]> Okta

aaron@parecki.com https://aaronparecki.com

emelia@brandedcode.com https://thisismissem.social

Security Web Authorization Protocol oauth This specification defines a mechanism through which an OAuth client can identify itself to authorization servers, without prior dynamic client registration or other existing registration. This is through the usage of a URL as a client_id in an OAuth flow, where the URL refers to a document containing the necessary client metadata, enabling the authorization server to fetch the metadata about the client as needed. 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 In order for an OAuth 2.0 client to utilize an OAuth 2.0 authorization server, the client needs to establish a unique identifier, and needs to to provide the server with metadata about the application, such as the application name, icon and redirect URIs. In cases where a client is interacting with authorization servers that it has no relationship with, manual registration is impossible. While Dynamic Client Registration can provide a method for a previously unknown client to establish itself at an authorization server and obtain a client identifier, this is not always practical in some deployments and can create additional challenges around management of the registration data and cleanup of inactive clients. This specification describes how an OAuth 2.0 client can publish its own registration information and avoid the need for pre-registering at each authorization server.

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.

Client Identifier This specification defines the client identifier as a URL with the following restrictions. Client identifier URLs MUST have an "https" scheme, MUST contain a path component, MUST NOT contain single-dot or double-dot path segments, MUST NOT contain a fragment component and MUST NOT contain a username or password Client identifier URLs SHOULD NOT include a query string component, and MAY contain a port. This specification places no restrictions on what URL is used as a client identifier. A short URL is RECOMMENDED, since the URL may be displayed to the end user in the authorization interface or in management interfaces. Usage of a stable URL that does not frequently change for the client is also RECOMMENDED.

Client Information Discovery One purpose of registering clients at the authorization server is so that the authorization server has additional information about the client that can be used during an OAuth flow, such as presenting information about the client to the user in an authorization consent screen, for example the client name and logo. The authorization server SHOULD fetch the document indicated by the client_id to retrieve the client registration information.

Client Metadata The client metadata document URL is a JSON document containing the metadata of the client. The client metadata values are the values defined in the OAuth Dynamic Client Registration Metadata OAuth Parameters registry https://www.iana.org/assignments/oauth-parameters/oauth-parameters.xhtml#client-metadata. The client metadata document MUST contain a client_id property whose value MUST compare and match the URL of the document using simple string comparison as defined in Section 6.2.1. The client metadata document MAY define additional properties in the response. The client metadata document MAY also be served with more specific content types as long as the response is JSON and conforms to application/<AS-defined>+json. As there is no way to establish a shared secret to be used with client metadata documents, the following restrictions apply on the contents of the client metadata document:

• the token_endpoint_auth_method property MUST NOT include client_secret_post or client_secret_basic
• the client_secret and client_secret_expires_at properties MUST NOT be used

See for more details. Other specifications MAY place additional restrictions on the contents of the client metadata document accepted by authorization servers implementing their specification, for instance, preventing the registration of confidential clients by requiring the token_endpoint_auth_method property be set to "none". TBD: We may want a property such as client_id_expires_at for indicating that the client is ephemeral and not valid after a given timestamp, especially for documents issued by a service for development purposes.

Metadata Discovery Errors If fetching the metadata document fails, the authorization server SHOULD abort the authorization request.

Metadata Caching The authorization server MAY cache the client metadata it discovers at the client metadata document URL. TBD: recommend a cache lifetime? considerations about stale data? The authorization server MUST NOT cache error responses. The authorization server also MUST NOT cache documents which are invalid or malformed. TBD: Do we want to define an endpoint through which a document can be validated by an authorization server, such that a developer can assert that their document is valid?

Redirect URL Registration According to , the authorization server MUST require registration of redirect URIs, and MUST ensure that the redirect URI in a request is an exact match of a registered redirect URI. This method of client information discovery establishes a registered redirect URI with the authorization server which is used when comparing the redirect URI in an authorization request against the registered redirect URIs.

Authorization Server Metadata Authorization servers that publish Authorization Server Metadata MUST include the following property to signal support for client metadata documents as described in this specification.

client_id_metadata_document_supported:

OPTIONAL. Boolean value specifying whether the authorization server supports retrieving client metadata from a client_id URL as described in this specification.

This enables clients to avoid sending the user to a dead end, by only redirecting the user to an authorization server that supports this specification. Otherwise, the client would redirect the user and the user would be met with an error about an invalid client as described by Section 4.1.2.1 of .

Security Considerations In addition to the security considerations in OAuth 2.0 Core , and OAuth 2.0 Threat Model and Security Considerations , and the additional considerations apply.

Client ID Metadata Documents for Development Purposes When developing applications against a service that uses Client ID Metadata Documents, developers often encounter the issue of "how do I serve a Client ID Metadata Document at a https URL whilst developing my application?". For this purpose, it is recommended to either host a document on a webserver somewhere that describes the application under development (e.g., using localhost redirect URIs), or to use a service which can generate and host a Client ID Metadata Document for you. Such a service should issue URLs that are stable.

Client Authentication Since the client establishes its own registration data at the authorization server, prior coordination of client credentials is not possible. However, clients MAY establish credentials at the authorization server by using authentication methods that use public/private key pairs, by publishing the public key in their metadata document. For example, the client MAY include the following properties in its metadata document to establish a public key and the private_key_jwt authentication method defined in : This establishes this client as a confidential client, and any communication with the authorization server MUST include client authentication of the registered type.

OAuth Phishing Attacks Authorization servers SHOULD fetch the client_id metadata document provided in the authorization request in order to provide users with additional information about the request, such as the application name and logo. If the server does not fetch the client metadata document, then it SHOULD take additional measures to ensure the user is provided with as much information as possible about the request. The authorization server SHOULD display the hostname of the client_id on the authorization interface, in addition to displaying the fetched client information if any. Displaying the hostname helps users know that they are authorizing the expected application. If fetching the client metadata document fails for any reason, the client_id URL is the only piece of information the user has as an indication of which application they are authorizing.

Server Side Request Forgery (SSRF) Attacks Authorization servers fetching the client metadata document and resolving URLs located in the metadata document should be aware of possible SSRF attacks. Authorization servers SHOULD avoid fetching any URLs using private or loopback addresses and consider network policies or other measures to prevent making requests to these addresses. Authorization servers SHOULD also be aware of the possibility that URLs might be non-http-based URI schemes which can lead to other possible SSRF attack vectors.

Maximum Response Size for Client Metadata Documents Authorization servers SHOULD limit the response size when fetching the client metadata document, as to avoid denial of service attacks against the authorization server by consuming excessive resources (memory, disk, database). The recommended maximum response size for client metadata documents is 5 kilobytes.

Displaying Logos to End-Users Authorization servers that wish to make use of the logo_uri property within client metadata document SHOULD prefetch the file at logo_uri and cache it for the cache duration of the client metadata document. This allows for moderation tools to verify the file contents (e.g., preventing usage of logos that look like other logos), as well as preventing the logo from being dynamically changed to confuse an end-user. Caching of the logo_uri response can additionally prevent cross-domain tracking through the logo_uri being requested by the client, since the cached file would be served not from the remote URI but instead from a URI that the Authorization server trusts.

IANA Considerations

OAuth Authorization Server Metadata Registry The following authorization server metadata value is defined by this specification and registered in the IANA "OAuth Authorization Server Metadata" registry established in OAuth 2.0 Authorization Server Metadata .

• Metadata Name: client_id_metadata_document_supported:
• Metadata Description: JSON boolean value specifying whether the authorization server supports retrieving client metadata from a client_id URL.
• Change Controller: IETF
• Specification Document: of [draft-parecki-oauth-client-id-metadata-document-02]

References Normative References A Uniform Resource Identifier (URI) is a compact sequence of characters that identifies an abstract or physical resource. This specification defines the generic URI syntax and a process for resolving URI references that might be in relative form, along with guidelines and security considerations for the use of URIs on the Internet. The URI syntax defines a grammar that is a superset of all valid URIs, allowing an implementation to parse the common components of a URI reference without knowing the scheme-specific requirements of every possible identifier. This specification does not define a generative grammar for URIs; that task is performed by the individual specifications of each URI scheme. [STANDARDS-TRACK] 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 document gives additional security considerations for OAuth, beyond those in the OAuth 2.0 specification, based on a comprehensive threat model for the OAuth 2.0 protocol. This document is not an Internet Standards Track specification; it is published for informational purposes. This specification defines mechanisms for dynamically registering OAuth 2.0 clients with authorization servers. Registration requests send a set of desired client metadata values to the authorization server. The resulting registration responses return a client identifier to use at the authorization server and the client metadata values registered for the client. The client can then use this registration information to communicate with the authorization server using the OAuth 2.0 protocol. This specification also defines a set of common client metadata fields and values for clients to use during registration. This specification defines a metadata format that an OAuth 2.0 client can use to obtain the information needed to interact with an OAuth 2.0 authorization server, including its endpoint locations and authorization server capabilities. SPRIND Yubico Independent Researcher Authlete This document describes best current security practice for OAuth 2.0. It updates and extends the threat model and security advice given in RFC 6749, RFC 6750, and RFC 6819 to incorporate practical experiences gathered since OAuth 2.0 was published and covers new threats relevant due to the broader application of OAuth 2.0. Further, it deprecates some modes of operation that are deemed less secure or even insecure. 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 Inrupt NAT.Consulting Yubico Self-Issued Consulting Google Disney independent Self-Issued Consulting Sikt Yubico independent Connect2id

Acknowledgments The idea of using URIs as the client_id in OAuth based authorization requests is not new, and has previously been specified in varying ways by , , and . This specification is largely inspired by the work of Aaron Coburn, elf Pavlik, and Dmitri Zagidulin in their specification which defined dereferenceable Client Identifier Documents. The authors would like to thank the following people for their contributions and reviews of this specification: Dick Hardt, Matthieu Sieben.

Document History (This appendix to be deleted by the RFC editor in the final specification.) -02

• Added security consideration around displaying logos to end users
• Changed query string parameters in Client ID Metadata Document URLs to "SHOULD NOT", since this encourages bad security practices (e.g., minting documents based on query string parameters)
• Added prohibition on the client_secret_expires_at property, as it is not relevant for Client ID Metadata Documents.
• Added security consideration for development use-cases.

-01

• Added recommendation of max metadata document size
• Changed metadata property reference to IANA registry instead of Dynamic Client Registration

-00

• Initial draft