OAuth Working Group W. Denniss
Internet-Draft Google
Intended status: Best Current Practice J. Bradley
Expires: April 15, 2017 Ping Identity
October 12, 2016
OAuth 2.0 for Native Apps
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 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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provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on April 15, 2017.
Copyright Notice
Copyright (c) 2016 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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include Simplified BSD License text as described in Section 4.e of
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Notational Conventions . . . . . . . . . . . . . . . . . . . 3
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 4
4.1. Authorization Flow for Native Apps Using the Browser . . 5
5. Using Inter-app URI Communication for OAuth . . . . . . . . . 6
6. Initiating the Authorization Request from a Native App . . . 6
7. Receiving the Authorization Response in a Native App . . . . 7
7.1. App-declared Custom URI Scheme Redirection . . . . . . . 7
7.2. App-claimed HTTPS URI Redirection . . . . . . . . . . . . 8
7.3. Loopback URI Redirection . . . . . . . . . . . . . . . . 8
8. Security Considerations . . . . . . . . . . . . . . . . . . . 9
8.1. Embedded User-Agents . . . . . . . . . . . . . . . . . . 9
8.2. Protecting the Authorization Code . . . . . . . . . . . . 10
8.3. Loopback Redirect Considerations . . . . . . . . . . . . 11
8.4. Registration of App Redirection URIs . . . . . . . . . . 11
8.5. OAuth Implicit Flow . . . . . . . . . . . . . . . . . . . 11
8.6. Phishability of In-App Browser Tabs . . . . . . . . . . . 12
8.7. Limitations of Non-verifiable Clients . . . . . . . . . . 12
8.8. Non-Browser External User Agents . . . . . . . . . . . . 12
8.9. Client Authentication . . . . . . . . . . . . . . . . . . 13
8.10. Cross-App Request Forgery Protections . . . . . . . . . . 13
8.11. Authorization Server Mix-Up Mitigation . . . . . . . . . 13
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 14
10.1. Normative References . . . . . . . . . . . . . . . . . . 14
10.2. Informative References . . . . . . . . . . . . . . . . . 14
Appendix A. Server Support Checklist . . . . . . . . . . . . . . 15
Appendix B. Operating System Specific Implementation Details . . 15
B.1. iOS Implementation Details . . . . . . . . . . . . . . . 16
B.2. Android Implementation Details . . . . . . . . . . . . . 16
B.3. Windows Implementation Details . . . . . . . . . . . . . 16
B.4. macOS Implementation Details . . . . . . . . . . . . . . 17
B.5. Linux Implementation Details . . . . . . . . . . . . . . 17
Appendix C. Acknowledgements . . . . . . . . . . . . . . . . . . 17
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 18
1. Introduction
The OAuth 2.0 [RFC6749] authorization framework documents two
approaches in Section 9 for native apps to interact with the
authorization endpoint: an embedded user-agent, or an external user-
agent.
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This best current practice recommends that only external user-agents
like the browser are used for OAuth by native apps. It documents how
native apps can implement authorization flows using the browser as
the preferred external user-agent, and the requirements for
authorization servers to support such usage.
This practice is also known as the AppAuth pattern, in reference to
open source libraries that implement it.
2. Notational Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in Key
words for use in RFCs to Indicate Requirement Levels [RFC2119]. If
these words are used without being spelled in uppercase then they are
to be interpreted with their normal natural language meanings.
3. Terminology
In addition to the terms defined in referenced specifications, this
document uses the following terms:
"native app" An application that is installed by the user to their
device, as distinct from a web app that runs in the browser
context only. Apps implemented using web-based technology but
distributed as a native app, so-called hybrid apps, are considered
equivalent to native apps for the purpose of this specification.
"OAuth" In this document, OAuth refers to OAuth 2.0 [RFC6749].
"app" Shorthand for "native app".
"app store" An ecommerce store where users can download and purchase
apps.
"authz" Abbreviation of "authorization".
"browser" The operating system's default browser, pre-installed as
part of the operating system, or installed and set as default by
the user.
"browser tab" An open page of the browser. Browser typically have
multiple "tabs" representing various open pages.
"in-app browser tab" A full page browser with limited navigation
capabilities that is displayed inside a host app, but retains the
full security properties and authentication state of the browser.
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Has different platform-specific product names, such as
SFSafariViewController on iOS, and Chrome Custom Tab on Android.
"inter-app communication" Communication between two apps on a
device.
"claimed HTTPS URL" Some platforms allow apps to claim a HTTPS URL
after proving ownership of the domain name. URLs claimed in such
a way are then opened in the app instead of the browser.
"custom URI scheme" A URI scheme (as defined by [RFC3986]) that the
app creates and registers with the OS (and is not a standard URI
scheme like "https:" or "tel:"). Requests to such a scheme
results in the app which registered it being launched by the OS.
"web-view" A web browser UI component that can be embedded in apps
to render web pages, used to create embedded user-agents.
"reverse domain name notation" A naming convention based on the
domain name system, but where where the domain components are
reversed, for example "app.example.com" becomes "com.example.app".
4. Overview
The best current practice for authorizing users in native apps is to
perform the OAuth authorization request in a browser (an external
user-agent), rather than web-view (an embedded user-agent).
Previously it was common for native apps to use web-views for OAuth
authorization requests. That approach has many drawbacks, typically
including the host app being able to copy user credentials and
cookies, and the user needing to authenticate from scratch in each
app. See Section 8.1 for a deeper analysis of using embedded user-
agents for OAuth.
Native app authorization requests that use the browser are more
secure and can take advantage of the user's authentication state.
Being able to use the existing authentication session in the browser
enables single sign-on, as users don't need to authenticate to the
authorization server each time they use a new app (unless required by
authorization server policy).
Supporting authorization flows between a native app and the browser
is possible without changing the OAuth protocol itself, as the
authorization request and response are already defined in terms of
URIs, which emcompasses URIs that can be used for inter-process
communication. Some OAuth server implementations that assume all
clients are confidential web-clients will need to add an
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understanding of native app OAuth clients and the types of redirect
URIs they use to support this best practice.
4.1. Authorization Flow for Native Apps Using the Browser
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~+
| User Device |
| |
| +---------------------------+ | +-----------+
| | | | (5) Authz Code | |
| | Client App |----------------------->| Token |
| | |<-----------------------| Endpoint |
| +---------------------------+ | (6) Access Token, | |
| | ^ | Refresh Token +-----------+
| | | |
| | | |
| | (1) | (4) |
| | Authz | Authz |
| | Request | Code |
| | | |
| | | |
| v | |
| +---------------------------+ | +---------------+
| | | | (2) Authz Request | |
| | Browser |--------------------->| Authorization |
| | |<---------------------| Endpoint |
| +---------------------------+ | (3) Authz Code | |
| | +---------------+
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~+
Figure 1: Native App Authorization via External User-agent
Figure 1 illustrates the interaction of the native app with the
system browser to authorize the user via an external user-agent.
1) The client app opens a browser tab with the authorization request.
2) Authorization endpoint receives the authorization request,
authenticates the user and obtains authorization. Authenticating
the user may involve chaining to other authentication systems.
3) Authorization server issues an authorization code to the redirect
URI.
4) Client receives the authorization code from the redirect URI.
5) Client app presents the authorization code at the token endpoint.
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6) Token endpoint validates the authorization code and issues the
tokens requested.
5. Using Inter-app URI Communication for OAuth
Just as URIs are used for OAuth 2.0 [RFC6749] on the web to initiate
the authorization request and return the authorization response to
the requesting website, URIs can be used by native apps to initiate
the authorization request in the device's browser and return the
response to the requesting native app.
By applying the same principles from the web to native apps, we gain
similar benefits like the usability of a single sign-on session, and
the security of a separate authentication context. It also reduces
the implementation complexity by reusing the same flows as the web,
and increases interoperability by relying on standards-based web
flows that are not specific to a particular platform.
Native apps MUST use an external user-agent to perform OAuth
authentication requests. This is achieved by opening the
authorization request in the browser (detailed in Section 6), and
using a redirect URI that will return the authorization response back
to the native app, as defined in Section 7.
This best practice focuses on the browser as the RECOMMENDED external
user-agent for native apps. Other external user-agents, such as a
native app provided by the authorization server may meet the criteria
set out in this best practice, including using the same redirection
URI properties, but their use is out of scope for this specification.
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 user's browser 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 response to the OAuth
client's server, the redirect URI used by a native app returns the
response to the app. The various options for a redirect URI that
will return the code to the native app are documented in Section 7.
Any redirect URI that allows the app to receive the URI and inspect
its parameters is viable.
Some platforms support a browser feature known as in-app browser
tabs, where an app can present a tab of the browser within the app
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context without switching apps, but still retain key benefits of the
browser such as a shared authentication state and security context.
On platforms where they are supported, it is RECOMMENDED for
usability reasons that apps use in-app browser tabs for the
Authorization Request.
7. Receiving the Authorization Response in a Native App
There are several redirect URI options available to native apps for
receiving the authorization response from the browser, the
availability and user experience of which varies by platform.
To fully support this best practice, authorization servers MUST
support the following three redirect URI options. Native apps MAY
use whichever redirect option suits their needs best, taking into
account platform specific implementation details.
7.1. App-declared Custom URI Scheme Redirection
Many mobile and desktop computing platforms support inter-app
communication via URIs by allowing apps to register custom URI
schemes, like "com.example.app:". When the browser or another app
attempts to load a URI with a custom scheme, the app that registered
it is launched to handle the request.
To perform an OAuth 2.0 Authorization Request on a supported
platform, the native app launches the browser with a normal OAuth 2.0
Authorization Request, but provides a redirection URI that utilizes a
custom URI scheme registered with the operating system by the calling
app.
When the authentication server completes the request, it redirects to
the client's redirection URI like it would any redirect URI, but as
the redirection URI uses a custom scheme, this results in the OS
launching the native app passing in the URI. The native app then
processes the authorization response like any OAuth client.
7.1.1. Custom URI Scheme Namespace Considerations
When choosing a URI scheme to associate with the app, apps MUST use a
URI scheme based on a domain name under their control, expressed in
reverse order, as recommended by Section 3.8 of [RFC7595] for
private-use URI schemes.
For example, an app that controls the domain name "app.example.com"
can use "com.example.app:/" as their custom scheme. Some
authorization servers assign client identifiers based on domain
names, for example "client1234.usercontent.example.net", which can
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also be used as the domain name for the custom scheme, when reversed
in the same manner, for example "net.example.usercontent.client1234".
URI schemes not based on a domain name (for example "myapp:/") MUST
NOT be used, as they are not collision resistant, and don't comply
with Section 3.8 of [RFC7595].
Care must be taken when there are multiple apps by the same publisher
that each URI scheme is unique within that group. On platforms that
use app identifiers that are also based on reverse order domain
names, those can be re-used as the custom URI scheme for the OAuth
redirect.
In addition to the collision resistant properties, basing the URI
scheme off a domain name that is under the control of the app can
help to prove ownership in the event of a dispute where two apps
claim the same custom scheme (such as if an app is acting
maliciously). For example, if two apps claimed "com.example.app:",
the owner of "example.com" could petition the app store operator to
remove the counterfeit app. This petition is harder to prove if a
generic URI scheme was used.
7.2. App-claimed HTTPS URI Redirection
Some operating systems allow apps to claim HTTPS URLs in their
domains. When the browser encounters a claimed URL, instead of the
page being loaded in the browser, the native app is launched instead
with the URL supplied as input.
App-claimed HTTPS redirect URIs have some advantages in that the
identity of the destination app is guaranteed by the operating
system. Due to this reason, they SHOULD be used over the other
redirect choices for native apps where possible.
App-claimed HTTPS redirect URIs function 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 that use app-claimed HTTPS redirect
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
Desktop operating systems allow native apps to listen on a local port
for HTTP redirects. This can be used by native apps to receive OAuth
authorization responses on compatible platforms.
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Loopback redirect URIs take the form of the loopback IP, any port
(dynamically provided by the client), and a path component.
Specifically: "http://127.0.0.1:{port}/{path}", and
"http://[::1]:{port}/{path}".
For loopback IP redirect URIs, the authorization server MUST allow
any port to be specified at the time of the request, to accommodate
clients that obtain an available port from the operating system at
the time of the request. Other than that, the redirect is be treated
like any other.
8. Security Considerations
8.1. Embedded User-Agents
Embedded user-agents, 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 browser, requiring the
user to login for every authorization request and leading to a poor
user experience.
Native apps MUST NOT use embedded user-agents for OAuth to third-
parties.
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Authorization servers MAY take steps to detect and block
authorization requests in third-party embedded user-agents.
8.2. Protecting the Authorization Code
The redirect URI options documented in Section 7 share the benefit
that only a native app on the same device can receive the
authorization code, 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 7.1.1 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
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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.
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Tokens granted via the implicit flow also cannot be refreshed without
user interaction making the code flow, with refresh tokens the more
practical option for native app authorizations that require
refreshing.
8.6. Phishability of In-App Browser Tabs
While in-app browser tabs provide a secure authentication context, as
the user initiates the flow from a native app, it is possible for
that native app to completely fake an in-app browser tab.
This can't be prevented directly - once the user is in the native
app, that app is fully in control of what it can render, however
there are several mitigating factors.
Importantly, such an attack that uses a web-view to fake an in-app
browser tab will always start with no authentication state. If all
native apps use the techniques described in this best practice, users
will not need to sign-in frequently and thus should be suspicious of
any sign-in request when they should have already been signed-in.
This is the case even for authorization servers that require
occasional or frequent re-authentication, as such servers can
preserve some user identifiable information from the old session,
like the email address or profile picture and display that on the re-
authentication.
Users who are particularly concerned about their security may also
take the additional step of opening the request in the browser from
the in-app browser tab, and completing the authorization there, as
most implementations of the in-app browser tab pattern offer such
functionality.
8.7. Limitations of Non-verifiable Clients
As stated in Section 10.2 of OAuth 2.0 [RFC6749], the authorization
server SHOULD NOT process authorization requests automatically
without user consent or interaction, except when the identity of the
client can be assured. Measures such as claimed HTTPS redirects can
be used by native apps to prove their identity to the authorization
server, and some operating systems may offer alternative platform-
specific identity features which may be used, as appropriate.
8.8. Non-Browser External User Agents
This best practice recommends a particular type of external user-
agent, the user's browser. Other external user-agents patterns may
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also be viable for secure and usable OAuth. This document makes no
comment on those patterns.
8.9. Client Authentication
Secrets that are statically included as part of an app distributed to
multiple users should not be treated as confidential secrets, as one
user may inspect their copy and learn the shared secret. For this
reason, and those stated in Section 5.3.1 of [RFC6819], it is NOT
RECOMMENDED for authorization servers to require client
authentication of native apps using a shared secret, as this serves
little value beyond client identification which is already provided
by the "client_id" request parameter.
Authorization servers that still require a shared secret for native
app clients MUST treat the client as a public client, and not treat
the secret as proof of the client's identity. In those cases, it is
NOT RECOMMENDED to automatically issue tokens on the basis that the
user has previously granted access to the same client, as there is no
guarantee that the client is not counterfeit.
8.10. Cross-App Request Forgery Protections
Section 5.3.5 of [RFC6819] recommends using the 'state' parameter to
link client requests and responses to prevent CSRF attacks.
It is similarly RECOMMENDED for native apps to include a high entropy
secure random number in the 'state' parameter of the authorization
request, and reject any incoming authorization responses without a
state value that matches a pending outgoing authorization request.
8.11. Authorization Server Mix-Up Mitigation
To protect against a compromised or malicious authorization server
attacking another authorization server used by the same app, it is
RECOMMENDED that a unique redirect URI is used for each different
authorization server used by the app (for example, by varying the
path component), and that authorization responses are rejected if the
redirect URI they were received on doesn't match the redirect URI in
a pending outgoing authorization request.
Authorization servers SHOULD allow the registration of a specific
redirect URI, including path components, and reject authorization
requests that specify a redirect URI that doesn't exactly match the
one that was registered.
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9. IANA Considerations
[RFC Editor: please do not remove this section.]
Section 7.1 specifies how private-use URI schemes are used for inter-
app communication in OAuth protocol flows. This document requires in
Section 7.1.1 that such schemes are based on domain names owned or
assigned to the app, as recommended in Section 3.8 of [RFC7595]. Per
section 6 of [RFC7595], registration of domain based URI schemes with
IANA is not required. Therefore, this document has no IANA actions.
10. References
10.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, DOI 10.17487/RFC3986, January 2005,
<http://www.rfc-editor.org/info/rfc3986>.
[RFC6749] Hardt, D., Ed., "The OAuth 2.0 Authorization Framework",
RFC 6749, DOI 10.17487/RFC6749, October 2012,
<http://www.rfc-editor.org/info/rfc6749>.
[RFC7595] Thaler, D., Ed., Hansen, T., and T. Hardie, "Guidelines
and Registration Procedures for URI Schemes", BCP 35,
RFC 7595, DOI 10.17487/RFC7595, June 2015,
<http://www.rfc-editor.org/info/rfc7595>.
[RFC7636] Sakimura, N., Ed., Bradley, J., and N. Agarwal, "Proof Key
for Code Exchange by OAuth Public Clients", RFC 7636,
DOI 10.17487/RFC7636, September 2015,
<http://www.rfc-editor.org/info/rfc7636>.
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>.
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[AppAuth.iOSmacOS]
Wright, S., Denniss, W., and others, "AppAuth for iOS and
macOS", February 2016, <https://github.com/openid/AppAuth-
iOS>.
[AppAuth.Android]
McGinniss, I., Denniss, W., and others, "AppAuth for
Android", February 2016, <https://github.com/openid/
AppAuth-Android>.
[SamplesForWindows]
Denniss, W., "OAuth for Apps: Samples for Windows", July
2016, <https://github.com/googlesamples/oauth-apps-for-
windows>.
Appendix A. Server Support Checklist
OAuth servers that support native apps should:
1. Support custom URI-scheme redirect URIs. This is required to
support mobile operating systems. See Section 7.1.
2. Support HTTPS redirect URIs for use with public native app
clients. This is used by apps on advanced mobile operating
systems that allow app-claimed HTTPS URIs. See Section 7.2.
3. Support loopback IP redirect URIs. This is required to support
desktop operating systems. See Section 7.3.
4. Not assume native app clients can keep a secret. If secrets are
distributed to multiple installs of the same native app, they
should not be treated as confidential. See Section 8.9.
5. Support PKCE. Recommended to protect authorization code grants
transmitted to public clients over inter-app communication
channels. See Section 8.2
Appendix B. Operating System Specific Implementation Details
Most of this document defines best practices in an generic manner,
referencing techniques commonly available in a variety of
environments. This non-normative section contains OS-specific
implementation details for the generic pattern, that are considered
accurate at the time of publishing, but may change over time.
It is expected that this OS-specific information will change, but
that the overall principles described in this document for using
external user-agents will remain valid.
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B.1. iOS Implementation Details
Apps can initiate an authorization request in the browser without the
user leaving the app, through the SFSafariViewController class which
implements the browser-view pattern. Safari can be used to handle
requests on old versions of iOS without SFSafariViewController.
To receive the authorization response, both custom URI scheme
redirects and claimed HTTPS links (known as Universal Links) are
viable choices, and function the same whether the request is loaded
in SFSafariViewController or the Safari app. Apps can claim Custom
URI schemes with the "CFBundleURLTypes" key in the application's
property list file "Info.plist", and HTTPS links using the Universal
Links feature with an entitlement file and an association file on the
domain.
Universal Links are the preferred choice on iOS 9 and above due to
the ownership proof that is provided by the operating system.
A complete open source sample is included in the AppAuth for iOS and
macOS [AppAuth.iOSmacOS] library.
B.2. Android Implementation Details
Apps can initiate an authorization request in the browser without the
user leaving the app, through the Android Custom Tab feature which
implements the browser-view pattern. The user's default browser can
be used to handle requests when no browser supports Custom Tabs.
Android browser vendors should support the Custom Tabs protocol (by
providing an implementation of the "CustomTabsService" class), to
provide the in-app browser tab user experience optimization to their
users. Chrome is one such browser that implements Custom Tabs.
To receive the authorization response, custom URI schemes are broadly
supported through Android Implicit Intends. Claimed HTTPS redirect
URIs through Android App Links are available on Android 6.0 and
above. Both types of redirect URIs are registered in the
application's manifest.
A complete open source sample is included in the AppAuth for Android
[AppAuth.Android] library.
B.3. Windows Implementation Details
Apps can initiate an authorization request in the user's default
browser using platform APIs for this purpose.
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The custom URI scheme redirect is a good choice for Universal Windows
Platform (UWP) apps as it will open the app returning the user right
back where they were. Known on UWP as URI Activation, the scheme is
limited to 39 characters, but may include the "." character, making
short reverse domain name based schemes (as recommended in
Section 7.1.1) possible.
The loopback redirect is the common choice for traditional desktop
apps, listening on a loopback interface port is permitted by default
Windows firewall rules.
A complete open source sample is available [SamplesForWindows].
B.4. macOS Implementation Details
Apps can initiate an authorization request in the user's default
browser using platform APIs for this purpose.
To receive the authorization response, custom URI schemes are are a
good redirect URI choice on macOS, as the user is returned right back
to the app they launched the request from. These are registered in
the application's bundle information property list using the
"CFBundleURLSchemes" key. Loopback IP redirects are another viable
option, and listening on the loopback interface is allowed by default
firewall rules.
A complete open source sample is included in the AppAuth for iOS and
macOS [AppAuth.iOSmacOS] library.
B.5. Linux Implementation Details
Opening the Authorization Request in the user's default browser
requires a distro-specific command, "xdg-open" is one such tool.
The loopback redirect is the recommended redirect choice for desktop
apps on Linux to receive the authorization response.
Appendix C. Acknowledgements
The author would like to acknowledge the work of Marius Scurtescu,
and Ben Wiley Sittler whose design for using custom URI schemes in
native OAuth 2.0 clients formed the basis of Section 7.1.
The following individuals contributed ideas, feedback, and wording
that shaped and formed the final specification:
Andy Zmolek, Steven E Wright, Brian Campbell, Paul Madsen, Nat
Sakimura, Iain McGinniss, Rahul Ravikumar, Eric Sachs, Breno de
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Medeiros, Hannes Tschofenig, Ashish Jain, Erik Wahlstrom, Bill
Fisher, Sudhi 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
Email: wdenniss@google.com
URI: http://wdenniss.com/appauth
John Bradley
Ping Identity
Phone: +1 202-630-5272
Email: ve7jtb@ve7jtb.com
URI: http://www.thread-safe.com/p/appauth.html
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