Internet-Draft OAuth 2.0 Client Discovery November 2022
Looker & Sivasamy Expires 12 May 2023 [Page]
OAuth Working Group
Intended Status:
T. Looker
K. Sivasamy

OAuth 2.0 Client Discovery


This specification defines a mechanism for an authorization server to obtain the metadata of a client, including its endpoint locations and capabilities without the need for a registration process.

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Table of Contents

1. Introduction

In the traditional OAuth 2.0 model [RFC6749], the authorization server (AS) registers and assigns an identifier to a client through a registration process, during which the authorization server records certain characteristics about the client, commonly known as its metadata.

This requirement for registration greatly reduces how dynamic the relationship between a client and authorization server can be. For instance, a client that is updating the capabilities it supports must update its registration(s) with affected authorization servers for this change to be recognized. This requirement also affects deployments that feature many clients and authorization servers whereby requiring the client to be registered with and maintain this registration with an authorization server is costly.

To enable a more dynamic relationship between a client and an authorization server, dynamic client registration via [RFC7591] was introduced. This model allows a client to register dynamically with a supporting authorization server by sending a registration request. Although this mechanism does provide some benefits it also introduces new operational challenges for both the client and AS. For instance clients that interface with many authorization servers are burdened with having to manage a client identifier per authorization server and in some cases forced to re-register the same client instance multiple times due to local storage limitations. Furthermore, protecting the authorization servers registration endpoint forces other design tradeoffs, typically either the authorization server enforces some form of authentication (e.g a "registration_token") for registration requests, which is often problematic for public clients to manage/obtain. Or the authorization server permits any registration request and has to mitigate potential spam/malicious registration requests via some other mechanism.

Instead of requiring a registration process, this specification describes a model where a client identifies itself to the authorization server with its client_uri, which can be resolved to its metadata in a similar way to how an authorization server makes its metadata available to a client via [RFC8414].

The metadata for a client is retrieved from a .well-known location as a JSON [RFC8259] document, which declares its endpoint locations and client capabilities, this process is described in Obtaining Client Metadata (Section 3). Once the client metadata is retrieved and processed by the authorization server, the client can interact with the authorization server like any other client.

This specification defines a new request parameter 'client_discovery' to indicate that the interacting client has no prior registration with the authorization server and instead has resolvable metadata that describes its endpoint locations and capabilities.

This specification uses the metadata elements defined in the client registration specification [RFC7591] and no additional metadata fields or formats are defined in this specification.

1.1. Conventions and Terminology

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.

This specification uses the terms "access token", "refresh token", "authorization server", "resource server", "authorization endpoint", "authorization request", "authorization response", "token endpoint", "grant type", "access token request", "access token response", "client", "public client", and "confidential client" defined by The OAuth 2.0 Authorization Framework [RFC6749].

The terms "request", "response", "header field", and "target URI" are imported from [RFC9110].

2. Client Metadata

Clients can have metadata described in their configuration. Examples of existing registered metadata fields that a client can make use of can be found at the OAuth 2.0 dynamic client registration metadata IANA registry [RFC 7591].

The client's published metadata MUST include the client_uri field as defined in section 2 of RFC7591 [RFC7591]. The value of this field MUST be a URI as defined in RFC3986 [RFC3986] with a scheme component that MUST be https, a host component, and optionally, port and path components and no query or fragment components. Additionally, host names MUST be domain names and MUST NOT be IPv4 or IPv6 addresses.

3. Obtaining Client Metadata

A Client supporting metadata discovery MUST make a JSON document containing metadata as specified in RFC7591 [RFC7591] available at a path formed by inserting a well-known URI string into the client_uri between the host component and the path component, if any. By default, the well-known URI string used is "/.well-known/oauth-client". This path MUST use the "https" scheme. The syntax and semantics of ".well-known" are defined in RFC 5785 [RFC5785]. The well-known URI suffix used MUST be registered in the IANA "Well-Known URIs" registry.

Different clients utilizing OAuth 2.0 in application-specific ways may define and register different well-known URI suffixes used to publish client metadata as used by those applications, for example using a well-known URI string such as "/.well-known/example-configuration". Alternatively, many such clients will use the default well-known URI string "/.well-known/oauth-client", which is the right choice for general-purpose OAuth 2.0 applications.

An OAuth 2.0 client using this specification MUST specify what well-known URI suffix it will use for this purpose. The same client MAY choose to publish its metadata at multiple well-known locations derived from its client_uri, for example, publishing metadata at both "/.well-known/example-configuration" and "/.well-known/oauth-client".

Some OAuth 2.0 applications will choose to use the well-known URI suffix "openid-federation", as described in Compatibility Notes (Section 8).

3.1. Client Metadata Request

A client metadata document MUST be queried using an HTTP "GET" request at the previously specified path. The OAuth 2.0 authorization server would make the following request when the client_uri is "" and the well-known URI suffix is "oauth-client" to obtain the metadata, since the client_uri contains no path component:

GET /.well-known/oauth-client HTTP/1.1

If the client_uri value contains a path component, any terminating "/" MUST be removed before inserting "/.well-known/" and the well-known URI suffix between the host component and the path component. The OAuth 2.0 authorization server would make the following request when the client_uri is "" and the well-known URI suffix is "oauth-client" to obtain the metadata, since the client_uri contains a path component:

GET /.well-known/oauth-client/client1 HTTP/1.1

Using path components enables supporting multiple clients per host. This is required in some complex client configurations. This use of ".well-known" is for supporting multiple clients per host; unlike its use in RFC 5785 [RFC5785], it does not provide general information about the host.

3.2. Client Metadata Response

The response is a set of metadata values describing client's configuration, including all valid redirection URIs and features supported by the client. A successful response MUST use the 200 OK HTTP status code and return a JSON object using the "application/json" content type that contains a set of metadata fields and values as defined in Client Metadata (Section 2). Other metadata fields MAY also be returned.

Metadata fields that return multiple values are represented as JSON arrays. Metadata fields with no values MUST be omitted from the response.

An error response uses the applicable HTTP status code value.

The following is a non-normative example response:

HTTP/1.1 200 OK
Content-Type: application/json

    "client_uri": "",
    "client_name": "My Example Client",
    "redirect_uris": [
    "logo_uri": "",
    "jwks_uri": "",
    "example_extension_parameter": "example_value"

3.3. Client Metadata Validation

The client_uri value returned in the client metadata response MUST be identical to the client_uri value into which the well-known URI string was inserted to create the URL used to retrieve the metadata. If these values are not identical, the data contained in the response MUST NOT be used.

The following sections describe the mechanism through which a client communicates its metadata discovery url.

4. Authorization Request Using Client Discovery

A client can indicate to an authorization server that it has discoverable metadata in an authorization request via the "client_discovery" request parameter. Presence of this parameter in an authorization request with a value of "true" indicates to the authorization server that the "client_id" value of the authorization request is a URL encoded value corresponding to the "client_uri" for the client and if the authorization server does not already have the metadata for the identified client it can retrieve the metadata by following the procedure outlined in Client Metadata Section (Section 2).

The following is a non-normative example request of a client making an authorization request to an authorization server with the "client_discovery" parameter:

GET /authorize?response_type=code

The value of the "client_id" parameter in the authorization request MUST represent the URL encoded form of the "client_uri" value for the corresponding client. The "client_id" value MUST be URL decoded by the authorization server to obtain the "client_uri" value which can be used to resolve the client metadata as described in the Obtaining Client Metadata (Section 3) section.

TODO stipulate new error responses

5. Token Request Using Client Discovery

A client can indicate to an authorization server that it has discoverable metadata in an token request via the "client_discovery" request parameter. Presence of this parameter in a token request with a value of "true" indicates to the authorization server that the "client_id" value of the token request represents then "client_uri" for the client and if the authorization server does not already have the metadata for the identified client it can retrieve the metadata by following the procedure outlined in Client Metadata Section (Section 2).

The following is a non-normative example request of a client making an token request using "client_discovery" parameter:

POST /token
Content-type: application/x-www-form-urlencoded
Accept: application/json


The "client_id" parameter is passed to the token request during client authentication (as described in the Section 3.2.1 of [RFC6749]). Clients in possession of a client password MAY use the HTTP Basic authentication scheme as defined in RFC 2617 [RFC2617] or MAY include the client credentials in the request-body to authenticate with the authorization server.

In case of any errors, error response is returned (as described in the Section 5.2 of [RFC6749]). TODO expand

6. String Operations

Processing some OAuth 2.0 messages requires comparing values in the messages to known values. For example, the member names in the metadata response might be compared to specific member names such as "client_uri". Comparing Unicode [UNICODE] strings, however, has significant security implications.

Therefore, comparisons between JSON strings and other Unicode strings MUST be performed as specified below:

  1. Remove any JSON-applied escaping to produce an array of Unicode code points.
  2. Unicode Normalization [USA15] MUST NOT be applied at any point to either the JSON string or the string it is to be compared against.
  3. Comparisons between the two strings MUST be performed as a Unicode code-point-to-code-point equality comparison.

Note that this is the same equality comparison procedure described in ( Section 8.3 of [RFC8259]).

7. Security Considerations

7.1. TLS Requirements

Implementations MUST support TLS. Which version(s) ought to be implemented will vary over time and depend on the widespread deployment and known security vulnerabilities at the time of implementation. The client MUST support TLS version 1.2 [RFC5246] and MAY support additional TLS mechanisms meeting its security requirements. When using TLS, the authorization server MUST perform a TLS/SSL server certificate check, per RFC 6125 [RFC6125]. Implementation security considerations can be found in "Recommendations for Secure Use of Transport Layer Security (TLS) and Datagram Transport Layer Security (DTLS)" [BCP195].

To protect against information disclosure and tampering, confidentiality protection MUST be applied using TLS with a ciphersuite that provides confidentiality and integrity protection.

7.2. Impersonation Attacks

TLS certificate checking MUST be performed by the authorization server, as described in Section 7.1, when making a client metadata request. Checking that the server certificate is valid for the "client_uri" URL prevents man-in-middle and DNS-based attacks. These attacks could cause a authorization server to be tricked into using an attacker's keys and endpoints, which would enable impersonation of the legitimate client. If an attacker can accomplish this, they can access the resources that the affected client has access to by impersonating their profile.

An attacker may also attempt to impersonate a client by publishing a metadata document that contains a "client_uri" claim using the "client_uri" URL of the client being impersonated, but with its own endpoints and signing keys. This would enable it to impersonate that client, if accepted by the authorization server. To prevent this, the authorization server MUST ensure that the "client_uri" URL it is using as the prefix for the metadata request exactly matches the value of the "client_uri" metadata value in the client's metadata document received by the authorization server.

8. Compatibility Notes


9. IANA Considerations

The following IANA registration requests are made by this document.

9.1. OAuth Parameters Registry

This specification registers the following parameters in the IANA "OAuth Parameters" registry defined in OAuth 2.0 RFC 6749 [RFC6749]

client_discovery - Authorization request

  • Parameter name: client_discovery
  • Parameter usage location: authorization request
  • Change controller: IESG
  • Specification document(s): RFC XXXX (this document)

client_discovery - Token request

  • Parameter name: client_discovery
  • Parameter usage location: token request
  • Change controller: IESG
  • Specification document(s): RFC XXXX (this document)

9.2. Well-Known URI Registry

This specification registers the well-known URI defined in Obtaining Client Metadata (Section 3) in the (IANA "Well-Known URIs" registry) established by RFC 5785 [RFC5785]

9.2.1. Registry Contents

  • URI suffix: oauth-client
  • Change controller: IESG
  • Specification document: Section 3 of RFC 8414
  • Related information: (none)

<< TODO - IANA registration - >>

10. Normative References

Sheffer, Y., Holz, R., and P. Saint-Andre, "Recommendations for Secure Use of Transport Layer Security (TLS) and Datagram Transport Layer Security (DTLS)", BCP 195, RFC 7525, .
Moriarty, K. and S. Farrell, "Deprecating TLS 1.0 and TLS 1.1", BCP 195, RFC 8996, .
Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <>.
Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S., Leach, P., Luotonen, A., and L. Stewart, "HTTP Authentication: Basic and Digest Access Authentication", RFC 2617, DOI 10.17487/RFC2617, , <>.
Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform Resource Identifier (URI): Generic Syntax", STD 66, RFC 3986, DOI 10.17487/RFC3986, , <>.
Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS) Protocol Version 1.2", RFC 5246, DOI 10.17487/RFC5246, , <>.
Nottingham, M. and E. Hammer-Lahav, "Defining Well-Known Uniform Resource Identifiers (URIs)", RFC 5785, DOI 10.17487/RFC5785, , <>.
Saint-Andre, P. and J. Hodges, "Representation and Verification of Domain-Based Application Service Identity within Internet Public Key Infrastructure Using X.509 (PKIX) Certificates in the Context of Transport Layer Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, , <>.
Hardt, D., Ed., "The OAuth 2.0 Authorization Framework", RFC 6749, DOI 10.17487/RFC6749, , <>.
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, , <>.
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, , <>.
Bray, T., Ed., "The JavaScript Object Notation (JSON) Data Interchange Format", STD 90, RFC 8259, DOI 10.17487/RFC8259, , <>.
Jones, M., Sakimura, N., and J. Bradley, "OAuth 2.0 Authorization Server Metadata", RFC 8414, DOI 10.17487/RFC8414, , <>.
Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke, Ed., "HTTP Semantics", STD 97, RFC 9110, DOI 10.17487/RFC9110, , <>.


TODO acknowledge.

Authors' Addresses

Tobias Looker
Karthik Sivasamy