OAuth Working Group W. Denniss
Internet-Draft Google
Intended status: Best Current Practice J. Bradley
Expires: January 23, 2016 Ping Identity
July 22, 2015
OAuth 2.0 for Native Apps
draft-wdenniss-oauth-native-apps-00
Abstract
OAuth 2.0 authorization requests from native apps should only be made
through external user-agents such as the system 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 January 23, 2016.
Copyright Notice
Copyright (c) 2015 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 January 23, 2016 [Page 1]
Internet-Draft oauth_mobile July 2015
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Authorization Flow for Native Apps . . . . . . . . . . . 3
2. Notational Conventions . . . . . . . . . . . . . . . . . . . 4
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. The External User-Agent . . . . . . . . . . . . . . . . . . . 5
5. Redirection URIs for Native Apps . . . . . . . . . . . . . . 5
5.1. App-claimed HTTPS URI Redirection . . . . . . . . . . . . 5
5.2. App-declared Custom URI Scheme Redirection . . . . . . . 6
6. Security Considerations . . . . . . . . . . . . . . . . . . . 8
6.1. Embedded User-Agents . . . . . . . . . . . . . . . . . . 8
6.2. Protecting the Authorization Code . . . . . . . . . . . . 9
6.3. Claimed URLs and Phishing . . . . . . . . . . . . . . . . 10
6.4. Always Prompting for User Interaction . . . . . . . . . . 10
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
7.1. Normative References . . . . . . . . . . . . . . . . . . 10
7.2. Informative References . . . . . . . . . . . . . . . . . 11
Appendix A. Operating System Specific Implementation Details . . 12
A.1. iOS Implementation Details . . . . . . . . . . . . . . . 12
A.2. Android Implementation Details . . . . . . . . . . . . . 12
Appendix B. Acknowledgements . . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 13
1. Introduction
The OAuth 2.0 [RFC6749] authorization framework, documents two ways
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 (such as the system
browser) as the only secure and usable choice for OAuth2. It
documents how native apps can implement authorization flows with such
agents, and the additional requirements of authorization servers
needed to support such usage.
Many native apps today are using an embedded user-agent in the form
of a web-view. This approach suffers from several security and
usability issues including allowing the client app to eavesdrop user
credentials, and forcing users 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. Operating systems are increasingly
making the system browser even more viable for OAuth by allowing apps
Denniss & Bradley Expires January 23, 2016 [Page 2]
Internet-Draft oauth_mobile July 2015
to show a browser window within the active app, removing the only
usability benefit of using embedded browsers in the first place (not
wanting to send the user to another app).
Inter-app communication (such as that between a native OAuth client
and the system browser) can be achieved through app-specific custom
URI schemes and/or claimed HTTPS URLs. For example, an app can
launch the system browser with a HTTPS request (such as an OAuth
request), the browser can process the request and return control to
the app by simply following a URI using a scheme that the app
registered (for example "com.example.app:/oauth2callback?code=..."),
or a HTTPS path that the app claimed. Parameters can be passed
through these URIs, allowing complete use of OAuth flows, while
minimizing the added complexity for authorization servers to support
native apps.
1.1. Authorization Flow for Native Apps
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~+
| 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 | | 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 achieve authorization via an external user-agent.
Denniss & Bradley Expires January 23, 2016 [Page 3]
Internet-Draft oauth_mobile July 2015
1) The client app launches the system browser or browser-view 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 [PKCE]
proof of possession verifier
5) Server issues the tokens requested
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 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 product users.
"system browser" The operating system's native default browser,
typically pre-installed as part of the operating system, or
installed and set as default by the user. For example mobile
Safari on iOS, and Chrome on Android.
"web-view" A web browser UI component that can be embedded in apps
to render web pages, used to create embedded user-agents.
"browser-view" 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
Denniss & Bradley Expires January 23, 2016 [Page 4]
Internet-Draft oauth_mobile July 2015
browser. Goes by different names on different platforms, such as
SFSafariViewController on iOS 9, and Chrome Custom Tab in Chrome
for Android.
"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 apps on a
device.
4. The External User-Agent
The external user-agent for native apps can be the system browser, or
a native app provided by the authorization server.
Both the system browser and authorization server app affords several
advantages for OAuth over embedded web-view based user-agents,
including the security of a separate process, and usability of a
shared authentication session.
The system browser is the RECOMMENDED external user-agent choice for
most authorization servers, as it reduces implementation complexity
by reusing the web authorization endpoint, and is often needed as a
fallback even when an authorization server app is available.
5. Redirection URIs for Native Apps
5.1. App-claimed HTTPS URI Redirection
Several operating systems support a method for an app to claim a
regular HTTPS URL. When such a URL is loaded in the browser, instead
of the request being made and the page loaded, the native app is
launched instead.
On operating systems that support app-claimed HTTPS URIs, these URIs
SHOULD be used with OAuth, 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,
or lack it altogether MUST fallback to using custom URI schemes.
Denniss & Bradley Expires January 23, 2016 [Page 5]
Internet-Draft oauth_mobile July 2015
The authorization server MUST allow the registration of HTTPS
redirect URIs for non-confidential native clients to support app-
claimed HTTPS redirect URIs.
5.2. 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.
5.2.1. 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.
5.2.2. 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
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.
Denniss & Bradley Expires January 23, 2016 [Page 6]
Internet-Draft oauth_mobile July 2015
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.
5.2.3. Registration of App Redirection URIs
As recommended in Section 3.1.2.2 of [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.
Denniss & Bradley Expires January 23, 2016 [Page 7]
Internet-Draft oauth_mobile July 2015
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 5.2.2 that apps follow naming guidelines for URI schemes.
6. Security Considerations
6.1. 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.
The only use-case where it is reasonable to use an embedded user-
agent is when the app itself is a trusted and secure first-party app
that acts as the external user-agent for other apps. Use of embedded
user-agents by first party apps other than those that act as an
external user-agent themselves is NOT RECOMMENDED, as it increases
development complexity and the potential to introduce security
issues, and hampers the potential for usability improvements through
taking advantage of the shared authentication context.
Denniss & Bradley Expires January 23, 2016 [Page 8]
Internet-Draft oauth_mobile July 2015
Authorization servers SHOULD consider taking steps to detect and
block logins via embedded user-agents that are not their own, where
possible.
6.2. 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]), being the domain name system, which does not
allow multiple entities to own a single 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). Figure 1 of [PKCE]
demonstrates the code interception attack. This attack vector
applies to public clients (clients that are unable to maintain a
client secret) which is typical of most installed apps.
While Section 5.2.2 mentions ways that this can be mitigated through
policy enforcement (by being able to request that the offending app
is removed), 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) [PKCE]
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.
Both the client and the Authorization Server MUST support PKCE [PKCE]
to use custom URI schemes. Authorization Servers SHOULD reject
requests that use a custom scheme in the redirection URI if the
required PKCE parameters are not also present, returning the error
message as defined in Section 4.4.1 of [PKCE]
PKCE provides proof of possession by the client generating 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.
Denniss & Bradley Expires January 23, 2016 [Page 9]
Internet-Draft oauth_mobile July 2015
6.3. Claimed URLs and Phishing
While using a claimed HTTPS URI for redirection in the system browser
guarantees the identity of the receiving app, it is still possible
for a bad app to put the user through an authentication flow in an
embedded user-agent of their own, and observe the redirect URI.
We can't directly prevent this, however it can be mitigated through
user contextual awareness. Such an attack necessarily starts with no
authentication state, meaning that the user will be prompted to sign-
in. If all native apps are using the techniques described here,
users should not be signing-in frequently, and thus should treat any
password request event with more suspicion. Sophisticated users will
be able to recognise the UI treatment of the browser-view or full
system browser, and shouldn't sign-in anywhere else. Users who are
particularly security conscious can also use the "open in browser"
functionality from the browser-view to gain even more assurances
about where they are entering their credentials.
6.4. Always Prompting for User Interaction
Due to the fact that the identity of non-confidential clients cannot
be assured, tokens SHOULD NOT be issued to such clients without user
consent or interaction, even if the the user has consented to the
scopes and approved the client previously.
7. References
7.1. 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>.
[PKCE] Sakimura, N., Ed., Bradley, J., and N. Agarwal, "The Proof
Key for Code Exchange by OAuth Public Clients", February
2015, <https://tools.ietf.org/html/draft-ietf-oauth-spop>.
[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>.
Denniss & Bradley Expires January 23, 2016 [Page 10]
Internet-Draft oauth_mobile July 2015
7.2. 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", 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>.
Denniss & Bradley Expires January 23, 2016 [Page 11]
Internet-Draft oauth_mobile July 2015
[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>.
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 valid 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.
Denniss & Bradley Expires January 23, 2016 [Page 12]
Internet-Draft oauth_mobile July 2015
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 5.2.
The following individuals contributed ideas, feedback, and wording
that shaped and formed the final specification:
Naveen Agarwal, John Bradley, Brian Campbell, Adam Dawes, Ashish
Jain, Paul Madsen, Breno de Medeiros, Eric Sachs, Nat Sakimura, Steve
Wright.
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: +44 20 8133 3718
Email: ve7jtb@ve7jtb.com
URI: http://www.thread-safe.com/
Denniss & Bradley Expires January 23, 2016 [Page 13]