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Security OAuth Working Group OAuth Discovery Metadata Discovery Metadata Configuration Information Resource Server Protected Resource Resource Identifier JavaScript Object Notation JSON JSON Web Token JWT This specification defines a metadata format that an OAuth 2.0 client or authorization server can use to obtain the information needed to interact with an OAuth 2.0 protected resource.

This specification defines a metadata format enabling OAuth 2.0 clients and authorization servers to obtain information needed to interact with an OAuth 2.0 protected resource. The structure and content of this specification is intentionally as parallel as possible to that of "OAuth 2.0 Dynamic Client Registration Protocol", which enables a client to provide metadata about itself to an OAuth 2.0 authorization server and to OAuth 2.0 Authorization Server Metadata, which enables a client to obtain metadata about an OAuth 2.0 authorization server. The metadata for a protected resource is retrieved from a well-known location as a JSON document, which declares information about its capabilities and optionally, its relationships to other services. This process is described in . This metadata can either be communicated in a self-asserted fashion or as a set of signed metadata values represented as claims in a JSON Web Token (JWT) . In the JWT case, the issuer is vouching for the validity of the data about the protected resource. This is analogous to the role that the Software Statement plays in OAuth Dynamic Client Registration . Each protected resource publishing metadata about itself makes its own metadata document available at a well-known location deterministically derived from the protected resource's URL, even when the resource server implements multiple protected resources. This prevents attackers from publishing metadata supposedly describing the protected resource, but that is not actually authoritative for the protected resource, as described in . The means by which the client obtains the location of the protected resource is out of scope. In some cases, the location may be manually configured into the client. In other cases, it may be dynamically discovered, for instance, through the use of WebFinger, in a manner related to the description in Section 2 of "OpenID Connect Discovery 1.0". defines metadata values that a protected resource can publish, which includes things like which scopes are supported, how a client can present an access token, and more. These values may be used by other specifications, such as the jwks_uri used to publish public keys the resource server uses to sign resource responses, for instance, as described in .

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. All uses of JSON Web Signature (JWS) and JSON Web Encryption (JWE) data structures in this specification utilize the JWS Compact Serialization or the JWE Compact Serialization; the JWS JSON Serialization and the JWE JSON Serialization are not used.

This specification uses the terms "Access Token", "Authorization Code", "Authorization Endpoint", "Authorization Grant", "Authorization Server", "Client", "Client Authentication", "Client Identifier", "Client Secret", "Grant Type", "Protected Resource", "Redirection URI", "Refresh Token", "Resource Owner", "Resource Server", "Response Type", and "Token Endpoint" defined by OAuth 2.0, the terms "Claim Name", "Claim Value", and "JSON Web Token (JWT)" defined by JSON Web Token (JWT). This specification defines the following term: The Protected resource's resource identifier, which is a URL that uses the https scheme and has no query or fragment components. Protected resource metadata is published at a .well-known location derived from this resource identifier, as described in .

Protected resources can have metadata describing their configuration. The following protected resource metadata values are used by this specification and are registered in the IANA "OAuth Protected Resource Metadata" registry established in : REQUIRED. The protected resource's resource identifier, which is a URL that uses the https scheme and has no query or fragment components. Using these well-known resources is described in . OPTIONAL. JSON array containing a list of OAuth authorization server issuer identifiers, as defined in , for authorization servers that can be used with this protected resource. Protected resources MAY choose not to advertise some supported authorization servers even when this parameter is used. In some use cases, the set of authorization servers will not be enumerable, in which case this metadata parameter would not be used. OPTIONAL. URL of the protected resource's JSON Web Key (JWK) Set document. This contains public keys belonging to the protected resource, such as signing key(s) that the resource server uses to sign resource responses. This URL MUST use the https scheme. When both signing and encryption keys are made available, a use (public key use) parameter value is REQUIRED for all keys in the referenced JWK Set to indicate each key's intended usage. RECOMMENDED. JSON array containing a list of the OAuth 2.0 scope values that are used in authorization requests to request access to this protected resource. Protected resources MAY choose not to advertise some scope values supported even when this parameter is used. OPTIONAL. JSON array containing a list of the supported methods of sending an OAuth 2.0 Bearer Token to the protected resource. Defined values are ["header", "body", "query"], corresponding to Sections 2.1, 2.2, and 2.3 of RFC 6750. OPTIONAL. JSON array containing a list of the JWS signing algorithms (alg values) supported by the protected resource for signing resource responses, for instance, as described in . No default algorithms are implied if this entry is omitted. The value none MUST NOT be used. OPTIONAL. URL of a page containing human-readable information that developers might want or need to know when using the protected resource OPTIONAL. URL that the protected resource provides to read about the protected resource's requirements on how the client can use the data provided by the protected resource OPTIONAL. URL that the protected resource provides to read about the protected resource's terms of service Additional protected resource metadata parameters MAY also be used.

In addition to JSON elements, metadata values MAY also be provided as a signed_metadata value, which is a JSON Web Token (JWT) that asserts metadata values about the protected resource as a bundle. A set of claims that can be used in signed metadata are defined in . The signed metadata MUST be digitally signed or MACed using JSON Web Signature (JWS) and MUST contain an iss (issuer) claim denoting the party attesting to the claims in the signed metadata. Consumers of the metadata MAY ignore the signed metadata if they do not support this feature. If the consumer of the metadata supports signed metadata, metadata values conveyed in the signed metadata MUST take precedence over the corresponding values conveyed using plain JSON elements. Signed metadata is included in the protected resource metadata JSON object using this OPTIONAL member: A JWT containing metadata values about the protected resource as claims. This is a string value consisting of the entire signed JWT. A signed_metadata metadata value SHOULD NOT appear as a claim in the JWT.

Protected resources supporting metadata MUST make a JSON document containing metadata as specified in available at a path formed by inserting a well-known URI string into the protected resource's resource identifier between the host component and the path component, if any. By default, the well-known URI string used is /.well-known/oauth-protected-resource. The syntax and semantics of .well-known are defined in . The well-known URI path suffix used MUST be registered in the IANA "Well-Known URIs" registry . Examples of this construction can be found in . The term "application", as used below (and as used in ), encompasses all the components used to accomplish the task for the use case. That can include OAuth clients, authorization servers, protected resources, and non-OAuth components, inclusive of the code running in each of them. Applications are built to solve particular problems and may utilize many components and services. Different applications utilizing OAuth protected resources in application-specific ways MAY define and register different well-known URI path suffixes for publishing protected resource metadata used by those applications. For instance, if the Example application uses an OAuth protected resource in an Example-specific way, and there are Example-specific metadata values that it needs to publish, then it might register and use the example-protected-resource URI path suffix and publish the metadata document at the path formed by inserting /.well-known/example-protected-resource between the host and path components of the protected resource's resource identifier. Alternatively, many such applications will use the default well-known URI string /.well-known/oauth-protected-resource, which is the right choice for general-purpose OAuth protected resources, and not register an application-specific one. An OAuth 2.0 application using this specification MUST specify what well-known URI suffix it will use for this purpose. The same protected resource MAY choose to publish its metadata at multiple well-known locations derived from its resource identifier, for example, publishing metadata at both /.well-known/example-protected-resource and /.well-known/oauth-protected-resource.

A protected resource metadata document MUST be queried using an HTTP GET request at the previously specified path. The consumer of the metadata would make the following request when the resource identifier is https://resource.example.com and the well-known URI path suffix is oauth-protected-resource to obtain the metadata, since the resource identifier contains no path component:

If the resource identifier value contains a path component, any terminating / MUST be removed before inserting /.well-known/ and the well-known URI path suffix between the host component and the path component. The consumer of the metadata would make the following request when the resource identifier is https://resource.example.com/resource1 and the well-known URI path suffix is oauth-protected-resource to obtain the metadata, since the resource identifier contains a path component:

Using path components enables supporting multiple resources per host. This is required in some multi-tenant hosting configurations. This use of .well-known is for supporting multiple resources per host; unlike its use in , it does not provide general information about the host.

The response is a set of claims about the protected resource's configuration. 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 claims as its members that are a subset of the metadata values defined in . Other claims MAY also be returned. Claims that return multiple values are represented as JSON arrays. Claims with zero elements MUST be omitted from the response. An error response uses the applicable HTTP status code value.

The following is a non-normative example response:

The resource value returned MUST be identical to the protected resource's resource identifier value into which the well-known URI path suffix 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. If the protected resource metadata was retrieved from a URL returned by the protected resource via the WWW-Authenticate resource_metadata parameter, then the resource value returned MUST be identical to the URL that the client used to make the request to the resource server. If these values are not identical, the data contained in the response MUST NOT be used. These validation actions can thwart impersonation attacks, as described in .

To support use cases in which the set of legitimate protected resources to use with the authorization server is fixed and enumerable, this specification defines the authorization server metadata value protected_resources, which enables the authorization server to explicitly list the protected resources. Note that if the set of legitimate authorization servers to use with a protected resource is also fixed and enumerable, lists in the authorization server metadata and protected resource metadata should be cross-checked against one another for consistency when these lists are used by the application profile. The following authorization server metadata value is defined by this specification and is registered in the IANA "OAuth Authorization Server Metadata" registry established in OAuth 2.0 Authorization Server Metadata. OPTIONAL. JSON array containing a list of resource identifiers for OAuth protected resources for protected resources that can be used with this authorization server. Authorization servers MAY choose not to advertise some supported protected resources even when this parameter is used. In some use cases, the set of protected resources will not be enumerable, in which case this metadata parameter will not be present.

A protected resource MAY use a WWW-Authenticate response to return a URL to its protected resource metadata to the client. The client can then retrieve protected resource metadata as described in . The client might then, for instance, determine what authorization server to use for the resource based on protected resource metadata retrieved. A typical end-to-end flow doing so is as follows. Note that while this example uses the OAuth 2.0 Authorization Code flow, a similar sequence could also be implemented with any other OAuth flow.

Sequence Diagram | | | Without Access Token | | | | | | | | | WWW-Authenticate | | | <---------------------- | | | | | | | | | Fetch RS Metadata | | | ----------------------> | | | | | | | | | RS Metadata Response | | | <---------------------- | | | | | +---------+-----------+ | | |Validate RS Metadata | | | |Build AS Metadata URL| | | +---------+-----------+ | | | | | | Fetch AS Metadata | | | ------------------------+----------------> | | | | | | | | AS Metadata Response | | | <-----------------------+----------------- | | | | +-+-------------------------+------------------+-+ | OAuth Authorization Code Flow | | Client Obtains Access Token | +-+-------------------------+------------------+-+ | | | | Resource Request | | | ----------------------> | | | With Access Token | | | | | | | | | Resource Response | | | <---------------------- | | | | | ]]>

The client makes a request to a protected resource without presenting an access token. The resource server responds with a WWW-Authenticate header including the URL of the protected resource metadata. The client fetches the protected resource metadata from this URL. The resource server responds with the protected resource metadata according to . The client validates the protected resource metadata, as described in . The client builds the authorization server metadata URL from an issuer identifier in the resource metadata according to and makes a request to fetch the authorization server metadata. The authorization server responds with the authorization server metadata document according to . The client directs the user agent to the authorization server to begin the authorization flow. The authorization exchange is completed and the authorization server returns an access token to the client. The client repeats the resource request from step 1, presenting the newly obtained access token. The resource server returns the requested protected resource.

This specification introduces a new parameter in the WWW-Authenticate response to indicate the protected resource metadata URL: The URL of the protected resource metadata.

The response below is an example of a WWW-Authenticate header that includes the resource identifier.

The HTTP status code and error string in the response are defined by . The resource_metadata parameter MAY be combined with other parameters defined in other extensions, such as the max_age parameter defined by .

At any point, for any reason determined by the protected resource, the protected resource MAY respond with a new WWW-Authenticate challenge that includes a value for the protected resource metadata URL to indicate that its metadata MAY have changed. If the client receives such a WWW-Authenticate response, it SHOULD retrieve the updated protected resource metadata and use the new metadata values obtained. Among other things, this enables a resource server to change which authorization servers it uses without any other coordination with clients.

The way in which the client identifier is established at the authorization server is out of scope of this specification. This specification is intended to be deployed in scenarios where the client has no prior knowledge about the resource server, and the resource server might or might not have prior knowledge about the client. There are some existing methods by which an unrecognized client can make use of an authorization server, such as using Dynamic Client Registration to register the client prior to initiating the authorization flow. Future extensions might define alternatives, such as using URLs to identify clients.

Resource servers MAY return other WWW-Authenticate headers indicating various authentication schemes. This allows the resource server to support clients that may or may not implement this specification, and allows clients to choose their preferred authentication scheme.

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 resource. Comparing Unicode strings, however, has significant security implications. Therefore, comparisons between JSON strings and other Unicode strings MUST be performed as specified below: Remove any JSON applied escaping to produce an array of Unicode code points. Unicode Normalization MUST NOT be applied at any point to either the JSON string or to the string it is to be compared against. 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 .

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. Implementations SHOULD follow the guidance in BCP 195 , which provides recommendations and requirements for improving the security of deployed services that use TLS. To protect against information disclosure and tampering, confidentiality protection MUST be applied using TLS with a ciphersuite that provides confidentiality and integrity protection.

The scopes_supported parameter is the list of scopes the resource server is willing to disclose that it supports. It is not meant to indicate that an OAuth client should request all scopes in the list. The client SHOULD still follow OAuth best practices and request tokens with as limited scope as possible for the given operation, as described in Section 2.3 of OAuth 2.0 Security Best Current Practice .

TLS certificate checking MUST be performed by the client, as described in , when making a protected resource metadata request. Checking that the server certificate is valid for the resource identifier URL prevents man-in-middle and DNS-based attacks. These attacks could cause a client to be tricked into using an attacker's resource server, which would enable impersonation of the legitimate protected resource. If an attacker can accomplish this, they can access the resources that the affected client has access to using the protected resource that they are impersonating. An attacker may also attempt to impersonate a protected resource by publishing a metadata document that contains a resource claim using the resource identifier URL of the protected resource being impersonated, but containing information of the attacker's choosing. This would enable it to impersonate that protected resource, if accepted by the client. To prevent this, the client MUST ensure that the resource identifier URL it is using as the prefix for the metadata request exactly matches the value of the resource metadata value in the protected resource metadata document received by the client, as described in .

If a client expects to interact with multiple resource servers, the client SHOULD request audience-restricted access tokens using , and the authorization server SHOULD support audience-restricted access tokens. Without audience-restricted access tokens, a malicious resource server (RS1) may be able to use the WWW-Authenticate header to get a client to request an access token with a scope used by a legitimate resource server (RS2), and after the client sends a request to RS1, then RS1 could re-use the access token at RS2. While this attack is not explicitly enabled by this specification, and is possible in a plain OAuth 2.0 deployment, it is made somewhat more likely by the use of dynamically-configured clients. As such, the use of audience-restricted access tokens and Resource Indicators is RECOMMENDED when using the features in this specification.

Publishing information about the protected resource in a standard format makes it easier for both legitimate clients and attackers to use the protected resource. Whether a protected resource publishes its metadata in an ad-hoc manner or in the standard format defined by this specification, the same defenses against attacks that might be mounted that use this information should be applied.

To support use cases in which the set of legitimate authorization servers to use with the protected resource is fixed and enumerable, this specification defines the authorization_servers metadata value, which enables explicitly listing them. Note that if the set of legitimate protected resources to use with an authorization server is also fixed and enumerable, lists in the protected resource metadata and authorization server metadata should be cross-checked against one another for consistency when these lists are used by the application profile. Secure determination of appropriate authorization servers to use with a protected resource for all use cases is out of scope of this specification. This specification assumes that the client has a means of determining appropriate authorization servers to use with a protected resource and that the client is using the correct metadata for each protected resource. Implementers need to be aware that if an inappropriate authorization server is used by the client, that an attacker may be able to act as a man-in-the-middle proxy to a valid authorization server without it being detected by the authorization server or the client. The ways to determine the appropriate authorization servers to use with a protected resource are in general, application-dependent. For instance, some protected resources are used with a fixed authorization server or set of authorization servers, the locations of which may be well known, or which could be published as metadata values by the protected resource. In other cases, the set of authorization servers that can be used with a protected resource can by dynamically changed by administrative actions or by changes to the set of authorization servers adhering to a trust framework. Many other means of determining appropriate associations between protected resources and authorization servers are also possible.

The OAuth client is expected to fetch the authorization server metadata based on the value of the issuer in the resource server metadata. Since this specification enables clients to interoperate with RSs and ASs it has no prior knowledge of, this opens a risk for SSRF attacks by malicious users or malicious resource servers. Clients SHOULD take appropriate precautions against SSRF attacks, such as blocking requests to internal IP address ranges. Further recommendations can be found in the OWASP SSRF Prevention Cheat Sheet .

This specification may be deployed in a scenario where the desired HTTP resource is identified by a user-selected URL. If this resource is malicious or compromised, it could mislead the user into revealing their account credentials or authorizing unwanted access to OAuth-controlled capabilities. This risk is reduced, but not eliminated, by following best practices for OAuth user interfaces, such as providing clear notice to the user, displaying the authorization server's domain name, supporting origin-bound phishing-resistant authenticators, supporting the use of password managers, and applying heuristic checks such as domain reputation.

Unsigned metadata is integrity protected by use of TLS at the site where it is hosted. This means that its security is dependent upon the Internet Public Key Infrastructure (PKI) . Signed metadata is additionally integrity protected by the JWS signature applied by the issuer, which is not dependent upon the Internet PKI. When using unsigned metadata, the party issuing the metadata is the protected resource itself, which is represented by the resource value in the metadata. Whereas, when using signed metadata, the party issuing the metadata is represented by the iss (issuer) claim in the signed metadata. When using signed metadata, applications can make trust decisions based on the issuer that performed the signing -- information that is not available when using unsigned metadata. How these trust decisions are made is out of scope for this specification.

The following registration procedure is used for the registry established by this specification. Values are registered on a Specification Required basis after a two-week review period on the oauth-ext-review@ietf.org mailing list, on the advice of one or more Designated Experts. However, to allow for the allocation of values prior to publication, the Designated Experts may approve registration once they are satisfied that such a specification will be published. Registration requests sent to the mailing list for review should use an appropriate subject (e.g., "Request to register OAuth Protected Resource Metadata: example"). Within the review period, the Designated Experts will either approve or deny the registration request, communicating this decision to the review list and IANA. Denials should include an explanation and, if applicable, suggestions as to how to make the request successful. Registration requests that are undetermined for a period longer than 21 days can be brought to the IESG's attention (using the iesg@ietf.org mailing list) for resolution. Criteria that should be applied by the Designated Experts includes determining whether the proposed registration duplicates existing functionality, determining whether it is likely to be of general applicability or whether it is useful only for a single application, and whether the registration makes sense. IANA must only accept registry updates from the Designated Experts and should direct all requests for registration to the review mailing list. It is suggested that multiple Designated Experts be appointed who are able to represent the perspectives of different applications using this specification, in order to enable broadly-informed review of registration decisions. In cases where a registration decision could be perceived as creating a conflict of interest for a particular Expert, that Expert should defer to the judgment of the other Experts.

This specification establishes the IANA "OAuth Protected Resource Metadata" registry for OAuth 2.0 protected resource metadata names. The registry records the protected resource metadata member and a reference to the specification that defines it.

The name requested (e.g., "resource"). This name is case-sensitive. Names may not match other registered names in a case-insensitive manner unless the Designated Experts state that there is a compelling reason to allow an exception. Brief description of the metadata (e.g., "Resource identifier URL"). For Standards Track RFCs, list the "IETF". For others, give the name of the responsible party. Other details (e.g., postal address, email address, home page URI) may also be included. Reference to the document or documents that specify the parameter, preferably including URIs that can be used to retrieve copies of the documents. An indication of the relevant sections may also be included but is not required.

Metadata Name: resource Metadata Description: Protected resource's resource identifier URL Change Controller: IETF Specification Document(s): of [[ this specification ]] Metadata Name: authorization_servers Metadata Description: JSON array containing a list of OAuth authorization server issuer identifiers Change Controller: IETF Specification Document(s): of [[ this specification ]] Metadata Name: jwks_uri Metadata Description: URL of the protected resource's JWK Set document Change Controller: IETF Specification Document(s): of [[ this specification ]] Metadata Name: scopes_supported Metadata Description: JSON array containing a list of the OAuth 2.0 scope values that are used in authorization requests to request access this protected resource Change Controller: IETF Specification Document(s): of [[ this specification ]] Metadata Name: bearer_methods_supported Metadata Description: JSON array containing a list of the OAuth 2.0 Bearer Token presentation methods that this protected resource supports Change Controller: IETF Specification Document(s): of [[ this specification ]] Metadata Name: resource_signing_alg_values_supported Metadata Description: JSON array containing a list of the JWS signing algorithms (alg values) supported by the protected resource for signed content Change Controller: IETF Specification Document(s): of [[ this specification ]] Metadata Name: resource_documentation Metadata Description: URL of a page containing human-readable information that developers might want or need to know when using the protected resource Change Controller: IETF Specification Document(s): of [[ this specification ]] Metadata Name: resource_policy_uri Metadata Description: URL that the protected resource provides to read about the protected resource's requirements on how the client can use the data provided by the protected resource Change Controller: IETF Specification Document(s): of [[ this specification ]] Metadata Name: resource_tos_uri Metadata Description: URL that the protected resource provides to read about the protected resource's terms of service Change Controller: IETF Specification Document(s): of [[ this specification ]] Metadata Name: signed_metadata Metadata Description: Signed JWT containing metadata values about the protected resource as claims Change Controller: IETF Specification Document(s): of [[ this specification ]]

The following authorization server metadata value is registered in the IANA "OAuth Authorization Server Metadata" registry established in OAuth 2.0 Authorization Server Metadata.

Metadata Name: protected_resources Metadata Description: JSON array containing a list of resource identifiers for OAuth protected resources Change Controller: IETF Specification Document(s): of [[ this specification ]]

This specification registers the well-known URI defined in in the IANA "Well-Known URIs" registry .

URI suffix: oauth-protected-resource Change controller: IETF Specification document: of [[ this specification ]] Related information: (none)

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The authors of this specification would like to thank the attendees of the IETF 115 OAuth and HTTP API Working Group meetings and the attendees of subsequent OAuth Working Group meetings for their input on this specification. We would would also like to thank Brian Campbell, Deb Cooley, Vladimir Dzhuvinov, George Fletcher, Pieter Kasselman, Tony Nadalin, Rifaat Shekh-Yusef, Filip Skokan, and Atul Tulshibagwale for their contributions to the specification.

[[ to be removed by the RFC Editor before publication as an RFC ]] -08 Added Security Considerations about the differences between unsigned and signed metadata, as suggested by Deb Cooley. Updated obsolete references. -07 Removed extraneous paragraph about downgrade attacks discussing an issue that's already addressed elsewhere in the specification. -06 Addressed shepherd review comments by Rifaat Shekh-Yusef. -05 Added SVG diagram -04 Applied working group last call suggestions by Atul Tulshibagwale. Better described the purpose of resource_signing_alg_values_supported and removed resource_encryption_alg_values_supported and resource_encryption_enc_values_supported, per WGLC comments by Vladimir Dzhuvinov and Brian Campbell. Applied suggestions by Pieter Kasselman. -03 Applied correction by Filip Skokan. -02 Switched from concatenating .well-known to the end of the resource identifier to inserting it between the host and path components of it. Have WWW-Authenticate return resource_metadata rather than resource. -01 Renamed scopes_provided to scopes_supported. Added security consideration for scopes_supported. Use BCP 195 for TLS recommendations. Clarified that resource metadata can be used by clients and authorization servers. Updated references. Added security consideration recommending audience-restricted access tokens. Mention FAPI Message Signing as a use case for publishing signing keys. -00 Initial working group version based on draft-jones-oauth-resource-metadata-04.