| Internet-Draft | OAuth 2.0 Protected Resource Metadata | September 2023 |
| Jones, et al. | Expires 9 March 2024 | [Page] |
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 protected resource.¶
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This specification defines a metadata format enabling OAuth 2.0 clients to obtain information needed to interact with an OAuth 2.0 protected resource. This specification is intentionally as parallel as possible to "OAuth 2.0 Dynamic Client Registration Protocol" [RFC7591], which enables a client to provide metadata about itself to an OAuth 2.0 authorization server and to OAuth 2.0 Authorization Server Metadata [OAuth.AuthorizationMetadata], 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 [RFC7159] document, which declares information about its capabilities and optionally, its relationships to other services. This process is described in Section 3.¶
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) [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 [RFC7591].¶
Each protected resource publishing metadata about itself makes its own metadata document available at a well-known location rooted at the protect 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 Section 7.2.¶
The means by which the client obtains the location of the protected resource metadata document 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 [RFC7033], in a manner related to the description in Section 2 of "OpenID Connect Discovery 1.0" [OpenID.Discovery].¶
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.¶
All uses of JSON Web Signature (JWS) [JWS] and JSON Web Encryption (JWE) [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 [RFC6749], the terms "Claim Name", "Claim Value", and "JSON Web Token (JWT)" defined by JSON Web Token (JWT) [JWT].¶
This specification defines the following term:¶
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 Section 8.1:¶
https scheme and has no fragment components.
This is the location where
.well-known [RFC5785] resources
containing information about the protected resource are published.
Using these well-known resources is described in Section 3.¶
use (public key use) parameter
value is REQUIRED for all keys in the referenced JWK Set
to indicate each key's intended usage.¶
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 provided
even when this parameter is used.¶
["header", "fragment", "query"],
corresponding to Sections 2.1, 2.2, and 2.3 of RFC 6750.¶
alg values) [JWA]
supported by the protected resource
for signed content.
The value none MAY be included.¶
alg values) [JWA]
supported by the protected resource
for encrypted content.¶
enc values) [JWA]
supported by the protected resource
for encrypted content.¶
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) [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 Section 2.
The signed metadata MUST be digitally signed or MACed
using JSON Web Signature (JWS) [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 those conveyed using plain JSON elements.¶
Signed metadata is included in the protected resource metadata JSON object using this OPTIONAL member:¶
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 Section 2
available at a path formed by
concatenating a well-known URI string such as
/.well-known/oauth-protected-resource to the
protected resource's resource identifier.
The syntax and semantics of .well-known
are defined in [RFC5785].
The well-known URI path suffix used MUST be registered in the IANA
"Well-Known URIs" registry [IANA.well-known].¶
Different applications utilizing OAuth protected resources in application-specific ways
may define and register different well-known URI path suffixes
used to publish protected resource metadata as 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-resource-configuration URI path suffix and publish
the metadata document at the path formed by concatenating
/.well-known/example-resource-configuration to the
protected resource's resource identifier.¶
An OAuth 2.0 application using this specification MUST specify
what well-known URI string it will use for this purpose.
The same protected resource MAY choose to publish its metadata at multiple
well-known locations relative to its resource identifier,
for example, publishing metadata at both
/.well-known/example-resource-configuration 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:¶
GET /.well-known/oauth-protected-resource HTTP/1.1 Host: resource.example.com¶
If the
resource identifier value contains a path component, any terminating
/ MUST be removed before appending
/.well-known/ and the well-known URI path suffix.
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:¶
GET /resource1/.well-known/oauth-protected-resource HTTP/1.1 Host: resource.example.com¶
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
[RFC5785], 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
Section 2.
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:¶
HTTP/1.1 200 OK
Content-Type: application/json
{
"resource":
"https://resource.example.com",
"authorization_servers":
["https://as1.example.com",
"https://as2.example.net"],
"bearer_methods_supported":
["header", "body"],
"resource_documentation":
"http://resource.example.com/resource_documentation.html"
}
¶
The resource value returned MUST be identical to
the protected resource's resource identifier value that was
concatenated with the well-known URI path suffix 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.¶
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 protected_resources
metadata value, which enables explicitly listing them.
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 [OAuth.AuthorizationMetadata].¶
A protected resource MAY use a WWW-Authenticate response to return its resource identifier to the client.
The client can then retrieve protected resource metadata using the resource identifier as described in Section 3.
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.¶
WWW-Authenticate header including the resource identifier of the protected resource.¶
.well-known/oauth-protected-resource location
derived from the resource identifier according to Section 3.¶
This specification introduces a new parameter in the WWW-Authenticate response
to indicate the resource identifier of the protected resource:¶
The response below is an example of a WWW-Authenticate header that includes the resource identifier.¶
HTTP/1.1 400 Bad Request
WWW-Authenticate: Bearer error="invalid_request",
error_description="No access token was provided in this request",
resource="https://resource.example.com"
¶
The HTTP status code and error string in the response are defined by [RFC6750].¶
The issuer parameter MAY be combined with other parameters defined in other extensions,
such as the max_age parameter defined by [I-D.ietf-oauth-step-up-authn-challenge].¶
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 resource identifier to indicate that its metadata MAY have changed.
If the client receives such a WWW-Authenticate response,
it is expected retrieve the protected resource metadata again,
and SHOULD 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 [RFC7591], 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 [UNICODE] strings,
however, has significant security implications.¶
Therefore, comparisons between JSON strings and other Unicode strings MUST be performed as specified below:¶
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 protected resource MUST support TLS version 1.2 [RFC5246] and MAY support additional transport-layer security mechanisms meeting its security requirements. When using TLS, the client MUST perform a TLS/SSL server certificate check, per [RFC6125]. Implementation security considerations can be found in Recommendations for Secure Use of TLS and 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.¶
TLS certificate checking MUST be performed by the client, as described in Section 7.1, 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.¶
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.¶
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.¶
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.¶
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 [OWASP.SSRF].¶
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 the use of password managers, and applying heuristic checks such as domain reputation.¶
The following registration procedure is used for the registry established by this specification.¶
Values are registered on a Specification Required [RFC5226] 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.¶
resource¶
authorization_servers¶
jwks_uri¶
scopes_provided¶
scope values that
are used in authorization requests to request access this protected resource¶
bearer_methods_supported¶
resource_signing_alg_values_supported¶
alg values)
supported by the protected resource
for signed content¶
resource_encryption_alg_values_supported¶
alg values)
supported by the protected resource
for encrypted content¶
resource_encryption_enc_values_supported¶
enc values)
supported by the protected resource
for encrypted content¶
resource_documentation¶
resource_policy_uri¶
The following authorization server metadata value is registered in the IANA "OAuth Authorization Server Metadata" registry established in OAuth 2.0 Authorization Server Metadata [OAuth.AuthorizationMetadata].¶
This specification registers the well-known URI defined in Section 3 in the IANA "Well-Known URIs" registry [IANA.well-known] established by [RFC5785].¶
The authors of this specification would like to thank the attendees of the IETF 115 OAuth and HTTP API Working Groups and the attendees of the IETF 116 OAuth Working Group for their input on this specification. We would would also like to thank George Fletcher and Tony Nadalin for their contributions to the specification.¶
[[ to be removed by the RFC Editor before publication as an RFC ]]¶
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