Internet DRAFT - draft-wdenniss-oauth-native-apps
draft-wdenniss-oauth-native-apps
OAuth Working Group W. Denniss
Internet-Draft Google
Intended status: Best Current Practice J. Bradley
Expires: August 7, 2016 Ping Identity
February 04, 2016
OAuth 2.0 for Native Apps
draft-wdenniss-oauth-native-apps-02
Abstract
OAuth 2.0 authorization requests from native apps should only be made
through external user-agents such as the system browser (including
via an in-app browser tab). 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 August 7, 2016.
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
Denniss & Bradley Expires August 7, 2016 [Page 1]
Internet-Draft oauth_mobile February 2016
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Notational Conventions . . . . . . . . . . . . . . . . . 3
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
1.3. Overview . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Using Inter-app URI Communication for OAuth . . . . . . . . . 6
3. Initiating the Authorization Request . . . . . . . . . . . . 6
4. Receiving the Authorization Response . . . . . . . . . . . . 7
4.1. App-declared Custom URI Scheme Redirection . . . . . . . 7
4.2. App-claimed HTTPS URI Redirection . . . . . . . . . . . . 9
4.3. Localhost-based URI Redirection . . . . . . . . . . . . . 9
5. Security Considerations . . . . . . . . . . . . . . . . . . . 10
5.1. Embedded User-Agents . . . . . . . . . . . . . . . . . . 10
5.2. Protecting the Authorization Code . . . . . . . . . . . . 11
5.3. Phishing . . . . . . . . . . . . . . . . . . . . . . . . 12
5.4. Limitations of Non-verifiable Clients . . . . . . . . . . 12
6. Other External User Agents . . . . . . . . . . . . . . . . . 12
7. Client Authentication . . . . . . . . . . . . . . . . . . . . 13
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 13
8.1. Normative References . . . . . . . . . . . . . . . . . . 13
8.2. Informative References . . . . . . . . . . . . . . . . . 13
Appendix A. Operating System Specific Implementation Details . . 15
A.1. iOS Implementation Details . . . . . . . . . . . . . . . 15
A.2. Android Implementation Details . . . . . . . . . . . . . 15
Appendix B. Acknowledgements . . . . . . . . . . . . . . . . . . 15
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 16
1. Introduction
The OAuth 2.0 [RFC6749] authorization 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.
This document recommends external user-agents like in-app browser
tabs as the only secure and usable choice for OAuth. It documents
how native apps can implement authorization flows with such agents,
and the additional requirements of authorization servers needed to
support such usage.
Denniss & Bradley Expires August 7, 2016 [Page 2]
Internet-Draft oauth_mobile February 2016
1.1. 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.
1.2. Terminology
In addition to the terms defined in referenced specifications, this
document uses the following terms:
"app" A native application, such as one on a mobile device or
desktop operating system.
"app store" An ecommerce store where users can download and purchase
apps. Typically with quality-control measures to protect users
from malicious developers.
"system 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, and Chrome Custom Tab on Android.
"Claimed HTTPS URL" Some platforms allow apps to claim a domain name
by hosting a file that proves the link between site and app.
Typically this means that URLs opened by the system will be 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
Denniss & Bradley Expires August 7, 2016 [Page 3]
Internet-Draft oauth_mobile February 2016
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 apps on a
device.
"OAuth" In this document, OAuth refers to OAuth 2.0 [RFC6749].
1.3. Overview
At the time of writing, many native apps are still using web-views, a
type of embedded user-agent, for OAuth. That approach has multiple
drawbacks, including the client app being able to eavesdrop user
credentials, and is a suboptimal user experience as the
authentication session can't be shared, and users need to sign-in to
each app separately.
OAuth flows between a native app and the system browser (or another
external user-agent) are more secure, and take advantage of the
shared authentication state to enable single sign-on. The in-app
browser tab pattern makes this approach even more viable, as apps can
present the system browser without the user switching context
something that could previously only be achieved by a web-view on
most platforms.
Inter-process communication, such as OAuth 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
OAuth flows between RP and IDP websites, OAuth can be used for native
app auth with very little modification.
1.3.1. Authorization Flow for Native Apps
Denniss & Bradley Expires August 7, 2016 [Page 4]
Internet-Draft oauth_mobile February 2016
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~+
| User Device |
| |
| +---------------------------+ | +-----------+
| | | | (4) Authz Grant | |
| | Client App |----------------------->| Authz |
| | |<-----------------------| Server |
| +---------------------------+ | (5) Access Token | |
| | ^ | +-----------+
| | | |
| | | |
| | (1) | (3) |
| | Authz | Authz |
| | Request | Grant |
| | "https://" | "app:/" |
| | | |
| v | |
| +---------------------------+ | +-----------+
| | | | (2) User | |
| | System Browser Tab | | authenticated | Identity |
| | |<---------------------->| Provider |
| +---------------------------+ | | |
| | +-----------+
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~+
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 system browser with the authorization
request (e.g. https://idp.example.com/oauth2/auth...)
2) Server authenticates the end-user, potentially chaining to another
authentication system, and issues Authorization Code Grant on
success
3) Browser switches focus back to the client app using a URI with a
custom scheme or claimed HTTPS URL, passing the code as a URI
parameter.
4) Client presents the OAuth 2.0 authorization code and PKCE
[RFC7636] proof of possession verifier.
5) Server issues the tokens requested.
Denniss & Bradley Expires August 7, 2016 [Page 5]
Internet-Draft oauth_mobile February 2016
2. 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 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 apps use the URI-based communication
functionality of the operating system to perform OAuth flows in an
external user-agent, typically the system browser.
For usability, it is RECOMMENDED that native apps perform OAuth using
the system browser by presenting an in-app browser tab where
possible. This affords the benefits of the system browser, while
allowing the user to remain in the app.
It is possible to create an external user-agent for OAuth that is a
native app provided by the authorization server, as opposed to the
system browser. This approach shares a lot of similarity with using
the system browser as both 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.
3. Initiating the Authorization Request
The authorization request is created as per OAuth 2.0 [RFC6749], and
opened in the system browser. Where the operating system supports
in-app browser tabs, those should be preferred over switching to the
system browser, to improve usability.
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 to the OAuth client's
server, it returns it to the native app. The various options for a
redirect URI that will return the code to the native app are
Denniss & Bradley Expires August 7, 2016 [Page 6]
Internet-Draft oauth_mobile February 2016
documented in Section 4. Any redirect URI that allows the app to
receive the URI and inspect its parameters is viable.
4. Receiving the Authorization Response
There are three main approaches to redirection URIs for native apps:
custom URI schemes, app-claimed HTTP URI schemes, and
http://localhost redirects.
4.1. App-declared Custom URI Scheme Redirection
Most major mobile and desktop computing platforms support inter-app
communication via URIs by allowing apps to register custom URI
schemes. When the system browser or another app attempts to follow 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 the
mobile platforms Android [Android.URIScheme], iOS [iOS.URIScheme],
and Windows Phone [WindowsPhone.URIScheme]. Desktop operating
systems Windows [Windows.URIScheme] and OS X [OSX.URIScheme] also
support custom URI schemes.
4.1.1. 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 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 the Authorization Code like a regular
OAuth 2.0 client.
4.1.2. Custom URI Scheme Namespace Considerations
When selecting which 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 a scheme can be based on
Denniss & Bradley Expires August 7, 2016 [Page 7]
Internet-Draft oauth_mobile February 2016
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 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 as the custom scheme. If an app
has a bundle id or package name that doesn't match a domain name
under the control of the app, the app SHOULD NOT register that as a
scheme, and instead create a URI scheme based off one of their domain
names.
For example, an app whose publisher owns the top level domain name
"example.com" can register "com.example.app:/" as their custom
scheme. An app whose authorization server issues client identifiers
that are also valid domain names, for example
"client1234.usercontent.idp.com", can use the reverse domain name
notation of that domain as the scheme, i.e.
"com.idp.usercontent.client1234:/". Each of these examples are URI
schemes which are likely to be unique, and where the publisher can
assert ownership.
As a counter-example, using a simple custom scheme like "myapp:/" is
not guaranteed to be unique and is NOT RECOMMENDED.
In addition to uniqueness, basing the URI scheme off a name that is
under the control of the app's publisher can help to prove ownership
in the event of a dispute where two apps register the same custom
scheme (such as if an app is acting maliciously). For example, if
two apps registered "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.
4.1.3. 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
Denniss & Bradley Expires August 7, 2016 [Page 8]
Internet-Draft oauth_mobile February 2016
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 4.1.2 that apps follow naming guidelines for URI schemes.
4.2. App-claimed HTTPS URI Redirection
Some operating systems allow apps to claim HTTPS URLs of their
domains. When the browser sees such a claimed URL, instead of the
page being loaded in the browser, the native app is launched instead
with the URL given as input.
Where the operating environment provided app-claimed HTTPS URIs in a
usable fashion, these URIs should be used as the OAuth redirect, as
they allow the identity of the destination app to be guaranteed by
the operating system.
Apps on platforms that allow the user 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 the registration of HTTPS
redirect URIs for non-confidential native clients to support app-
claimed HTTPS redirect URIs.
4.3. Localhost-based URI Redirection
More applicable to desktop operating systems, some environments allow
the app to create a local server and listen for redirect URIs that.
This is an acceptable redirect URI choice for native apps on
compatible platforms.
Authorization servers SHOULD support redirect URIs on the localhost
host, and HTTP scheme, that is redirect URIs beginning with
http://localhost (NB. in this case, HTTP is acceptable, as the
request never leaves the device).
Denniss & Bradley Expires August 7, 2016 [Page 9]
Internet-Draft oauth_mobile February 2016
When an app is registered with such a redirect, it SHOULD be able to
specify any port in the authorization request, meaning that a request
with http://localhost:*/* as the redirect should be considered valid.
5. Security Considerations
5.1. 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 other identity 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, 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.
Authorization servers SHOULD consider taking steps to detect and
block logins via embedded user-agents that are not their own, where
possible.
Denniss & Bradley Expires August 7, 2016 [Page 10]
Internet-Draft oauth_mobile February 2016
5.2. Protecting the Authorization Code
A limitation of custom URI schemes 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 4.1.2 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 localhost redirects.
Authorization Servers SHOULD reject authorization requests using a
custom scheme, or localhost 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.
Denniss & Bradley Expires August 7, 2016 [Page 11]
Internet-Draft oauth_mobile February 2016
5.3. Phishing
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 even for authorization servers that require frequent or
occasional re-authentication, as such servers can preserve some user
identifiable information from the old request, like the email address
or avatar. To help mitigate against phishing, it is RECOMMENDED to
show the user some hint that they were previously logged in, as an
attacking app would not be capable of doing this.
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.
5.4. Limitations of Non-verifiable Clients
As stated in Section 10.2 of RFC 6749, 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
identity features which may be used, as appropriate.
6. Other External User Agents
This best practice recommends a particular type of external user-
agent: the in-app browser tab. Other external user-agents patterns
may also be viable for secure and usable OAuth. This document makes
no comment on those patterns.
Denniss & Bradley Expires August 7, 2016 [Page 12]
Internet-Draft oauth_mobile February 2016
7. 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. For
this reason it is NOT RECOMMENDED for authorization servers to
require client authentication of native apps using a secret shared by
multiple installs of the app, as this serves no value beyond client
identification which is already provided by the client_id request
parameter. If an authorization server requires a client secret for
native apps, it MUST NOT assume that it is actually secret, unless
some method is being used to dynamically provision a unique secret to
each installation.
8. References
8.1. Normative References
[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>.
[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>.
[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>.
8.2. 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>.
Denniss & Bradley Expires August 7, 2016 [Page 13]
Internet-Draft oauth_mobile February 2016
[iOS.URIScheme]
"Inter-App Communication", February 2015, <https://develop
er.apple.com/library/ios/documentation/iPhone/Conceptual/
iPhoneOSProgrammingGuide/Inter-AppCommunication/Inter-
AppCommunication.html>.
[OSX.URIScheme]
"Launch Services Concepts", February 2015, <https://develo
per.apple.com/library/mac/documentation/Carbon/Conceptual/
LaunchServicesConcepts/LSCConcepts/LSCConcepts.html#//appl
e_ref/doc/uid/TP30000999-CH202-CIHFEEAD>.
[Android.URIScheme]
"Intents and Intent Filters", February 2015,
<http://developer.android.com/guide/components/
intents-filters.html#ires>.
[WindowsPhone.URIScheme]
"Auto-launching apps using file and URI associations for
Windows Phone 8", February 2015,
<https://msdn.microsoft.com/en-us/library/windows/apps/
jj206987(v=vs.105).aspx>.
[Windows.URIScheme]
"Registering an Application to a URI Scheme", February
2015, <https://msdn.microsoft.com/en-us/library/ie/
aa767914%28v=vs.85%29.aspx>.
[IANA.URISchemes]
"Uniform Resource Identifier (URI) Schemes", February
2015, <http://www.iana.org/assignments/uri-schemes/
uri-schemes.xhtml>.
[ChromeCustomTab]
"Chrome Custom Tabs", July 2015,
<https://developer.chrome.com/multidevice/android/
customtabs>.
[SFSafariViewController]
"SafariServices Changes", July 2015, <https://developer.ap
ple.com/library/prerelease/ios/releasenotes/General/
iOS90APIDiffs/frameworks/SafariServices.html>.
[Android.AppLinks]
"App Links", July 2015,
<https://developer.android.com/preview/features/app-
linking.html>.
Denniss & Bradley Expires August 7, 2016 [Page 14]
Internet-Draft oauth_mobile February 2016
Appendix A. Operating System Specific Implementation Details
Most of this document attempts to lay out best practices in an
generic manner, referencing technology available on most operating
systems. This non-normative section contains OS-specific
implementation details that are accurate at the time of authorship.
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 for longer.
A.1. iOS Implementation Details
From iOS 9, apps can invoke the system browser without the user
leaving the app through SFSafariViewController
[SFSafariViewController], which implements the browser-view pattern.
This class has all the properties of the system browser, and is
considered an 'external user-agent', even though it is presented
within the host app. Regardless of whether the system browser is
opened, or SFSafariViewController, the return of the token goes
through the same system.
A.2. Android Implementation Details
Chrome 45 introduced the concept of Chrome Custom Tab
[ChromeCustomTab], which follows the browser-view pattern and allows
authentication without the user leaving the app.
The return of the token can go through the custom URI scheme or
claimed HTTPS URI (including those registered with the App Link
[Android.AppLinks] system), or the navigation events can be observed
by the host app. It is RECOMMENDED that the custom URI, or claimed
HTTPS URI options be used for better portability, to allow the user
to open the authorization request in the Chrome app, and to prevent
accidental observation of intermediate tokens on URI parameters.
At the time of writing, Android does allow apps to claim HTTPs links
(App Links), but not in a way that is usable for OAuth, the native
app is only opened if the intent is fired from outside the browser.
Appendix B. 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 4.1.
The following individuals contributed ideas, feedback, and wording
that shaped and formed the final specification:
Denniss & Bradley Expires August 7, 2016 [Page 15]
Internet-Draft oauth_mobile February 2016
Naveen Agarwal, John Bradley, Brian Campbell, Adam Dawes, Hannes
Tschofenig, Ashish Jain, Paul Madsen, Breno de Medeiros, Eric Sachs,
Nat Sakimura, Steve Wright, Erik Wahlstrom, Andy Zmolek.
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/
John Bradley
Ping Identity
Phone: +1 202-630-5272
Email: ve7jtb@ve7jtb.com
URI: http://www.thread-safe.com/
Denniss & Bradley Expires August 7, 2016 [Page 16]