Cross-Device Flows: Security Best Current Practice
draft-ietf-oauth-cross-device-security-06
| Document | Type | Active Internet-Draft (oauth WG) | |
|---|---|---|---|
| Authors | Pieter Kasselman , Daniel Fett , Filip Skokan | ||
| Last updated | 2024-04-04 | ||
| Replaces | draft-kasselman-cross-device-security | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Intended RFC status | (None) | ||
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| Additional resources | Mailing list discussion | ||
| Stream | WG state | WG Document | |
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| Send notices to | (None) |
draft-ietf-oauth-cross-device-security-06
Web Authorization Protocol P. Kasselman
Internet-Draft Microsoft
Intended status: Best Current Practice D. Fett
Expires: 6 October 2024 Authlete
F. Skokan
Okta
4 April 2024
Cross-Device Flows: Security Best Current Practice
draft-ietf-oauth-cross-device-security-06
Abstract
This document describes threats against cross-device flows along with
practical mitigations, protocol selection guidance, and a summary of
formal analysis results identified as relevant to the security of
cross-device flows. 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 6 October 2024.
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Copyright Notice
Copyright (c) 2024 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
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. Best Practices . . . . . . . . . . . . . . . . . . . . . . . 7
3. Cross-Device Flow Patterns . . . . . . . . . . . . . . . . . 7
3.1. Cross-Device Authorization . . . . . . . . . . . . . . . 8
3.1.1. User-Transferred Session Data Pattern . . . . . . . . 9
3.1.2. Backchannel-Transferred Session Pattern . . . . . . . 10
3.1.3. User-Transferred Authorization Data Pattern . . . . . 11
3.2. Cross-Device Session Transfer . . . . . . . . . . . . . . 12
3.2.1. Cross-Device Session Transfer Pattern . . . . . . . . 12
3.3. Examples of Cross-Device Flows . . . . . . . . . . . . . 13
3.3.1. Example A1: Authorize Access to a Video Streaming
Service (User-Transferred Session Data Pattern) . . . 14
3.3.2. Example A2: Authorize Access to Productivity Services
(User-Transferred Session Data Pattern) . . . . . . . 14
3.3.3. Example A3: Authorize Use of a Bike Sharing Scheme
(User-Transferred Session Data Pattern) . . . . . . . 14
3.3.4. Example A4: Authorize a Financial Transaction
(Backchannel-Transferred Session Pattern) . . . . . . 14
3.3.5. Example A5: Add a Device to a Network (Session Transfer
Pattern) . . . . . . . . . . . . . . . . . . . . . . 15
3.3.6. Example A6: Remote Onboarding (User-Transferred Session
Data Pattern) . . . . . . . . . . . . . . . . . . . . 15
3.3.7. Example A7: Application Bootstrap (Session Transfer
Pattern) . . . . . . . . . . . . . . . . . . . . . . 15
3.3.8. Example A8: Access a Productivity Application
(User-Transferred Authorization Data Pattern) . . . . 15
3.3.9. Example A9: Administer a System
(Backchannel-Transferred Session Pattern) . . . . . . 16
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4. Cross-Device Flow Exploits . . . . . . . . . . . . . . . . . 16
4.1. Cross-Device Authorization Flow Exploits . . . . . . . . 16
4.1.1. User-Transferred Session Data Pattern Exploits . . . 17
4.1.2. Backchannel-Transferred Session Pattern Exploits . . 19
4.1.3. User-Transferred Authorization Data Pattern
Exploits . . . . . . . . . . . . . . . . . . . . . . 21
4.2. Cross-Device Session Transfer Exploits . . . . . . . . . 23
4.3. Examples of Cross-Device Flow Exploits . . . . . . . . . 25
4.3.1. Example B1: Illicit Access to a Video Streaming Service
(User-Transferred Session Data Pattern) . . . . . . . 25
4.3.2. Example B2: Illicit Access to Productivity Services
(User-Transferred Session Data Pattern) . . . . . . . 26
4.3.3. Example B3: Illicit Access to Physical Assets
(User-Transferred Session Data Pattern) . . . . . . . 26
4.3.4. Example B4.1: Illicit Transaction Authorization
(Backchannel-Transferred Session Pattern) . . . . . . 26
4.3.5. Example B4.2: Fake Helpdesk (Backchannel-Transferred
Session Pattern) . . . . . . . . . . . . . . . . . . 27
4.3.6. Example B5: Illicit Network Join (Session Transfer
Pattern Exploit) . . . . . . . . . . . . . . . . . . 27
4.3.7. Example B6: Illicit Onboarding (User-Transferred
Session Data Pattern) . . . . . . . . . . . . . . . . 27
4.3.8. Example B7: Illicit Application Bootstrap (Session
Transfer Pattern Exploit) . . . . . . . . . . . . . . 28
4.3.9. Example B8: Account Takeover (User-Transferred Session
Data Pattern) . . . . . . . . . . . . . . . . . . . . 28
4.3.10. Example B9: Illicit Access to Administration
Capabilities Through Consent Request Overload
(Backchannel-Transferred Session Pattern) . . . . . . 28
4.3.11. Out of Scope . . . . . . . . . . . . . . . . . . . . 28
5. Cross-Device Protocols and Standards . . . . . . . . . . . . 29
6. Mitigating Against Cross-Device Flow Attacks . . . . . . . . 30
6.1. Practical Mitigations . . . . . . . . . . . . . . . . . . 31
6.1.1. Establish Proximity . . . . . . . . . . . . . . . . . 31
6.1.2. Short Lived/Timebound QR or User Codes . . . . . . . 33
6.1.3. One-Time or Limited Use Codes . . . . . . . . . . . . 33
6.1.4. Unique Codes . . . . . . . . . . . . . . . . . . . . 34
6.1.5. Content Filtering . . . . . . . . . . . . . . . . . . 34
6.1.6. Detect and Remediate . . . . . . . . . . . . . . . . 35
6.1.7. Trusted Devices . . . . . . . . . . . . . . . . . . . 35
6.1.8. Trusted Networks . . . . . . . . . . . . . . . . . . 36
6.1.9. Limited Scopes . . . . . . . . . . . . . . . . . . . 36
6.1.10. Short Lived Tokens . . . . . . . . . . . . . . . . . 37
6.1.11. Rate Limits . . . . . . . . . . . . . . . . . . . . . 37
6.1.12. Sender-Constrained Tokens . . . . . . . . . . . . . . 37
6.1.13. User Education . . . . . . . . . . . . . . . . . . . 38
6.1.14. User Experience . . . . . . . . . . . . . . . . . . . 38
6.1.15. Authenticate-then-Inititiate . . . . . . . . . . . . 39
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6.1.16. Request Initiation Verification . . . . . . . . . . . 40
6.1.17. Request Binding with Out-of-Band Data . . . . . . . . 40
6.1.18. Practical Mitigation Summary . . . . . . . . . . . . 41
6.2. Protocol Selection . . . . . . . . . . . . . . . . . . . 43
6.2.1. IETF OAuth 2.0 Device Authorization Grant RFC8628: . 43
6.2.2. OpenID Foundation Client Initiated Back-Channel
Authentication (CIBA): . . . . . . . . . . . . . . . 44
6.2.3. FIDO2/WebAuthn . . . . . . . . . . . . . . . . . . . 45
6.2.4. Protocol Selection Summary . . . . . . . . . . . . . 46
6.3. Foundational Pillars . . . . . . . . . . . . . . . . . . 47
7. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . 49
8. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 49
9. Informative References . . . . . . . . . . . . . . . . . . . 49
Appendix A. Document History . . . . . . . . . . . . . . . . . . 52
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 54
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
authorization or not. However, the QR code or PIN are transferred
via an unauthorized channel, leaving it up to the user to 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
This document provides guidance to implementers to defend against
Cross-Device Consent Phishing and Cross-Device Session Phishing
attacks. This guidance includes:
1. Practical mitigations for susceptible protocols (Section 6.1).
2. Protocol selection guidance to avoid using susceptible protocols
(Section 6.2).
3. Results from formal analysis of susceptible protocols
(Section 6.3).
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. Best Practices
This section describes the set of security mechanisms and measures to
secure cross-device protools against Cross-Device Consent Phishing
and Cross-Device Session Phishing attacks that the OAuth working
group considers best practices at the time of writing.
1. Implementers MUST perform a risk assessment before implementing
cross-device flows, weighing the risks from Cross-Device Consent
Phishing and Cross-Device Session Phishing attacks against
benefits for users.
2. Implementers SHOULD avoid cross-device flows if risks cannot be
sufficiently mitigated.
3. Implementers SHOULD follow the guidance provided in Section 6.2
for protocol selection.
4. Implementers MUST implement practical mitigations as listed in
Section 6.1 that are appropriate for the use case, architecture,
and selected protocols.
5. Implementers SHOULD implement proximity checks as defined in
Section 6.1.1 if possible.
These best practices apply to the Device Authorization Grant
([RFC8628]) as well as other cross-device protocols such as the
Client Initiated Backchannel Authentication [CIBA], Self-Issued
OpenID Provider v2 [OpenID.SIOPV2], OpenID for Verifiable
Presentations [OpenID.VP], the Pre-Authorized Code Flow in
([OpenID.VCI]) and other cross-device protocols that rely on the user
to authenticate the channel between devices.
Section 3 provides details about susceptible protocols and Section 4
provides attack descriptions. Section 6.1 provides details about the
security mechanisms and mitigations, (protocol-selection) provides
protocol selection guidance and Section 6.3 provides details from
formal analysis of protocols that apply to cross device flows.
3. 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.).
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3.1. 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.
* *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
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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.
3.1.1. 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 by either entering
data manually or scanning a QR code. The figure below shows a
typical example of this flow:
(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.
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* (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.
3.1.2. 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.
(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.
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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.
3.1.3. 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.
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* (E) The Authorization Server issues tokens or grants authorization
to the Consumption Device to access the user's resources.
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.
3.2. 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]).
3.2.1. 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.
3.3. Examples of Cross-Device Flows
Examples of cross-device flow scenarios include:
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3.3.1. 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.
3.3.2. 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.
3.3.3. 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.
3.3.4. 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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3.3.5. 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.
3.3.6. 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.
3.3.7. 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.
3.3.8. 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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3.3.9. 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.
4. 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.
4.1. 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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4.1.1. 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
the user code by suggesting that doing so would grant them a
reward through a loyalty program or prevent the loss of their
data.
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* (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.
4.1.2. 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.
* (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.
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4.1.3. 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.
* (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).
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* (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.
4.2. 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.
* (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.
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* (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).
4.3. 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.
4.3.1. 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.
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.
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4.3.2. 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.
4.3.3. 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.
4.3.4. 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.
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4.3.5. 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.
4.3.6. 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.
4.3.7. 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.
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4.3.8. 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.
4.3.9. 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.
4.3.10. 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.
4.3.11. 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.
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This document only considers scenarios where a user does not collude
with an attacker.
5. 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. I.e., 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:
* *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 - see [OpenID.VCI]) or present
credentials (Presentation - see [OpenID.VP]). The QR code is
presented on a device that initiates the flow.
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* *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.
6. 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.
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6.1. 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.
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.
6.1.1. 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.
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* 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].
* 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.
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*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.
6.1.2. 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].
6.1.3. 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.
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6.1.4. 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.
*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.
6.1.5. 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.
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6.1.6. 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.
*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.
6.1.7. 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.
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*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.
6.1.8. 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
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.
6.1.9. 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.
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6.1.10. 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.
6.1.11. 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.
6.1.12. 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
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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.
6.1.13. 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. In [PCRSM2023], it is proposed to advise users to
verify the trustworthiness of the source of a QR code, for instance
by checking that the connection is protected through TLS or by
verifying that the URL really belongs to the Authorization Server.
*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.
6.1.14. 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
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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
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.
6.1.15. 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
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(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)).
6.1.16. 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
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.
6.1.17. 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]).
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*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 authorization
request. The additional step could also have a negative impact on
the usability level of the solution.
6.1.18. 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
Table: Practical Mitigation Summary
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6.2. 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.
6.2.1. IETF OAuth 2.0 Device Authorization Grant [RFC8628]:
6.2.1.1. 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).
6.2.1.2. 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]).
6.2.1.3. 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.
6.2.1.4. 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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6.2.1.5. 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).
6.2.2. OpenID Foundation Client Initiated Back-Channel Authentication
(CIBA):
6.2.2.1. 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.
6.2.2.2. 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)).
6.2.2.3. 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.
6.2.2.4. 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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6.2.2.5. 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).
6.2.3. FIDO2/WebAuthn
6.2.3.1. 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.
6.2.3.2. 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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6.2.3.3. 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.
6.2.3.4. 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.
6.2.3.5. 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].
6.2.4. 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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6.3. 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.
The following works have been identified as relevant to the analysis
of cross-device flows:
* In "Formal analysis of self-issued OpenID providers" [Bauer2022],
the protocol of [OpenID.SIOPV2] was analyzed using the Web
Infrastructure Model (WIM). The WIM is 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 to a degree that is hard to match in
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automated models. In previous works, previously unknown flaws in
OAuth, OpenID Connect, and FAPI were discovered using the WIM. In
the analysis of a cross-device SIOP V2 flow in [Bauer2022], the
request replay attack already described in Section 13.3 of
[OpenID.SIOPV2] was confirmed in the model. A mitigation was
implemented based on a so-called Cross-Device Stub, essentially a
component that serves to link the two devices before the protocol
flow starts. This can be seen as an implementation of a trusted
device relationship as described in Section 6.1.7. The mitigation
was shown to be effective in the model.
* In "Security analysis of the Grant Negotiation and Authorization
Protocol" [Helmschmidt2022], an analysis of a draft of the Grant
Negotiation and Authorization Protocol (GNAP)
[I-D.ietf-gnap-core-protocol] was performed using the Web
Infrastructure Model. The same attack as in [Bauer2022] was found
to apply to GNAP as well. In this case, a model of a "careful
user" (see Section 6.1.13) was used to show that the attack can be
prevented (at least in theory) by the user.
* In "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" [MPRCS2020], Pernpruner et al. formally
analysed an authentication protocol relying on push notifications
delivered to an out-of-band device to approve the authentication
attempt on the primary device (Backchannel-Transferred Session
Pattern, Section 3.1.2). The analysis was performed using the
specification language ASLan++ and the model checker SATMC.
According to the results of the analysis, they identified and
defined the category of _implicit attacks_, which manage to
deceive users into approving a malicious authentication attempt
through social engineering techniques, thus not compromising all
the authentication factors involved; these attacks are aligned
with the definition of Cross-Device Consent Phishing (CDCP)
attacks.
* In "An Automated Multi-Layered Methodology to Assist the Secure
and Risk-Aware Design of Multi-Factor Authentication Protocols"
[PCRSM2023], Pernpruner et al. defined a multi-layered methodology
to analyze multi-factor authentication protocols at different
levels of granularity. They leveraged their methodology to
formally analyze a protocol relying on a QR code that has to be
scanned on a secondary device to approve the authentication
attempt on the primary device (User-Transferred Session Data
Pattern, Section 3.1.1). Given the results of the analysis, they
proposed some practical mitigations either to prevent or reduce
the risk of successful attacks, such as those described in
Section 6.1.13 and Section 6.1.17.
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7. 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.
8. 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 Würtele, 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.
9. Informative References
[Baki2023] Baki, S. and R. M. Verma, "Sixteen Years of Phishing User
Studies: What Have We Learned?", Volume 20, Number 2,
Pages 1200-1212, 2023,
<https://doi.org/10.1109/TDSC.2022.3151103>.
[Bauer2022]
Bauer, C., "Formal analysis of self-issued OpenID
providers", 2022,
<https://elib.uni-stuttgart.de/handle/11682/12417>.
[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/>.
[Helmschmidt2022]
Helmschmidt, F., "Security analysis of the Grant
Negotiation and Authorization Protocol", 2022,
<https://elib.uni-stuttgart.de/handle/11682/12220>.
[I-D.ietf-gnap-core-protocol]
Richer, J. and F. Imbault, "Grant Negotiation and
Authorization Protocol", Work in Progress, Internet-Draft,
draft-ietf-gnap-core-protocol-20, 19 March 2024,
<https://datatracker.ietf.org/doc/html/draft-ietf-gnap-
core-protocol-20>.
[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,
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<https://standards.ieee.org/standard/802_15_4-2020.html>.
This standard defines the physical layer and media access
control for low-rate wireless personal area networks (LR-
WPANs) and is maintained by the IEEE 802.15 working group.
[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>.
[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-4-verifiable-credential-
issuance-1_0.html>.
[OpenID.VP]
Terbu, O., Lodderstedt, T., Yasuda, K., and T. Looker,
"OpenID for Verifiable Credential Presentations", November
2023, <https://openid.net/specs/openid-4-verifiable-
presentations-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
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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>.
[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 typos.
-05
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* Added section to provide actionable guidance to implementers on
how to use this document.
* Expanded section on formal analysis to include completed research
projects.
* Added reference to OpenID for Verifiable Presentations.
-04
* 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
* 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
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* 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
* 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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