wdiff draft-ietf-oauth-native-apps-03.txt draft-ietf-oauth-native-apps-04.txt

OAuth Working Group W. Denniss
Internet-Draft Google
Intended status: Best Current Practice J. Bradley
Expires: January 21, April 15, 2017 Ping Identity
July 20,
October 12, 2016

OAuth 2.0 for Native Apps
draft-ietf-oauth-native-apps-03
draft-ietf-oauth-native-apps-04

Abstract

OAuth 2.0 authorization requests from native apps should only be made
through external user-agents, primarily the system 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

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This Internet-Draft will expire on January 21, April 15, 2017.

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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Notational Conventions . . . . . . . . . . . . . . . . . . . 3
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 4
4.1. Authorization Flow for Native Apps Using App-Claimed URI
Schemes . . . . . . . . . . . . . . . . . . . . . . . the Browser . . 4 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 . . . . . . . . . . . . 9 8
7.3. Loopback URI Redirection . . . . . . . . . . . . . . . . 9 8
8. Security Considerations . . . . . . . . . . . . . . . . . . . 10 9
8.1. Embedded User-Agents . . . . . . . . . . . . . . . . . . 10 9
8.2. Protecting the Authorization Code . . . . . . . . . . . . 11 10
8.3. Loopback Redirect Considerations . . . . . . . . . . . . 11
8.4. Registration of App Redirection URIs . . . . . . . . . . 11
8.5. OAuth Implicit Flow . . . . . . . . . . . . . . . . . . . 11
8.6. Phishability of In-App Browser Tabs . . . . . . . . . . . 12
8.4.
8.7. Limitations of Non-verifiable Clients . . . . . . . . . . 12
9. Other
8.8. Non-Browser External User Agents . . . . . . . . . . . . 12
8.9. Client Authentication . . . . . . . . . . . 12
10. Client Authentication . . . . . . . 13
8.10. Cross-App Request Forgery Protections . . . . . . . . . . 13
8.11. Authorization Server Mix-Up Mitigation . . . . . . . . . 13
11.
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 13
11.1. 14
10.1. Normative References . . . . . . . . . . . . . . . . . . 13
11.2. 14
10.2. Informative References . . . . . . . . . . . . . . . . . 13 14
Appendix A. Server Support Checklist . . . . . . . . . . . . . . 15
Appendix B. Operating System Specific Implementation Details . . 15
A.1.
B.1. iOS Implementation Details . . . . . . . . . . . . . . . 15
A.2. 16
B.2. Android Implementation Details . . . . . . . . . . . . . 15
A.3. 16
B.3. Windows Implementation Details . . . . . . . . . . . . . 16
A.4.
B.4. macOS Implementation Details . . . . . . . . . . . . . . 16 17
B.5. Linux Implementation Details . . . . . . . . . . . . . . 17
Appendix B. C. Acknowledgements . . . . . . . . . . . . . . . . . . 17
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 17 18

1. Introduction

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

This document best current practice recommends that only external user-agents
like in-app browser
tabs as the only secure and usable choice browser are used for OAuth. OAuth by native apps. It documents how
native apps can implement authorization flows with such agents, using the browser as
the preferred external user-agent, and the additional requirements of for
authorization servers needed 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:

"app" A native application, such

"native app" An application that is installed by the user to their
device, as one on distinct from a mobile device or
desktop operating system. 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].

"app" Shorthand for "native app".

"app store" An ecommerce store where users can download and purchase
apps. Typically with quality-control measures to protect users
from malicious developers.

"authz" Abbreviation of "authorization".

"system browser"

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

"browser tab" An open page of the system 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 system browser.

Has different platform-specific product names, such as
SFSafariViewController on iOS 9, iOS, and Chrome Custom Tab on Android.

"Claimed

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

"claimed HTTPS URL" Some platforms allow apps to claim a domain name
by hosting a file that proves HTTPS URL
after proving ownership of the link between site and app.
Typically this means that domain name. URLs opened by the system will be claimed in such
a way are then opened in the app instead of the browser.

"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".

"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.
For example, "myapp:", "com.example.myapp:" are both custom URI
schemes.

"inter-app communication" Communication between two

"web-view" A web browser UI component that can be embedded in apps on a
device.

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

At the time of writing, many

The best current practice for authorizing users in native apps are still using web-views, is to
perform the OAuth authorization request in a
type of browser (an external
user-agent), rather than web-view (an embedded user-agent, user-agent).

Previously it was common for OAuth. native apps to use web-views for OAuth
authorization requests. That approach has multiple many drawbacks, typically
including the client host app being able to eavesdrop copy user
credentials, credentials and is a suboptimal user experience as the
authentication session can't be shared,
cookies, and users need to sign-in the user needing to authenticate from scratch in each app separately.

OAuth flows between
app. See Section 8.1 for a native deeper analysis of using embedded user-
agents for OAuth.

Native app and authorization requests that use the system browser (or another
external user-agent) are more secure,
secure and can take advantage of the
shared user's authentication state state.
Being able to enable use the existing authentication session in the browser
enables single sign-on.

Inter-process communication, such sign-on, as OAuth 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 system browser can be achieved through URI-based
communication. As this
is exactly how OAuth works for web-based possible without changing the OAuth flows between RP protocol itself, as the
authorization request and IDP websites, OAuth 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 auth with very little modification. OAuth clients and the types of redirect
URIs they use to support this best practice.

4.1. Authorization Flow for Native Apps Using App-Claimed URI Schemes 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.

2) Authorization endpoint receives the authorization request, and
processes it, typically by authenticating
authenticates the end-user user and
obtaining an authorization decision. How the authorization server
authenticates obtains authorization. Authenticating
the end-user is out of scope for this specification,
but can potentially user may involve chaining to other authentication
systems using various authentication protocols. 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 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 system 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 by 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.

It is RECOMMENDED that native

Native apps MUST use the URI-based communication
functionality of the operating system an external user-agent to perform OAuth flows
authentication requests. This is achieved by opening the
authorization request in an
external user-agent, typically the system browser.

Some platforms support a browser feature known as in-app browser
tabs, where an app can present (detailed in Section 6), and
using a tab of redirect URI that will return the browser within authorization response back
to the app
context without switching apps, but still retain key benefits of native app, as defined in Section 7.

This best practice focuses on 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.

It is possible to create an RECOMMENDED external
user-agent for OAuth that is native apps. Other external user-agents, such as a
native app provided by the authorization server, as opposed to server may meet the
system browser. This approach shares a lot of similarity with criteria
set out in this best practice, including using the system browser as both same redirection
URI properties, but their use URIs for inter-app communication and
is able to provide a secure, shared authentication session, and thus
MAY be used for secure native OAuth, applying most of the techniques
described here. However it is NOT RECOMMENDED due to the increased
complexity and requirement for the user to have the AS app installed.
While much of the advice and security considerations are applicable
to such clients, they are out of scope for this specification.
options for inter-app communication, offering similar security

6. Initiating the Authorization Request from a Native App

The authorization request is created as per OAuth 2.0 [RFC6749], and
opened in the system browser. Where the operating system supports
in-app user's browser tabs, those should be preferred over switching to the
system browser, to improve usability. using platform-specific APIs for that
purpose.

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 code response to the OAuth
client's server, it the redirect URI used by a native app returns it the
response to the native 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 three main approaches several redirect URI options available to redirection URIs for native apps:
custom URI schemes, app-claimed HTTPS URI schemes, apps for
receiving the authorization response from the browser, the
availability and loopback
redirects. 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

Most major

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

In particular, the custom URI scheme pattern is supported on Android
[Android.URIScheme], iOS [iOS.URIScheme], Windows Universal Platform
(UWP) [WindowsUWP.URIScheme] and macOS [macOS.URIScheme].

7.1.1. Using Custom URI Schemes for Redirection

To perform an OAuth 2.0 Authorization Request on a supported
platform, the native app launches the system browser with a normal OAuth 2.0
Authorization Request, but provides a redirection URI that utilizes a
custom URI scheme that is 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 extracts
the code from the query parameters from the URI just like a web
client would, and exchanges then
processes the Authorization Code authorization response like a regular any OAuth 2.0 client.

7.1.2.

7.1.1. Custom URI Scheme Namespace Considerations

When selecting which choosing a URI scheme to associate with the app, apps
SHOULD pick a scheme that is globally unique, and which they can
assert ownership over.

To avoid clashing with existing schemes in use, using a scheme that
follows the reverse domain name pattern applied to a domain under the
app publishers control is RECOMMENDED. Such MUST use a
URI scheme can be based on a domain they control, or the OAuth client identifier in cases where
the authorization server issues client identifiers that are also
valid DNS subdomains. The chosen scheme MUST NOT clash with any IANA
registered scheme [IANA.URISchemes]. You SHOULD also ensure that no
other app by the same publisher uses the same scheme.

Schemes using reverse domain name notation are hardened against
collision. They are unlikely to clash with an officially registered
scheme [IANA.URISchemes] or unregistered de-facto scheme, as these
generally don't include a period character, and are unlikely to match
your domain name in any case. They are guaranteed not to clash with
any OAuth client following these naming guidelines under their control, expressed in full.

Some platforms use globally unique bundle or package names that
follow the
reverse domain name notation pattern. In these cases, the
app SHOULD register that bundle id order, as the custom scheme. If an app
has a bundle id or package name that doesn't match a domain name
under the control recommended by Section 3.8 of the app, the app SHOULD NOT register that as a
scheme, and instead create a [RFC7595] for
private-use URI scheme based off one of their domain
names. schemes.

For example, an app whose publisher owns that controls the top level domain name
"example.com" "app.example.com"
can register use "com.example.app:/" as their custom scheme. An app whose Some
authorization server issues servers assign client identifiers
that are also valid based on domain
names, for example
"client1234.usercontent.idp.com", "client1234.usercontent.example.net", which can use
also be used as the reverse domain name
notation of that domain as for the custom scheme, i.e.
"com.idp.usercontent.client1234:/". Each of these examples are when reversed
in the same manner, for example "net.example.usercontent.client1234".

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

Care must be taken when there are multiple apps by the same publisher can
assert ownership.

As a counter-example, using a simple custom
that each URI scheme like "myapp:/" is
not guaranteed to be unique and is NOT RECOMMENDED. 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 uniqueness, the collision resistant properties, basing the URI
scheme off a domain name that is under the control of the app's publisher app can
help to prove ownership in the event of a dispute where two apps register
claim the same custom scheme (such as if an app is acting
maliciously). For example, if two apps registered claimed "com.example.app:",
the true 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 chosen.

7.1.3. Registration of App Redirection URIs

As recommended in Section 3.1.2.2 of OAuth 2.0 [RFC6749], the
authorization server SHOULD require the client to pre-register the
redirection URI. This remains true for app redirection URIs that use
custom schemes.

Additionally, 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.

Authorizations servers SHOULD support the ability for native apps to
register Redirection URIs that utilize custom URI schemes.
Authorization servers SHOULD enforce the recommendation in
Section 7.1.2 that apps follow naming guidelines for URI schemes. used.

7.2. App-claimed HTTPS URI Redirection

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

Where the operating environment provided app-claimed

App-claimed HTTPS redirect URIs have some advantages in a
usable fashion, these URIs should be used as the OAuth redirect, as
they allow that the
identity of the destination app to be is guaranteed by the operating
system.

Apps on platforms that allow the user Due to disable this functionality,
present it in a user-unfriendly way, or lack it altogether MUST
fallback to using custom URI schemes.

The authorization server MUST allow reason, they SHOULD be used over the registration of other
redirect choices for native apps where possible.

App-claimed HTTPS redirect URIs for non-confidential function exactly as normal HTTPS
redirects from the perspective of the authorization server, though it
is RECOMMENDED that the authorization server is able to distinguish
between public native app clients to support app-
claimed that use app-claimed HTTPS redirect URIs.
URIs and confidential web clients. A flag in the client registration
information that indicates a public native app client is one such
method for distinguishing client types.

7.3. Loopback URI Redirection

More applicable to desktop

Desktop operating systems, some environments systems allow native apps to create listen on a local port
for HTTP listener on a random port, and receive
URI redirects that way. redirects. This is an acceptable redirect URI choice
for can be used by native apps to receive OAuth
authorization responses on compatible platforms.

Authorization servers SHOULD support

Loopback redirect URIs on take the loopback IP
address and HTTP scheme, that is, redirect URIs beginning with
http://127.0.0.1[:port]/, http://::1[:port]/, and
http://localhost[:port]/. Authorization servers supporting this class form of redirect URI MUST allow the client to specify a loopback IP, any port of their
choice, and SHOULD allow
(dynamically provided by the client to use an arbitrary client), and a path component.

While both the
Specifically: "http://127.0.0.1:{port}/{path}", and
"http://[::1]:{port}/{path}".

For loopback IP and localhost variants SHOULD be supported
by redirect URIs, the authorization server for completeness, it is RECOMMENDED that
apps primarily use MUST allow
any port to be specified at the loopback IP variant, as it is less susceptible time of the request, to misconfigured routing and client side firewalls Note accommodate
clients that obtain an available port from the HTTP
scheme is acceptable for this category operating system at
the time of redirect URIs, as the
request never leaves request. Other than that, the device. redirect is be treated
like any other.

8. Security Considerations

8.1. Embedded User-Agents

Embedded user-agents, commonly implemented with web-views, are an
alternative method for authorizing native apps. They are however
unsafe for use by third-parties by definition. They involve the user
signing in with their full login credentials, only to have them
downscoped to less powerful OAuth credentials.

Even when used by trusted first-party apps, embedded user-agents
violate the principle of least privilege by obtaining more powerful
credentials than they need, potentially increasing the attack
surface.

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.

Encouraging users to enter credentials in an embedded web-view
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
are, it trains them that it's OK to enter credentials without
validating the site first.

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

Due to the above,

Native apps MUST NOT use of embedded user-agents is NOT RECOMMENDED,
except where a trusted first-party app acts as the external user-
agent for other apps, or provides single sign-on for multiple first-
party apps. OAuth to third-
parties.

Authorization servers SHOULD consider taking MAY take steps to detect and block logins via
authorization requests in third-party embedded user-agents that are not their own, where
possible. 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, however code interception by a native app other
than the intended recipient may 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
Grant. This is not an issue for HTTPS redirection URIs (i.e.
standard web URLs) due to the fact the HTTPS URI scheme is enforced
by the authority (as defined by [RFC3986]), the domain name system,
which does not allow multiple entities to own the same domain.

If multiple apps register the same scheme, it is possible that the
authorization code will be sent to the wrong app (generally the
operating system makes no guarantee of which app will handle the URI
when multiple register the same scheme). PKCE [RFC7636] details how
this limitation can be used to execute a code interception attack
(see Figure 1). This attack vector applies to public clients
(clients that are unable to maintain a client secret) which is
typical of most native apps.

While Section 7.1.2 7.1.1 details ways that this can be mitigated through
policy enforcement (through being able to report and have removed any
offending apps), we can also protect the authorization code grant
from being used in cases where it was intercepted.

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.

Both the client and the Authorization Server MUST support PKCE
[RFC7636] to use custom URI schemes, or loopback IP redirects.
Authorization Servers SHOULD reject authorization requests using a
custom scheme, or loopback IP as part of the redirection URI if the
required PKCE parameters are not present, returning the error message
as defined in Section 4.4.1 of PKCE [RFC7636]. It is RECOMMENDED to
use PKCE [RFC7636] for app-claimed HTTPS redirect URIs, even though
these are not generally subject to interception, to protect against
attacks on inter-app communication.

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 App Redirection URIs

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.

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 true the case even for authorization servers that require frequent or
occasional or frequent re-authentication, as such servers can
preserve some user identifiable information from the old request, session,
like the email address or avatar. To help mitigate against phishing, it is RECOMMENDED to
show the user some hint profile picture and display that they were previously logged in, as an
attacking app would not be capable of doing this. on the re-
authentication.

Users who are particularly concerned about their security may also
take the additional step of opening the request in the system 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. This is not expected to be common user behavior,
however.

8.4.

8.7. Limitations of Non-verifiable Clients

As stated in Section 10.2 of RFC 6749, OAuth 2.0 [RFC6749], the authorization
server SHOULD NOT process authorization requests automatically
without user consent or interaction, except when the identity of the
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 platform-
specific identity features which may be used, as appropriate.

9. Other

8.8. Non-Browser External User Agents

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

10.

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 secret of all users. shared secret. For this reason
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 secret shared by
multiple installs of the app, secret, as this serves
little value beyond client identification which is already provided
by the client_id "client_id" request parameter. If an authorization server requires

Authorization servers that still require a client shared secret for native apps, it
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 assume RECOMMENDED to automatically issue tokens on the basis that it the
user has previously granted access to the same client, as there is actually
secret, unless some method no
guarantee that the client is being used not counterfeit.

8.10. Cross-App Request Forgery Protections

Section 5.3.5 of [RFC6819] recommends using the 'state' parameter to dynamically provision
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 secret to redirect URI is used for each installation.

11. References

11.1. Normative References

[RFC6749] Hardt, D., Ed., "The OAuth 2.0 different
authorization server used by the app (for example, by varying the
path component), and that authorization responses are rejected if the
redirect URI they were received on doesn't match the redirect URI in
a pending outgoing authorization request.

Authorization Framework",
RFC 6749, DOI 10.17487/RFC6749, October 2012,
<http://www.rfc-editor.org/info/rfc6749>.

[RFC7636] Sakimura, N., Ed., Bradley, J., servers SHOULD allow the registration of a specific
redirect URI, including path components, and N. Agarwal, "Proof Key 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 Code Exchange by inter-
app communication in OAuth Public Clients", RFC 7636,
DOI 10.17487/RFC7636, September 2015,
<http://www.rfc-editor.org/info/rfc7636>. 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>.

11.2.

[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>.

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>.

[iOS.URIScheme]
"Inter-App Communication", July 2016, <https://developer.a
pple.com/library/ios/documentation/iPhone/Conceptual/
iPhoneOSProgrammingGuide/Inter-AppCommunication/Inter-
AppCommunication.html>.

[macOS.URIScheme]
"Launch Services Concepts", July 2016, <https://developer.
apple.com/library/mac/documentation/Carbon/Conceptual/Laun
chServicesConcepts/LSCConcepts/LSCConcepts.html#//apple_re
f/doc/uid/TP30000999-CH202-CIHFEEAD>.

[Android.URIScheme]
"Intents

[AppAuth.iOSmacOS]
Wright, S., Denniss, W., and Intent Filters", July 2016,
<http://developer.android.com/guide/components/
intents-filters.html#ires>.

[WindowsUWP.URIScheme]
"Handle URI activation", July 2016,
<https://msdn.microsoft.com/en-us/windows/uwp/launch-
resume/handle-uri-activation>.

[IANA.URISchemes]
"Uniform Resource Identifier (URI) Schemes", July 2016,
<http://www.iana.org/assignments/uri-schemes/
uri-schemes.xhtml>.

[ChromeCustomTab]
"Chrome Custom Tabs", July 2016,
<https://developer.chrome.com/multidevice/android/
customtabs>.

[SFSafariViewController]
"SafariServices Changes", July others, "AppAuth for iOS and
macOS", February 2016,
<https://developer.apple.com/library/ios/documentation/
SafariServices/Reference/SFSafariViewController_Ref/>.

[Android.AppLinks]
"App Links", July 2015,
<https://developer.android.com/preview/features/app-
linking.html>.

[CustomTabsService]
"CustomTabsService", July <https://github.com/openid/AppAuth-
iOS>.

[AppAuth.Android]
McGinniss, I., Denniss, W., and others, "AppAuth for
Android", February 2016,
<https://developer.android.com/reference/android/support/
customtabs/CustomTabsService.html>.

[UniversalLinks]
"Universal Links", <https://github.com/openid/
AppAuth-Android>.

[SamplesForWindows]
Denniss, W., "OAuth for Apps: Samples for Windows", July
2016, <https://developer.apple.com
/library/ios/documentation/General/Conceptual/AppSearch/
UniversalLinks.html>. <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

Appendix B. Operating System Specific Implementation Details

Most of this document attempts to lay out defines best practices in an generic manner,
referencing technology techniques commonly available on most operating
systems. 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 authorship. 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.

A.1.

B.1. iOS Implementation Details

Claimed HTTPS and

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 both
viable choices
for OAuth on iOS. Developers 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 [UniversalLinks], available since iOS 9, feature with an entitlement file and can use custom URI
scheme [iOS.URIScheme] redirects for backwards compatibility.
Clients SHOULD use an association file on the
domain.

Universal Links for authorization requests are the preferred choice on iOS 9 and beyond, with the custom URI scheme redirect substituted on
older versions. In both cases, above due to
the app claims ownership proof that is provided by the redirect operating system.

A complete open source sample is included in the
application manifest.

As a user experience optimisation, since AppAuth for iOS 9, apps and
macOS [AppAuth.iOSmacOS] library.

B.2. Android Implementation Details

Apps can invoke initiate an authorization request in the
system browser without the
user leaving the app app, through
SFSafariViewController [SFSafariViewController], the Android Custom Tab feature which
implements the browser-view pattern. This class has all the properties of The user's default browser can
be used to handle requests when no browser supports Custom Tabs.

Android browser vendors should support the
system browser, and is Custom Tabs protocol (by
providing an 'external user-agent', even though it is
presented within the host app. Regardless implementation of whether the user
completes the request in "CustomTabsService" class), to
provide the system in-app browser (as is tab user experience optimization to their choice), or
the SFSafariViewController, the return of the token via custom URI
scheme or claimed HTTPS link
users. Chrome is one such browser that implements Custom Tabs.

To receive the same.

A.2. Android Implementation Details

Claimed HTTPS and authorization response, custom URI scheme redirects schemes are both viable choices
for OAuth on Android. Developers can claim broadly
supported through Android Implicit Intends. Claimed HTTPS links using redirect
URIs through Android App Links [Android.AppLinks], are available since on Android 6.0 though browser
support varies, and custom URI scheme [Android.URIScheme] redirects
are broadly supported. Clients SHOULD support custom URI scheme
redirects for broad compatibility and MAY upgrade to using claimed
HTTPs redirects in supported environments. For both redirect
options, the app claims the
above. Both types of redirect URIs are registered in the application
application's manifest.

As a user experience optimisation, apps SHOULD try to launch the
authorization request in a Custom Tab. Custom Tab

A complete open source sample is an
implementation of the browser-view pattern, providing a secure
browser tab displayed included in the context of the app. Chrome is an
example of a browser that supports [ChromeCustomTab] CustomTabs. AppAuth for Android Browser vendors SHOULD implement the CustomTabsService
[CustomTabsService] to provide this functionality to their users.

A.3.
[AppAuth.Android] library.

B.3. Windows Implementation Details

Apps written on the Universal Windows Platform (UWP) can claim initiate an authorization request in the user's default
browser using platform APIs for this purpose.

The custom URI schemes [WindowsUWP.URIScheme] in their application manifest.
This scheme redirect is a good choice for Universal Windows
Platform (UWP) apps as it will also open the app when returning the user taps right
back where they were. Known on UWP as URI Activation, the
link. The scheme is
limited to 39 characters, and but may include the
`.` character.

UWP apps can launch the authorization request "." character, making
short reverse domain name based schemes (as recommended in the user's default
browser like so:

Uri authorizationRequest = ...
var success = Windows.System.Launcher.LaunchUriAsync(authorizationRequest)
Section 7.1.1) possible.

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

Traditional apps

A complete open source sample is available [SamplesForWindows].

B.4. macOS Implementation Details

Apps can launch the URI initiate an authorization request in the user's default
browser
like so:

string authorizationRequest = ...
System.Diagnostics.Process.Start(authorizationRequest);

When using platform APIs for this purpose.

To receive the "Process.Start" method, care must be taken that the
input is authorization response, custom URI schemes are are a valid URL, including correct
good redirect URI encoding of choice on macOS, as the
parameters. This user is especially important when returned right back
to the URL includes user-
supplied app they launched the request from. These are registered in
the application's bundle information such as a login hint.

A.4. macOS Implementation Details

Both property list using the loopback
"CFBundleURLSchemes" key. Loopback IP and custom URI scheme redirect choices redirects are another viable
option, and listening on macOS. Custom URI schemes [macOS.URIScheme] are registered the loopback interface is allowed by default
firewall rules.

A complete open source sample is included in the application manifest. Listening on 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 IP typically
does not require any firewall changes.

Apps can launch redirect is the recommended redirect choice for desktop
apps on Linux to receive the authorization request like so:

NSURL *authorizationRequest = ...
BOOL success = [[NSWorkspace sharedWorkspace] openURL:authorizationRequest]; response.

Appendix B. 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:

Naveen Agarwal,

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

Authors' Addresses

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

Phone: +1 650-253-0000

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

John Bradley
Ping Identity

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