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Versions: (draft-wdenniss-oauth-native-apps) 00 01 02 03 04 05 06 07 08 09

OAuth Working Group                                           W. Denniss
Internet-Draft                                                    Google
Intended status: Best Current Practice                        J. Bradley
Expires: May 17, 2017                                      Ping Identity
                                                       November 13, 2016


                       OAuth 2.0 for Native Apps
                    draft-ietf-oauth-native-apps-06

Abstract

   OAuth 2.0 authorization requests from native apps should only be made
   through external user-agents, primarily the user's browser.  This
   specification details the security and usability reasons why this is
   the case, and how native apps and authorization servers can implement
   this best practice.

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 http://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 May 17, 2017.

Copyright Notice

   Copyright (c) 2016 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
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of




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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Notational Conventions  . . . . . . . . . . . . . . . . . . .   3
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Overview  . . . . . . . . . . . . . . . . . . . . . . . . . .   4
     4.1.  Authorization Flow for Native Apps Using the Browser  . .   5
   5.  Using Inter-app URI Communication for OAuth . . . . . . . . .   6
   6.  Initiating the Authorization Request from a Native App  . . .   6
   7.  Receiving the Authorization Response in a Native App  . . . .   7
     7.1.  App-declared Custom URI Scheme Redirection  . . . . . . .   7
     7.2.  App-claimed HTTPS URI Redirection . . . . . . . . . . . .   8
     7.3.  Loopback URI Redirection  . . . . . . . . . . . . . . . .   9
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
     8.1.  Embedded User-Agents  . . . . . . . . . . . . . . . . . .   9
     8.2.  Protecting the Authorization Code . . . . . . . . . . . .  10
     8.3.  Loopback Redirect Considerations  . . . . . . . . . . . .  11
     8.4.  Registration of Native App Clients  . . . . . . . . . . .  11
     8.5.  OAuth Implicit Flow . . . . . . . . . . . . . . . . . . .  12
     8.6.  Phishability of In-App Browser Tabs . . . . . . . . . . .  12
     8.7.  Limitations of Non-verifiable Clients . . . . . . . . . .  12
     8.8.  Non-Browser External User-Agents  . . . . . . . . . . . .  13
     8.9.  Client Authentication . . . . . . . . . . . . . . . . . .  13
     8.10. Cross-App Request Forgery Protections . . . . . . . . . .  13
     8.11. Authorization Server Mix-Up Mitigation  . . . . . . . . .  13
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  14
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  14
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  14
     10.2.  Informative References . . . . . . . . . . . . . . . . .  15
   Appendix A.  Server Support Checklist . . . . . . . . . . . . . .  15
   Appendix B.  Operating System Specific Implementation Details . .  16
     B.1.  iOS Implementation Details  . . . . . . . . . . . . . . .  16
     B.2.  Android Implementation Details  . . . . . . . . . . . . .  16
     B.3.  Windows Implementation Details  . . . . . . . . . . . . .  17
     B.4.  macOS Implementation Details  . . . . . . . . . . . . . .  17
     B.5.  Linux Implementation Details  . . . . . . . . . . . . . .  17
   Appendix C.  Acknowledgements . . . . . . . . . . . . . . . . . .  18
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  18

1.  Introduction

   The OAuth 2.0 [RFC6749] authorization framework documents two
   approaches in Section 9 for native apps to interact with the
   authorization endpoint: an embedded user-agent, or an external user-
   agent.



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   This best current practice recommends that only external user-agents
   like the browser are used for OAuth by native apps.  It documents how
   native apps can implement authorization flows using the browser as
   the preferred external user-agent, and the requirements for
   authorization servers to support such usage.

   This practice is also known as the AppAuth pattern, in reference to
   open source libraries that implement it.

2.  Notational Conventions

   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 Key
   words for use in RFCs to Indicate Requirement Levels [RFC2119].  If
   these words are used without being spelled in uppercase then they are
   to be interpreted with their normal natural language meanings.

3.  Terminology

   In addition to the terms defined in referenced specifications, this
   document uses the following terms:

   "native app"  An application that is installed by the user to their
      device, as distinct from a web app that runs in the browser
      context only.  Apps implemented using web-based technology but
      distributed as a native app, so-called hybrid apps, are considered
      equivalent to native apps for the purpose of this specification.

   "OAuth"  In this document, OAuth refers to OAuth 2.0 [RFC6749].

   "external user-agent"  A user-agent capable of handling the
      authorization request that is a separate entity to the native app
      making the request (such as a browser), such that the app cannot
      access the cookie storage or modify the page content.

   "embedded user-agent"  A user-agent hosted inside the native app
      itself (such as via a web-view), with which the app has control
      over to the extent it is capable of accessing the cookie storage
      and/or modify the page content.

   "app"  Shorthand for "native app".

   "app store"  An ecommerce store where users can download and purchase
      apps.






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   "browser"  The operating system's default browser, pre-installed as
      part of the operating system, or installed and set as default by
      the user.

   "browser tab"  An open page of the browser.  Browser typically have
      multiple "tabs" representing various open pages.

   "in-app browser tab"  A full page browser with limited navigation
      capabilities that is displayed inside a host app, but retains the
      full security properties and authentication state of the browser.
      Has different platform-specific product names, such as
      SFSafariViewController on iOS, and Chrome Custom Tab on Android.

   "inter-app communication"  Communication between two apps on a
      device.

   "claimed HTTPS URL"  Some platforms allow apps to claim a HTTPS URL
      after proving ownership of the domain name.  URLs claimed in such
      a way are then opened in the app instead of the browser.

   "custom URI scheme"  A URI scheme (as defined by [RFC3986]) that the
      app creates and registers with the OS (and is not a standard URI
      scheme like "https:" or "tel:").  Requests to such a scheme
      results in the app which registered it being launched by the OS.

   "web-view"  A web browser UI component that can be embedded in apps
      to render web pages, used to create embedded user-agents.

   "reverse domain name notation"  A naming convention based on the
      domain name system, but where where the domain components are
      reversed, for example "app.example.com" becomes "com.example.app".

4.  Overview

   The best current practice for authorizing users in native apps is to
   perform the OAuth authorization request in an external user-agent
   (typically the browser), rather than an embedded user-agent (such as
   one implemented with web-views).

   Previously it was common for native apps to use embedded user-agents
   (commonly implemented with web-views) for OAuth authorization
   requests.  That approach has many drawbacks, including the host app
   being able to copy user credentials and cookies, and the user needing
   to authenticate from scratch in each app.  See Section 8.1 for a
   deeper analysis of using embedded user-agents for OAuth.

   Native app authorization requests that use the browser are more
   secure and can take advantage of the user's authentication state.



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   Being able to use the existing authentication session in the browser
   enables single sign-on, as users don't need to authenticate to the
   authorization server each time they use a new app (unless required by
   authorization server policy).

   Supporting authorization flows between a native app and the browser
   is possible without changing the OAuth protocol itself, as the
   authorization request and response are already defined in terms of
   URIs, which emcompasses URIs that can be used for inter-process
   communication.  Some OAuth server implementations that assume all
   clients are confidential web-clients will need to add an
   understanding of native app OAuth clients and the types of redirect
   URIs they use to support this best practice.

4.1.  Authorization Flow for Native Apps Using the Browser

  +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~+
  |          User Device           |
  |                                |
  | +---------------------------+  |                     +-----------+
  | |                           |  | (5) Authz Code      |           |
  | |        Client App         |----------------------->|  Token    |
  | |                           |<-----------------------|  Endpoint |
  | +---------------------------+  | (6) Access Token,   |           |
  |    |              ^            |     Refresh Token   +-----------+
  |    |              |            |
  |    |              |            |
  |    | (1)          | (4)        |
  |    | Authz        | Authz      |
  |    | Request      | Code       |
  |    |              |            |
  |    |              |            |
  |    v              |            |
  | +---------------------------+  |                   +---------------+
  | |                           |  | (2) Authz Request |               |
  | |          Browser          |--------------------->| Authorization |
  | |                           |<---------------------| Endpoint      |
  | +---------------------------+  | (3) Authz Code    |               |
  |                                |                   +---------------+
  +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~+

        Figure 1: Native App Authorization via External User-agent

   Figure 1 illustrates the interaction of the native app with the
   system browser to authorize the user via an external user-agent.

   (1)  The client app opens a browser tab with the authorization
        request.



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   (2)  Authorization endpoint receives the authorization request,
        authenticates the user and obtains authorization.
        Authenticating the user may involve chaining to other
        authentication systems.

   (3)  Authorization server issues an authorization code to the
        redirect URI.

   (4)  Client receives the authorization code from the redirect URI.

   (5)  Client app presents the authorization code at the token
        endpoint.

   (6)  Token endpoint validates the authorization code and issues the
        tokens requested.

5.  Using Inter-app URI Communication for OAuth

   Just as URIs are used for OAuth 2.0 [RFC6749] on the web to initiate
   the authorization request and return the authorization response to
   the requesting website, URIs can be used by native apps to initiate
   the authorization request in the device's browser and return the
   response to the requesting native app.

   By applying the same principles from the web to native apps, we gain
   similar benefits like the usability of a single sign-on session, and
   the security of a separate authentication context.  It also reduces
   the implementation complexity by reusing the same flows as the web,
   and increases interoperability by relying on standards-based web
   flows that are not specific to a particular platform.

   Native apps MUST use an external user-agent to perform OAuth
   authentication requests.  This is achieved by opening the
   authorization request in the browser (detailed in Section 6), and
   using a redirect URI that will return the authorization response back
   to the native app, as defined in Section 7.

   This best practice focuses on the browser as the RECOMMENDED external
   user-agent for native apps.  Other external user-agents, such as a
   native app provided by the authorization server may meet the criteria
   set out in this best practice, including using the same redirection
   URI properties, but their use is out of scope for this specification.

6.  Initiating the Authorization Request from a Native App

   The authorization request is created as per OAuth 2.0 [RFC6749], and
   opened in the user's browser using platform-specific APIs for that
   purpose.



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   The function of the redirect URI for a native app authorization
   request is similar to that of a web-based authorization request.
   Rather than returning the authorization response to the OAuth
   client's server, the redirect URI used by a native app returns the
   response to the app.  The various options for a redirect URI that
   will return the code to the native app are documented in Section 7.
   Any redirect URI that allows the app to receive the URI and inspect
   its parameters is viable.

   Some platforms support a browser feature known as in-app browser
   tabs, where an app can present a tab of the browser within the app
   context without switching apps, but still retain key benefits of the
   browser such as a shared authentication state and security context.
   On platforms where they are supported, it is RECOMMENDED for
   usability reasons that apps use in-app browser tabs for the
   Authorization Request.

7.  Receiving the Authorization Response in a Native App

   There are several redirect URI options available to native apps for
   receiving the authorization response from the browser, the
   availability and user experience of which varies by platform.

   To fully support this best practice, authorization servers MUST
   support the following three redirect URI options.  Native apps MAY
   use whichever redirect option suits their needs best, taking into
   account platform specific implementation details.

7.1.  App-declared Custom URI Scheme Redirection

   Many mobile and desktop computing platforms support inter-app
   communication via URIs by allowing apps to register custom URI
   schemes, like "com.example.app:".  When the browser or another app
   attempts to load a URI with a custom scheme, the app that registered
   it is launched to handle the request.

   To perform an OAuth 2.0 Authorization Request with a custom URI
   scheme-based redirect URI, the native app launches the browser with a
   normal OAuth 2.0 Authorization Request, but provides a redirection
   URI that utilizes a custom URI scheme registered with the operating
   system by the calling app.

   When the authentication server completes the request, it redirects to
   the client's redirection URI like it would any redirect URI, but as
   the redirection URI uses a custom scheme, this results in the OS
   launching the native app passing in the URI.  The native app then
   processes the authorization response like any OAuth client.




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7.1.1.  Custom URI Scheme Namespace Considerations

   When choosing a URI scheme to associate with the app, apps MUST use a
   URI scheme based on a domain name under their control, expressed in
   reverse order, as recommended by Section 3.8 of [RFC7595] for
   private-use URI schemes.

   For example, an app that controls the domain name "app.example.com"
   can use "com.example.app:" as their custom scheme.  Some
   authorization servers assign client identifiers based on domain
   names, for example "client1234.usercontent.example.net", which can
   also be used as the domain name for the custom scheme, when reversed
   in the same manner, for example "net.example.usercontent.client1234".

   URI schemes not based on a domain name (for example "myapp:") MUST
   NOT be used, as they are not collision resistant, and don't comply
   with Section 3.8 of [RFC7595].

   Care must be taken when there are multiple apps by the same publisher
   that each URI scheme is unique within that group.  On platforms that
   use app identifiers that are also based on reverse order domain
   names, those can be re-used as the custom URI scheme for the OAuth
   redirect.

   In addition to the collision resistant properties, basing the URI
   scheme off a domain name that is under the control of the app can
   help to prove ownership in the event of a dispute where two apps
   claim the same custom scheme (such as if an app is acting
   maliciously).  For example, if two apps claimed "com.example.app:",
   the owner of "example.com" could petition the app store operator to
   remove the counterfeit app.  This petition is harder to prove if a
   generic URI scheme was used.

7.2.  App-claimed HTTPS URI Redirection

   Some operating systems allow apps to claim HTTPS URLs in their
   domains.  When the browser encounters a claimed URL, instead of the
   page being loaded in the browser, the native app is launched with the
   URL supplied as a launch parameter.

   App-claimed HTTPS redirect URIs have some advantages in that the
   identity of the destination app is guaranteed by the operating
   system.  Due to this reason, they SHOULD be used over the other
   redirect choices for native apps where possible.

   App-claimed HTTPS redirect URIs function as normal HTTPS redirects
   from the perspective of the authorization server, though it is
   RECOMMENDED that the authorization server is able to distinguish



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   between public native app clients that use app-claimed HTTPS redirect
   URIs and confidential web clients.  A configuration option in the
   client registration (as documented in Section 8.4) is one method for
   distinguishing client types.

7.3.  Loopback URI Redirection

   Desktop operating systems allow native apps to listen on a local port
   for HTTP redirects.  This can be used by native apps to receive OAuth
   authorization responses on compatible platforms.

   Loopback redirect URIs take the form of the loopback IP, any port
   (dynamically provided by the client), and a path component.
   Specifically: "http://127.0.0.1:{port}/{path}" for IPv4, and
   "http://[::1]:{port}/{path}" for IPv6.

   For loopback IP redirect URIs, the authorization server MUST allow
   any port to be specified at the time of the request, to accommodate
   clients that obtain an available port from the operating system at
   the time of the request.  Other than that, the redirect is be treated
   like any other.

8.  Security Considerations

8.1.  Embedded User-Agents

   Embedded user-agents are an alternative method for authorizing native
   apps.  They are however unsafe for use by third-parties to the
   authorization server by definition, as the app that hosts the
   embedded user-agent can access the user's full authentication
   credential, not just the OAuth authorization grant that was intended
   for the app.

   In typical web-view based implementations of embedded user-agents,
   the host application can: log every keystroke entered in the form to
   capture usernames and passwords; automatically submit forms and
   bypass user-consent; copy session cookies and use them to perform
   authenticated actions as the user.

   Even when used by trusted apps belonging to the same party as the
   authorization server, embedded user-agents violate the principle of
   least privilege by having access to more powerful credentials than
   they need, potentially increasing the attack surface.

   Encouraging users to enter credentials in an embedded user-agent
   without the usual address bar and visible certificate validation
   features that browsers have makes it impossible for the user to know
   if they are signing in to the legitimate site, and even when they



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   are, it trains them that it's OK to enter credentials without
   validating the site first.

   Aside from the security concerns, embedded user-agents do not share
   the authentication state with other apps or the browser, requiring
   the user to login for every authorization request and leading to a
   poor user experience.

   Native apps MUST NOT use embedded user-agents to perform
   authorization requests.

   Authorization endpoints MAY take steps to detect and block
   authorization requests in embedded user-agents.

8.2.  Protecting the Authorization Code

   The redirect URI options documented in Section 7 share the benefit
   that only a native app on the same device can receive the
   authorization code which limits the attack surface, however code
   interception by a native app other than the intended app may still be
   possible.

   A limitation of using custom URI schemes for redirect URIs is that
   multiple apps can typically register the same scheme, which makes it
   indeterminate as to which app will receive the Authorization Code.
   PKCE [RFC7636] details how this limitation can be used to execute a
   code interception attack (see Figure 1).

   Loopback IP based redirect URIs may be susceptible to interception by
   other apps listening on the same loopback interface.

   As most forms of inter-app URI-based communication sends data over
   insecure local channels, eavesdropping and interception of the
   authorization response is a risk for native apps.  App-claimed HTTPS
   redirects are hardened against this type of attack due to the
   presence of the URI authority, but they are still public clients and
   the URI is still transmitted over local channels with unknown
   security properties.

   The Proof Key for Code Exchange by OAuth Public Clients (PKCE
   [RFC7636]) standard was created specifically to mitigate against this
   attack.  It is a Proof of Possession extension to OAuth 2.0 that
   protects the code grant from being used if it is intercepted.  It
   achieves this by having the client generate a secret verifier which
   it passes in the initial authorization request, and which it must
   present later when redeeming the authorization code grant.  An app
   that intercepted the authorization code would not be in possession of
   this secret, rendering the code useless.



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   Public native app clients MUST protect the authorization request with
   PKCE [RFC7636].  Authorization servers MUST support PKCE [RFC7636]
   for public native app clients.  Authorization servers SHOULD reject
   authorization requests from native apps that don't use PKCE by
   returning an error message as defined in Section 4.4.1 of PKCE
   [RFC7636].

8.3.  Loopback Redirect Considerations

   Loopback interface redirect URIs use the "http" scheme (i.e. without
   TLS).  This is acceptable for loopback interface redirect URIs as the
   HTTP request never leaves the device.

   Clients should open the loopback port only when starting the
   authorization request, and close it once the response is returned.

   While redirect URIs using localhost (i.e.  "http://localhost:{port}/"
   function similarly to loopback IP redirects described in Section 7.3,
   the use of "localhost" is NOT RECOMMENDED.  Opening a port on the
   loopback interface is more secure as only apps on the local device
   can connect to it.  It is also less susceptible to misconfigured
   routing, and interference by client side firewalls.

8.4.  Registration of Native App Clients

   Authorization Servers SHOULD have a way to distinguish public native
   app clients from confidential web-clients, as the lack of client
   authentication means they are often handled differently.  A
   configuration option to indicate a public native app client is one
   such popular method for achieving this.

   As recommended in Section 3.1.2.2 of OAuth 2.0 [RFC6749], the
   authorization server SHOULD require the client to pre-register the
   complete redirection URI.  This applies and is RECOMMENDED for all
   redirection URIs used by native apps.

   For Custom URI scheme based redirects, authorization servers SHOULD
   enforce the requirement in Section 7.1.1 that clients use reverse
   domain name based schemes.

   Authorization servers MAY request the inclusion of other platform-
   specific information, such as the app package or bundle name, or
   other information used to associate the app that may be useful for
   verifying the calling app's identity, on operating systems that
   support such functions.






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8.5.  OAuth Implicit Flow

   The OAuth 2.0 Implicit Flow as defined in Section 4.2 of OAuth 2.0
   [RFC6749] generally works with the practice of performing the
   authorization request in the browser, and receiving the authorization
   response via URI-based inter-app communication.  However, as the
   Implicit Flow cannot be protected by PKCE (which is a recommended in
   Section 7.1.1), the use of the Implicit Flow with native apps is NOT
   RECOMMENDED.

   Tokens granted via the implicit flow also cannot be refreshed without
   user interaction making the code flow, with refresh tokens the more
   practical option for native app authorizations that require
   refreshing.

8.6.  Phishability of In-App Browser Tabs

   While in-app browser tabs provide a secure authentication context, as
   the user initiates the flow from a native app, it is possible for
   that native app to completely fake an in-app browser tab.

   This can't be prevented directly - once the user is in the native
   app, that app is fully in control of what it can render, however
   there are several mitigating factors.

   Importantly, such an attack that uses a web-view to fake an in-app
   browser tab will always start with no authentication state.  If all
   native apps use the techniques described in this best practice, users
   will not need to sign-in frequently and thus should be suspicious of
   any sign-in request when they should have already been signed-in.

   This is the case even for authorization servers that require
   occasional or frequent re-authentication, as such servers can
   preserve some user identifiable information from the old session,
   like the email address or profile picture and display that on the re-
   authentication.

   Users who are particularly concerned about their security may also
   take the additional step of opening the request in the browser from
   the in-app browser tab, and completing the authorization there, as
   most implementations of the in-app browser tab pattern offer such
   functionality.

8.7.  Limitations of Non-verifiable Clients

   As stated in Section 10.2 of OAuth 2.0 [RFC6749], the authorization
   server SHOULD NOT process authorization requests automatically
   without user consent or interaction, except when the identity of the



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   client can be assured.  Measures such as claimed HTTPS redirects can
   be used by native apps to prove their identity to the authorization
   server, and some operating systems may offer alternative platform-
   specific identity features which may be used, as appropriate.

8.8.  Non-Browser External User-Agents

   This best practice recommends a particular type of external user-
   agent, the user's browser.  Other external user-agent patterns may
   also be viable for secure and usable OAuth.  This document makes no
   comment on those patterns.

8.9.  Client Authentication

   Secrets that are statically included as part of an app distributed to
   multiple users should not be treated as confidential secrets, as one
   user may inspect their copy and learn the shared secret.  For this
   reason, and those stated in Section 5.3.1 of [RFC6819], it is NOT
   RECOMMENDED for authorization servers to require client
   authentication of native apps using a shared secret, as this serves
   little value beyond client identification which is already provided
   by the "client_id" request parameter.

   Authorization servers that still require a shared secret for native
   app clients MUST treat the client as a public client, and not treat
   the secret as proof of the client's identity.  In those cases, it is
   NOT RECOMMENDED to automatically issue tokens on the basis that the
   user has previously granted access to the same client, as there is no
   guarantee that the client is not counterfeit.

8.10.  Cross-App Request Forgery Protections

   Section 5.3.5 of [RFC6819] recommends using the 'state' parameter to
   link client requests and responses to prevent CSRF attacks.

   It is similarly RECOMMENDED for native apps to include a high entropy
   secure random number in the 'state' parameter of the authorization
   request, and reject any incoming authorization responses without a
   state value that matches a pending outgoing authorization request.

8.11.  Authorization Server Mix-Up Mitigation

   To protect against a compromised or malicious authorization server
   attacking another authorization server used by the same app, it is
   RECOMMENDED that a unique redirect URI is used for each different
   authorization server used by the app (for example, by varying the
   path component), and that authorization responses are rejected if the




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   redirect URI they were received on doesn't match the redirect URI in
   a pending outgoing authorization request.

   Authorization servers SHOULD allow the registration of a specific
   redirect URI, including path components, and reject authorization
   requests that specify a redirect URI that doesn't exactly match the
   one that was registered.

9.  IANA Considerations

   [RFC Editor: please do not remove this section.]

   Section 7.1 specifies how private-use URI schemes are used for inter-
   app communication in OAuth protocol flows.  This document requires in
   Section 7.1.1 that such schemes are based on domain names owned or
   assigned to the app, as recommended in Section 3.8 of [RFC7595].  Per
   section 6 of [RFC7595], registration of domain based URI schemes with
   IANA is not required.  Therefore, this document has no IANA actions.

10.  References

10.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,
              <http://www.rfc-editor.org/info/rfc3986>.

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

   [RFC7595]  Thaler, D., Ed., Hansen, T., and T. Hardie, "Guidelines
              and Registration Procedures for URI Schemes", BCP 35,
              RFC 7595, DOI 10.17487/RFC7595, June 2015,
              <http://www.rfc-editor.org/info/rfc7595>.

   [RFC7636]  Sakimura, N., Ed., Bradley, J., and N. Agarwal, "Proof Key
              for Code Exchange by OAuth Public Clients", RFC 7636,
              DOI 10.17487/RFC7636, September 2015,
              <http://www.rfc-editor.org/info/rfc7636>.





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10.2.  Informative References

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

   [AppAuth.iOSmacOS]
              Wright, S., Denniss, W., and others, "AppAuth for iOS and
              macOS", February 2016, <https://github.com/openid/AppAuth-
              iOS>.

   [AppAuth.Android]
              McGinniss, I., Denniss, W., and others, "AppAuth for
              Android", February 2016, <https://github.com/openid/
              AppAuth-Android>.

   [SamplesForWindows]
              Denniss, W., "OAuth for Apps: Samples for Windows", July
              2016, <https://github.com/googlesamples/oauth-apps-for-
              windows>.

Appendix A.  Server Support Checklist

   OAuth servers that support native apps should:

   1.  Support custom URI-scheme redirect URIs.  This is required to
       support mobile operating systems.  See Section 7.1.

   2.  Support HTTPS redirect URIs for use with public native app
       clients.  This is used by apps on advanced mobile operating
       systems that allow app-claimed HTTPS URIs.  See Section 7.2.

   3.  Support loopback IP redirect URIs.  This is required to support
       desktop operating systems.  See Section 7.3.

   4.  Not assume native app clients can keep a secret.  If secrets are
       distributed to multiple installs of the same native app, they
       should not be treated as confidential.  See Section 8.9.

   5.  Support PKCE.  Recommended to protect authorization code grants
       transmitted to public clients over inter-app communication
       channels.  See Section 8.2








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Appendix B.  Operating System Specific Implementation Details

   Most of this document defines best practices in an generic manner,
   referencing techniques commonly available in a variety of
   environments.  This non-normative section contains OS-specific
   implementation details for the generic pattern, that are considered
   accurate at the time of publishing, but may change over time.

   It is expected that this OS-specific information will change, but
   that the overall principles described in this document for using
   external user-agents will remain valid.

B.1.  iOS Implementation Details

   Apps can initiate an authorization request in the browser without the
   user leaving the app, through the SFSafariViewController class which
   implements the browser-view pattern.  Safari can be used to handle
   requests on old versions of iOS without SFSafariViewController.

   To receive the authorization response, both custom URI scheme
   redirects and claimed HTTPS links (known as Universal Links) are
   viable choices, and function the same whether the request is loaded
   in SFSafariViewController or the Safari app.  Apps can claim Custom
   URI schemes with the "CFBundleURLTypes" key in the application's
   property list file "Info.plist", and HTTPS links using the Universal
   Links feature with an entitlement file and an association file on the
   domain.

   Universal Links are the preferred choice on iOS 9 and above due to
   the ownership proof that is provided by the operating system.

   A complete open source sample is included in the AppAuth for iOS and
   macOS [AppAuth.iOSmacOS] library.

B.2.  Android Implementation Details

   Apps can initiate an authorization request in the browser without the
   user leaving the app, through the Android Custom Tab feature which
   implements the browser-view pattern.  The user's default browser can
   be used to handle requests when no browser supports Custom Tabs.

   Android browser vendors should support the Custom Tabs protocol (by
   providing an implementation of the "CustomTabsService" class), to
   provide the in-app browser tab user experience optimization to their
   users.  Chrome is one such browser that implements Custom Tabs.

   To receive the authorization response, custom URI schemes are broadly
   supported through Android Implicit Intends.  Claimed HTTPS redirect



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   URIs through Android App Links are available on Android 6.0 and
   above.  Both types of redirect URIs are registered in the
   application's manifest.

   A complete open source sample is included in the AppAuth for Android
   [AppAuth.Android] library.

B.3.  Windows Implementation Details

   Apps can initiate an authorization request in the user's default
   browser using platform APIs for this purpose.

   The custom URI scheme redirect is a good choice for Universal Windows
   Platform (UWP) apps as it will open the app returning the user right
   back where they were.  Known on UWP as URI Activation, the scheme is
   limited to 39 characters, but may include the "." character, making
   short reverse domain name based schemes (as recommended in
   Section 7.1.1) possible.

   The loopback redirect is the common choice for traditional desktop
   apps, listening on a loopback interface port is permitted by default
   Windows firewall rules.

   A complete open source sample is available [SamplesForWindows].

B.4.  macOS Implementation Details

   Apps can initiate an authorization request in the user's default
   browser using platform APIs for this purpose.

   To receive the authorization response, custom URI schemes are are a
   good redirect URI choice on macOS, as the user is returned right back
   to the app they launched the request from.  These are registered in
   the application's bundle information property list using the
   "CFBundleURLSchemes" key.  Loopback IP redirects are another viable
   option, and listening on the loopback interface is allowed by default
   firewall rules.

   A complete open source sample is included in the AppAuth for iOS and
   macOS [AppAuth.iOSmacOS] library.

B.5.  Linux Implementation Details

   Opening the Authorization Request in the user's default browser
   requires a distro-specific command, "xdg-open" is one such tool.

   The loopback redirect is the recommended redirect choice for desktop
   apps on Linux to receive the authorization response.



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Appendix C.  Acknowledgements

   The author would like to acknowledge the work of Marius Scurtescu,
   and Ben Wiley Sittler whose design for using custom URI schemes in
   native OAuth 2.0 clients formed the basis of Section 7.1.

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

   Andy Zmolek, Steven E Wright, Brian Campbell, Paul Madsen, Nat
   Sakimura, Iain McGinniss, Rahul Ravikumar, Eric Sachs, Breno de
   Medeiros, Adam Dawes, Naveen Agarwal, Hannes Tschofenig, Ashish Jain,
   Erik Wahlstrom, Bill Fisher, Sudhi Umarji, Michael B. Jones, Vittorio
   Bertocci, Dick Hardt, David Waite, and Ignacio Fiorentino.

Authors' Addresses

   William Denniss
   Google
   1600 Amphitheatre Pkwy
   Mountain View, CA  94043
   USA

   Email: wdenniss@google.com
   URI:   http://wdenniss.com/appauth


   John Bradley
   Ping Identity

   Phone: +1 202-630-5272
   Email: ve7jtb@ve7jtb.com
   URI:   http://www.thread-safe.com/p/appauth.html


















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