]> Raiffeisen Bank International

yaron.zehavi@rbinternational.com

Security Web Authorization Protocol native-apps oauth app2app browserless This document describes a protocol connecting 2 native apps via the OAuth pattern, to achieve native user navigation (no web browser required), regardless of any number of OAuth brokers federating the request across trust domains. About This Document The latest revision of this draft can be found at . Status information for this document may be found at . Discussion of this document takes place on the Web Authorization Protocol Working Group mailing list (), which is archived at . Subscribe at . Source for this draft and an issue tracker can be found at .

Introduction This document, OAuth 2.0 App2App Browserless Flow (Native App2App), presents a protocol enabling native OAuth browser-less navigation across apps. It addresses the challenges presented when using a web browser to navigate through one or more Brokering Authorization Servers:

• Such OAuth Brokers are required when Client App is not an OAuth client of the User-Interacting Authorization Server and so they federate the flow across trust domains.
• Since no app owns OAuth Brokers' urls, App2App flows involving brokers require a web browser, which degrades the user experience.

This document specifies:

• A new parameter to the authorization endpoint: native_callback_uri.
• A new scope value: app2app.
• A new error_description value: native_callback_uri_not_claimed.

Difference from OpenID.Native-SSO also offers a native SSO flow across apps. However, it is limited to apps:

• Published by the same issuer, therefore can securely share information.
• Using the same Authorization Server.

Terminology In addition to the terms defined in referenced specifications, this document uses the following terms:

"OAuth":

In this document, "OAuth" refers to OAuth 2.0, and as well as , both in their authorization code flow.

"PKCE":

Proof Key for Code Exchange (PKCE) , a mechanism to prevent various attacks on OAuth authorization codes.

"OAuth Broker":

A component acting as an Authorization Server for its clients, as well as an OAuth Client towards Downstream Authorization Servers. Brokers are used to facilitate a trust relationship when there is no direct relation between an OAuth Client and the final Authorization Server where end-user authenticates and authorizes. This pattern is in current use to establish trust in federation use cases in Academia and in the business world, across corporations. Note: It is possible OAuth Brokers will be superseded in the future by , offering dynamic trust establishment.

"Client App":

A Native app implementing "OAuth 2.0 for Native Apps" as an OAuth client of Initial Authorization Server. Client's redirect_uri is claimed by the app.

"Initial Authorization Server":

The Authorization Server of Client App which acts as an OAuth Broker as an OAuth client of a Downstream Authorization Server.

"Downstream Authorization Server":

An Authorization Server which may be an OAuth Broker or a User-Interacting Authorization Server.

"User-Interacting Authorization Server":

The Authorization Server which interacts with end-user to perform authentication and authorization.

"User-Interacting App":

Native App of User-Interacting Authorization Server.

"Deep Link":

A url claimed by a native application.

"Native Callback uri":

Client App's redirect_uri, claimed as a deep link. This deep link is invoked by User-Interacting App to natively return to Client App.

Conventions and Definitions 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 when, and only when, they appear in all capitals, as shown here.

Challenge of App2App with OAuth Brokers

App2App with OAuth Brokers requires a web browser

App2App with brokers and browser (redirects back to Client not depicted) | Initial |------>| Authorization|------>| Authenticating | | | | Authorization| | Broker | Auth. | Servers | Auth. | Authorization | | | | Request | +---------+ Req. +--------------+ Req. | Server | | | | | +---------+----------+ | | | +----------------------------------------------------|-------------+ | | +--------------+ | | | | User- | | | | |Authenticating|<--------------------------------------------------+ | | | App | Deep Link | | +--------------+ | +------------------------------------------------------------------------------------+ Mobile Device ]]>

Since no native app claims OAuth Brokers' urls, OAuth requests and redirect_uri responses to and from OAuth Brokers are handled by a web browser as the User Agent.

Impact of using a web browser Using a web browser may degrade the user experience in several ways:

• Some browser's support for deep links is limited by design, or by the settings used.
• The browser may prompt end-user for consent before opening deep links, introducing additional friction.
• Browser's loading of urls and redirecting may be noticeable by end-user, rendering the UX less smooth.
• App developers cannot control which browser will handle the response redirect_uri. This poses a risk that necessary cookies used to bind session identifiers to the user agent (nonce, state or PKCE verifier) will be unavailable, which may break the flow.
• After the flow is complete, "orphan" browser tabs might remain. While they do not directly impact the flow, they can be regarded as unnecessary "clutter".

App2Web with brokers

App2Web with brokers (redirects back to Client not depicted) | Initial |------>| Authorization|------>| Authenticating | | | | Authorization| | Broker | Auth. | Servers | Auth. | Authorization | | | | Request | +---------+ Req. +--------------+ Req. | Server | | | | | +-+----------------+-+ | | | | | User- | | | | | | Authenticating | | | | | | Web UI | | | | | +----------------+ | | | +------------------------------------------------------------------+ | +------------------------------------------------------------------------------------+ Mobile Device ]]>

End-user's device may not have an app claiming User-Interacting Authorization Server's urls, when:

• User-Interacting Authorization Server offers no native app.
• Or such an app is offered, but is not installed on the end-user's device.

In such case interacting with User-Interacting Authorization Server's MUST use the browser as user agent. This is similar to the flow described in "OAuth 2.0 for Native Apps" , and referred to in as App2Web, and is therefore not discussed further in this document.

Browser-less App2App with OAuth Brokers

Flow Diagram

Browser-less App2App with Brokers | Initial | | | | | Authorization | | | +--------------+ (1) | Server | | | | Client |------------->| | | | | App |<-------------| (OAuth Broker)| | | | | (2) +---------------+ | | +--------------+ | | ^ | ^ ^ | | | | | | (3) +---------------+ | | | | | +---------------->| | | | | | +------------------>| Downstream | | | | | (7) | Authorization | | | | | | Servers | | | | | | | | |(6)| |(4) +---------------+ | | | v | | +----------------+ +---------------+ | | | | | User- | | | | User- |<---------->| Authenticating| | | | Authenticating | (5) | Authorization | | | | App | | Server | | | +----------------+ +---------------+ | +--------------------------------------------------+ Mobile Device ]]>

• (1) Client App presents an authorization request to Initial Authorization Server, indicating app2app flow using new scope value app2app.
• (2) Initial Authorization Server returns an authorization request for Downstream Authorization Server, including Client App's redirect_uri as native_callback_uri.
• (3) Client App checks if the returned authorization request url is claimed by an app on the device and if so proceeds to the next step. Otherwise it loops through Downstream Authorization Servers, calling their authorization endpoints and processing their HTTP 3xx redirect responses, until a url claimed by an app on the device is reached.
• (4) Client App natively invokes User-Interacting App.
• (5) User-Interacting App authenticates user and authorizes the request.
• (6) User-Interacting App natively invokes native_callback_uri (overriding the request's redirect_uri), providing as a parameter the redirect_uri with its response parameters.
• (7) Client App loops through Authorization Servers in reverse order, starting from the redirect_uri it received from the User-Interacting App. It calls the first redirect_uri and any subsequent uri obtained as 3xx redirect directive, until it obtains a location header indicating its own redirect_uri.
• (8) Client App exchanges code for tokens and the flow is complete.

New Parameters and Values The protocol described in this document requires User-Interacting App to natively navigate end-user back to Client App, for which it requires Client App's native_callback_uri. Therefore this document defines new parameters and values.

"app2app":

New scope value, used by Client App to request an app2app flow from Initial Authorization Server.

Initial Authorization Server, processing an app2app flow according to this document, MUST provide Client App's redirect_uri as Native Callback uri to Downstream Authorization Server using one of the following options:

"native_callback_uri":

OPTIONAL. New authorization endpoint request parameter. When native_callback_uri is provided, structured scope app2app:native_callback_uri MUST NOT be provided.

"app2app:{native_callback_uri}":

OPTIONAL. New structured scope value including the app2app flag as well as the Client's native_callback_uri, separated by a colon. When structured scope app2app:{native_callback_uri} is provided, native_callback_uri MUST NOT be provided.

native_callback_uri accepts the following query parameter when invoked by User-Interacting Authorization Server's App:

"redirect_uri":

url-encoded OAuth redirect_uri with its response parameters.

Downstream Authorization Server, processing an app2app flow according to this document:

• MUST retain the native_callback_uri in downstream authorization requests created.
• MAY validate native_callback_uri.

Validation of native_callback_uri Validation of native_callback_uri by User-Interacting Authorization Server and its App is RECOMMENDED, to mitiagte open redirection attacks. A validating Authorization Server MAY use various mechanisms outside the scope of this document. For example, validation using is possible:

• Strip url path from native_callback_uri (retaining the DNS domain).
• Add the url path /.well-known/openid-federation and perform trust chain resolution.
• Inspect Client's metadata for redirect_uri's and validate native_callback_uri is included.

Protocol Flow

Client App calls Initial Authorization Server Client App calls Initial Authorization Server's authorization_endpoint to initiate an authorization code flow and indicates App2App flow using the scope: app2app.

Initial Authorization Server returns authorization request to Downstream Authorization Server Initial Authorization Server SHALL process Client's request and return an HTTP 3xx response with a Location header containing an authorization request url towards Downstream Authorization Server's authorization_endpoint. The request SHALL include Client's redirect_uri as native_callback_uri in one of the methods specified in this document.

Client App invokes app of User-Interacting Authorization Server Client App SHALL use OS mechanisms to locate an app installed on the device claiming the authorization request url. If so, Client App SHALL natively invoke the app claiming the url to process the authorization request. This achieves native navigation across applications. If an app handling the authorization request url is not found, Client App SHALL use HTTP to call the authorization request url and process the response:

• If the response is successful (HTTP Code 2xx), it is assumed to be the User-Interacting Authorization Server. This means the Client App "over-stepped" and MUST downgrade to App2Web.
• If the response is a redirect instruction (HTTP Code 3xx + Location header), Client App SHALL repeat the logic previously described:
  • Check if an app owns the obtained url, and if so natively invoke it.
  • Otherwise use HTTP to call the obtained url and analyze the response.
• Handle error response (HTTP 4xx / 5xx) for example by displaying the error.

As the Client App traverses through Brokers, it SHALL maintain a list of all the DNS domains it traverses, which serves later as the Allowlist when traversing the response.

Downgrade to App2Web If Client App reaches a User-Interacting Authorization Server but failed to locate an app claiming its urls, it may be impossible to relaunch the last authorization request on the browser as it might have included a single-use "OAuth 2.0 Pushed Authorization Requests" request_uri which by now has been used and is therefore invalid. In such a case the Client App MUST start over, generating a new authorization request without the app2app scope, which is then launched on the browser. The remaining flow follows "OAuth 2.0 for Native Apps" and is therefore not elaborated further in this document.

Processing by User-Interacting Authorization Server's App: The User-Interacting Authorization Server SHALL handle the authorization request using its native app:

• Authenticates end-user and authorizes the request.
• SHALL use native_callback_uri to override the request's original redirect_uri:
  • User-Interacting Authorization Server's app validates that an app claiming native_callback_uri is on the device
  • If so it natively invokes native_callback_uri with the redirect url and its response parameters as the url-encoded query parameter redirect_uri.
  • If such an app does not exist on the device, the flow terminates and User-Interacting Authorization Server's app redirects to redirect_uri with:
    • error=invalid_request.
    • error_description=native_callback_uri_not_claimed.

Client App traverses OAuth Brokers in reverse order Client App is natively invoked by User-Interacting Authorization Server App, with the request's redirect_uri. Client App MUST validate this url, and any url subsequently obtained via a 3xx redirect instruction, against the Allowlist it previously generated, and MUST fail if any url is not included in the Allowlist. Client App SHALL invoke the url it received using HTTP GET:

• If the response is a redirect instruction (HTTP Code 3xx + Location header), Client App SHALL repeat the logic and proceed to call obtained urls until reaching its own redirect_uri (native_callback_uri).
• SHALL handle any other HTTP code (2xx / 4xx / 5xx) as a failure.

Client App obtains response Once Client App's own redirect_uri is returned in a redirect 3xx directive, the traversal of OAuth Brokers is complete. Client App SHALL proceed according to OAuth to exchange code for tokens, or handle error responses.

Detecting Presence of Native Apps claiming Urls Native Apps on iOS and Android MAY use OS SDK's to detect if an app owns a url. The general method is the same - App calls an SDK to open the url as deep link and handles an exception thrown if no matching app is found.

Android App SHALL invoke Android method with FLAG_ACTIVITY_REQUIRE_NON_BROWSER, which throws ActivityNotFoundException if no matching app is found.

iOS App SHALL invoke iOS method with options which ensures URLs must be universal links and have an app configured to open them. Otherwise the method returns false in completion.success

Security Considerations

OAuth request forgery and manipulation It is RECOMMENDED that Client App acts as a confidential OAuth client.

Secure Native application communication If Client App uses a Backend it is RECOMMENDED to communicate with it securely:

• Use TLS in up to date versions and ciphers.
• Use DNSSEC.
• Perform certificate pinning.

Deep link hijacking It is RECOMMENDED that all apps in this specification shall use https-scheme deep links (Android App Links / iOS universal links). Apps SHOULD implement the most specific package identifiers mitigating deep link hijacking by malicious apps.

Open redirection by Client App Client App SHALL construct an Allowlist of DNS domains it traverses while processing the request, used to enforce all urls it later traverses during response processing. This mitigates open redirection attacks as urls not in this Allowlist SHALL be rejected. In addition Client App MUST ignore any invocation for response processing which is not in the context of a request it initiated. It is RECOMMENDED the Allowlist be managed as a single-use object, destructed after each protocol flow ends. It is RECOMMENDED Client App allows only one OAuth request processing at a time.

Open redirection by User-Interacting Authorization Server's App It is RECOMMENDED that User-Interacting Authorization Server's App establishes trust in native_callback_uri to mitigate open redirection attacks and reject untrusted urls.

Authorization code theft and injection It is RECOMMENDED that PKCE is used and that the code_verifier is tied to the Client App instance.

Handling of Cookies It can be assumed that Authorization Servers will use Cookies to bind security elements (state, nonce, PKCE) to the user agent, which will break the flow if these cookies are not present in subsequent HTTP requests. Therefore, Client App MUST handle Cookies:

• Store cookies it obtains on HTTP responses.
• Send cookies on subsequent HTTP requests to Authorization Servers that returned such cookies.

IANA Considerations This document has no IANA actions.

References Normative References The OAuth 2.0 authorization framework enables a third-party application to obtain limited access to an HTTP service, either on behalf of a resource owner by orchestrating an approval interaction between the resource owner and the HTTP service, or by allowing the third-party application to obtain access on its own behalf. This specification replaces and obsoletes the OAuth 1.0 protocol described in RFC 5849. [STANDARDS-TRACK] This specification describes how to use bearer tokens in HTTP requests to access OAuth 2.0 protected resources. Any party in possession of a bearer token (a "bearer") can use it to get access to the associated resources (without demonstrating possession of a cryptographic key). To prevent misuse, bearer tokens need to be protected from disclosure in storage and in transport. [STANDARDS-TRACK] OAuth 2.0 public clients utilizing the Authorization Code Grant are susceptible to the authorization code interception attack. This specification describes the attack as well as a technique to mitigate against the threat through the use of Proof Key for Code Exchange (PKCE, pronounced "pixy"). 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. This document defines the pushed authorization request (PAR) endpoint, which allows clients to push the payload of an OAuth 2.0 authorization request to the authorization server via a direct request and provides them with a request URI that is used as reference to the data in a subsequent call to the authorization endpoint. In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. Informative References n.d. n.d. n.d.

Acknowledgments The authors would like to thank the following individuals who contributed ideas, feedback, and wording that shaped and formed the final specification: Henrik Kroll, Grese Hyseni. As well as the attendees of the OAuth Security Workshop 2025 session in which this topic was discussed for their ideas and feedback.