OAuth Working Group W. Denniss
Internet-Draft Google
Intended status: Best Current Practice J. Bradley
Expires: September 3, 2017 Ping Identity
March 2, 2017
OAuth 2.0 for Native Apps
draft-ietf-oauth-native-apps-08
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.
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This Internet-Draft will expire on September 3, 2017.
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document authors. All rights reserved.
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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 . . . . . . . . . . . . . . . . 9
8. Security Considerations . . . . . . . . . . . . . . . . . . . 9
8.1. Embedded User-Agents . . . . . . . . . . . . . . . . . . 9
8.2. Non-Browser External User-Agents . . . . . . . . . . . . 10
8.3. Phishability of In-App Browser Tabs . . . . . . . . . . . 10
8.4. Protecting the Authorization Code . . . . . . . . . . . . 11
8.5. OAuth Implicit Flow . . . . . . . . . . . . . . . . . . . 12
8.6. Loopback Redirect Considerations . . . . . . . . . . . . 12
8.7. Registration of Native App Clients . . . . . . . . . . . 13
8.8. Client Authentication . . . . . . . . . . . . . . . . . . 13
8.9. Client Impersonation . . . . . . . . . . . . . . . . . . 14
8.10. Cross-App Request Forgery Protections . . . . . . . . . . 14
8.11. Authorization Server Mix-Up Mitigation . . . . . . . . . 14
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 15
10.1. Normative References . . . . . . . . . . . . . . . . . . 15
10.2. Informative References . . . . . . . . . . . . . . . . . 15
Appendix A. Server Support Checklist . . . . . . . . . . . . . . 16
Appendix B. Operating System Specific Implementation Details . . 16
B.1. iOS Implementation Details . . . . . . . . . . . . . . . 17
B.2. Android Implementation Details . . . . . . . . . . . . . 17
B.3. Windows Implementation Details . . . . . . . . . . . . . 18
B.4. macOS Implementation Details . . . . . . . . . . . . . . 18
B.5. Linux Implementation Details . . . . . . . . . . . . . . 19
Appendix C. Acknowledgements . . . . . . . . . . . . . . . . . . 19
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 19
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, and an external user-
agent.
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This best current practice requires that only external user-agents
like the browser are used for OAuth by native apps. It documents how
native apps can implement authorization flows using the browser as
the preferred external user-agent, and the requirements for
authorization servers to support such usage.
This practice is also known as the AppAuth pattern, in reference to
open source libraries that implement it.
2. Notational Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in Key
words for use in RFCs to Indicate Requirement Levels [RFC2119]. If
these words are used without being spelled in uppercase then they are
to be interpreted with their normal natural language meanings.
3. Terminology
In addition to the terms defined in referenced specifications, this
document uses the following terms:
"native app" An application that is installed by the user to their
device, as distinct from a web app that runs in the browser
context only. Apps implemented using web-based technology but
distributed as a native app, so-called hybrid apps, are considered
equivalent to native apps for the purpose of this specification.
"OAuth" In this document, OAuth refers to OAuth 2.0 [RFC6749].
"external user-agent" A user-agent capable of handling the
authorization request that is a separate entity to the native app
making the request (such as a browser), such that the app cannot
access the cookie storage or modify the page content.
"embedded user-agent" A user-agent hosted inside the native app
itself (such as via a web-view), with which the app has control
over to the extent it is capable of accessing the cookie storage
and/or modify the page content.
"app" Shorthand for "native app".
"app store" An ecommerce store where users can download and purchase
apps.
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"browser" The operating system's default browser, pre-installed as
part of the operating system, or installed and set as default by
the user.
"browser tab" An open page of the browser. Browser typically have
multiple "tabs" representing various open pages.
"in-app browser tab" A full page browser with limited navigation
capabilities that is displayed inside a host app, but retains the
full security properties and authentication state of the browser.
Has different platform-specific product names, such as
SFSafariViewController on iOS, and Custom Tabs 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 private-use URI scheme defined by the app and
registered with the operating system. URI requests to such
schemes trigger the app which registered it to be launched to
handle the request.
"web-view" A web browser UI component that can be embedded in apps
to render web pages, used to create embedded user-agents.
"reverse domain name notation" A naming convention based on the
domain name system, but where where the domain components are
reversed, for example "app.example.com" becomes "com.example.app".
4. Overview
The best current practice for authorizing users in native apps is to
perform the OAuth authorization request in an external user-agent
(typically the browser), rather than an embedded user-agent (such as
one implemented with web-views).
Previously it was common for native apps to use embedded user-agents
(commonly implemented with web-views) for OAuth authorization
requests. That approach has many drawbacks, including the host app
being able to copy user credentials and cookies, and the user needing
to authenticate from scratch in each app. See Section 8.1 for a
deeper analysis of using embedded user-agents for OAuth.
Native app authorization requests that use the browser are more
secure and can take advantage of the user's authentication state.
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Being able to use the existing authentication session in the browser
enables single sign-on, as users don't need to authenticate to the
authorization server each time they use a new app (unless required by
authorization server policy).
Supporting authorization flows between a native app and the browser
is possible without changing the OAuth protocol itself, as the
authorization request and response are already defined in terms of
URIs, which emcompasses URIs that can be used for inter-process
communication. Some OAuth server implementations that assume all
clients are confidential web-clients will need to add an
understanding of public native app clients and the types of redirect
URIs they use to support this best practice.
4.1. Authorization Flow for Native Apps Using the Browser
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~+
| User Device |
| |
| +---------------------------+ | +-----------+
| | | | (5) Authz Code | |
| | Client App |----------------------->| Token |
| | |<-----------------------| Endpoint |
| +---------------------------+ | (6) Access Token, | |
| | ^ | Refresh Token +-----------+
| | | |
| | | |
| | (1) | (4) |
| | Authz | Authz |
| | Request | Code |
| | | |
| | | |
| v | |
| +---------------------------+ | +---------------+
| | | | (2) Authz Request | |
| | Browser |--------------------->| Authorization |
| | |<---------------------| Endpoint |
| +---------------------------+ | (3) Authz Code | |
| | +---------------+
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~+
Figure 1: Native App Authorization via External User-agent
Figure 1 illustrates the interaction of the native app with the
system browser to authorize the user via an external user-agent.
(1) The client app opens a browser tab with the authorization
request.
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(2) Authorization endpoint receives the authorization request,
authenticates the user and obtains authorization.
Authenticating the user may involve chaining to other
authentication systems.
(3) Authorization server issues an authorization code to the
redirect URI.
(4) Client receives the authorization code from the redirect URI.
(5) Client app presents the authorization code at the token
endpoint.
(6) Token endpoint validates the authorization code and issues the
tokens requested.
5. Using Inter-app URI Communication for OAuth
Just as URIs are used for OAuth 2.0 [RFC6749] on the web to initiate
the authorization request and return the authorization response to
the requesting website, URIs can be used by native apps to initiate
the authorization request in the device's browser and return the
response to the requesting native app.
By applying the same principles from the web to native apps, we gain
benefits seen on the web 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 similar flows
as the web, and increases interoperability by relying on standards-
based web flows that are not specific to a particular platform.
Native apps MUST use an external user-agent to perform OAuth
authentication requests. This is achieved by opening the
authorization request in the browser (detailed in Section 6), and
using a redirect URI that will return the authorization response back
to the native app, as defined in Section 7.
This best practice focuses on the browser as the RECOMMENDED external
user-agent for native apps. Other external user-agents, such as a
native app provided by the authorization server may meet the criteria
set out in this best practice, including using the same redirection
URI properties, but their use is out of scope for this specification.
6. Initiating the Authorization Request from a Native App
The authorization request is created as per OAuth 2.0 [RFC6749], and
opened in the user's browser using platform-specific APIs for that
purpose.
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The function of the redirect URI for a native app authorization
request is similar to that of a web-based authorization request.
Rather than returning the authorization response to the OAuth
client's server, the redirect URI used by a native app returns the
response to the app. The various options for a redirect URI that
will return the code to the native app are documented in Section 7.
Any redirect URI that allows the app to receive the URI and inspect
its parameters is viable.
Some platforms support a browser feature known as in-app browser
tabs, where an app can present a tab of the browser within the app
context without switching apps, but still retain key benefits of the
browser such as a shared authentication state and security context.
On platforms where they are supported, it is RECOMMENDED for
usability reasons that apps use in-app browser tabs for the
Authorization Request.
7. Receiving the Authorization Response in a Native App
There are several redirect URI options available to native apps for
receiving the authorization response from the browser, the
availability and user experience of which varies by platform.
To fully support this best practice, authorization servers MUST
support the following three redirect URI options. Native apps MAY
use whichever redirect option suits their needs best, taking into
account platform specific implementation details.
7.1. App-declared Custom URI Scheme Redirection
Many mobile and desktop computing platforms support inter-app
communication via URIs by allowing apps to register private-use
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.
As the custom URI scheme does not have a naming authority (as defined
by [RFC3986]), there is only a single slash ("/") after the scheme
component. The following is a complete example of a redirect URI
utilizing a custom URI scheme:
com.example.app:/oauth2redirect/example-provider
To perform an OAuth 2.0 Authorization Request with a custom URI
scheme redirect URI, the native app launches the browser with a
normal OAuth 2.0 Authorization Request, but provides a redirection
URI that utilizes a custom URI scheme it registered with the
operating system.
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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 it results in the operating
system launching the native app, passing in the URI as a launch
parameter. 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 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. Such a 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 URL paths in domains
they control. When the browser encounters a claimed URL, instead of
the page being loaded in the browser, the native app is launched with
the URL supplied as a launch parameter.
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Such claimed HTTPS URIs can be used as OAuth redirect URIs. They are
indistinguishable from OAuth redirects of web-based clients. An
example is:
https://app.example.com/oauth2redirect/example-provider
App-claimed HTTPS redirect URIs have some advantages in that the
identity of the destination app is guaranteed by the operating
system. Due to this reason, they SHOULD be used over the other
redirect choices for native apps where possible.
App-claimed HTTPS redirect URIs function as normal HTTPS redirects
from the perspective of the authorization server, though as stated in
Section 8.7, it REQUIRED that the authorization server is able to
distinguish between public native app clients that use app-claimed
HTTPS redirect URIs and confidential web clients.
7.3. Loopback URI Redirection
Native apps that are able to open a port on the loopback network
interface without needing special permissions (typically, those on
desktop operating systems) can use the loopback network interface to
receive the OAuth redirect.
Loopback redirect URIs use the HTTP scheme and are constructed with
the loopback IP literal and whatever port the client is listening on.
That is, "http://127.0.0.1:{port}/{path}" for IPv4, and
"http://[::1]:{port}/{path}" for IPv6. An complete example of such a
redirect with a randomly assigned port:
http://127.0.0.1:56861/oauth2redirect/example-provider
The authorization server MUST allow any port to be specified at the
time of the request for loopback IP redirect URIs, to accommodate
clients that obtain an available port from the operating system at
the time of the request.
8. Security Considerations
8.1. Embedded User-Agents
Embedded user-agents are an alternative method for authorizing native
apps. They are however unsafe for use by third-parties to the
authorization server by definition, as the app that hosts the
embedded user-agent can access the user's full authentication
credential, not just the OAuth authorization grant that was intended
for the app.
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In typical web-view based implementations of embedded user-agents,
the host application can: log every keystroke entered in the form to
capture usernames and passwords; automatically submit forms and
bypass user-consent; copy session cookies and use them to perform
authenticated actions as the user.
Even when used by trusted apps belonging to the same party as the
authorization server, embedded user-agents violate the principle of
least privilege by having access to more powerful credentials than
they need, potentially increasing the attack surface.
Encouraging users to enter credentials in an embedded user-agent
without the usual address bar and visible certificate validation
features that browsers have makes it impossible for the user to know
if they are signing in to the legitimate site, and even when they
are, it trains them that it's OK to enter credentials without
validating the site first.
Aside from the security concerns, embedded user-agents do not share
the authentication state with other apps or the browser, requiring
the user to login for every authorization request and leading to a
poor user experience.
Native apps MUST NOT use embedded user-agents to perform
authorization requests.
Authorization endpoints MAY take steps to detect and block
authorization requests in embedded user-agents.
8.2. Non-Browser External User-Agents
This best practice recommends a particular type of external user-
agent, the user's browser. Other external user-agent patterns may
also be viable for secure and usable OAuth. This document makes no
comment on those patterns.
8.3. 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
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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.
8.4. Protecting the Authorization Code
The redirect URI options documented in Section 7 share the benefit
that only a native app on the same device can receive the
authorization code which limits the attack surface, however code
interception by a native app other than the intended app may still be
possible.
A limitation of using custom URI schemes for redirect URIs is that
multiple apps can typically register the same scheme, which makes it
indeterminate as to which app will receive the Authorization Code.
PKCE [RFC7636] details how this limitation can be used to execute a
code interception attack (see Figure 1).
Loopback IP based redirect URIs may be susceptible to interception by
other apps listening on the same loopback interface.
As most forms of inter-app URI-based communication sends data over
insecure local channels, eavesdropping and interception of the
authorization response is a risk for native apps. App-claimed HTTPS
redirects are hardened against this type of attack due to the
presence of the URI authority, but they are still public clients and
the URI is still transmitted over local channels with unknown
security properties.
The Proof Key for Code Exchange by OAuth Public Clients (PKCE
[RFC7636]) standard was created specifically to mitigate against this
attack. It is a Proof of Possession extension to OAuth 2.0 that
protects the code grant from being used if it is intercepted. It
achieves this by having the client generate a secret verifier which
it passes in the initial authorization request, and which it must
present later when redeeming the authorization code grant. An app
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that intercepted the authorization code would not be in possession of
this secret, rendering the code useless.
Public native app clients MUST protect the authorization request with
PKCE [RFC7636]. Authorization servers MUST support PKCE [RFC7636]
for public native app clients. Authorization servers SHOULD reject
authorization requests from native apps that don't use PKCE by
returning an error message as defined in Section 4.4.1 of PKCE
[RFC7636].
8.5. 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 required in
Section 8.4), the use of the Implicit Flow with native apps is NOT
RECOMMENDED.
Tokens granted via the implicit flow also cannot be refreshed without
user interaction, making the code flow which can issue refresh tokens
the more practical option for native app authorizations that require
refreshing.
8.6. 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 network port only when starting the
authorization request, and close it once the response is returned.
Clients should listen on the loopback network interface only, to
avoid interference by other network actors.
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. Specifying a redirect URI with the loopback IP literal
rather than localhost avoids inadvertently listening on network
interfaces other than the loopback interface. It is also less
susceptible to client side firewalls, and misconfigured host name
resolution on the user's device.
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8.7. Registration of Native App Clients
Native apps, except when using a mechanism like Dynamic Client
Registration [RFC7591] to provision per-instance secrets, are
classified as public clients, as defined by Section 2.1 of OAuth 2.0
[RFC6749] and MUST be registered with the authorization server as
such. Authorization servers MUST record the client type in the
client registration details in order to identify and process requests
accordingly.
Authorization servers MUST require clients to register their complete
redirect URI (including the path component), and reject authorization
requests that specify a redirect URI that doesn't exactly match the
one that was registered, with the exception of loopback redirects,
where an exact match is required except for the port URI component.
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.
8.8. 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 public native apps clients 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 statically included shared
secret for native app clients MUST treat the client as a public
client (as defined by Section 2.1 of OAuth 2.0 [RFC6749]), and not
accept the secret as proof of the client's identity. Without
additional measures, such clients are subject to client impersonation
(see Section 8.9).
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8.9. Client Impersonation
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. This includes the case where the user has
previously approved an authorization request for a given client id -
unless the identity of the client can be proven, the request SHOULD
be processed as if no previous request had been approved.
Measures such as claimed HTTPS redirects MAY be accepted by
authorization servers as identity proof. Some operating systems may
offer alternative platform-specific identity features which MAY be
accepted, as appropriate.
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
REQUIRED that a unique redirect URI is used for each 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 outgoing
authorization request.
The native app MUST store the redirect uri used in the authorization
request with the authorization session data (i.e. along with "state"
and other related data), and MUST verify that the URI on which the
authorization response was received exactly matches it.
The requirements of Section 8.7 that authorization servers reject
requests with URIs that don't match what was registered are also
required to prevent such attacks.
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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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[RFC7591] Richer, J., Ed., Jones, M., Bradley, J., Machulak, M., and
P. Hunt, "OAuth 2.0 Dynamic Client Registration Protocol",
RFC 7591, DOI 10.17487/RFC7591, July 2015,
<http://www.rfc-editor.org/info/rfc7591>.
[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 must:
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 8.8.
5. Support PKCE [RFC7636]. Required to protect authorization code
grants sent to public clients over inter-app communication
channels. See Section 8.4
Appendix B. Operating System Specific Implementation Details
This document primarily defines best practices in an generic manner,
referencing techniques commonly available in a variety of
environments. This non-normative section documents operating system
specific implementation details of the best practice.
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The implementation details herein are considered accurate at the time
of publishing but will likely change over time. It is hoped that
such change won't invalidate the generic principles in the rest of
the document, and those principles should take precedence in the
event of a conflict.
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 in-app browser tab 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 in-app browser tab 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.
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A complete open source sample is included in the AppAuth for Android
[AppAuth.Android] library.
B.3. Windows Implementation Details
Universal Windows Platform (UWP) apps can use the Web Authentication
Broker API in SSO mode as an external user-agent for authorization
flows, and all app types can open an authorization request in the
user's default browser using platform APIs for opening URIs in the
browser.
The Web Authentication Broker when used in SSO mode is an external
user-agent with an authentication context that is shared with all
invocations of the broker but not the user's browser. Note that if
not used in SSO mode, the broker is an embedded user-agent, hence
only operation in SSO mode is RECOMMENDED.
To use the Web Authentication Broker in SSO mode, the redirect URI
must be of the form "msapp://{appSID}" where "appSID" is the app's
SID, which can be found in the app's registration information. While
Windows enforces the URI authority on such redirects, ensuring only
the app with the matching SID can receive the response on Windows,
the URI scheme could be claimed by apps on other platforms without
the same authority present, thus this redirect type should be treated
similar to custom URI scheme redirects for security purposes.
Both traditional and Universal Windows Platform (UWP) apps can
perform authorization requests in the user's browser. Traditional
apps typically use a loopback redirect to receive the authorization
response, and listening on the loopback interface is allowed by
default firewall rules. Universal Windows Platform (UWP) apps can
use custom URI scheme redirects to receive the authorization
response, which will bring the app to the foreground. Known on the
platform as "URI Activation", the URI scheme is limited to 39
characters in length, and may include the "." character, making short
reverse domain name based schemes (as recommended in Section 7.1.1)
possible.
An open source sample demonstrating these patterns is available
[SamplesForWindows].
B.4. macOS Implementation Details
Apps can initiate an authorization request in the user's default
browser using platform APIs for opening URIs in the browser.
To receive the authorization response, custom URI schemes are are a
good redirect URI choice on macOS, as the user is returned right back
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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
Medeiros, Adam Dawes, Naveen Agarwal, Hannes Tschofenig, Ashish Jain,
Erik Wahlstrom, Bill Fisher, Sudhi Umarji, Michael B. Jones, Vittorio
Bertocci, Dick Hardt, David Waite, and Ignacio Fiorentino.
Authors' Addresses
William Denniss
Google
1600 Amphitheatre Pkwy
Mountain View, CA 94043
USA
Email: wdenniss@google.com
URI: http://wdenniss.com/appauth
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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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