mitigating Against Cross-Device Flow Attacks  . . . . . . . .  31
     5.1.  Practical Mitigations . . . . . . . . . . . . . . . . . .  31
       5.1.1.  Establish Proximity . . . . . . . . . . . . . . . . .  32
       5.1.2.  Short Lived/Timebound QR or User Codes  . . . . . . .  34
       5.1.3.  One-Time or Limited Use Codes . . . . . . . . . . . .  34
       5.1.4.  Unique Codes  . . . . . . . . . . . . . . . . . . . .  34
       5.1.5.  Content Filtering . . . . . . . . . . . . . . . . . .  35
       5.1.6.  Detect and Remediate  . . . . . . . . . . . . . . . .  35
       5.1.7.  Trusted Devices . . . . . . . . . . . . . . . . . . .  36
       5.1.8.  Trusted Networks  . . . . . . . . . . . . . . . . . .  36
       5.1.9.  Limited Scopes  . . . . . . . . . . . . . . . . . . .  37
       5.1.10. Short Lived Tokens  . . . . . . . . . . . . . . . . .  37
       5.1.11. Rate Limits . . . . . . . . . . . . . . . . . . . . .  38
       5.1.12. Sender-Constrained Tokens . . . . . . . . . . . . . .  38
       5.1.13. User Education  . . . . . . . . . . . . . . . . . . .  39
       5.1.14. User Experience . . . . . . . . . . . . . . . . . . .  39
       5.1.15. Authenticate-then-Inititiate  . . . . . . . . . . . .  40
       5.1.16. Request Initiation Verification . . . . . . . . . . .  40

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       5.1.17. Request Binding with Out-of-Band Data . . . . . . . .  41
       5.1.18. Practical Mitigation Summary  . . . . . . . . . . . .  41
     5.2.  Protocol Selection  . . . . . . . . . . . . . . . . . . .  43
       5.2.1.  IETF OAuth 2.0 Device Authorization Grant RFC8628:  .  43
       5.2.2.  OpenID Foundation Client Initiated Back-Channel
               Authentication (CIBA):  . . . . . . . . . . . . . . .  44
       5.2.3.  FIDO2/WebAuthn  . . . . . . . . . . . . . . . . . . .  45
       5.2.4.  Protocol Selection Summary  . . . . . . . . . . . . .  46
     5.3.  Foundational Pillars  . . . . . . . . . . . . . . . . . .  47
   6.  Conclusion  . . . . . . . . . . . . . . . . . . . . . . . . .  48
   7.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  48
   8.  Informative References  . . . . . . . . . . . . . . . . . . .  48
   Appendix A.  Document History . . . . . . . . . . . . . . . . . .  51
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  53

1.  Introduction

   Protocol flows that span multiple end-user devices are in widespread
   use today.  These flows are often referred to as cross-device flows.
   A common example is a user that uses their mobile phone to scan a QR
   code from their SmartTV, giving an app on the TV access to their
   video streaming service.  Besides QR codes, other mechanisms are
   often used such as PIN codes that the user has to enter on one of the
   devices, or push notifications to a mobile app that the user has to
   approve.

   In all cases, it is up to the user to decide whether to grant the
   authorization or not.  However, the transfer the QR code or PIN are
   transferred via an unauthorized channel and the user cannot always
   decide in which context an authorization is requested.  This may be
   exploited by attackers to gain unauthorized access to a user's
   resources.

   To accommodate the various nuances of cross-device flows, this
   document distinguished between cases where the cross-device flow is
   used to authorize access to a resource (cross-device authorization
   flows) and cases where the cross-device flow is used to transfer an
   existing session (cross-device session transfer flows).

1.1.  Cross-Device Authorization

   Cross-device authorization flows enable a user to initiate an
   authorization flow on one device (the Consumption Device) and then
   use a second, personally trusted, device (Authorization Device) to
   authorize the Consumption Device to access a resource (e.g., access
   to a service).  The Device Authorization Grant [RFC8628] and Client-
   Initiated Backchannel Authentication [CIBA] are two examples of
   popular cross-device authorization flows.

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   In these flows, it is the user's decision whether to continue the
   session by scanning a QR code, entering a user code, or accepting an
   authorization request pushed to their Authorization Device.

   Cross-Device Consent Phishing (CDCP) attacks exploit the
   unauthenticated channel between the Consumption Device and
   Authorization Device using social engineering techniques to gain
   unauthorized access to the user's data.  Several publications have
   emerged in the public domain ([Exploit1], [Exploit2], [Exploit3],
   [Exploit4], [Exploit5], [Exploit6]), describing how the
   unauthenticated channel can be exploited using social engineering
   techniques borrowed from phishing.  Unlike traditional phishing
   attacks, these attacks don't harvest credentials.  Instead, they skip
   the step of collecting credentials by persuading users to grant
   authorization using their Authorization Devices.

   Once the user grants authorization, the attacker has access to the
   user's resources and in some cases is able to collect access and
   refresh tokens.  Once in possession of the access and refresh tokens,
   the attacker may use these tokens to execute lateral attacks and gain
   additional access, or monetize the tokens by selling them.  These
   attacks are effective even when multi-factor authentication is
   deployed, since the attacker's aim is not to capture and replay the
   credentials, but rather to persuade the user to grant authorization.

1.2.  Cross-Device Session Transfer

   Session transfer flows enable a user to transfer access to a service
   or network from a device on which the user is already authenticated
   to a second device such as a mobile phone.  In these flows, the user
   is authenticated and then authorizes the session transfer on one
   device, referred to as the Authorization Device (e.g., a personal
   computer, web portal or application), and transfers the session to
   the device where they will continue to consume the session, referred
   to as the Consumption Device (e.g., a mobile phone or portable
   device).

   The session may be transferred by showing the user a session transfer
   code on the Authorization Device, which is then entered on the
   Consumption Device.  This flow may be streamlined by rendering the
   session transfer code as a QR code on the Authorization Device and
   scanned by the Consumption Device.

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   The session transfer preserves state information, including
   authentication state, at the second device to avoid additional
   configuration and optimize the user experience.  These flows are
   often used to add new devices to a network, onboard customers to a
   mobile application, or provision new credentials (e.g.,
   [OpenID.SIOPV2]).

   In these cross-device session transfer flows, the channel between the
   Authorization Device and the Consumption Device is unauthenticated.

   Cross-Device Session Phishing (CDSP) attacks exploit the
   unauthenticated channel between the Authorization Device and
   Consumption Device by using social engineering techniques to convince
   the user to send the session transfer code to the attacker.  These
   attacks borrow techniques from traditional phishing attacks, but
   instead of collecting passwords, attackers collect session transfer
   codes and other artefacts that allow them to setup a session and then
   use it to access a user's data.

1.3.  Defending Against Cross-Device Attacks

   In order to defend against Cross-Device Consent Phishing and Cross-
   Device Session Phishing attacks, this document outlines three
   responses:

   1.  For protocols that are susceptible to these exploits, deploy
       practical mitigations (Section 5.1).

   2.  Select protocols that are more resistant to these exploits when
       possible (Section 5.2).

   3.  Conduct formal analysis of cross-device flows to assess
       susceptibility to these attacks and the effectiveness of the
       proposed mitigations (Section 5.3).

1.4.  Conventions and 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.

   This specification uses the terms "access token", "refresh token",
   "authorization server", "resource server", "authorization endpoint",
   "authorization request", and "client" defined by The OAuth 2.0
   Authorization Framework [RFC6749].

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2.  Cross-Device Flow Patterns

   Cross-device flows allow a user to start a flow on one device (e.g.,
   a SmartTV) and then transfer the session to continue it on a second
   device (e.g., a mobile phone).  The second device may be used to
   access the service that was running on the first device, or to
   perform an action such as authenticating or granting authorization
   before potentially passing control back to the first device.

   These flows typically involve using a mobile phone to scan a QR code
   or enter a user code displayed on the first device (e.g., Smart TV,
   Kiosk, Personal Computer etc.).

2.1.  Cross-Device Authorization

   In a cross-device authorization flow, a user attempts to access a
   service on one device, referred to as the Consumption Device, (e.g.,
   a smart TV) and then uses a second device, referred to as the
   Authorization Device (e.g., a smartphone), to authorize access to a
   resource (e.g., access to a streaming service) on the Consumption
   Device.

   Cross-device authorization flows have several benefits, including:

   *  Authorization on devices with limited input capabilities: End-
      users can authorize devices with limited input capabilities to
      access content (e.g., smart TVs, digital whiteboards, printers,
      etc).

   *  Secure authentication on shared or public devices: End-users can
      perform authentication and authorization using a personally
      trusted device, without risk of disclosing their credentials to a
      public or shared device.

   *  Ubiquitous multi-factor authentication: Enables a user to use
      multi-factor authentication, independent of the device on which
      the service is being accessed (e.g., a kiosk, smart TV or shared
      Personal Computer).

   *  Convenience of a single, portable, credential store: Users can
      keep all their credentials in a mobile wallet or mobile phone that
      they already carry with them.

   There are three cross-device flow patterns for transferring the
   authorization request between the Consumption Device to the
   Authorization Device.

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   *  *User-Transferred Session Data Pattern:* In the first pattern, the
      user initiates the authorization process with the authorization
      server by copying information from the Consumption Device to the
      Authorization Device, before authorizing an action.  By
      transferring the data from the Consumption Device to the
      Authorization Device, the user transfers the authorization
      session.  For example the user may read a code displayed on the
      Consumption Device and enter it on the Authorization Device, or
      they may scan a QR code displayed on the Consumption Device with
      the Authorization Device.  The Device Authorization Grant
      ([RFC8628]) is an example of a cross-device flow that follow this
      pattern.

   *  *Backchannel-Transferred Session Pattern:* In the second pattern,
      the OAuth client on the Consumption Device is responsible for
      transferring the session and initiating authorization on the
      Authorization Device via a backchannel with the Authorization
      Server.  For example the user may attempt an online purchase on a
      Consumption Device (e.g., a personal computer) and receive an
      authorization request on their Authentication Device (e.g., mobile
      phone).  The Client Initiated Backchannel Authentication [CIBA] is
      an example of a cross-device flow that follow this pattern.

   *  *User-Transferred Authorization Data Pattern:* In the third
      pattern, the OAuth client on the Consumption Device triggers the
      authorization request via a backchannel with the Authorization
      Server.  Authorization data (e.g., a 6 digit authorization code)
      is displayed on the Authorization Device, which the user transfers
      to Consumption Device (e.g., by manually entering it).  For
      example the user may attempt to access data in an enterprise
      application and receive a 6 digit authorization code on their
      Authentication Device (e.g., mobile phone) that they enter on
      Consumption Device.

   In all of these flows, it is the user's decision whether to continue
   the session by scanning a QR code, entering a user code, or accepting
   an authorization request pushed to their Authorization Device.

2.1.1.  User-Transferred Session Data Pattern

   The Device Authorization Grant ([RFC8628]) is an example of a cross-
   device flow that relies on the user copying information from the
   Consumption Device to the Authorization Device.  The figure below
   shows a typical example of this flow:

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                                 (B) Consumption Device
                +--------------+     Get QR/User Code  +---------------+
   (A)User  +---|  Consumption |<--------------------->|               |
      Start |   |   Device     |(E) Grant Authorization| Authorization |
      Flow  +-->|              |<--------------------->|     Server    |
                +--------------+                       |               |
                       |                               |               |
                       | (C) Scan QR code              |               |
                       |         or                    |               |
                       |   enter User Code             |               |
                       v                               |               |
                +--------------+                       |               |
                | Authorization|                       |               |
                |    Device    |<--------------------->|               |
                |              |(D) User Authenticates |               |
                |              | and Authorize Access  |               |
                +--------------+                       +---------------+

    Figure 1: Cross-Device Flows: User-Transferred Session Data Pattern

   *  (A) The user takes an action on the Consumption Device by starting
      a purchase, adding a device to a network or connecting a service
      to the Consumption Device.

   *  (B) The Consumption Device retrieves a QR code or user code from
      an Authorization Server.

   *  (C) The QR code or user code is displayed on the Consumption
      Device where the user scans the QR code or enters the user code on
      the Authorization Device.

   *  (D) The user authenticates to the Authorization Server before
      granting authorization.

   *  (E) The Authorization Server issues tokens or grants authorization
      to the Consumption Device to access the user's resources.

2.1.2.  Backchannel-Transferred Session Pattern

   The Client Initiated Backchannel Authentication [CIBA] transfers the
   session on the backchannel with the Authorization Server to request
   authorization on the Authorization Device.  The figure below shows an
   example of this flow.

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                                 (B) Backchannel Authorization
                +--------------+     Request           +---------------+
   (A)User  +---|  Consumption |<--------------------->|               |
      Start |   |   Device     |(E) Grant Authorization| Authorization |
      Flow  +-->|              |<--------------------->|     Server    |
                +--------------+                       |               |
                                                       |               |
                                                       |               |
                                                       |               |
                                                       |               |
   (D)User                                             |               |
     Authorize  +--------------+                       |               |
     Action +---| Authorization|                       |               |
            |   |    Device    |<--------------------->|               |
            +-->|              |(C) Request User       |               |
                |              |    Authorization      |               |
                +--------------+                       +---------------+

   Figure 2: Cross-Device Flows: Backchannel-Transferred Session Pattern

   *  (A) The user takes an action on the Consumption Device by starting
      a purchase, adding a device to a network or connecting a service
      to the Consumption Device.

   *  (B) The client on the Consumption Device requests user
      authorization on the backchannel from the Authorization Server.

   *  (C) The Authorization Server requests the authorization from the
      user on the user's Authorization Device.

   *  (D) The user authenticates to the Authorization Server before
      using their device to grant authorization.

   *  (E) The Authorization Server issues tokens or grants authorization
      to the Consumption Device to access the user's resources.

   The Authorization Server may use a variety of mechanisms to request
   user authorization, including a push notification to a dedicated app
   on a mobile phone, or sending a text message with a link to an
   endpoint where the user can authenticate and authorize an action.

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2.1.3.  User-Transferred Authorization Data Pattern

   Examples of the user-transferred authorization data pattern include
   flows in which the Consumption Device requests the Authorization
   Server to send authorization data (e.g., a 6 digit authorization code
   in a text message or e-mail) to the Authorization Device.  Once the
   Authorization Device receives the authorization data, the user enters
   it on the Consumption Device.  The Consumption Device sends the
   authorization data back to the Authorization Server for validation
   before gaining access to the user's resources.  The figure below
   shows an example of this flow.

                                 (B) Backchannel Authorization
                +--------------+     Request           +---------------+
   (A)User  +---|  Consumption |<--------------------->|               |
      Start |   |   Device     |(E) Grant Authorization| Authorization |
      Flow  +-->|              |<--------------------->|     Server    |
                +--------------+                       |               |
                       ^                               |               |
                       | (D)User Enters                |               |
                       |    Authorization Data         |               |
                       |                               |               |
                       |                               |               |
                +--------------+                       |               |
                | Authorization|                       |               |
                |    Device    |<--------------------->|               |
                |              |(C) Send Authorization |               |
                |              |    Data               |               |
                +--------------+                       +---------------+

      Figure 3: Cross-Device Flow: User-Transferred Authorization Data
                                  Pattern

   *  (A) The user takes an action on the Consumption Device by starting
      a purchase, adding a device to a network or connecting a service
      to the Consumption Device.

   *  (B) The client on the Consumption Device requests user
      authorization on the backchannel from the Authorization Server.

   *  (C) The Authorization Server sends authorization data (e.g., a 6
      digit authorization code) to the Authorization Device.

   *  (D) The user enters the authorization data (e.g., the 6 digit
      authorization code) on the Consumption Device.

   *  (E) The Authorization Server issues tokens or grants authorization
      to the Consumption Device to access the user's resources.

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   The Authorization Server may choose to authenticate the user before
   sending the authorization data.  The authorization data may be
   delivered as a text message or through a mobile application.

2.2.  Cross-Device Session Transfer

   Session transfer flows enable a user to transfer access to a service
   or network from a device on which the user is already authenticated
   to a second device such as a mobile phone.  In these flows, the user
   is authenticated and then authorizes the session transfer on one
   device, referred to as the Authorization Device (e.g., a personal
   computer, web portal or application), and transfers the session to
   the device where they will continue to consume the session, referred
   to as the Consumption Device (e.g., a mobile phone or portable
   device).

   The session transfer preserves state information, including
   authentication state, at the second device to avoid additional
   configuration and optimize the user experience.  These flows are
   often used to add new devices to a network, onboard customers to a
   mobile application, or provision new credentials (e.g.,
   [OpenID.SIOPV2]).

2.2.1.  Cross-Device Session Transfer Pattern

   In this flow, the user is authenticated and starts the flow by
   authorizing the transfer of the session on the Authorization Device.
   The Authorization Device requests a session transfer code that may be
   rendered as a QR code on the Authorization Device.  When the user
   scans the QR code or enters it on the Consumption Device where they
   would like the session to continue, the Consumption Device presents
   it to the Authorization Server.  The Authorization Server then
   transfers the session to the Consumption Device.  This may include
   transferring authentication and authorization state to optimize the
   user experience.  This type of flow is used, for example, for adding
   new devices to networks, bootstrapping new applications, or
   provisioning new credentials.  The Pre-Authorized Code Flow in
   ([OpenID.VCI]) is an instance of using this pattern to provision a
   new credential.  The figure below shows a typical flow.

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                                 (B) Session Transfer
                +--------------+     Request           +---------------+
   (A)User  +---| Authorization|---------------------->|               |
      Start |   |   Device     |(C) Session Transfer   |               |
      Flow  |   |              |    Code               | Authorization |
            +-->|              |<----------------------|     Server    |
                +--------------+                       |               |
                       |                               |               |
                       | (D) Scan QR code              |               |
                       |      or enter                 |               |
                       | Session Transfer Code         |               |
                       |                               |               |
                       v         (E) Present Session   |               |
                +--------------+     Transfer Code     |               |
                | Consumption  |---------------------->|               |
   (G)User  +---|    Device    |                       |               |
   Resumes  |   |              | (F) Return Session    |               |
   Session  |   |              |     Context           |               |
            +-->|              |<----------------------|               |
                +--------------+                       +---------------+

           Figure 4: Cross-Device Flows: Session Transfer Pattern

   *  (A) The user is authenticated on the Authorization Device and
      authorizes the transfer of the session to the Consumption device.

   *  (B) The client on the Authorization Device requests a session
      transfer code from the Authorization Server.

   *  (C) The Authorization Server responds with a session transfer
      code, which may be rendered as a QR code on the Authorization
      Device.

   *  (D) The user scans the QR code with the Consumption Device (e.g.,
      their mobile phone), or enters the session transfer code on the
      target Consumption Device.

   *  (E) The client on the Consumption Device presents the session
      transfer code to the Authorization Server.

   *  (F) The Authorization Server verifies the session transfer code
      and retrieves the session context information needed to resume the
      session on the Consumption Device.

   *  (G) The user resumes the session and is able to access the
      information on the Consumption Device that they authorized on the
      Authorization Device.

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2.3.  Examples of Cross-Device Flows

   Examples of cross-device flow scenarios include:

2.3.1.  Example A1: Authorize Access to a Video Streaming Service (User-
        Transferred Session Data Pattern)

   An end-user sets up a new smart TV and wants to connect it to their
   favorite streaming service.  The TV displays a QR code that the user
   scans with their mobile phone.  The user is redirected to the
   streaming service provider's web page and asked to enter their
   credentials to authorize the smart TV to access the streaming
   service.  The user enters their credentials and grants authorization,
   after which the streaming service is available on the smart TV.

2.3.2.  Example A2: Authorize Access to Productivity Services (User-
        Transferred Session Data Pattern)

   An employee wants to access their files on an interactive whiteboard
   in a conference room.  The interactive whiteboard displays a URL and
   a code.  The user enters the URL on their personal computer and is
   prompted for the code.  Once they enter the code, the user is asked
   to authenticate and authorize the interactive whiteboard to access
   their files.  The user enters their credentials and authorizes the
   transaction and the interactive whiteboard retrieves their files and
   allows the user to interact with the content.

2.3.3.  Example A3: Authorize Use of a Bike Sharing Scheme (User-
        Transferred Session Data Pattern)

   An end-user wants to rent a bicycle from a bike sharing scheme.  The
   bicycles are locked in bicylce racks on sidewalks throughout a city.
   To unlock and use a bicycle, the user scans a QR code on the bicycle
   using their mobile phone.  Scanning the QR code redirects the user to
   the bicycle sharing scheme's authorization page where the user
   authenticates and authorizes payment for renting the bicycle.  Once
   authorized, the bicycle sharing service unlocks the bicycle, allowing
   the user to use it to cycle around the city.

2.3.4.  Example A4: Authorize a Financial Transaction (Backchannel-
        Transferred Session Pattern)

   An end-user makes an online purchase.  Before completing the
   purchase, they get a notification on their mobile phone, asking them
   to authorize the transaction.  The user opens their app and
   authenticates to the service before authorizing the transaction.

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2.3.5.  Example A5: Add a Device to a Network (Session Transfer Pattern)

   An employee is issued with a personal computer that is already joined
   to a network.  The employee wants to add their mobile phone to the
   network to allow it to access corporate data and services (e.g.,
   files and e-mail).  The employee is logged-in on the personal
   computer where they initiate the process of adding their mobile phone
   to the network.  The personal computer displays a QR code which
   authorizes the user to join their mobile phone to the network.  The
   employee scans the QR code with their mobile phone and the mobile
   phone is joined to the network.  The employee can start accessing
   corporate data and services on their mobile device.

2.3.6.  Example A6: Remote Onboarding (User-Transferred Session Data
        Pattern)

   A new employee is directed to an onboarding portal to provide
   additional information to confirm their identity on their first day
   with their new employer.  Before activating the employee's account,
   the onboarding portal requests that the employee present a government
   issued ID, proof of a background check and proof of their
   qualifications.  The onboarding portal displays a QR code, which the
   user scans with their mobile phone.  Scanning the QR code invokes the
   employee's digital wallet on their mobile phone, and the employee is
   asked to present digital versions of an identity document (e.g., a
   driving license), proof of a background check by an identity
   verifier, and proof of their qualifications.  The employee authorizes
   the release of the credentials and after completing the onboarding
   process, their account is activated.

2.3.7.  Example A7: Application Bootstrap (Session Transfer Pattern)

   An employee is signed into an application on their personal computer
   and wants to bootstrap the mobile application on their mobile phone.
   The employee initiates the cross-device flow and is shown a QR code
   in their application.  The employee launches the mobile application
   on their phone and scans the QR code which results in the user being
   signed into the application on the mobile phone.

2.3.8.  Example A8: Access a Productivity Application (User-Transferred
        Authorization Data Pattern)

   A user is accessing a Computer Aid Design (CAD) application.  When
   accessing the application, authorization data in the form of a 6
   digit authorization code is sent to the user's mobile phone.  The
   user views the 6 digit authorization code on their phone and enters
   it in the CAD application, after which the CAD application displays
   the user's most recent designs.

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2.3.9.  Example A9: Administer a System (Backchannel-Transferred Session
        Pattern)

   A network administrator wants to access an adminstration portal used
   to configure network assets and deploy new applications.  When
   attempting to access the service, the network administrator receives
   a notification in an app on their mobile device, requesting them to
   confirm access to the portal.  The network administrator approves the
   request on their mobile phone and is granted access to the portal.

3.  Cross-Device Flow Exploits

   Attackers exploit the absence of an authenticated channel between the
   two devices used in a cross-device flow by using social engineering
   techniques typicaly used in phishing attacks.

   In cross-device authorization flows the attacker uses these social
   engineering techniques by changing the context in which the
   authorization request is presented to convince the user to grant
   authorization when they shouldn't.  These attacks are also known as
   Cross-Device Consent Phishing (CDCP) attacks.

   In cross-device session transfer flows the attacker uses these social
   engineering techniques to convince the user to initiate a session
   transfer and send them a session transfer code.  Once the attacker is
   in posession of this session transfer code, they present it to the
   Authorization Server to transfer the session and access the users
   resources.  These attacks are referred to as Cross-Device Session
   Phishing (CDSP) attacks.

3.1.  Cross-Device Authorization Flow Exploits

   Attackers exploit cross-device authorization flows by initiating an
   authorization flow on the Consumption Device and then use social
   engineering techniques to change the context in which the request is
   presented to the user in order to convince them to grant
   authorization on the Authorization Device.  The attacker is able to
   change the context of the authorization request because the channel
   between the Consumption Device and the Authorization Device is
   unauthenticated.  These attacks are also known as Cross-Device
   Consent Phishing (CDCP) attacks.

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3.1.1.  User-Transferred Session Data Pattern Exploits

   A common action in cross-device flows is to present the user with a
   QR code or a user code on the Consumption Device (e.g., Smart TV)
   which is then scanned or entered on the Authorization Device (the
   mobile phone).  When the user scans the code or copies the user code,
   they do so without any proof that the QR code or user code is being
   displayed in the place or context intended by the service provider.
   It is up to the user to decide whether they should trust the QR code
   or user code.  In effect the user is asked to compensate for the
   absence of an authenticated channel between the Consumption Device
   (e.g., smart TV) and the Authorization Device (e.g., the mobile
   phone).

   Attackers exploit this absence of an authenticated channel between
   the two devices by obtaining QR codes or user codes (e.g., by
   initiating the authorization flows).  They then use social
   engineering techniques to change the context in which authorization
   is requested to convince end-users to scan the QR code or enter it on
   their Authorization Device (e.g., mobile phone).  Once the end-user
   performs the authorization on the mobile device, the attacker who
   initiated the authentication or authorization request obtains access
   to the users resources.  The figure below shows an example of such an
   attack.

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                              (B) Consumption Device
              +--------------+     Get QR/User Code  +---------------+
              |  Attacker's  |<--------------------->|               |
              |  Consumption |(G) Grant Authorization| Authorization |
              |   Device     |<--------------------->|     Server    |
              +--------------+                       |               |
                ^   | (C) Attacker Copy              |               |
   (A) Attacker |   |     QR or User Code            |               |
       Start    |   |                                |               |
       Flow     |   V                                |               |
              +--------------+                       |               |
              |              |                       |               |
              |   Attacker   |                       |               |
              |              | (D) Attacker Change   |               |
              |              |     QR Code/User Code |               |
              |              |     Context           |               |
              +--------------+                       |               |
                     | (E) User is convinced by the  |               |
                     |     attacker and scans QR code|               |
                     |     or enters User Code       |               |
                     v                               |               |
              +--------------+                       |               |
              |   End User   |                       |               |
              | Authorization|                       |               |
              |    Device    |<--------------------->|               |
              |              |(F) User Authenticates |               |
              |              | and Authorize Access  |               |
              +--------------+                       +---------------+

     Figure 5: Cross-Device Consent Phishing: User-Transferred Session
                           Data Pattern Exploits

   *  (A) The attacker initiates the protocol on the Consumption Device
      (or mimicks the Consumption Device) by starting a purchase, adding
      a device to a network or connecting a service to the Consumption
      Device.

   *  (B) The Consumption Device retrieves a QR code or user code from
      an Authorization Server.

   *  (C) The attacker copies the QR code or user code.

   *  (D) The attacker changes the context in which the QR code or user
      code is displayed in such a way that the user is likely to scan
      the QR code or use the user code when completing the
      authorization.  For example, the attacker could craft an e-mail
      that includes the user code or QR code and send it to the user.
      The e-mail might encourage the user to scan the QR code or enter

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      the user code by suggesting that doing so would grant them a
      reward through a loyalty program or prevent the loss of their
      data.

   *  (E) The QR code or user code is displayed to the user in a context
      chosen by the attacker.  The user is convinced by the attacker's
      effort and scans the QR code or enters the user code on the
      Authorization Device.

   *  (F) The user authenticates to the Authorization Server before
      granting authorization.

   *  (G) The Authorization Server issues tokens or grants authorization
      to the Consumption Device, which is under the attacker's control,
      to access the user's resources.  The attacker gains access to the
      resources and any authorization artifacts (like access and refresh
      tokens) which may be used in future exploits.

3.1.2.  Backchannel-Transferred Session Pattern Exploits

   In the backchannel-transferred session pattern, the client requests
   the authorization server to authenticate the user and obtain
   authorization for an action.  This may happen as a result of user
   interaction with the Consumption Device, but may also be triggered
   without the users direct interaction with the Consumption Device,
   resulting in an authorization request presented to the user without
   context of why or who triggered the request.

   Attackers exploit this lack of context by using social engineering
   techniques to prime the user for an authorization request and thereby
   convince them to granting authorization.  The social engineering
   techniques range in sophistication from messages misrepresenting the
   reason for receiving an authorization request, to triggering a large
   volume of requests at an inconvenient time for the user, in the hope
   that the user will grant authorization to make the requests stop.
   The figure below shows an example of such an attack.

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                                 (C) Backchannel Authorization
                +--------------+     Request           +---------------+
                |  Attacker's  |<--------------------->|               |
                |  Consumption |(F) Grant Authorization| Authorization |
                |  Device      |<--------------------->|     Server    |
                +--------------+                       |               |
                  ^                                    |               |
     (B) Attacker |                                    |               |
         Starts   |                                    |               |
         Flow     |                                    |               |
                +--------------+                       |               |
                |              |                       |               |
                |   Attacker   |                       |               |
                |              |                       |               |
                |              |                       |               |
                |              |                       |               |
                +--------------+                       |               |
                       |  (A) Attacker Sends           |               |
                       |       Social Engineering      |               |
                       |       Message to User         |               |
                       |                               |               |
   (E)User             v                               |               |
     Authorize  +--------------+                       |               |
     Action +---| Authorization|                       |               |
            |   |    Device    |<--------------------->|               |
            +-->|              |(D) Request User       |               |
                |              |    Authorization      |               |
                +--------------+                       +---------------+

      Figure 6: Cross-Device Consent Phishing: Backchannel-Transferred
                          Session Pattern Exploits

   *  (A) The attacker sends a social engineering message to prepare the
      user for the upcoming authorization (optional).

   *  (B) The attacker initiates the protocol on the Consumption Device
      (or by mimicking the Consumption Device) by starting a purchase,
      adding a device to a network or accessing a service on the
      Consumption Device.

   *  (C) The client on the Consumption Device requests user
      authorization on the backchannel from the Authorization Server.

   *  (D) The Authorization Server requests the authorization from the
      user on the user's device.

   *  (E) The user authenticates to the authorization server before
      granting authorization on their device.

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   *  (G) The Authorization Server issues tokens or grants authorization
      to the Consumption Device, which is under the attacker's control.
      The attacker gains access to the user's resources and possibly any
      authorization artifacts like access and refresh tokens.

3.1.3.  User-Transferred Authorization Data Pattern Exploits

   In cross-device flows that follow the user-transferred authorization
   data pattern, the client on the Consumption Device initiates the
   authorization request, but the user still has to transfer the
   authorization data to the Consumption Device.  The authorization data
   may take different forms, including a numerical value such as a 6
   digit authorization code.  The authorization request may happen as a
   result of user interaction with the Consumption Device, but may also
   be triggered without the user's direct interaction with the
   Consumption Device.

   Attackers exploit the user-transferred authorization data pattern by
   combining the social engineering techniques used to set context for
   users and convincing users to providing them with authorization data
   sent to their Authorization Devices (e.g., mobile phones).  These
   attacks are very similar to phishing attacks, except that the
   attacker also has the ability to trigger the authorization request to
   be sent to the user directly by the Authorization Server.

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                                 (C) Backchannel Authorization
                +--------------+     Request           +---------------+
                |  Attacker's  |<--------------------->|               |
                |  Consumption |(G) Grant Authorization| Authorization |
                |  Device      |<--------------------->|     Server    |
                +--------------+                       |               |
                  ^       ^                            |               |
     (B) Attacker |       | (F) Attacker Forwards      |               |
         Starts   |       |     Authorization Data     |               |
         Flow     |       |                            |               |
                +--------------+                       |               |
                |              |                       |               |
                |   Attacker   |                       |               |
                |              |                       |               |
                |              |                       |               |
                |              |                       |               |
                +--------------+                       |               |
   (A) Attacker    |       ^   (E) User                |               |
       Sends       |       |       Sends               |               |
       Social      |       |       Authorization Data  |               |
       Engineering |       |                           |               |
       Message     |       |                           |               |
                   v       |                           |               |
                +--------------+                       |               |
                | Authorization|                       |               |
                |    Device    |<--------------------->|               |
                |              |(D) Send Authorization |               |
                |              |    Data               |               |
                +--------------+                       +---------------+

         Figure 7: Cross-Device Consent Phishing: User-Transferred
                    Authorization Data Pattern Exploits

   *  (A) The attacker sends a social engineering message to prime the
      user for the authorization request they are about to receive,
      including instructions on what to do with the authorization data
      once they receive it.

   *  (B) The attacker initiates the protocol on the Consumption Device
      (or by mimicking the Consumption Device) by starting a purchase,
      adding a device to a network or accessing a service on the
      Consumption Device.

   *  (C) The client on the Consumption Device requests user
      authorization on the backchannel from the Authorization Server.

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   *  (D) The Authorization Server sends authorization data (e.g., a 6
      digit authorization code) to the user's Authorization Device (the
      authorization data may be presented as a QR code, or text
      message).

   *  (E) The user is convinced by the social engineering message
      received in step (A) and forwards the authorization data (e.g., a
      6 digit authorization code) to the attacker.

   *  (F) The attacker enters the authorization data (e.g., a 6 digit
      authorization code) on the Consumption Device.

   *  (G) The Authorization Server grants authorization and issues
      access and refresh tokens to the Consumption Device, which is
      under the attacker's control.  On completion of the exploit, the
      attacker gains access to the user's resources.

   The unauthenticated channel may also be exploited in variations of
   the above scenario where the user (as opposed to the attacker)
   initiates the flow and is then convinced using social engineering
   techniques into sending the authorization data (e.g., a 6 digit
   authorization code) to the attacker.  In these flows, the user is
   already authenticated and they request authorization data to transfer
   a session or obtain some other privilege such as joining a device to
   a network.  The authorization data may be represented as a QR code or
   text string (e.g., 6 digit authorization code).  The attacker then
   proceeds to exploit the unauthenticated channel by using social
   engineering techniques to convince the user to send the QR code or
   user code to the attacker.  The attacker then use the authorization
   data to obtain the privileges that would have been assigned to the
   user.

3.2.  Cross-Device Session Transfer Exploits

   Attackers exploit cross-device session transfer flows by using social
   engineering techniques typically used in phishing attacks to convince
   the user to authorize the transfer of a session and then send the
   session transfer code or QR code to the attacker.  The absence of an
   authenticated channel between these two devices enables the attacker
   to use the session transfer code on their own device to obtain access
   to the session and access the users data.  These attacks are referred
   to as Cross-Device Session Phishing (CDSP) attacks.

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                                 (C) Session Transfer
                +--------------+     Request           +---------------+
   (B)User  +---| Authorization|---------------------->|               |
      Start |   |   Device     |(D) Session Transfer   |               |
      Flow  |   |              |    Code               | Authorization |
            +-->|              |<----------------------|     Server    |
                +--------------+                       |               |
   (A)Attacker    ^          |                         |               |
      Sends Social|          | (E) User sends QR code  |               |
      Engineering |          |     or Session Transfer |               |
      Message     |          v     Code to Attacker    |               |
                +--------------+                       |               |
                |              |                       |               |
                |   Attacker   |                       |               |
                |              |                       |               |
                |              |                       |               |
                |              |                       |               |
                +--------------+                       |               |
   (F)Attacker scans   |                               |               |
      QR code or enters|                               |               |
      Session Transfer |                               |               |
      Code             v         (G) Present Session   |               |
                +--------------+     Transfer Code     |               |
                |  Attacker's  |---------------------->|               |
   (I)      +---|  Consumption |                       |               |
    Attacker|   |    Device    | (H) Return Session    |               |
    Resumes |   |              |     Context           |               |
    Session +-->|              |<----------------------|               |
                +--------------+                       +---------------+

       Figure 8: Cross-Device Flows: Session Transfer Pattern Exploit

   *  (A) The attacker sends a social engineering message to that
      convinces the user that they should authorize a session transfer
      including instructions on what to do with the QR code or session
      transfer code once they receive it.

   *  (B) The user is authenticated on their Authorization Device and
      authorizes the transfer of the session to the Consumption Device.

   *  (C) The client on the Authorization Device requests a session
      transfer code from the Authorization Server.

   *  (D) The Authorization Server responds with a session transfer
      code, which may be rendered as a QR code on the Authorization
      Device.

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   *  (E) The user sends the QR code or session transfer code to the
      attacker, following the instructions they received in step (A).

   *  (F) Once the attacker receives the QR code, they scan it or enter
      it on their own Consumption Device.

   *  (G) The client on the Consumption Device presents the session
      transfer code to the Authorization Server.

   *  (H) The Authorization Server verifies the session transfer code
      and retrieves the session context information needed to resume the
      session on the Consumption Device.

   *  (I) The attacker resumes the session on their own Consumption
      Device and is able to access the information that the user
      authorized on their Authorization Device in step (B).

3.3.  Examples of Cross-Device Flow Exploits

   The following examples illustrate these attacks in practical settings
   and show how the unauthenticated channel is exploited by attackers
   who can copy the QR codes and user codes, change the context in which
   they are presented using social engineering techniques and mislead
   end-users into granting consent to avail of services, access data and
   make payments.

3.3.1.  Example B1: Illicit Access to a Video Streaming Service (User-
        Transferred Session Data Pattern)

   An attacker obtains a smart TV and attempts to access an online
   streaming service.  The smart TV obtains a QR code from the
   authorization server and displays it on screen.  The attacker copies
   the QR code and embeds it in an e-mail that is sent to a large number
   of recipients.  The e-mail contains a message stating that the
   streaming service wants to thank them for their loyal support and by
   scanning the QR code, they will be able to add a bonus device to
   their account for no charge.  One of the recipients open the e-mail
   and scan the QR code to claim the loyalty reward.  The user performs
   multi-factor authentication, and when asked if they want a new device
   to be added to their account, they authorize the action.  The
   attacker's device is now authorized to access the content and obtains
   an access and refresh token.  The access token allows the attacker to
   access content and the refresh token allows the attacker to obtain
   fresh tokens whenever the access token expires.

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   The attacker scales up the attack by emulating a new smart TV,
   obtaining multiple QR codes and widening the audience it sends the QR
   code to.  Whenever a recipient scans the QR code and authorizes the
   addition of a new device, the attacker obtains an access and refresh
   token, which they sell for a profit.

3.3.2.  Example B2: Illicit Access to Productivity Services (User-
        Transferred Session Data Pattern)

   An attacker emulates an enterprise application (e.g., an interactive
   whiteboard) and initiates a cross-device flow by requesting a user
   code and URL from the authorization server.  The attacker obtains a
   list of potential victims and sends an e-mail informing users that
   their files will be deleted within 24 hours if they don't follow the
   link, enter the user code and authenticate.  The e-mail reminds them
   that this is the third time that they have been notified and their
   last opportunity to prevent deletion of their work files.  One or
   more employees respond by following the URL, entering the code and
   performing multi-factor authentication.  Throughout the
   authentication experience, the user is interacting with a trusted
   user experience, re-enforcing the legitimacy of the request.  Once
   these employees authorized access, the attacker obtains access and
   refresh tokens from the authorization server and uses it to access
   the users' files, perform lateral attacks to obtain access to other
   information and continuously refresh the session by requesting new
   access tokens.  These tokens may be exfiltrated and sold to third
   parties.

3.3.3.  Example B3: Illicit Access to Physical Assets (User-Transferred
        Session Data Pattern)

   An attacker copies a QR code from a bicycle locked in a bicycle rack
   in a city, prints it on a label and places the label on a bicycle at
   the other end of the bicycle rack.  A customer approaches the bicycle
   that contains the replicated QR code and scans the code and
   authenticates before authorizing payment for renting the bicycle.
   The bicycle rack unlocks the bicycle containing the original QR code
   and the attacker removes the bicycle before cycling down the street
   while the customer is left frustrated that the bicycle they were
   trying to use is not being unlocked [NYC.Bike].  The customer
   proceeds to unlock another bicycle and lodges a complaint with the
   bicycle renting company.

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3.3.4.  Example B4.1: Illicit Transaction Authorization (Backchannel-
        Transferred Session Pattern)

   An attacker obtains a list of user identifiers for a financial
   institution and triggers a transaction request for each of the users
   on the list.  The financial institution's authorization server sends
   push notifications to each of the users, requesting authorization of
   a transaction.  The vast majority of users ignore the request to
   authorize the transaction, but a small percentage grants
   authorization by approving the transaction.

3.3.5.  Example B4.2: Fake Helpdesk (Backchannel-Transferred Session
        Pattern)

   An attacker obtains the contact information for a user and contacts
   them, pretending to be a representative of the user's financial
   institution.  The attacker informs the user that there were a number
   of fraudulent transactions against their account and asks them to
   review these transactions by approving or rejecting them.  The
   attacker then triggers a sequence of transactions.  The user receives
   an authorization request for each transaction and declines them as
   they do not recognize them.  The attacker then informs the user that
   they need to close the users account and transfer all the funds to a
   new account to prevent further fraudulent transactions.  The user
   receives another authorization request which they approve, or provide
   additional authorization information to the attacker which enables
   the attacker to complete their attack and defraud the user.

3.3.6.  Example B5: Illicit Network Join (Session Transfer Pattern
        Exploit)

   An attacker creates a message to all employees of a company, claiming
   to be from a trusted technology provider investigating a suspected
   security breach.  They ask employees to send them the QR code
   typically used to join a new device to the network, along with
   detailed steps on how to obtain the QR code.  The employee, eager to
   assist, initiates the process to add a new mobile device to the
   network.  They authenticate to the network and obtain a QR code.
   They send the QR code to the attacker.  The attacker scans the QR
   code and adds their own device to the network.  They use this device
   access as an entry point and perform lateral moves to obtain
   additional privileges and access to restricted resources.

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3.3.7.  Example B6: Illicit Onboarding (User-Transferred Session Data
        Pattern)

   An attacker initiates an employee onboarding flow and obtains a QR
   code from the onboarding portal to invoke a digital wallet and
   present a verifiable credential attesting to a new employee's
   identity.  The attacker obtains a list of potential new employees and
   sends an e-mail informing them that it is time to present proof of
   their background check or government issued ID.  The new employee
   scans the QR code, invokes their digital wallet and presents their
   credentials.  Once the credentials are presented, the employee's
   account is activated.  The employee portal accessed by the attacker
   to obtain the QR code displays a message to the attacker with
   instructions on how to access their account.

3.3.8.  Example B7: Illicit Application Bootstrap (Session Transfer
        Pattern Exploit)

   An attacker creates a message to all employees of a company, claiming
   to be from the company's IT service provider.  They claim that they
   are trying to resolve an application performance issue and ask
   employees to send them the QR code typically used to transfer a
   session.  The employee, eager to assist, initiates the process to
   transfer a session.  They authenticate and obtain a QR code and then
   send the QR code to the attacker.  The attacker scans the QR code
   with their mobile phone and access the users data and resources.

3.3.9.  Example B8: Account Takeover (User-Transferred Session Data
        Pattern)

   An attacker wants to use some website which requires presentation of
   a verifiable credential for authentication.  The attacker creates a
   phishing website which will in real time capture log-in QR Codes from
   the original website and present these to the user.  The attacker
   tries to get the user to use the phishing website using an e-mail
   campaign etc.  The user scans the QR code on the phishing website,
   invokes their digital wallet and presents their credentials.  Once
   the credentials are presented, the original session from the
   attackers device is authorized with the user's credentials.

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3.3.10.  Example B9: Illicit Access to Administration Capabilities
         Through Consent Request Overload (Backchannel-Transferred
         Session Pattern)

   An attacker attempts to access an adminstration portal repeatedly,
   generating a stream of authorization requests to the network
   administrator.  The attempts are timed to occur while the
   administrator is asleep.  The administrator is woken by the incoming
   requests on their phone, and, in an attempt to stop the
   notifications, they accidentally approve access and the attacker
   gains access to the portal.

3.3.11.  Out of Scope

   In all of the attack scenarios listed above, a user is misled or
   exploited.  For other attacks, where the user is willingly colluding
   with the attacker, the threat model, security implications and
   potential mitigations are very different.  For example, a cooperating
   user can bypass software mitigations on their device, share access to
   hardware tokens with the attacker, and install additional devices to
   forward radio signals to circumvent proximity checks.

   This document only considers scenarios where a user does not collude
   with an attacker.

4.  Cross-Device Protocols and Standards

   Cross-device flows that are subject to the attacks described earlier
   typically share the following characteristics:

   1.  The attacker can initiate the flow and manipulate the context of
       an authorization request.  E.g., the attacker can obtain a QR
       code or user code, or can request an authentication/authorization
       decision from the user.

   2.  The interaction between the Consumption Device and Authorization
       Device is unauthenticated.  E.g., it is left to the user to
       decide if the QR code, user code or authentication request is
       being presented in a legitimate context

   A number of protocols that have been standardized, or are in the
   process of being standardized that share these characteristics
   include:

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   *  *IETF OAuth 2.0 Device Authorization Grant ([RFC8628]):* A
      standard to enable authorization on devices with constrained input
      capabilities (smart TVs, printers, kiosks).  In this protocol, the
      user code or QR code is displayed on the Consumption Device and
      entered on a second device (e.g., a mobile phone).

   *  *Open ID Foundation Client Initiated Back-Channel Authentication
      (CIBA) [CIBA]:* A standard developed in the OpenID Foundation that
      allows a device or service (e.g., a personal computer, Smart TV,
      Kiosk) to request the OpenID Provider to initiate an
      authentication flow if it knows a valid identifier for the user.
      The user completes the authentication flow using a second device
      (e.g., a mobile phone).  In this flow the user does not scan a QR
      code or obtain a user code from the Consumption Device, but is
      instead contacted by the OpenID Provider to complete the
      authentication using a push notification, e-mail, text message or
      any other suitable mechanism.

   *  *OpenID for Verifiable Credential Protocol Suite (Issuance,
      Presentation):* The OpenID for Verifiable Credentials enables
      cross-device scenarios by allowing users to scan QR codes to
      retrieve credentials (Issuance) or present credentials
      (Presentation).  The QR code is presented on a device that
      initiates the flow.

   *  *Self-Issued OpenID Provider v2 (SIOP V2):* A standard that allows
      end-user to present self-attested or third party attested
      attributes when used with OpenID for Verifiable Credential
      protocols.  The user scans a QR code presented by the relying
      party to initiate the flow.

   Cross-device protocols SHOULD not be used for same-device scenarios.
   If the Consumption Device and Authorization Device are the same
   device, protocols like OpenID Connect Core [OpenID.Core] and OAuth
   2.0 Authorization Code Grant as defined in [RFC6749] are more
   appropriate.  If a protocol supports both same-device and cross-
   device modes (e.g., [OpenID.SIOPV2]), the cross-device mode SHOULD
   not be used for same-device scenarios.  If an implementor decides to
   use a cross-device protocol or a protocol with a cross-device mode in
   a same-device scenario, the mitigations recommended in this document
   SHOULD be implemented to reduce the risks that the unauthenticated
   channel is exploited.

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5.  Mitigating Against Cross-Device Flow Attacks

   The unauthenticated channel between the Consumption Device and the
   Authorization Device allows attackers to change the context in which
   the authorization request is presented to the user.  This shifts
   responsibility of authenticating the channel between the two devices
   to the end-user.  End-users have "expertise elsewhere" and are
   typically not security experts and don't understand the protocols and
   systems they interact with.  As a result, end-users are poorly
   equipped to authenticate the channel between the two devices.
   Mitigations should focus on:

   1.  Minimizing reliance on the user to make decisions to authenticate
       the channel.

   2.  Providing better information with which to make decisions to
       authenticate the channel.

   3.  Recovering from incorrect channel authentication decisions by
       users.

   To achieve the above outcomes, mitigating against Cross-Device
   Consent Phishing attacks require a three-pronged approach:

   1.  Reduce risks of deployed protocols with practical mitigations.

   2.  Adopt or develop protocols that are less susceptible to these
       attacks where possible.

   3.  Provide analytical tools to assess vulnerabilities and
       effectiveness of mitigations.

5.1.  Practical Mitigations

   A number of protocols that enable cross-device flows that are
   susceptible to Cross-Device Consent Phishing attacks are already
   deployed.  The security profile of these protocols can be improved
   through practical mitigations that provide defense in depth that
   either:

   1.  Prevents the attack from being initiated.

   2.  Disrupts the attack once it is initiated.

   3.  Remediates or reduces the impact if the attack succeeds.

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   It is RECOMMENDED that one or more of the mitigations are applied
   whenever implementing a cross-device flow.  Every mitigation provides
   an additional layer of security that makes it harder to initiate the
   attack, disrupts attacks in progress or reduces the impact of a
   successful attack.

5.1.1.  Establish Proximity

   The unauthenticated channel between the Consumption Device and
   Authorization Device allows attackers to obtain a QR code or user
   code in one location and display it in another location.
   Consequently, proximity-enforced cross-device flows are more
   resistant to Cross-Device Consent Phishing attacks than proximity-
   less cross-device flows.  Establishing proximity between the location
   of the Consumption Device and the Authorization Device limits an
   attacker's ability to launch attacks by sending the user or QR codes
   to large numbers of users that are geographically distributed.  There
   are a couple of ways to establish proximity:

   *  Physical connectivity: This is a good indicator of proximity, but
      requires specific ports, cables and hardware and may be
      challenging from a user experience perspective or may not be
      possible in certain settings (e.g., when USB ports are blocked or
      removed for security purposes).  Physical connectivity may be
      better suited to dedicated hardware like FIDO devices that can be
      used with protocols that are resistant to the exploits described
      in this document.

   *  Wireless proximity: Near Field Communications (NFC), Bluetooth Low
      Energy (BLE), and Ultra Wideband (UWB) services can be used to
      prove proximity between the two devices.  NFC technology is widely
      deployed in mobile phones as part of payment solutions, but NFC
      readers are less widely deployed.  BLE presents another
      alternative for establishing proximity, but may present user
      experience challenges when setting up.  UWB standards such as IEEE
      802.15.4 and the IEEE 802.15.4z-2020 Amendment 1 enable secure
      ranging between devices and allow devices to establish proximity
      relative to each other [IEEE802154].

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   *  Shared network: Device proximity can be inferred by verifying that
      both devices are on the same network.  This check may be performed
      by the authorization server by comparing the network addresses of
      the device where the code is displayed (Consumption Device) with
      that of the Authorization Device.  Alternatively the check can be
      performed on the device, provided that the network address is
      available.  This could be achieved if the authorization server
      encodes the Consumption Device's network address in the QR code
      and uses a digital signature to prevent tampering with the code.
      This does require the wallet to be aware of the countermeasure and
      effectively enforce it.

   *  Geo-location: Proximity can be established by comparing geo-
      location information derived from global navigation satellite-
      system (GNSS) co-ordinates or geolocation lookup of IP addresses
      and comparing proximity.  Due to inaccuracies, this may require
      restrictions to be at a more granular level (e.g., same city,
      country, region or continent).  Similar to the shared network
      checks, these checks may be performed by the authorization server
      or on the users device, provided that the information encoded in a
      QR code is integrity protected using a digital signature.

   Depending on the risk profile and the threat model in which a system
   is operating, it MAY be necessary to use more than one mechanism to
   establish proximity to raise the bar for any potential attackers.

   Note: There are scenarios that require that an authorization takes
   place in a different location than the one in which the transaction
   is authorized.  For example, there may be a primary and secondary
   credit card holder and both can initiate transactions, but only the
   primary holder can authorize it.  There is no guarantee that the
   primary and secondary holders are in the same location at the time of
   the authorization.  In such cases, proximity (or lack of proximity)
   may be an indicator of risk and the system may deploy additional
   controls (e.g., transaction value limits, transaction velocity
   limits) or use the proximity information as input to a risk
   management system.

   *Limitations:* Proximity mechanisms make it harder to perform Cross-
   Device Consent Phishing (CDCP) attacks.  However, depending on how
   the proximity check is performed, an attacker may be able to
   circumvent the protection: The attacker can use a VPN to simulate a
   shared network or spoof a GNSS position.  For example, the attacker
   can try to request the location of the end-user's Authorization
   Device through browser APIs and then simulate the same location on
   their Consumption Device using standard debugging features available
   on many platforms.

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5.1.2.  Short Lived/Timebound QR or User Codes

   The impact of an attack can be reduced by making QR or user codes
   short lived.  If an attacker obtains a short lived code, the duration
   during which the unauthenticated channel can be exploited is reduced,
   potentially increasing the cost of a successful attack.

   *Limitations:* There is a practical limit to how short a user code
   can be valid due to network latency and user experience limitations
   (time taken to enter a code, or incorrectly entering a code).  More
   sophisticated Cross-Device Consent Phishing attacks counter the
   effectiveness of short lived codes by convincing a user to respond to
   a phishing e-mail and only request the QR or user code once the user
   clicks on the link in the phishing e-mail [Exploit6].

5.1.3.  One-Time or Limited Use Codes

   By enforcing one-time use or limited use of user or QR codes, the
   authorization server can limit the impact of attacks where the same
   user code or QR code is sent to multiple victims.  One-time use may
   be achieved by including a nonce or date-stamp in the user code or QR
   code which is validated by the authorization server when the user
   scans the QR code against a list of previously issued codes.

   *Limitations:* Enforcing one-time use may be difficult in large
   globally distributed systems with low latency requirements, in which
   case short lived tokens may be more practical.  One-time use codes
   may also have an impact on the user experience.  For example, a user
   may enter a code, but their session may be interrupted before the
   access request is completed.  If the code is a one-time use code,
   they would need to restart the session and obtain a new code since
   they won't be allowed to enter the same code a second time.  To avoid
   this, implementers MAY allow the same code to be presented a small
   number of times.

5.1.4.  Unique Codes

   By issuing unique user or QR codes, an authorization server can
   detect if the same codes are being repeatedly submitted.  This may be
   interpreted as anomalous behavior and the authorization server MAY
   choose to decline issuing access and refresh tokens if it detects the
   same codes being presented repeatedly.  This may be achieved by
   maintaining a deny list that contains QR codes or user codes that
   were previously used.  The authorization server MAY use a sliding
   window equal to the lifetime of a token if short lived/timebound
   tokens are used (see Short Lived/Timebound Codes (#Short Lived/
   Timebound Codes)).  This will limit the size of the deny list.

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   *Limitations:* Maintaining a deny list of previously redeemed codes,
   even for a sliding window, may have an impact on the latency of
   globally distributed systems.  One alternative is to segment user
   codes by geography or region and maintain local deny lists.

5.1.5.  Content Filtering

   Attackers exploit the unauthenticated channel by changing the context
   of the user code or QR code and then sending a message to a user
   (e-mail, text, instant messaging etc).  By deploying content
   filtering (e.g., anti-spam filter), these messages can be blocked and
   prevented from reaching the end-users.  It may be possible to fine-
   tune content filtering solutions to detect artefacts like QR codes or
   user codes that are included in a message that is sent to multiple
   recipients in the expectation that at least one of the recipients
   will be convinced by the message and grant authorization to access
   restricted resources.

   *Limitations:* Some scenarios may require legitimate re-transmission
   of user, QR and authorization data (e.g., retries).  To prevent the
   disruption of legitimate scenarios, content filters may use a
   threshold and allow a limited number of messages with the same QR or
   user codes to be transmitted before interrupting the delivery of
   those messages.  Content filtering may also be fragmented across
   multiple communications systems and channels (e-mail, messaging, text
   etc), making it harder to detect or interrupt attacks that are
   executed over multiple channels, unless here is a high degree of
   integration between content filtering systems.

5.1.6.  Detect and Remediate

   The authorization server may be able to detect misuse of the codes
   due to repeated use as described in Unique Codes (#Unique Codes), as
   an input from a content filtering engine as described in Content
   Filtering (#Content Filtering), or through other mechanisms such as
   reports from end-users.  If an authorization server determines that a
   user code or QR code is being used in an attack it may choose to
   invalidate all tokens issued in response to these codes and make that
   information available through a token introspection endpoint (see
   [RFC7662]).  In addition it may notify resource servers to stop
   accepting these tokens or to terminate existing sessions associated
   with these tokens using Continuous Access Evaluation Protocol (CAEP)
   messages [CAEP] using the Shared Signals Framework (SSF) [SSF]
   framework or an equivalent notification system.

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   *Limitations:* Detection and remediation requires that resource
   servers are integrated with security eventing systems or token
   introspection services.  This may not always be practical for
   existing systems and may need to be targeted to the most critical
   resource services in an environment.

5.1.7.  Trusted Devices

   If an attacker is unable to initiate the protocol, they are unable to
   obtain a QR code or user code that can be leveraged for the attacks
   described in this document.  By restricting the protocol to only be
   executed on devices trusted by the authorization server, it prevents
   attackers from using arbitrary devices, or by mimicking devices to
   initiate the protocol.

   Authorization Servers MAY use different mechanisms to establish which
   devices it trusts.  This includes limiting cross-device flows to
   specific device types such as intractive whiteboards or smart TVs,
   pre-registering devices with the authorization server or only allow
   cross-device flows on devices managed through device management
   systems.  Device management systems may enforce policies that govern
   patching, version updates, on-device anti-malware deployment,
   revocation status and device location amongst others.  Trusted
   devices MAY have their identities rooted in hardware (e.g., a TPM or
   equivalent technology).

   By only allowing trusted devices to initiate cross-device flows, it
   requires the attacker to have access to such a device and maintain
   access in a way that does not result in the device's trust status
   from being revoked.

   *Limitations:* An attacker may still be able to obtain access to a
   trusted device and use it to initiate authorization requests, making
   it necessary to apply additional controls and integrating with other
   threat detection and management systems that can detect suspicious
   behaviour such as repeated requests to initiate authorization or high
   volume of service activation on the same device.

5.1.8.  Trusted Networks

   An attacker can be prevented from initiating a cross-device flow
   protocol by only allowing the protocol to be initiated on a trusted
   network or within a security perimeter (e.g., a corporate network).
   A trusted network may be defined as a set of IP addresses and joining
   the network is subject to security controls managed by the network
   operator, which may include only allowing trusted devices on the
   network, device management, user authentication and physical access
   policies and systems.  By limiting protocol initiation to a specific

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   network, the attacker needs to have access to a device on the
   network.

   *Limitations:* Network level controls may not always be feasible,
   especially when dealing with consumer scenarios where the network may
   not be under control of the service provider.  Even if it is possible
   to deploy network level controls, it SHOULD be used in conjunction
   with other controls outlined in this document to achieve defence in-
   depth.

5.1.9.  Limited Scopes

   Authorization servers MAY choose to limit the scopes they include in
   access tokens issued through cross-device flows where the
   unauthenticated channel between two devices are susceptible to being
   exploited.  Including limited scopes lessens the impact in case of a
   successful attack.  The decision about which scopes are included may
   be further refined based on whether the protocol is initiated on a
   trusted device or the user's location relative to the location of the
   Consumption Device.

   *Limitations:* Limiting scopes reduces the impact of a compromise,
   but does not avoid it.  It SHOULD be used in conjunction with other
   mitigations described in this document.

5.1.10.  Short Lived Tokens

   Another mitigation strategy includes limiting the life of the access
   and refresh tokens.  The lifetime can be lengthened or shortened,
   depending on the user's location, the resources they are trying to
   access or whether they are using a trusted device.  Short lived
   tokens do not prevent or disrupt the attack, but serve as a remedial
   mechanism in case the attack succeeded.

   *Limitations:* Short lived tokens reduces the time window during
   which an attacker can benefit from a successful attack.  This is most
   effective for access tokens.  However, once an attacker obtains a
   refresh token, they can continue to request new access tokens, as
   well as refresh tokens.  Forcing the expiry of refresh tokens may
   cause the user to re-authorize an action more frequently, which
   results in a negative user experience.

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5.1.11.  Rate Limits

   An attacker that engages in a scaled attack may need to request a
   large number of user codes (see exploit Example B1 (#Example B1:
   Illicit access to a video streaming service (User-Transferred Session
   Data Pattern))) or initiate a large number of authorization requests
   (see exploit Example B4 (#Example B4: Illicit Transaction
   Authorization (Backchannel-Transferred Session Pattern))) in a short
   period of time.  An authorization server MAY apply rate limits to
   minimize the number of requests it would accept from a client in a
   limited time period.

   *Limitations:* Rate limits are effective at slowing an attacker down
   and help to degrade scaled attacks, but do not prevent more targeted
   attacks that are executed with lower volumes and velocity.
   Therefore, it should be used along with other techniques to provide a
   defence-in-depth defence against cross-device attacks.

5.1.12.  Sender-Constrained Tokens

   Sender-constrained tokens limit the impact of a successful attack by
   preventing the tokens from being moved from the device on which the
   attack was successfully executed.  This makes attacks where an
   attacker gathers a large number of access and refresh tokens on a
   single device and then sells them for profit more difficult, since
   the attacker would also have to export the cryptographic keys used to
   sender-constrain the tokens or be able to access them and generate
   signatures for future use.  If the attack is being executed on a
   trusted device to a device with anti-malware, any attempts to
   exfiltrate tokens or keys may be detected and the device's trust
   status may be changed.  Using hardware keys for sender-constraining
   tokens will further reduce the ability of the attacker to move tokens
   to another device.

   *Limitations:* Sender-constrained tokens, especially sender-
   constrained tokens that require proof-of-posession, raise the bar for
   executing the attack and profiting from exfiltrating tokens.
   Although a software proof-of-posession key is better than no proof-
   of-posession key, an attacker may still exfiltrate the software key.
   Hardware keys are harder to exfiltrate, but come with additional
   implementation complexity.  An attacker that controls the Consumption
   Device may still be able to excercise the key, even if it is in
   hardware.  Consequently the main protection derived from sender-
   constrained tokens is preventing tokens from being moved from the
   Consumption Device to another device, thereby making it harder sell
   stolen tokens and profit from the attack.

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5.1.13.  User Education

   Research shows that user education is effective in reducing the risk
   of phishing attacks [Baki2023].  The service provider MAY educate
   users on the risks of cross-device consent phishing and provide out-
   of-band reinforcement to the user on the context and conditions under
   which an authorization grant may be requested.  For example, if the
   service provider does not send e-mails with QR codes requesting users
   to grant authorization, this may be reinforced in marketing messages
   and anti-fraud awareness campaigns.  The service provider MAY also
   choose to reinforce these user education messages through in-app
   experiences.

   *Limitations:* Although user education helps to raise awareness and
   reduce the overall risk to users, it is insufficient on its own to
   mitigate cross-device consent phishing attacks.  In particular,
   carefully designed phishing attacks can be practically
   indistinguishable from benign authorization flows even for well-
   trained users.  User education SHOULD therefore be used in
   conjunction with other controls described in this document.

5.1.14.  User Experience

   The user experience SHOULD preserve the context within which the
   protocols were initiated and communicate this clearly to the user
   when they are asked to authorize, authenticate or present a
   credential.  In preserving the context, it should be clear to the
   user who invoked the flow, why it was invoked and what the
   consequence of completing the authorization, authentication or
   credential presentation is.  The user experience SHOULD reinforce the
   message that unless the user initiated the authorization request, or
   was expecting it, they should decline the request.

   This information MAY be communicated graphically or in a simple
   message (e.g., "It looks like you are trying to access your files on
   a digital whiteboard in your city center office.  Click here to grant
   access to your files.  If you are not trying to access your files,
   you should decline this request and notify the security department").

   It SHOULD be clear to the user how to decline the request.  To avoid
   accidental authorization grants, the "decline" option SHOULD be the
   default option or given similar prominence in the user experience as
   the "grant" option.

   If the user uses an application on a mobile device to scan a QR code,
   the application MAY display information advising the user under which
   conditions they should expect to be asked to scan a QR code and under
   which circumstances they should never scan a QR code (e.g., display a

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   message that the QR code will only be displayed on kiosks within
   trusted locations or on trusted websites hosted on a specific domain,
   and never in e-mail or other media and locations).

   The user experience MAY include information to further educate the
   user on cross-device consent phishing attacks and reinforce the
   conditions under which authorization grants may be requested.

   *Limitations:* Improvements to user experience on their own is
   unlikely to be sufficient and SHOULD be used in conjunction with
   other controls described in this document.

5.1.15.  Authenticate-then-Inititiate

   By requiring a user to authenticate on the Consumption Device with a
   phishing resistant authentication method before initiating a cross-
   device flow, the server can prevent an attacker from initiating a
   cross-device flow and obtaining QR codes or user codes.  This
   prevents the attacker from obtaining a QR code or user code that they
   can use to mislead an unsuspecting user.  This requires that the
   Consumption Device has sufficient input capabilities to support a
   phishing resistant authentication mechanism, which may in itself
   negate the need for a cross-device flow.

   *Limitations:* Authenticating on the Consumption Device before
   starting a cross-device flow does not prevent the attacks described
   in Example B5: Illicit Network Join (#Example B5: Illicit Network
   Join (User-Transferred Authorization Data Pattern)) and Example B7:
   Illicit Session Transfer (#Example B7: Illicit session transfer
   (User-Transferred Authorization Data Pattern)) and it is RECOMMENDED
   that additional mitigations described in this document is used if the
   cross-device flows are used in scenarios such as Example A5: Add a
   device to a network (#Example A5: Add a device to a network (User-
   Transferred Authorization Data Pattern)) and Example A7: Transfer a
   session (#Example A7: Transfer a session (User-Transferred
   Authorization Data Pattern)).

5.1.16.  Request Initiation Verification

   The user MAY be asked to verify if they initiated an authentication
   or authorization request by sending a one-time password (OTP) or PIN
   to the user's Authorization Device and asking them to enter it on the
   Consumption Device to confirm the request.  If the request was
   initiated without the users' consent, they would receive an OTP or
   PIN out of context which may raise suspicion for the user.  In
   addition, they would not have information on where to enter the OTP
   or PIN.  The user experience on the Authorization Device MAY
   reinforce the risk of receiving an out-of-context OTP or PIN and

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   provide information to the user on how to report an unauthorized
   authentication or authorization request.

   *Limitations:* The additional verification step may reduce the
   overall usability of the system as it is one more thing users need to
   do right.  Attackers may combine traditional phishing attacks and
   target users who respond to those messages with an interactive attack
   that sets the expectation with the user that they will have to
   provide the OTP or PIN, in addition to granting authorization for the
   request.

5.1.17.  Request Binding with Out-of-Band Data

   In the User-Transferred Session Data Pattern, users MAY enter out-of-
   band information on the Consumption Device to start the authorization
   process.  The out-of-band data entered by the user MAY then be
   included in the QR code which is displayed on the Consumption Device.
   When the QR code is scanned by the Authorization Device, the out-of-
   band data is verified by the user or by the Authorization Device.
   The out-of-band data could be any attribute that the user or
   Authorization Device can retrieve during the authorization process.
   Examples include a serial number, one-time password or PIN, location
   or any other data that the user or the Authorization Device can
   recall or retrieve during the authorization process ([MPRCS2020],
   [PCRSM2023]).

   *Limitations:* A sophistacted attacker may include an additional step
   in their attack where they create a phishing attack that gathers the
   out-of-band data from the user before initiating the authorisation
   request.  The additional step could also have a negative impact on
   the usability level of the solution.

5.1.18.  Practical Mitigation Summary

   The practical mitigations described in this section can prevent the
   attacks from being initiated, disrupt attacks once they start or
   reduce the impact or remediate an attack if it succeeds.  When
   combining one or more of these mitigations the overall security
   profile of a cross-device flow improves significantly.  The following
   table provides a summary view of these mitigations:

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     +=================================+=========+=========+=========+
     | Mitigation                      | Prevent | Disrupt | Recover |
     +=================================+=========+=========+=========+
     | Establish Proximity             |    X    |    X    |         |
     +---------------------------------+---------+---------+---------+
     | Short Lived/Timebound Codes     |         |    X    |         |
     +---------------------------------+---------+---------+---------+
     | One-Time or Limited Use Codes   |         |    X    |         |
     +---------------------------------+---------+---------+---------+
     | Unique Codes                    |         |    X    |         |
     +---------------------------------+---------+---------+---------+
     | Content Filtering               |         |    X    |         |
     +---------------------------------+---------+---------+---------+
     | Detect and remediate            |         |         |    X    |
     +---------------------------------+---------+---------+---------+
     | Trusted Devices                 |    X    |         |         |
     +---------------------------------+---------+---------+---------+
     | Trusted Networks                |    X    |         |         |
     +---------------------------------+---------+---------+---------+
     | Limited Scopes                  |         |         |    X    |
     +---------------------------------+---------+---------+---------+
     | Short Lived Tokens              |         |         |    X    |
     +---------------------------------+---------+---------+---------+
     | Rate Limits                     |    X    |    X    |         |
     +---------------------------------+---------+---------+---------+
     | Sender-Constrained Tokens       |         |         |    X    |
     +---------------------------------+---------+---------+---------+
     | User Education                  |    X    |         |         |
     +---------------------------------+---------+---------+---------+
     | User Experience                 |    X    |         |         |
     +---------------------------------+---------+---------+---------+
     | Authenticate-then-Inititiate    |    X    |         |         |
     +---------------------------------+---------+---------+---------+
     | Request Initiation Verification |         |    X    |         |
     +---------------------------------+---------+---------+---------+
     | Request Binding with Out-of-    |         |    X    |         |
     | Band Data                       |         |         |         |
     +---------------------------------+---------+---------+---------+

                   Table 1: Practical Mitigation Summary

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5.2.  Protocol Selection

   Some cross-device protocols are more susceptible to the exploits
   described in this document than others.  In this section we will
   compare three different cross-device protocols in terms of their
   susceptibility to exploits focused on the unauthenticated channel,
   the prerequisites to implement and deploy them, along with guidance
   on when it is appropriate to use them.

5.2.1.  IETF OAuth 2.0 Device Authorization Grant [RFC8628]:

5.2.1.1.  Description

   A standard to enable authorization on devices with constrained input
   capabilities (smart TVs, printers, kiosks).  In this protocol, the
   user code or QR code is displayed or made available on the
   Consumption Device (smart TV) and entered on a second device (e.g., a
   mobile phone).

5.2.1.2.  Susceptibility

   There are several reports in the public domain outlining how the
   unauthenticated channel may be exploited to execute a Cross-Device
   Consent Phishing attack ([Exploit1], [Exploit2], [Exploit3],
   [Exploit4], [Exploit5], [Exploit6]).

5.2.1.3.  Device Capabilities

   There are no assumptions in the protocol about underlying
   capabilities of the device, making it a "least common denominator"
   protocol that is expected to work on the broadest set of devices and
   environments.

5.2.1.4.  Mitigations

   In addition to the security considerations section in the standard,
   it is RECOMMENDED that one or more of the mitigations outlined in
   this document be considered, especially mitigations that can help
   establish proximity or prevent attackers from obtaining QR or user
   codes.

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5.2.1.5.  When to use

   Only use this protocol if other cross-device protocols are not viable
   due to device or system constraints.  Avoid using if the protected
   resources are sensitive, high value, or business critical.  Always
   deploy additional mitigations like proximity or only allow with pre-
   registered devices.  Do not use for same-device scenarios (e.g., if
   the Consumption Device and Authorization Device is the same device).

5.2.2.  OpenID Foundation Client Initiated Back-Channel Authentication
        (CIBA):

5.2.2.1.  Description

   Client Initiated Back-Channel Authentication (CIBA) [CIBA]: A
   standard developed in the OpenID Foundation that allows a device or
   service (e.g., a personal computer, Smart TV, Kiosk) to request the
   OpenID Provider to initiate an authentication flow if it knows a
   valid identifier for the user.  The user completes the authentication
   flow using a second device (e.g., a mobile phone).  In this flow the
   user does not scan a QR code or obtain a user code from the
   Consumption Device, but is instead contacted by the OpenID Provider
   to complete the authentication using a push notification, e-mail,
   text message or any other suitable mechanism.

5.2.2.2.  Susceptibility

   Less susceptible to unauthenticated channel attacks, but still
   vulnerable to attackers who know or can guess the user identifier and
   initiate an attack as described in Example B4: Illicit Transaction
   Authorization (#Example B4: Illicit Transaction Authorization
   (Backchannel-Transferred Session Pattern)).

5.2.2.3.  Device Capabilities

   There is no requirement on the Consumption Device to support specific
   hardware.  The Authorization Device must be registered/associated
   with the user and it must be possible for the Authorization Server to
   trigger an authorization on this device.

5.2.2.4.  Mitigations

   In addition to the security considerations section in the standard,
   it is RECOMMENDED that one or more of the mitigations outlined in
   this document be considered, especially mitigations that can help
   establish proximity or prevent attackers from initiating
   authorization requests.

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5.2.2.5.  When to Use

   Use CIBA instead of Device Authorization Grant if it is possible for
   the Consumption Device to obtain a user identifier on the Consumption
   Device (e.g., through an input or selection mechanism) and if the
   Authorization Server can trigger an authorization on the
   Authorization Device.  Do not use for same-device scenarios (e.g., if
   the Consumption Device and Authorization Device is the same device).

5.2.3.  FIDO2/WebAuthn

5.2.3.1.  Description

   FIDO2/WebAuthn is a stack of standards developed in the FIDO Alliance
   and W3C respectively which allow for origin-bound, phishing-resistant
   user authentication using asymmetric cryptography that can be invoked
   from a web browser or native client.  Version 2.2 of the FIDO Client
   to Authenticator Protocol (CTAP) supports a new cross-device
   authentication protocol, called "hybrid", which enables an external
   device, such as a phone or tablet, to be used as a roaming
   authenticator for signing into the primary device, such as a personal
   computer.  This is commonly called FIDO Cross-Device Authentication
   (CDA).

   When a user wants to authenticate using their mobile device
   (authenticator) for the first time, they need to link their
   authenticator to their main device.  This is done using a scan of a
   QR code.  When the authenticator scans the QR code, the device sends
   an encrypted BLE advertisement containing keying material and a
   tunnel ID.  The main device and authenticator both establish
   connections to the web service, and the normal CTAP protocol exchange
   occurs.

   If the user chooses to keep their authenticator linked with the main
   device, the QR code link step is not necessary for subsequent use.
   The user will receive a push notification on the authenticator.

5.2.3.2.  Susceptibility

   The Cross-Device Authentication flow proves proximity by leveraging
   BLE advertisements for service establishment, significantly reducing
   the susceptibility to any of the exploits described in Examples 1-6.

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5.2.3.3.  Device Capabilities

   Both the Consumption Device and the authenticator require BLE
   support.  The Consumption Device must support both FIDO2/WebAuthn,
   specifically CTAP 2.2 with hybrid transport.  The mobile phone must
   support CTAP 2.2+ to be used as a cross-device authenticator.

5.2.3.4.  Mitigations

   FIDO Cross-Device Authentication (CDA) establishes proximity through
   the use of BLE, reducing the need for additional mitigations.  An
   implementer MAY still choose to implement additional mitigation as
   described in this document.

5.2.3.5.  When to Use

   FIDO2/WebAuthn SHOULD be used for cross-device authentication
   scenarios whenever the devices are capable of doing so.  It MAY be
   used as an authentication method with the Authorization Code Grant
   [RFC6749] and PKCE [RFC7663], to grant authorization to an
   Consumption Device (e.g., Smart TV or interactive whiteboard) using a
   mobile phone as the authenticating device.  This combination of
   FIDO2/WebAuthn and Authorization Code Flow with PKCE enables cross
   device authorization flows, without the risks posed by the Device
   Authorization Grant [RFC8628].

5.2.4.  Protocol Selection Summary

   The FIDO Cross-Device Authentication (CDA) flow provides the best
   protection against attacks on the unauthenticated channel for cross
   device flows.  It can be combined with OAuth 2.0 and OpenID Connect
   protocols for standards-based authorization and authentication flows.
   If FIDO2/WebAuthn support is not available, Client Initiated
   Backchannel Authentication (CIBA) provides an alternative, provided
   that there is a channel through which the authorization server can
   contact the end user.  Examples of such a channel include device push
   notifications, e-mail or text messages which the user can access from
   their device.  If CIBA is used, additional mitigations to enforce
   proximity and initiate transactions from trusted devices or trusted
   networks SHOULD be considered.  The OAuth 2.0 Device Authorization
   Grant provides the most flexibility and has the lowest requirements
   on devices used, but it is RECOMMENDED that it is only used when
   additional mitigations are deployed to prevent attacks that exploit
   the unauthenticated channel between devices.

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5.3.  Foundational Pillars

   Experience with web authorization and authentication protocols such
   as OAuth and OpenID Connect has shown that securing these protocols
   can be hard.  The major reason for this is that the landscape in
   which they are operating - the web infrastructure with browsers,
   servers, and the underlying network - is complex, diverse, and ever-
   evolving.

   As is the case with other kinds of protocols, it can be easy to
   overlook vulnerabilities in this environment.  One way to reduce the
   chances of hidden security problems is to use mathematical-logical
   models to describe the protocols, their environments and their
   security goals, and then use these models to try to prove security.
   This approach is what is usually subsumed as "formal security
   analysis".

   There are two major strengths of formal analysis: First, finding new
   vulnerabilities does not require creativity - i.e., new classes of
   attacks can be uncovered even if no one thought of these attacks
   before.  In a faithful model, vulnerabilities become clear during the
   proof process or even earlier.  Second, formal analysis can exclude
   the existence of any attacks within the boundaries of the model
   (e.g., the protocol layers modeled, the level of detail and
   functionalities covered, the assumed attacker capabilities, and the
   formalized security goals).  As a downside, there is usually a gap
   between the model (which necessarily abstracts away from details) and
   implementations.  In other words, implementations can introduce flaws
   where the model does not have any.  Nonetheless, for protocol
   standards, formal analysis can help to ensure that the specification
   is secure when implemented correctly.

   There are various different approaches to formal security analysis
   and each brings its own strengths and weaknesses.  For example,
   models differ in the level of detail in which they can capture a
   protocol (granularity, expressiveness), in the kind of statements
   they can produce, and whether the proofs can be assisted by tools or
   have to be performed manually.  One of the most successfully used
   approaches is the so-called Web Infrastructure Model (WIM), a model
   specifically designed for the analysis of web authentication and
   authorization protocols.  While it is a manual (pen-and-paper) model,
   it captures details of browsers and web interactions in unprecedented
   detail.  Using the WIM, previously unknown flaws in OAuth, OpenID
   Connect, and FAPI were discovered.

   To ensure secure cross-device interactions, a formal analysis using
   the WIM therefore seems to be in order.  Such an analysis should
   comprise a generic model for cross-device flows, potentially

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   including different kinds of interactions.  The aim of the analysis
   would be to evaluate the effectiveness of selected mitigation
   strategies.  To the best of our knowledge, this would be the first
   study of this kind.

6.  Conclusion

   Cross-device flows enable authorization on devices with limited input
   capabilities, allow for secure authentication when using public or
   shared devices, provide a path towards multi-factor authentication
   and, provide the convenience of a single, portable credential store.

   The popularity of cross-device flows attracted the attention of
   attackers that exploit the unauthenticated channel between the
   Consumption Device and Authorization Device using techniques commonly
   used in phishing attacks.  These Cross-Device Consent Phishing (CDCP)
   attacks allow attackers to obtain access and refresh tokens, rather
   than authentication credentials, resulting in access to resources
   even if the user used multi-factor authentication.

   To address these attacks, we propose a three pronged approach that
   includes the deployment of practical mitigations to safeguard
   protocols that are already deployed, provide guidance on when to use
   different protocols, including protocols that are not susceptible to
   these attacks, and the introduction of formal methods to evaluate the
   impact of mitigations and find additional issues.

7.  Contributors

   The authors would like to thank Tim Cappalli, Nick Ludwig, Adrian
   Frei, Nikhil Reddy Boreddy, Bjorn Hjelm, Joseph Heenan, Brian
   Campbell, Damien Bowden, Kristina Yasuda, Tim W├╝rtele, Karsten Meyer
   zu Selhausen, Maryam Mehrnezhad, Marco Pernpruner, Giada Sciarretta
   and others (please let us know, if you've been mistakenly omitted)
   for their valuable input, feedback and general support of this work.

8.  Informative References

   [Baki2023] Baki, S. and R. M. Verma, "Sixteen Years of Phishing User
              Studies: What Have We Learned?", Pages 1200-1212, 2023,
              <https://doi.org/10.1109/TDSC.2022.3151103>.

   [CAEP]     Tulshibagwale, A. and T. Cappalli, "OpenID Continuous
              Access Evaluation Profile 1.0 - draft 01", June 2021,
              <https://openid.net/specs/openid-caep-specification-
              1_0-01.html>.

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   [CIBA]     Fernandez, G., Walter, F., Nennker, A., Tonge, D., and B.
              Campbell, "OpenID Connect Client-Initiated Backchannel
              Authentication Flow - Core 1.0", September 2021,
              <https://openid.net/specs/openid-client-initiated-
              backchannel-authentication-core-1_0.html>.

   [Exploit1] Cooke, B., "The Art of the Device Code Phish", July 2021,
              <https://0xboku.com/2021/07/12/ArtOfDeviceCodePhish.html>.

   [Exploit2] "Microsoft 365 OAuth Device Code Flow and Phishing",
              August 2021, <https://www.optiv.com/insights/source-
              zero/blog/microsoft-365-oauth-device-code-flow-and-
              phishing>.

   [Exploit3] Syynimaa, N., "Introducing a new phishing technique for
              compromising Office 365 accounts", October 2020,
              <https://o365blog.com/post/phishing/#new-phishing-
              technique-device-code-authentication>.

   [Exploit4] Hwong, J., "New Phishing Attacks Exploiting OAuth
              Authentication Flows (DEFCON 29)", August 2021,
              <https://www.youtube.com/watch?v=9slRYvpKHp4>.

   [Exploit5] "OAuth's Device Code Flow Abused in Phishing Attacks",
              August 2021, <https://www.secureworks.com/blog/oauths-
              device-code-flow-abused-in-phishing-attacks>.

   [Exploit6] "SquarePhish: Advanced phishing tool combines QR codes and
              OAuth 2.0 device code flow", August 2022,
              <https://www.helpnetsecurity.com/2022/08/11/squarephish-
              video/>.

   [IEEE802154]
              Engineers, I. O. E. A. E., "IEEE Std 802.15.4-2020: IEEE
              Standard for Low-Rate Wireless Networks",
              IEEE 802.15.4-2020, 2020,
              <https://standards.ieee.org/standard/802_15_4-2020.html>.

   [MPRCS2020]
              Pernpruner, M., Carbone, R., Ranise, S., and G.
              Sciarretta, "The Good, the Bad and the (Not So) Ugly of
              Out-of-Band Authentication with eID Cards and Push
              Notifications: Design, Formal and Risk Analysis",
              IEEE Proceedings of the Tenth ACM Conference on Data and
              Application Security and Privacy (CODASPY '20), 2020,
              <https://doi.org/10.1145/3374664.3375727>.

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   [NYC.Bike] Byrne, K.J., "Citi Bikes being swiped by joyriding
              scammers who have cracked the QR code", August 2021,
              <https://nypost.com/2021/08/07/citi-bikes-being-swiped-by-
              joyriding-scammers-who-have-cracked-the-qr-code/>.

   [OpenID.Core]
              Sakimura, N., Bradley, J., Jones, M.B., Medeiros, B.d.,
              and C. Mortimore, "OpenID Connect Core 1.0", November
              2014,
              <http://openid.net/specs/openid-connect-core-1_0.html>.

   [OpenID.SIOPV2]
              Yasuda, K., Jones, M., and T. Lodderstedt, "Self-Issued
              OpenID Provider v2", November 2022,
              <https://bitbucket.org/openid/connect/src/master/openid-
              connect-self-issued-v2/openid-connect-self-issued-
              v2-1_0.md>.

   [OpenID.VCI]
              Lodderstedt, T., Yasuda, K., and T. Looker, "OpenID for
              Verifiable Credential Issuance", October 2023,
              <https://openid.net/specs/openid-connect-self-issued-
              v2-1_0.html>.

   [PCRSM2023]
              Pernpruner, M., Carbone, R., Sciarretta, G., and S.
              Ranise, "An Automated Multi-Layered Methodology to Assist
              the Secure and Risk-Aware Design of Multi-Factor
              Authentication Protocols", IEEE IEEE Transactions on
              Dependable and Secure Computing (TDSC), 2023,
              <https://doi.org/10.1109/TDSC.2023.3296210>.

   [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>.

   [RFC7662]  Richer, J., Ed., "OAuth 2.0 Token Introspection",
              RFC 7662, DOI 10.17487/RFC7662, October 2015,
              <https://www.rfc-editor.org/info/rfc7662>.

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   [RFC7663]  Trammell, B., Ed. and M. Kuehlewind, Ed., "Report from the
              IAB Workshop on Stack Evolution in a Middlebox Internet
              (SEMI)", RFC 7663, DOI 10.17487/RFC7663, October 2015,
              <https://www.rfc-editor.org/info/rfc7663>.

   [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>.

   [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/info/rfc8628>.

   [SSF]      Tulshibagwale, A., Cappalli, T., Scurtescu, M., Backman,
              A., and J. Bradley, "OpenID Shared Signals and Events
              Framework Specification 1.0", June 2021,
              <https://openid.net/specs/openid-sse-framework-
              1_0-01.html>.

Appendix A.  Document History

   [[ To be removed from the final specification ]]

   -latest

   *  Corrected formatting issue that prevented history from showing
      correctly.

   -03

   *  Introduced normative SHOULD, RECOMMENDED and MAY when applied to
      actions the Authorization Server, Resource Server or Client may
      implement.

   *  Added User Education as a standalone mitigation.

   *  Added Maryam Mehrnezhad, Marco Pernpruner and Giada Sciarretta to
      the contributors list.

   *  Added Request Binding with Out-of-Band Data as an additional
      mitigation.

   *  Adopted the OpenID Foundation terminology from [CIBA] and changed
      Initiating Device to Consumption Device

   *  Added Fake Helpdesk and Consent Request Overload examples

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   *  Replaced "Authenticated Flow" mitigation name with "Authenticate-
      then-Intitiate"

   *  Added Cross-Device Session Transfer pattern

   -02

   *  Fixed typos and grammar edits

   *  Capitalised Initiating Device and Authorization Device

   *  Introduced Cross-Device Consent Phishing as a label for the types
      of attacks described in this document.

   *  Updated labels for different types of flows (User-Transferred
      Session Data Pattern, Backchannel-Transferred Session Pattern,
      User-Transferred Authorization Data Pattern)

   *  Adopted consistent use of hyphenation in using "cross-device"

   *  Consistent use of "Authorization Device"

   *  Update Reference to Secure Signals Framework to reflect name
      change from Secure Signals and Events

   *  Described difference between proximity enforced and proximity-less
      cross-device flows

   *  General editorial pass

   -01

   *  Added additional diagrams and descriptions to distinguish between
      different cross-device flow patterns.

   *  Added short description on limitations of each mitigation.

   *  Added acknowledgement of additional contributors.

   *  Fixed document history format.

   -00 (Working Group Draft)

   *  Initial WG revision (content unchanged from draft-kasselman-cross-
      device-security-03)

   -03 draft-kasselman-cross-device-security

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   *  Minor edits and typos

   -02 draft-kasselman-cross-device-security

   *  Minor edits and typos

   *  Upload as draft-ietf-oauth-cross-device-security-best-practice-02

   -01 draft-kasselman-cross-device-security

   *  Updated draft based on feedback from version circulated to OAuth
      working group

   *  Upload as draft-ietf-oauth-cross-device-security-best-practice-01

   -00 draft-kasselman-cross-device-security

   *  Initial draft adopted from document circulated to the OAuth
      Security Workshop Slack Channel

   *  Upload as draft-ietf-oauth-cross-device-security-best-practice-00

Authors' Addresses

   Pieter Kasselman
   Microsoft
   Email: pieter.kasselman@microsoft.com

   Daniel Fett
   Authlete
   Email: mail@danielfett.de

   Filip Skokan
   Okta
   Email: panva.ip@gmail.com

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