OAuth 2.0 App2App Browserless Flow

Internet-Draft	Native OAuth App2App	May 2025
Zehavi, et al.	Expires 9 November 2025	[Page]

Abstract

This document describes a protocol enabling native apps from different app publishers, using the App2App pattern to act as OAuth Client And Authorization Server, native browser-less user navigation.¶

The native experience is also retained when the Client uses any number of brokers to federate across trust networks, while retaining highest levels of security.¶

This document, OAuth 2.0 App2App Browserless Flow (Native App2App), discusses the [App2App] pattern where two applications act as OAuth Client and Authorization Server respectively, to natively authenticate and authorize an end-user.¶

It addresses the challenges introduced when a web browser is involved in OAuth flows through one or more Brokering Authorization Servers, which are needed because Client App is not an OAuth client of the User-Interacting Authorization Server.¶

Since no app owns OAuth Brokers' urls, App2App flows involving brokers require using a web browser, which degrades the user experience.¶

This document presents a protocol enabling native App2App browser-less navigation, through any number of brokers, without compromising on any security property.¶

1.1. Difference from [OpenID.Native-SSO]

[OpenID.Native-SSO] also offers a native SSO flow across applications without requiring the browser. However, it is dealing with the specific sub-case when both apps are published by the same issuer and leverage this fact to share information.¶

1.2. 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, [RFC6749] and [RFC6750] as well as [OpenID], both in their authorization code flow.¶
"PKCE":: Proof Key for Code Exchange (PKCE) [RFC7636], 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. Brokers are an established pattern for establishing trust in federation use cases, such as in Academia and in the business world across corporations. Brokers may be replaced in the future with dynamic trust establishment leveraging [OpenID.Federation].¶
"Client App":: Native app implementing [RFC8252] as OAuth client of Primary Broker, and whose redirect_uri is claimed as a deep link.¶
"Primary Broker":: An OAuth Broker serving as Authorization Server of Client App. And also an OAuth client of a Downstream Authorization Server. Primary Broker performs additional handling for App2App use-case, covered in Section 5.2.¶
"Downstream Authorization Server":: An Authorization Server which may be a Secondary Broker or a User-Interacting Authorization Server.¶
"Secondary Broker":: A Broker redirecting the flow, which does not perform user authentication and authorization.¶
"User-Interacting Authorization Server":: The Authorization Server which interacts with end-user to perform authentication and authorization. May or may not offer App2App via a native app claiming it's urls as deep links. Such app may or may not be installed on end-user's device.¶

5. Browser-less App2App with Broker

5.1. Flow Diagram

Figure 3: Browser-less App2App with Broker

(1) Client App uses HTTP to call Primary Broker's authorization endpoint with an authorization request including app2app scope.¶
(2) Primary Broker prepares authorization request for Downstream Authorization Server including scope app2app:client_app_deep_link¶
(3) Client App loops through Brokers, natively calling their authorization endpoint over HTTP using previously obtained authorization request urls, and processing their HTTP 3xx redirect directives, until a deep link owned by an app on the device is reached.¶
(4) Client App natively invokes User-Authenticating App via its deep link.¶
(5) User-Authenticating App authenticates user and authorizes the request. It identifies app2app mode and uses client_app_deep_link to override the request's redirect_uri.¶
(6) User-Authenticating App natively invokes Client App using client_app_deep_link, handing it redirect_uri response.¶
(7) Client App loops through Brokers, starting from the redirect_uri it received from User-Authenticating App. It natively calls them using HTTP traversing through their redirect directives, until Primary Broker redirects to Client App's own deep link.¶
(8) Client App exchanges code for tokens.¶

5.2. Protocol Flow

5.2.1. Client App calls Primary Broker

Client App calls Primary Broker's authorization_endpoint to initiate an authorization code flow, it SHALL indicate App2App flow using the dedicated scope app2app.¶

Client App's redirect_uri SHALL be claimed as a deep link and will be referred to as client_app_deep_link.¶

5.2.2. Primary Broker returns authorization request to Downstream Authorization Server

Primary Broker SHALL validate Client's request and prepare an authorization request to Downstream Authorization Server's authorization_endpoint.¶
Primary Broker SHALL provide client_app_deep_link to Downstream Authorization Server as a suffix to the dedicated scope app2app. The combined scope is: app2app:client_app_deep_link.¶
Primary Broker SHALL respond with HTTP 302 with the authorization request url towards Downstream Authorization Server in the Location header.¶

5.2.3. Client App traverses Brokers with request

Client App SHALL use OS mechanisms to detect if the authorization request URL it received is handled by an app installed on the device. 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 natively call the authorization request URL using HTTP GET and processe 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), a Secondary Broker was reached and Client App SHALL repeat the logic previously described:¶
- Check if an app owns the obtained url, and if so natively invoke it.¶
- Otherwise natively 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 domains it traverses, which serves later as the Allowlist when traversing the response.¶

5.2.3.1. Secondary Brokers

Secondary Brokers engaged in the journey MUST retain structured scope app2app:client_app_deep_link in downstream authorization requests they create.¶

5.2.3.2. Downgrade to App2Web

If Client App reaches a User-Interacting Authorization Server with no app handling its urls, it may be impossible to relaunch the last authorization request URL on the browser as it might have included a single use 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 App2App indication, which is then launched on the browser. The remaining flow follows [RFC8252] and is therefore not further elaborated in this document.¶

5.2.4. Processing by User-Interacting Authorization Server:

The User-Interacting Authorization Server SHALL handle the authorization request using its native app:¶

Native app authenticates end user and authorizes the request.¶
The client_app_deep_link provided in the strcutured scope, SHALL override the request's original redirect_uri:¶
- User-Interacting Authorization Server's native app SHALL validate that an app owning client_app_deep_link is on the device¶
- If so it SHALL natively invoke it, handing it the redirect url with its response parameters¶
- If such an app does not exist it is an error and the flow SHALL terminate¶
To establish trust towards client_app_deep_link, User-Interacting Authorization Server MAY use mechanisms outside the scope of this document, or OpenID Federation:¶
- SHALL strip url path from client_app_deep_link (retaining the domain).¶
- SHALL add url path /.well-known/openid-federation and perform trust chain resolution.¶
- SHALL inspect Client's metadata for redirect_uri's and validate client_app_deep_link is included.¶

5.2.5. Client App traverses 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 through a 3xx redirect instruction from the brokers it traverses, against the Allowlist generated for this flow, 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 (client_app_deep_link).¶
SHALL handle any other HTTP code (2xx / 4xx / 5xx) as a failure.¶

5.2.6. Client App obtains response

Once Client App's own redirect_uri is returned in a redirect 3xx directive, the traversal is complete.¶

Client App SHALL proceed according to OAuth to exchange code for tokens, or handle error responses.¶

7. Security Considerations

7.1. OAuth request forgery and manipulation

It is RECOMMENDED Client App acts as a confidential OAuth client.¶

7.2. 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.¶

7.3. Deep link hijacking

It is RECOMMENDED that all apps in this specification shall protect their deep links using Android universal links / iOS App Links including the most specific package identifiers to prevent deep link hijacking by malicious apps.¶

7.4. Open redirection

Client App SHALL construct an Allowlist of domains it traverses through while processing the request, used to enforce all urls it later traverses through during response processing. This mitigates open redirection attacks as urls outside of this Allowlist will 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 is managed as a single-use object and destructed after each protocol flow ends.¶

It is RECOMMENDED Client App allows only one OAuth request processing at a time.¶

7.5. Authorization code theft and injection

It is RECOMMENDED that PKCE is used and that the code_verifier is tied to the Client App instance.¶

7.6. Handling of Cookies

It can be assumed that Authorization Servers will use Cookies to bind security elements (state, nonce, PKCE) to the user agent, and will break if these cookies are later missing.¶

Therefore, Client App MUST handle Cookies as a web browser would:¶

SHALL store cookies it obtains on HTTP responses.¶
SHALL send cookies on subsequent HTTP requests to servers that returned cookies.¶

9. References

9.1. Normative References

[OpenID]: Sakimura, N., Bradley, J., Jones, M. B., de Medeiros, B., and C. Mortimore, "OpenID Connect Core 1.0", November 2014, <https://openid.net/specs/openid-connect-core-1_0.html>.
[OpenID.Federation]: Hedberg, Ed, R., Jones, M. B., Solberg, A. A., Bradley, J., De Marco, G., and V. Dzhuvinov, "OpenID Federation 1.0", March 2025, <https://openid.net/specs/openid-federation-1_0.html>.
[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/rfc/rfc2119>.
[RFC6749]: Hardt, D., Ed., "The OAuth 2.0 Authorization Framework", RFC 6749, DOI 10.17487/RFC6749, October 2012, <https://www.rfc-editor.org/rfc/rfc6749>.
[RFC6750]: Jones, M. and D. Hardt, "The OAuth 2.0 Authorization Framework: Bearer Token Usage", RFC 6750, DOI 10.17487/RFC6750, October 2012, <https://www.rfc-editor.org/rfc/rfc6750>.
[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/rfc/rfc7636>.
[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/rfc/rfc8174>.
[RFC8252]: Denniss, W. and J. Bradley, "OAuth 2.0 for Native Apps", BCP 212, RFC 8252, DOI 10.17487/RFC8252, October 2017, <https://www.rfc-editor.org/rfc/rfc8252>.
[RFC9126]: Lodderstedt, T., Campbell, B., Sakimura, N., Tonge, D., and F. Skokan, "OAuth 2.0 Pushed Authorization Requests", RFC 9126, DOI 10.17487/RFC9126, September 2021, <https://www.rfc-editor.org/rfc/rfc9126>.

9.2. Informative References

[android.method.intent]: "Android Intent Method", n.d., <https://developer.android.com/reference/android/content/Intent>.
[App2App]: Heenan, J., "Guest Blog: Implementing App-to-App Authorisation in OAuth2/OpenID Connect", October 2019, <https://openid.net/guest-blog-implementing-app-to-app-authorisation-in-oauth2-openid-connect/>.
[iOS.method.openUrl]: "iOS open(_:options:completionHandler:) Method", n.d., <https://developer.apple.com/documentation/uikit/uiapplication/open(_:options:completionhandler:)>.
[iOS.option.universalLinksOnly]: "iOS method property universalLinksOnly", n.d., <https://developer.apple.com/documentation/uikit/uiapplication/openexternalurloptionskey/universallinksonly>.
[OpenID.Native-SSO]: Fletcher, G., "OpenID Connect Native SSO for Mobile Apps", November 2022, <https://openid.net/specs/openid-connect-native-sso-1_0.html>.