OAuth Working Group W. Denniss Internet-Draft Google Updates: 6749 (if approved) J. Bradley Intended status: Best Current Practice Ping Identity Expires: December 11, 2017 June 9, 2017 OAuth 2.0 for Native Apps draft-ietf-oauth-native-apps-12 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 Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on December 11, 2017. 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 (http://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 Denniss & Bradley Expires December 11, 2017 [Page 1]
Internet-Draft OAuth 2.0 for Native Apps June 2017 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. Private-use URI Scheme Redirection . . . . . . . . . . . 8 7.2. Claimed HTTPS 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 . . . . . . . . . . . 11 8.5. Client Authentication . . . . . . . . . . . . . . . . . . 12 8.6. Client Impersonation . . . . . . . . . . . . . . . . . . 12 8.7. Fake External User-Agent . . . . . . . . . . . . . . . . 13 8.8. Malicious External User-Agent . . . . . . . . . . . . . . 13 8.9. Cross-App Request Forgery Protections . . . . . . . . . . 14 8.10. Authorization Server Mix-Up Mitigation . . . . . . . . . 14 8.11. Non-Browser External User-Agents . . . . . . . . . . . . 14 8.12. Embedded User-Agents . . . . . . . . . . . . . . . . . . 14 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 16 10.1. Normative References . . . . . . . . . . . . . . . . . . 16 10.2. Informative References . . . . . . . . . . . . . . . . . 16 Appendix A. Server Support Checklist . . . . . . . . . . . . . . 17 Appendix B. Operating System Specific Implementation Details . . 17 B.1. iOS Implementation Details . . . . . . . . . . . . . . . 18 B.2. Android Implementation Details . . . . . . . . . . . . . 18 B.3. Windows Implementation Details . . . . . . . . . . . . . 19 B.4. macOS Implementation Details . . . . . . . . . . . . . . 19 B.5. Linux Implementation Details . . . . . . . . . . . . . . 20 Appendix C. Acknowledgements . . . . . . . . . . . . . . . . . . 20 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 20 1. Introduction The OAuth 2.0 [RFC6749] authorization framework documents two approaches in Section 9 for native apps to interact with the Denniss & Bradley Expires December 11, 2017 [Page 2]
Internet-Draft OAuth 2.0 for Native Apps June 2017 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, 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 [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 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 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" In this document, "app" means a "native app" unless further specified. "app store" An ecommerce store where users can download and purchase apps. "OAuth" In this document, OAuth refers to 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 as a browser), such that the app cannot access the cookie storage, nor inspect or modify page content. Denniss & Bradley Expires December 11, 2017 [Page 3]
Internet-Draft OAuth 2.0 for Native Apps June 2017 "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 modifying the 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 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 URI" Some platforms allow apps to claim a 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" 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 (user interface) 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 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 8.12 for a deeper analysis of using embedded user-agents for OAuth. Denniss & Bradley Expires December 11, 2017 [Page 4]
Internet-Draft OAuth 2.0 for Native Apps June 2017 Native app authorization requests that use the browser are more secure and can take advantage of the user's authentication state. Being able to use the existing authentication session in the browser enables single sign-on, as users don't need to authenticate to the authorization server each time they use a new app (unless required by authorization server policy). Supporting authorization flows between a native app and the browser is possible without changing the OAuth protocol itself, as the authorization request and response are already defined in terms of URIs, which 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 External User-agent Figure 1 illustrates the interaction of the native app with a browser external user-agent to authorize the user. Denniss & Bradley Expires December 11, 2017 [Page 5]
Internet-Draft OAuth 2.0 for Native Apps June 2017 (1) The client app opens a browser tab with the authorization request. (2) Authorization endpoint receives the authorization request, authenticates the user and obtains authorization. Authenticating the user may involve chaining to other authentication systems. (3) Authorization server issues an authorization code to the redirect URI. (4) Client receives the authorization code from the redirect URI. (5) Client app presents the authorization code at the token endpoint. (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. Re-using 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 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. 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. Denniss & Bradley Expires December 11, 2017 [Page 6]
Internet-Draft OAuth 2.0 for Native Apps June 2017 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, but 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 processing authorization requests capable of processing the request and redirect URIs in the same way 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. 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 following three redirect URI options to native apps. Native apps MAY use whichever redirect option suits their needs best, taking into account platform specific implementation details. Denniss & Bradley Expires December 11, 2017 [Page 7]
Internet-Draft OAuth 2.0 for Native Apps June 2017 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 custom 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 custom 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. Care must be taken when there are multiple apps by the same publisher that each scheme is unique within that group. On platforms that use app identifiers that are also based on reverse order domain names, those can be reused as the private-use URI scheme for the OAuth redirect to help avoid this problem. Following the requirements of [RFC3986] Section 3.2, 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: com.example.app:/oauth2redirect/example-provider When the authentication server completes the request, it redirects to the client's redirection URI as it would normally. 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 normal. Denniss & Bradley Expires December 11, 2017 [Page 8]
Internet-Draft OAuth 2.0 for Native Apps June 2017 7.2. Claimed HTTPS URI Redirection Some operating systems allow apps to claim HTTPS scheme [RFC7230] URIs in 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 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. 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://127.0.0.1:50719/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 Denniss & Bradley Expires December 11, 2017 [Page 9]
Internet-Draft OAuth 2.0 for Native Apps June 2017 clients that obtain an available ephemeral port from the operating system at the time of the request. Clients SHOULD NOT assume 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 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 redirects are less susceptible to URI interception due to the presence of the URI authority, but they are still public clients and the URI is sent using the operating system's URI dispatch handler 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, a hash of which it passes in the initial authorization request, and which it must present in full when redeeming the authorization code grant. An app that intercepted the authorization code would not be in possession of this secret, rendering the code useless. 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]. Denniss & Bradley Expires December 11, 2017 [Page 10]
Internet-Draft OAuth 2.0 for Native Apps June 2017 8.2. OAuth Implicit Grant Authorization Flow The OAuth 2.0 implicit grant authorization 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 [RFC7636] (which is a required in Section 8.1), 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 authorization code grant flow - which can issue refresh tokens - the more practical option for native app authorizations that require refreshing. 8.3. Loopback Redirect Considerations Loopback interface redirect URIs use the "http" scheme (i.e., without TLS). This is acceptable for loopback interface redirect URIs as the HTTP request never leaves the device. Clients should open the 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. 8.4. 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 Denniss & Bradley Expires December 11, 2017 [Page 11]
Internet-Draft OAuth 2.0 for Native Apps June 2017 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 private-use URI scheme based redirects, authorization servers SHOULD enforce the requirement in Section 7.1 that clients use reverse domain name based schemes. At a minimum, any 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 used to associate the app that may be useful for verifying the calling app's identity, on operating systems that support such functions. 8.5. 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 8.6). 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 Denniss & Bradley Expires December 11, 2017 [Page 12]
Internet-Draft OAuth 2.0 for Native Apps June 2017 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. 8.7. Fake External User-Agent The native app which 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. The advantage when all good actors are using external user-agents is that it is possible for security experts to detect bad actors, as anyone faking an external user-agent is provably bad. 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 malicious apps are detected, it may be possible to use this knowledge to blacklist the apps signatures 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. 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-Agent 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. Many operating systems mitigate this issue by requiring an explicit user action to change the default handler for HTTP URIs. This attack is not confined to OAuth for Native Apps, a malicious app configured Denniss & Bradley Expires December 11, 2017 [Page 13]
Internet-Draft OAuth 2.0 for Native Apps June 2017 in this way would present a general and ongoing risk to the user beyond OAuth usage. 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 using inter-app URI communication, 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 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 8.4 that authorization servers reject requests with URIs that don't match what was registered are 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 OAuth 2.0 [RFC6749] Section 9 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 Denniss & Bradley Expires December 11, 2017 [Page 14]
Internet-Draft OAuth 2.0 for Native Apps June 2017 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: 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 which is often considered an inferior user experience. 9. IANA Considerations [RFC Editor: please do NOT remove this section.] This document has no 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. Denniss & Bradley Expires December 11, 2017 [Page 15]
Internet-Draft OAuth 2.0 for Native Apps June 2017 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>. [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, <http://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, <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>. [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] Denniss, W., Wright, S., McGinniss, I., Ravikumar, R., and others, "AppAuth", May 22, <https://appauth.io>. Denniss & Bradley Expires December 11, 2017 [Page 16]
Internet-Draft OAuth 2.0 for Native Apps June 2017 [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 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 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 8.5. 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. Operating System 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 operating system specific implementation details of the best practice. 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 Denniss & Bradley Expires December 11, 2017 [Page 17]
Internet-Draft OAuth 2.0 for Native Apps June 2017 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 private-use URI scheme redirects (referred to as Custom URL Schemes) 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, private-use URI schemes are broadly supported through Android Implicit Intents. Claimed HTTPS redirect URIs through Android App Links are available on Android 6.0 and above. Both types of redirect URIs are registered in the application's manifest. A complete open source sample is included in the AppAuth for Android [AppAuth.Android] library. Denniss & Bradley Expires December 11, 2017 [Page 18]
Internet-Draft OAuth 2.0 for Native Apps June 2017 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 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) possible. UWP apps can alternatively use the Web Authentication Broker API in SSO (Single Sign-on) 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 sign-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. Personalisations the user has made to their browser, such as configuring a password manager may not 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 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 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 Denniss & Bradley Expires December 11, 2017 [Page 19]
Internet-Draft OAuth 2.0 for Native Apps June 2017 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. Apps SHOULD NOT set the "SO_REUSEPORT" or "SO_REUSEADDR" socket options, to prevent other apps binding to the same socket. Appendix C. Acknowledgements The author would like to acknowledge the work of Marius Scurtescu, and Ben Wiley Sittler whose design for using private-use URI schemes in native 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 USA Email: wdenniss@google.com URI: http://wdenniss.com/appauth Denniss & Bradley Expires December 11, 2017 [Page 20]
Internet-Draft OAuth 2.0 for Native Apps June 2017 John Bradley Ping Identity Phone: +1 202-630-5272 Email: ve7jtb@ve7jtb.com URI: http://www.thread-safe.com/p/appauth.html Denniss & Bradley Expires December 11, 2017 [Page 21]
