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BEST CURRENT PRACTICE
Errata ExistInternet Engineering Task Force (IETF) W. Denniss
Request for Comments: 8252 Google
BCP: 212 J. Bradley
Updates: 6749 Ping Identity
Category: Best Current Practice October 2017
ISSN: 2070-1721
OAuth 2.0 for Native Apps
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
This memo documents an Internet Best Current Practice.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
BCPs is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8252.
Copyright Notice
Copyright (c) 2017 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
(https://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
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Denniss & Bradley Best Current Practice [Page 1]
RFC 8252 OAuth 2.0 for Native Apps October 2017
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
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. Private-Use URI Scheme Redirection . . . . . . . . . . . 8
7.2. Claimed "https" Scheme URI Redirection . . . . . . . . . 9
7.3. Loopback Interface Redirection . . . . . . . . . . . . . 9
8. Security Considerations . . . . . . . . . . . . . . . . . . . 10
8.1. Protecting the Authorization Code . . . . . . . . . . . . 10
8.2. OAuth Implicit Grant Authorization Flow . . . . . . . . . 11
8.3. Loopback Redirect Considerations . . . . . . . . . . . . 11
8.4. Registration of Native App Clients . . . . . . . . . . . 12
8.5. Client Authentication . . . . . . . . . . . . . . . . . . 12
8.6. Client Impersonation . . . . . . . . . . . . . . . . . . 13
8.7. Fake External User-Agents . . . . . . . . . . . . . . . . 13
8.8. Malicious External User-Agents . . . . . . . . . . . . . 14
8.9. Cross-App Request Forgery Protections . . . . . . . . . . 14
8.10. Authorization Server Mix-Up Mitigation . . . . . . . . . 14
8.11. Non-Browser External User-Agents . . . . . . . . . . . . 15
8.12. Embedded User-Agents . . . . . . . . . . . . . . . . . . 15
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 16
10.1. Normative References . . . . . . . . . . . . . . . . . . 16
10.2. Informative References . . . . . . . . . . . . . . . . . 17
Appendix A. Server Support Checklist . . . . . . . . . . . . . . 18
Appendix B. Platform-Specific Implementation Details . . . . . . 18
B.1. iOS Implementation Details . . . . . . . . . . . . . . . 18
B.2. Android Implementation Details . . . . . . . . . . . . . 19
B.3. Windows Implementation Details . . . . . . . . . . . . . 19
B.4. macOS Implementation Details . . . . . . . . . . . . . . 20
B.5. Linux Implementation Details . . . . . . . . . . . . . . 21
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 21
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 21
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1. Introduction
Section 9 of the OAuth 2.0 authorization framework [RFC6749]
documents two approaches for native apps to interact with the
authorization endpoint: an embedded user-agent and an external user-
agent.
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 as well as the requirements for
authorization servers to support such usage.
This practice is also known as the "AppAuth pattern", in reference to
open-source libraries [AppAuth] 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
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
3. Terminology
In addition to the terms defined in referenced specifications, this
document uses the following terms:
"native app" An app or 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.
"app" A "native app" unless further specified.
"app store" An e-commerce store where users can download and
purchase apps.
"OAuth" Authorization protocol specified by the OAuth 2.0
Authorization Framework [RFC6749].
"external user-agent" A user-agent capable of handling the
authorization request that is a separate entity or security domain
to the native app making the request, such that the app cannot
access the cookie storage, nor inspect or modify page content.
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RFC 8252 OAuth 2.0 for Native Apps October 2017
"embedded user-agent" A user-agent hosted by the native app making
the authorization request that forms a part of the app or shares
the same security domain such that the app can access the cookie
storage and/or inspect or modify page content.
"browser" The default application launched by the operating system
to handle "http" and "https" scheme URI content.
"in-app browser tab" A programmatic instantiation of the browser
that is displayed inside a host app but that retains the full
security properties and authentication state of the browser. It
has different platform-specific product names, several of which
are detailed in Appendix B.
"web-view" A web browser UI (user interface) component that is
embedded in apps to render web pages under the control of the app.
"inter-app communication" Communication between two apps on a
device.
"claimed "https" scheme URI" Some platforms allow apps to claim an
"https" scheme URI after proving ownership of the domain name.
URIs claimed in such a way are then opened in the app instead of
the browser.
"private-use URI scheme" As used by this document, a URI scheme
defined by the app (following the requirements of Section 3.8 of
[RFC7595]) and registered with the operating system. URI requests
to such schemes launch the app that registered it to handle the
request.
"reverse domain name notation" A naming convention based on the
domain name system, but one where the domain components are
reversed, for example, "app.example.com" becomes
"com.example.app".
4. Overview
For authorizing users in native apps, the best current practice 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 as well as the user
needing to authenticate from scratch in each app. See Section 8.12
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for a deeper analysis of the drawbacks 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
the authorization server policy).
Supporting authorization flows between a native app and the browser
is possible without changing the OAuth protocol itself, as the OAuth
authorization request and response are already defined in terms of
URIs. This encompasses URIs that can be used for inter-app
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) Authorization +---------------+
| | | | Code | |
| | Client App |---------------------->| Token |
| | |<----------------------| Endpoint |
| +--------------------------+ | (6) Access Token, | |
| | ^ | Refresh Token +---------------+
| | | |
| | | |
| | (1) | (4) |
| | Authorizat- | Authoriza- |
| | ion Request | tion Code |
| | | |
| | | |
| v | |
| +---------------------------+ | (2) Authorization +---------------+
| | | | Request | |
| | Browser |--------------------->| Authorization |
| | |<---------------------| Endpoint |
| +---------------------------+ | (3) Authorization | |
| | Code +---------------+
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~+
Figure 1: Native App Authorization via an External User-Agent
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Figure 1 illustrates the interaction between a native app and the
browser to authorize the user.
(1) 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.
(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 adopting the same methods used on the web for OAuth, benefits seen
in the web context like the usability of a single sign-on session and
the security of a separate authentication context are likewise gained
in the native app context. Reusing the same approach also reduces
the implementation complexity and increases interoperability by
relying on standards-based web flows that are not specific to a
particular platform.
To conform to this best practice, native apps MUST use an external
user-agent to perform OAuth authorization 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 (defined in Section 7).
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6. Initiating the Authorization Request from a Native App
Native apps needing user authorization create an authorization
request URI with the authorization code grant type per Section 4.1 of
OAuth 2.0 [RFC6749], using a redirect URI capable of being received
by the native app.
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. Several options for a redirect URI that will
return the authorization response to the native app in different
platforms are documented in Section 7. Any redirect URI that allows
the app to receive the URI and inspect its parameters is viable.
Public native app clients MUST implement the Proof Key for Code
Exchange (PKCE [RFC7636]) extension to OAuth, and authorization
servers MUST support PKCE for such clients, for the reasons detailed
in Section 8.1.
After constructing the authorization request URI, the app uses
platform-specific APIs to open the URI in an external user-agent.
Typically, the external user-agent used is the default browser, that
is, the application configured for handling "http" and "https" scheme
URIs on the system; however, different browser selection criteria and
other categories of external user-agents MAY be used.
This best practice focuses on the browser as the RECOMMENDED external
user-agent for native apps. An external user-agent designed
specifically for user authorization and capable of processing
authorization requests and responses like a browser MAY also be used.
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.
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.
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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 offer
at least the three redirect URI options described in the following
subsections to native apps. Native apps MAY use whichever redirect
option suits their needs best, taking into account platform-specific
implementation details.
7.1. Private-Use URI Scheme Redirection
Many mobile and desktop computing platforms support inter-app
communication via URIs by allowing apps to register private-use URI
schemes (sometimes colloquially referred to as "custom URL schemes")
like "com.example.app". When the browser or another app attempts to
load a URI with a private-use URI scheme, the app that registered it
is launched to handle the request.
To perform an OAuth 2.0 authorization request with a private-use URI
scheme redirect, the native app launches the browser with a standard
authorization request, but one where the redirection URI utilizes a
private-use URI scheme it registered with the operating system.
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 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 scheme when reversed in the same manner. A
scheme such as "myapp", however, would not meet this requirement, as
it is not based on a domain name.
When there are multiple apps by the same publisher, care must be
taken so that each scheme is unique within that group. On platforms
that use app identifiers based on reverse-order domain names, those
identifiers can be reused as the private-use URI scheme for the OAuth
redirect to help avoid this problem.
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Following the requirements of Section 3.2 of [RFC3986], as there is
no naming authority for private-use URI scheme redirects, only a
single slash ("/") appears after the scheme component. A complete
example of a redirect URI utilizing a private-use URI scheme is:
com.example.app:/oauth2redirect/example-provider
When the authorization server completes the request, it redirects to
the client's redirection URI as it would normally. As the
redirection URI uses a private-use URI scheme, it results in the
operating system launching the native app, passing in the URI as a
launch parameter. Then, the native app uses normal processing for
the authorization response.
7.2. Claimed "https" Scheme URI Redirection
Some operating systems allow apps to claim "https" scheme [RFC7230]
URIs in the domains they control. When the browser encounters a
claimed URI, instead of the page being loaded in the browser, the
native app is launched with the URI supplied as a launch parameter.
Such URIs can be used as redirect URIs by native apps. They are
indistinguishable to the authorization server from a regular web-
based client redirect URI. An example is:
https://app.example.com/oauth2redirect/example-provider
As the redirect URI alone is not enough to distinguish public native
app clients from confidential web clients, it is REQUIRED in
Section 8.4 that the client type be recorded during client
registration to enable the server to determine the client type and
act accordingly.
App-claimed "https" scheme redirect URIs have some advantages
compared to other native app redirect options in that the identity of
the destination app is guaranteed to the authorization server by the
operating system. For this reason, native apps SHOULD use them over
the other options where possible.
7.3. Loopback Interface 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 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.
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That is, "http://127.0.0.1:{port}/{path}" for IPv4, and
"http://[::1]:{port}/{path}" for IPv6. An example redirect using the
IPv4 loopback interface with a randomly assigned port:
http://127.0.0.1:51004/oauth2redirect/example-provider
An example redirect using the IPv6 loopback interface with a randomly
assigned port:
http://[::1]:61023/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 ephemeral port from the operating
system at the time of the request.
Clients SHOULD NOT assume that the device supports a particular
version of the Internet Protocol. It is RECOMMENDED that clients
attempt to bind to the loopback interface using both IPv4 and IPv6
and use whichever is available.
8. Security Considerations
8.1. Protecting the Authorization Code
The redirect URI options documented in Section 7 share the benefit
that only a native app on the same device or the app's own website
can receive the authorization code, which limits the attack surface.
However, code interception by a different native app running on the
same device may be possible.
A limitation of using private-use 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. Section 1 of PKCE [RFC7636] details how this limitation can be
used to execute a code interception attack.
Loopback IP-based redirect URIs may be susceptible to interception by
other apps accessing the same loopback interface on some operating
systems.
App-claimed "https" scheme redirects are less susceptible to URI
interception due to the presence of the URI authority, but the app is
still a public client; further, the URI is sent using the operating
system's URI dispatch handler with unknown security properties.
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The PKCE [RFC7636] protocol was created specifically to mitigate this
attack. It is a proof-of-possession extension to OAuth 2.0 that
protects the authorization code from being used if it is intercepted.
To provide protection, this extension has the client generate a
secret verifier; it passes a hash of this verifier in the initial
authorization request, and must present the unhashed verifier when
redeeming the authorization code. An app that intercepted the
authorization code would not be in possession of this secret,
rendering the code useless.
Section 6 requires that both clients and servers use PKCE 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].
8.2. OAuth Implicit Grant Authorization Flow
The OAuth 2.0 implicit grant authorization flow (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 [RFC7636]
(which is required in Section 8.1), the use of the Implicit Flow with
native apps is NOT RECOMMENDED.
Access tokens granted via the implicit flow also cannot be refreshed
without user interaction, making the authorization code grant flow --
which can issue refresh tokens -- the more practical option for
native app authorizations that require refreshing of access tokens.
8.3. Loopback Redirect Considerations
Loopback interface redirect URIs use the "http" scheme (i.e., without
Transport Layer Security (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, in
order to avoid interference by other network actors.
While redirect URIs using localhost (i.e.,
"http://localhost:{port}/{path}") 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
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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.
8.4. Registration of Native App Clients
Except when using a mechanism like Dynamic Client Registration
[RFC7591] to provision per-instance secrets, native apps are
classified as public clients, as defined by Section 2.1 of OAuth 2.0
[RFC6749]; they 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; the exception is loopback redirects, where
an exact match is required except for the port URI component.
For private-use URI scheme-based redirects, authorization servers
SHOULD enforce the requirement in Section 7.1 that clients use
schemes that are reverse domain name based. At a minimum, any
private-use URI scheme that doesn't contain a period character (".")
SHOULD be rejected.
In addition to the collision-resistant properties, requiring a URI
scheme based on 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 private-use URI scheme (where one 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.
Authorization servers MAY request the inclusion of other platform-
specific information, such as the app package or bundle name, or
other information that may be useful for verifying the calling app's
identity on operating systems that support such functions.
8.5. 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
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RFC 8252 OAuth 2.0 for Native Apps October 2017
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 8.6).
8.6. 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" scheme redirects MAY be accepted by
authorization servers as identity proof. Some operating systems may
offer alternative platform-specific identity features that MAY be
accepted, as appropriate.
8.7. Fake External User-Agents
The native app that is initiating the authorization request has a
large degree of control over the user interface and can potentially
present a fake external user-agent, that is, an embedded user-agent
made to appear as an external user-agent.
When all good actors are using external user-agents, the advantage is
that it is possible for security experts to detect bad actors, as
anyone faking an external user-agent is provably bad. On the other
hand, if good and bad actors alike are using embedded user-agents,
bad actors don't need to fake anything, making them harder to detect.
Once a malicious app is detected, it may be possible to use this
knowledge to blacklist the app's signature in malware scanning
software, take removal action (in the case of apps distributed by app
stores) and other steps to reduce the impact and spread of the
malicious app.
Authorization servers can also directly protect against fake external
user-agents by requiring an authentication factor only available to
true external user-agents.
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Users who are particularly concerned about their security when using
in-app browser tabs may also take the additional step of opening the
request in the full browser from the in-app browser tab and complete
the authorization there, as most implementations of the in-app
browser tab pattern offer such functionality.
8.8. Malicious External User-Agents
If a malicious app is able to configure itself as the default handler
for "https" scheme URIs in the operating system, it will be able to
intercept authorization requests that use the default browser and
abuse this position of trust for malicious ends such as phishing the
user.
This attack is not confined to OAuth; a malicious app configured in
this way would present a general and ongoing risk to the user beyond
OAuth usage by native apps. Many operating systems mitigate this
issue by requiring an explicit user action to change the default
handler for "http" and "https" scheme URIs.
8.9. 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 (Cross-Site
Request Forgery) attacks.
To mitigate CSRF-style attacks over inter-app URI communication
channels (so called "cross-app request forgery"), it is similarly
RECOMMENDED that native apps 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.
8.10. 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 an 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.
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The requirement of Section 8.4, specifically that authorization
servers reject requests with URIs that don't match what was
registered, is also required to prevent such attacks.
8.11. 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.
8.12. Embedded User-Agents
Section 9 of OAuth 2.0 [RFC6749] documents two approaches for native
apps to interact with the authorization endpoint. This best current
practice requires that native apps MUST NOT use embedded user-agents
to perform authorization requests and allows that authorization
endpoints MAY take steps to detect and block authorization requests
in embedded user-agents. The security considerations for these
requirements are detailed herein.
Embedded user-agents are an alternative method for authorizing native
apps. These embedded user-agents are 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.
In typical web-view-based implementations of embedded user-agents,
the host application can record every keystroke entered in the login
form to capture usernames and passwords, automatically submit forms
to bypass user consent, and 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; even when they are, it
trains them that it's OK to enter credentials without validating the
site first.
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Aside from the security concerns, embedded user-agents do not share
the authentication state with other apps or the browser, requiring
the user to log in for every authorization request, which is often
considered an inferior user experience.
9. IANA Considerations
This document does not require any IANA actions.
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 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.
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,
<https://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,
<https://www.rfc-editor.org/info/rfc3986>.
[RFC6749] Hardt, D., Ed., "The OAuth 2.0 Authorization Framework",
RFC 6749, DOI 10.17487/RFC6749, October 2012,
<https://www.rfc-editor.org/info/rfc6749>.
[RFC7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Message Syntax and Routing",
RFC 7230, DOI 10.17487/RFC7230, June 2014,
<https://www.rfc-editor.org/info/rfc7230>.
[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,
<https://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,
<https://www.rfc-editor.org/info/rfc7636>.
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RFC 8252 OAuth 2.0 for Native Apps October 2017
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
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,
<https://www.rfc-editor.org/info/rfc6819>.
[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,
<https://www.rfc-editor.org/info/rfc7591>.
[AppAuth] OpenID Connect Working Group, "AppAuth", September 2017,
<https://openid.net/code/AppAuth>.
[AppAuth.iOSmacOS]
Wright, S., Denniss, W., et al., "AppAuth for iOS and
macOS", February 2016,
<https://openid.net/code/AppAuth-iOS>.
[AppAuth.Android]
McGinniss, I., Denniss, W., et al., "AppAuth for Android",
February 2016, <https://openid.net/code/AppAuth-Android>.
[SamplesForWindows]
Denniss, W., "OAuth for Apps: Samples for Windows", July
2016,
<https://openid.net/code/sample-oauth-apps-for-windows>.
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Appendix A. Server Support Checklist
OAuth servers that support native apps must:
1. Support private-use URI scheme redirect URIs. This is required
to support mobile operating systems. See Section 7.1.
2. Support "https" scheme redirect URIs for use with public native
app clients. This is used by apps on advanced mobile operating
systems that allow app-claimed "https" scheme 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 that 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 8.5.
5. Support PKCE [RFC7636]. Required to protect authorization code
grants sent to public clients over inter-app communication
channels. See Section 8.1
Appendix B. Platform-Specific Implementation Details
This document primarily defines best practices in a generic manner,
referencing techniques commonly available in a variety of
environments. This non-normative section documents implementation
details of the best practice for various operating systems.
The implementation details herein are considered accurate at the time
of publishing but will likely change over time. It is hoped that
such a change won't invalidate the generic principles in the rest of
the document and that 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
or its successor "SFAuthenticationSession", which implement the in-
app browser tab pattern. Safari can be used to handle requests on
old versions of iOS without in-app browser tab functionality.
To receive the authorization response, both private-use URI scheme
(referred to as "custom URL scheme") redirects and claimed "https"
scheme URIs (known as "Universal Links") are viable choices. Apps
can claim private-use URI schemes with the "CFBundleURLTypes" key in
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the application's property list file, "Info.plist", and "https"
scheme URIs using the Universal Links feature with an entitlement
file in the app and an association file hosted on the domain.
Claimed "https" scheme URIs are the preferred redirect 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, private-use URI schemes are
broadly supported through Android Implicit Intents. Claimed "https"
scheme 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
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. When creating the loopback network socket,
apps SHOULD set the "SO_EXCLUSIVEADDRUSE" socket option to prevent
other apps binding to the same socket.
UWP apps can use private-use URI scheme redirects to receive the
authorization response from the browser, which will bring the app to
the foreground. Known on the platform as "URI Activation", the URI
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RFC 8252 OAuth 2.0 for Native Apps October 2017
scheme is limited to 39 characters in length, and it may include the
"." character, making short reverse domain name based schemes (as
required in Section 7.1) possible.
UWP apps can alternatively use the Web Authentication Broker API in
Single Sign-on (SSO) mode, which is an external user-agent designed
for authorization flows. Cookies are shared between invocations of
the broker but not the user's preferred browser, meaning the user
will need to log in again, even if they have an active session in
their browser; but the session created in the broker will be
available to subsequent apps that use the broker. Personalizations
the user has made to their browser, such as configuring a password
manager, may not be available in the broker. To qualify as an
external user-agent, the broker MUST be used in SSO mode.
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
security identifier (SID), which can be found in the app's
registration information or by calling the
"GetCurrentApplicationCallbackUri" method. While Windows enforces
the URI authority on such redirects, ensuring that 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
similarly to private-use URI scheme redirects for security purposes.
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, private-use URI schemes 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.
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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. Apps SHOULD NOT
set the "SO_REUSEPORT" or "SO_REUSEADDR" socket options in order to
prevent other apps binding to the same socket.
Acknowledgements
The authors would like to acknowledge the work of Marius Scurtescu
and Ben Wiley Sittler, whose design for using private-use URI schemes
in native app OAuth 2.0 clients at Google 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, Nat Sakimura, Eric
Sachs, Paul Madsen, Iain McGinniss, Rahul Ravikumar, Breno de
Medeiros, Hannes Tschofenig, Ashish Jain, Erik Wahlstrom, Bill
Fisher, Sudhi Umarji, Michael B. Jones, Vittorio Bertocci, Dick
Hardt, David Waite, Ignacio Fiorentino, Kathleen Moriarty, and Elwyn
Davies.
Authors' Addresses
William Denniss
Google
1600 Amphitheatre Pkwy
Mountain View, CA 94043
United States of America
Email: rfc8252@wdenniss.com
URI: http://wdenniss.com/appauth
John Bradley
Ping Identity
Phone: +1 202-630-5272
Email: rfc8252@ve7jtb.com
URI: http://www.thread-safe.com/p/appauth.html
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