Internet Engineering Task Force (IETF) M.B. Jones
Request for Comments: 9728 Self-Issued Consulting
Category: Standards Track P. Hunt
ISSN: 2070-1721 Independent Identity, Inc.
A. Parecki
Okta
April 2025
OAuth 2.0 Protected Resource Metadata
Abstract
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.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9728.
Copyright Notice
Copyright (c) 2025 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction
1.1. Requirements Notation and Conventions
1.2. Terminology
2. Protected Resource Metadata
2.1. Human-Readable Resource Metadata
2.2. Signed Protected Resource Metadata
3. Obtaining Protected Resource Metadata
3.1. Protected Resource Metadata Request
3.2. Protected Resource Metadata Response
3.3. Protected Resource Metadata Validation
4. Authorization Server Metadata
5. Use of WWW-Authenticate for Protected Resource Metadata
5.1. WWW-Authenticate Response
5.2. Changes to Resource Metadata
5.3. Client Identifier and Client Authentication
5.4. Compatibility with Other Authentication Methods
6. String Operations
7. Security Considerations
7.1. TLS Requirements
7.2. Scopes
7.3. Impersonation Attacks
7.4. Audience-Restricted Access Tokens
7.5. Publishing Metadata in a Standard Format
7.6. Authorization Servers
7.7. Server-Side Request Forgery (SSRF)
7.8. Phishing
7.9. Differences Between Unsigned and Signed Metadata
7.10. Metadata Caching
8. IANA Considerations
8.1. OAuth Protected Resource Metadata Registry
8.1.1. Registration Template
8.1.2. Initial Registry Contents
8.2. OAuth Authorization Server Metadata Registry
8.2.1. Registry Contents
8.3. Well-Known URIs Registry
8.3.1. Registry Contents
9. References
9.1. Normative References
9.2. Informative References
Acknowledgements
Authors' Addresses
1. Introduction
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 are intentionally as parallel as
possible to (1) "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 (2) "OAuth 2.0 Authorization
Server Metadata" [RFC8414], which enables a client to obtain metadata
about an OAuth 2.0 authorization server.
The means by which the client obtains the location of the protected
resource is out of scope for this document. In some cases, the
location may be manually configured into the client; for example, an
email client could provide an interface for a user to enter the URL
of their JSON Meta Application Protocol (JMAP) server [RFC8620]. In
other cases, it may be dynamically discovered; for example, a user
could enter their email address into an email client, the client
could perform WebFinger discovery [RFC7033] (in a manner related to
the description in Section 2 of [OpenID.Discovery]) to find the
resource server, and the client could then fetch the resource server
metadata to find the authorization server to use to obtain
authorization to access the user's email.
The metadata for a protected resource is retrieved from a well-known
location as a JSON [RFC8259] document, which declares information
about its capabilities and and, optionally, its relationships to with other
services. This process is described in Section 3.
This metadata can be communicated either 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
deterministically derived from the protected resource's URL, even
when the resource server implements multiple protected resources.
This prevents attackers from publishing metadata that supposedly
describes the protected resource but that is not actually
authoritative for the protected resource, as described in
Section 7.3.
Section 2 defines metadata parameters 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, such as
the jwks_uri (see Section 2), may be used with other specifications;
for example, the public keys published in the jwks_uri can be used to
verify the signed resource responses, as described in
[FAPI.MessageSigning].
Section 5 describes the use of WWW-Authenticate by protected
resources to dynamically inform clients of the URL of their protected
resource metadata. This use of WWW-Authenticate can indicate that
the protected resource metadata may have changed.
1.1. Requirements Notation and 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
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
All applications of JSON Web Signature (JWS) data structures [JWS]
and JSON Web Encryption (JWE) data structures [JWE] as discussed 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. Choosing a single serialization is
intended to facilitate interoperability.
1.2. Terminology
This specification uses the terms "access token", "authorization
code", "authorization server", "client", "client authentication",
"client identifier", "protected resource", and "resource server"
defined by OAuth 2.0 [RFC6749], and the terms "Claim Name" and "JSON
Web Token (JWT)" defined by "JSON Web Token (JWT)" [JWT].
This specification defines the following term:
Resource Identifier:
The protected resource's resource identifier, which is a URL that
uses the https scheme and has no fragment component. As specified
in Section 2 of [RFC8707], it also SHOULD NOT include a query
component, but it is recognized that there are cases that make a
query component a useful and necessary part of a resource
identifier. Protected resource metadata is published at a .well-
known location [RFC8615] derived from this resource identifier, as
described in Section 3.
2. Protected Resource Metadata
Protected resources can have metadata describing their configuration.
The following protected resource metadata parameters are used by this
specification and are registered in the "OAuth Protected Resource
Metadata" registry established in Section 8.1:
resource
REQUIRED. The protected resource's resource identifier, as
defined in Section 1.2.
authorization_servers
OPTIONAL. JSON array containing a list of OAuth authorization
server issuer identifiers, as defined in [RFC8414], 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.
jwks_uri
OPTIONAL. URL of the protected resource's JSON Web Key (JWK) Set
[JWK] 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.
scopes_supported
RECOMMENDED. JSON array containing a list of scope values, as
defined in OAuth 2.0 [RFC6749], 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.
bearer_methods_supported
OPTIONAL. JSON array containing a list of the supported methods
of sending an OAuth 2.0 bearer token [RFC6750] to the protected
resource. Defined values are ["header", "body", "query"],
corresponding to Sections 2.1, 2.2, and 2.3 of [RFC6750]. The
empty array [] can be used to indicate that no bearer methods are
supported. If this entry is omitted, no default bearer methods
supported are implied, nor does its absence indicate that they are
not supported.
resource_signing_alg_values_supported
OPTIONAL. JSON array containing a list of the JWS [JWS] signing
algorithms (alg values) [JWA] supported by the protected resource
for signing resource responses, for instance, as described in
[FAPI.MessageSigning]. No default algorithms are implied if this
entry is omitted. The value none MUST NOT be used.
resource_name
Human-readable name of the protected resource intended for display
to the end user. It is RECOMMENDED that protected resource
metadata include this field. The value of this field MAY be
internationalized, as described in Section 2.1.
resource_documentation
OPTIONAL. URL of a page containing human-readable information
that developers might want or need to know when using the
protected resource. The value of this field MAY be
internationalized, as described in Section 2.1.
resource_policy_uri
OPTIONAL. URL of a page containing human-readable information
about the protected resource's requirements on how the client can
use the data provided by the protected resource. The value of
this field MAY be internationalized, as described in Section 2.1.
resource_tos_uri
OPTIONAL. URL of a page containing human-readable information
about the protected resource's terms of service. The value of
this field MAY be internationalized, as described in Section 2.1.
tls_client_certificate_bound_access_tokens
OPTIONAL. Boolean value indicating protected resource support for
mutual-TLS client certificate-bound access tokens [RFC8705]. If
omitted, the default value is false.
authorization_details_types_supported
OPTIONAL. JSON array containing a list of the authorization
details type values supported by the resource server when the
authorization_details request parameter [RFC9396] is used.
dpop_signing_alg_values_supported
OPTIONAL. JSON array containing a list of the JWS alg values
(from the "JSON Web Signature and Encryption Algorithms" registry
[IANA.JOSE]) supported by the resource server for validating
Demonstrating Proof of Possession (DPoP) proof JWTs [RFC9449].
dpop_bound_access_tokens_required
OPTIONAL. Boolean value specifying whether the protected resource
always requires the use of DPoP-bound access tokens [RFC9449]. If
omitted, the default value is false.
Additional protected resource metadata parameters MAY also be used.
2.1. Human-Readable Resource Metadata
Human-readable resource metadata values and resource metadata values
that reference human-readable content MAY be represented in multiple
languages and scripts. For example, the values of fields such as
resource_name, resource_documentation, resource_tos_uri, and
resource_policy_uri might have multiple locale-specific metadata
values to facilitate use in different locations.
To specify the languages and scripts, language tags [BCP47] are added
to resource metadata parameter names, delimited by a # character.
Since member names as discussed in JSON [RFC8259] are case sensitive,
it is RECOMMENDED that language tag values used in Claim Names be
spelled using the character case with which they are registered in
the "Language Subtag Registry" [IANA.Language]. In particular,
normally, language names are spelled with lowercase characters,
region names are spelled with uppercase characters, and languages are
spelled with mixed-case characters. However, since language tag
values are case insensitive per [BCP47], implementations SHOULD
interpret the language tag values supplied in a case-insensitive
manner. Per the recommendations in [BCP47], language tag values used
in metadata parameter names should only be as specific as is
necessary. For instance, using fr might be sufficient in many
contexts, rather than fr-CA or fr-FR.
For example, a resource could represent its name in English as
"resource_name#en": "My Resource" and its name in Italian as
"resource_name#it": "La mia bella risorsa" within its metadata. Any
or all of these names MAY be displayed to the end user, choosing
which names to display based on system configuration, user
preferences, or other factors.
If any human-readable field is sent without a language tag, parties
using it MUST NOT make any assumptions about the language, character
set, or script of the string value, and the string value MUST be used
as is wherever it is presented in a user interface. To facilitate
interoperability, it is RECOMMENDED that each kind of human-readable
metadata provided include an instance of its metadata parameter
without any language tags in addition to any language-specific
parameters, and it is RECOMMENDED that any human-readable fields sent
without language tags contain values suitable for display on a wide
variety of systems.
2.2. Signed Protected Resource Metadata
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 metadata parameters that can be used in signed metadata as
claims are defined in Section 2. The signed metadata MUST be
digitally signed or MACed (protected with a Message Authentication
Code) using a 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 the corresponding values
conveyed using plain JSON elements.
Signed metadata is included in the protected resource metadata JSON
object using this OPTIONAL metadata parameter:
signed_metadata
A JWT containing metadata parameters about the protected resource
as claims. This is a string value consisting of the entire signed
JWT. A signed_metadata parameter SHOULD NOT appear as a claim in
the JWT; it is RECOMMENDED to reject any metadata in which this
occurs.
3. Obtaining Protected Resource Metadata
Protected resources supporting metadata MUST make a JSON document
containing metadata as specified in Section 2 available at a URL
formed by inserting a well-known URI string into the protected
resource's resource identifier between the host component and the
path and/or query components, 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 [RFC8615]. The well-known
URI path suffix used MUST be registered in the "Well-Known URIs"
registry [IANA.well-known]. Examples of this construction can be
found in Section 3.1.
The term "application", as used below (and as used in [RFC8414]),
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 URL formed by
inserting /.well-known/example-protected-resource between the host
and path and/or query 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.
3.1. Protected Resource Metadata Request
A protected resource metadata document MUST be queried using an HTTP
GET request at the previously specified URL.
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 or query component,
any terminating slash (/) following the host component MUST be
removed before inserting /.well-known/ and the well-known URI path
suffix between the host component and the path and/or query
components. 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 /.well-known/oauth-protected-resource/resource1 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 [RFC8615], it does not provide general information
about the host.
3.2. Protected Resource Metadata Response
The response is a set of metadata parameters 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 metadata parameters as its
members that are a subset of the metadata parameters defined in
Section 2. Additional metadata parameters MAY be defined and used;
any metadata parameters that are not understood MUST be ignored.
Parameters with multiple values are represented as JSON arrays.
Parameters with zero 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
{
"resource":
"https://resource.example.com",
"authorization_servers":
["https://as1.example.com",
"https://as2.example.net"],
"bearer_methods_supported":
["header", "body"],
"scopes_supported":
["profile", "email", "phone"],
"resource_documentation":
"https://resource.example.com/resource_documentation.html"
}
3.3. Protected Resource Metadata Validation
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 Section 7.3.
The recipient MUST validate that any signed metadata was signed by a
key belonging to the issuer and that the signature is valid. If the
signature does not validate or the issuer is not trusted, the
recipient SHOULD treat this as an error condition.
4. Authorization Server Metadata
To support use cases in which the set of legitimate protected
resources to use with the authorization server is enumerable, this
specification defines the authorization server metadata parameter
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 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 parameter is defined by
this specification and is registered in the "OAuth Authorization
Server Metadata" registry established in "OAuth 2.0 Authorization
Server Metadata" [RFC8414].
protected_resources
OPTIONAL. JSON array containing a list of resource identifiers
for OAuth 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.
5. Use of WWW-Authenticate for Protected Resource Metadata
A protected resource MAY use the WWW-Authenticate HTTP response
header field, as discussed in [RFC9110], to return a URL to its
protected resource metadata to the client. The client can then
retrieve protected resource metadata 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.
+----------+ +----------+ +---------------+
| Client | | Resource | | Authorization |
| | | Server | | Server |
+----+-----+ +----+-----+ +-------+-------+
| | |
| 1. Resource Request | |
| ----------------------> | |
| Without Access Token | |
| | |
| | |
| 2. WWW-Authenticate | |
| <---------------------- | |
| | |
| | |
| 3. Fetch RS Metadata | |
| ----------------------> | |
| | |
| | |
| 4. RS Metadata Response | |
| <---------------------- | |
| | |
+---------+---------------+ | |
| 5. Validate RS Metadata | | |
| Build AS Metadata URL | | |
+---------+---------------+ | |
| | |
| 6. Fetch AS Metadata | |
| ------------------------+----------------> |
| | |
| | |
| 7. AS Metadata Response | |
| <-----------------------+----------------- |
| | |
+-+-------------------------+------------------+-+
| 8-9. OAuth Authorization Code Flow |
| Client Obtains Access Token |
+-+-------------------------+------------------+-+
| | |
| 10. Resource Request | |
| ----------------------> | |
| With Access Token | |
| | |
| | |
| 11. Resource Response | |
| <---------------------- | |
| | |
+----+-----+ +----+-----+ +-------+-------+
| Client | | Resource | | Authorization |
| | | Server | | Server |
+----------+ +----------+ +---------------+
Figure 1: Sequence Diagram
1. The client makes a request to a protected resource without
presenting an access token.
2. The resource server responds with a WWW-Authenticate header
including the URL of the protected resource metadata.
3. The client fetches the protected resource metadata from this
URL.
4. The resource server responds with the protected resource
metadata according to Section 3.2.
5. The client validates the protected resource metadata, as
described in Section 3.3, and builds the authorization server
metadata URL from an issuer identifier in the resource metadata
according to [RFC8414].
6. The client makes a request to fetch the authorization server
metadata.
7. The authorization server responds with the authorization server
metadata document according to [RFC8414].
8. The client directs the user agent to the authorization server to
begin the authorization flow.
9. The authorization exchange is completed and the authorization
server returns an access token to the client.
10. The client repeats the resource request from step 1, presenting
the newly obtained access token.
11. The resource server returns the requested protected resource.
5.1. WWW-Authenticate Response
This specification introduces a new parameter in the WWW-Authenticate
HTTP response header field to indicate the protected resource
metadata URL:
resource_metadata:
The URL of the protected resource metadata.
The response below is an example of a WWW-Authenticate header that
includes the resource identifier.
HTTP/1.1 401 Unauthorized
WWW-Authenticate: Bearer resource_metadata=
"https://resource.example.com/.well-known/oauth-protected-resource"
The HTTP status code in the example response above is defined by
[RFC6750].
This parameter MAY also be used in WWW-Authenticate responses using
authorization schemes other than "Bearer" [RFC6750], such as the DPoP
scheme defined by [RFC9449].
The resource_metadata parameter MAY be combined with other parameters
defined in other extensions, such as the max_age parameter defined by
[RFC9470].
5.2. Changes to Resource Metadata
At any point, for any reason determined by the resource server, 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,
after validating them as described in Section 3.3. Among other
things, this enables a resource server to change which authorization
servers it uses without any other coordination with clients.
5.3. Client Identifier and Client Authentication
The way in which the client identifier is established at the
authorization server is out of scope for this specification.
This specification is intended to be deployed in scenarios where the
client has no prior knowledge about the resource server and where 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 OAuth extensions might define
alternatives, such as using URLs to identify clients.
5.4. Compatibility with Other Authentication Methods
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.
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
resource. Comparing Unicode strings [UNICODE], 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 as that
described in Section 8.3 of [RFC8259].
7. Security Considerations
7.1. TLS Requirements
Implementations MUST support TLS. They MUST follow the guidance in
[BCP195], which provides recommendations and requirements for
improving the security of deployed services that use TLS.
The use of TLS at the protected resource metadata URLs protects
against information disclosure and tampering.
7.2. Scopes
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 a scope as possible for the given operation,
as described in Section 2.3 of "Best Current Practice for OAuth 2.0
Security" [RFC9700].
7.3. Impersonation Attacks
TLS certificate checking MUST be performed by the client as described
in [RFC9525] when making a protected resource metadata request.
Checking that the server certificate is valid for the resource
identifier URL prevents adversary-in-the-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 metadata
parameter using the resource identifier URL of the protected resource
being impersonated but that contains 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 parameter in the protected resource metadata
document received by the client, as described in Section 3.3.
7.4. Audience-Restricted Access Tokens
If a client expects to interact with multiple resource servers, the
client SHOULD request audience-restricted access tokens using
[RFC8707], 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 reuse 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
[RFC8707] is RECOMMENDED when using the features in this
specification.
7.5. Publishing Metadata in a Standard Format
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.
7.6. Authorization Servers
To support use cases in which the set of legitimate authorization
servers to use with the protected resource is enumerable, this
specification defines the authorization_servers metadata parameter,
which enables explicitly listing them. Note that if the set of
legitimate protected resources to use with an authorization server is
also 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 for 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, an
attacker may be able to act as an adversary-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 a set of authorization servers, the locations
of which may be known via out-of-band mechanisms. Alternatively, as
described in this specification, the locations of the authorization
servers could be published by the protected resource as metadata
values. In other cases, the set of authorization servers that can be
used with a protected resource can by be 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.
7.7. Server-Side Request Forgery (SSRF)
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 ASes it has no prior knowledge of, this opens a risk for
Server-Side Request Forgery (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 Open Worldwide Application Security Project (OWASP) SSRF
Prevention Cheat Sheet [OWASP.SSRF].
7.8. Phishing
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.
7.9. Differences Between Unsigned and Signed Metadata
Unsigned metadata is integrity protected by the use of TLS at the
site where it is hosted. This means that its security is dependent
upon the Internet Public Key Infrastructure using X.509 (PKIX), as
described in [RFC9525]. 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.
7.10. Metadata Caching
Protected resource metadata is retrieved using an HTTP GET request,
as specified in Section 3.1. Normal HTTP caching behaviors apply,
meaning that the GET request may retrieve a cached copy of the
content, rather than the latest copy. Implementations should utilize
HTTP caching directives such as Cache-Control with max-age, as
defined in [RFC9111], to enable caching of retrieved metadata for
appropriate time periods.
8. IANA Considerations
Values are registered via Specification Required [RFC8126].
Registration requests should be sent to <oauth-ext-review@ietf.org>
to initiate a two-week review period. However, to allow for the
allocation of values prior to publication of the final version of a
specification, the designated experts may approve registration once
they are satisfied that the specification will be completed and
published. However, if the specification is not completed and
published in a timely manner, as determined by the designated
experts, the designated experts may request that IANA withdraw the
registration.
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. If
the designated experts are not responsive, the registration
requesters should contact IANA to escalate the process.
Designated experts should apply the following criteria when reviewing
proposed registrations: They must be unique -- that is, they should
not duplicate existing functionality; they are likely generally
applicable, as opposed to being used for a single application; and
they are clear and fit the purpose of the registry.
IANA must only accept registry updates from the designated experts
and should direct all requests for registration to the review mailing
list.
In order to enable broadly informed review of registration decisions,
there should be multiple designated experts to represent the
perspectives of different applications using this specification. In
cases where registration may be perceived as a conflict of interest
for a particular expert, that expert should defer to the judgment of
the other experts.
The mailing list is used to enable public review of registration
requests, which enables both designated experts and other interested
parties to provide feedback on proposed registrations. Designated
experts may allocate values prior to publication of the final
specification. This allows authors to receive guidance from the
designated experts early, so any identified issues can be fixed
before the final specification is published.
8.1. OAuth Protected Resource Metadata Registry
This specification establishes the "OAuth Protected Resource
Metadata" registry for OAuth 2.0 protected resource metadata names.
The registry records the protected resource metadata parameter and a
reference to the specification that defines it.
8.1.1. Registration Template
Metadata Name:
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.
Metadata Description:
Brief description of the metadata (e.g., "Resource identifier
URL").
Change Controller:
For IETF Stream RFCs, list "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.
Specification Document(s):
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.
8.1.2. Initial Registry Contents
Metadata Name: resource
Metadata Description: Protected resource's resource identifier URL
Change Controller: IETF
Specification Document(s): Section 2 of RFC 9728
Metadata Name: authorization_servers
Metadata Description: JSON array containing a list of OAuth
authorization server issuer identifiers
Change Controller: IETF
Specification Document(s): Section 2 of RFC 9728
Metadata Name: jwks_uri
Metadata Description: URL of the protected resource's JWK Set
document
Change Controller: IETF
Specification Document(s): Section 2 of RFC 9728
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 to this protected resource
Change Controller: IETF
Specification Document(s): Section 2 of RFC 9728
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): Section 2 of RFC 9728
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): Section 2 of RFC 9728
Metadata Name: resource_name
Metadata Description: Human-readable name of the protected resource
Change Controller: IETF
Specification Document(s): Section 2 of RFC 9728
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): Section 2 of RFC 9728
Metadata Name: resource_policy_uri
Metadata Description: URL of a page containing human-readable
information 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): Section 2 of RFC 9728
Metadata Name: resource_tos_uri
Metadata Description: URL of a page containing human-readable
information about the protected resource's terms of service
Change Controller: IETF
Specification Document(s): Section 2 of RFC 9728
Metadata Name: tls_client_certificate_bound_access_tokens
Metadata Description: Boolean value indicating protected resource
support for mutual-TLS client certificate-bound access tokens
Change Controller: IETF
Specification Document(s): Section 2 of RFC 9728
Metadata Name: authorization_details_types_supported
Metadata Description: JSON array containing a list of the
authorization details type values supported by the resource server
when the authorization_details request parameter is used
Change Controller: IETF
Specification Document(s): Section 2 of RFC 9728
Metadata Name: dpop_signing_alg_values_supported
Metadata Description: JSON array containing a list of the JWS alg
values supported by the resource server for validating DPoP proof
JWTs
Change Controller: IETF
Specification Document(s): Section 2 of RFC 9728
Metadata Name: dpop_bound_access_tokens_required
Metadata Description: Boolean value specifying whether the protected
resource always requires the use of DPoP-bound access tokens
Change Controller: IETF
Specification Document(s): Section 2 of RFC 9728
Metadata Name: signed_metadata
Metadata Description: Signed JWT containing metadata parameters
about the protected resource as claims
Change Controller: IETF
Specification Document(s): Section 2.2 of RFC 9728
8.2. OAuth Authorization Server Metadata Registry
IANA has registered the following authorization server metadata
parameter in the "OAuth Authorization Server Metadata" registry
established in "OAuth 2.0 Authorization Server Metadata" [RFC8414].
8.2.1. Registry Contents
Metadata Name: protected_resources
Metadata Description: JSON array containing a list of resource
identifiers for OAuth protected resources
Change Controller: IETF
Specification Document(s): Section 4 of RFC 9728
8.3. Well-Known URIs Registry
This specification registers the well-known URI defined in Section 3
in the "Well-Known URIs" registry [IANA.well-known].
8.3.1. Registry Contents
URI Suffix: oauth-protected-resource
Reference: Section 3 of RFC 9728
Status: permanent
Change Controller: IETF
Related Information: (none)
9. References
9.1. Normative References
[BCP195] Best Current Practice 195,
<https://www.rfc-editor.org/info/bcp195>.
At the time of writing, this BCP comprises the following:
Moriarty, K. and S. Farrell, "Deprecating TLS 1.0 and TLS
1.1", BCP 195, RFC 8996, DOI 10.17487/RFC8996, March 2021,
<https://www.rfc-editor.org/info/rfc8996>.
Sheffer, Y., Saint-Andre, P., and T. Fossati,
"Recommendations for Secure Use of Transport Layer
Security (TLS) and Datagram Transport Layer Security
(DTLS)", BCP 195, RFC 9325, DOI 10.17487/RFC9325, November
2022, <https://www.rfc-editor.org/info/rfc9325>.
[BCP47] Best Current Practice 47,
<https://www.rfc-editor.org/info/bcp47>.
At the time of writing, this BCP comprises the following:
Phillips, A., Ed. and M. Davis, Ed., "Matching of Language
Tags", BCP 47, RFC 4647, DOI 10.17487/RFC4647, September
2006, <https://www.rfc-editor.org/info/rfc4647>.
Phillips, A., Ed. and M. Davis, Ed., "Tags for Identifying
Languages", BCP 47, RFC 5646, DOI 10.17487/RFC5646,
September 2009, <https://www.rfc-editor.org/info/rfc5646>.
[IANA.Language]
IANA, "Language Subtag Registry",
<https://www.iana.org/assignments/language-subtag-
registry>.
[JWA] Jones, M., "JSON Web Algorithms (JWA)", RFC 7518,
DOI 10.17487/RFC7518, May 2015,
<https://www.rfc-editor.org/info/rfc7518>.
[JWE] Jones, M. and J. Hildebrand, "JSON Web Encryption (JWE)",
RFC 7516, DOI 10.17487/RFC7516, May 2015,
<https://www.rfc-editor.org/info/rfc7516>.
[JWK] Jones, M., "JSON Web Key (JWK)", RFC 7517,
DOI 10.17487/RFC7517, May 2015,
<https://www.rfc-editor.org/info/rfc7517>.
[JWS] Jones, M., Bradley, J., and N. Sakimura, "JSON Web
Signature (JWS)", RFC 7515, DOI 10.17487/RFC7515, May
2015, <https://www.rfc-editor.org/info/rfc7515>.
[JWT] Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
(JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015,
<https://www.rfc-editor.org/info/rfc7519>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC6749] Hardt, D., Ed., "The OAuth 2.0 Authorization Framework",
RFC 6749, DOI 10.17487/RFC6749, October 2012,
<https://www.rfc-editor.org/info/rfc6749>.
[RFC6750] Jones, M. and D. Hardt, "The OAuth 2.0 Authorization
Framework: Bearer Token Usage", RFC 6750,
DOI 10.17487/RFC6750, October 2012,
<https://www.rfc-editor.org/info/rfc6750>.
[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,
<https://www.rfc-editor.org/info/rfc7591>.
[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/info/rfc8126>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8259] Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
Interchange Format", STD 90, RFC 8259,
DOI 10.17487/RFC8259, December 2017,
<https://www.rfc-editor.org/info/rfc8259>.
[RFC8414] Jones, M., Sakimura, N., and J. Bradley, "OAuth 2.0
Authorization Server Metadata", RFC 8414,
DOI 10.17487/RFC8414, June 2018,
<https://www.rfc-editor.org/info/rfc8414>.
[RFC8615] Nottingham, M., "Well-Known Uniform Resource Identifiers
(URIs)", RFC 8615, DOI 10.17487/RFC8615, May 2019,
<https://www.rfc-editor.org/info/rfc8615>.
[RFC8705] Campbell, B., Bradley, J., Sakimura, N., and T.
Lodderstedt, "OAuth 2.0 Mutual-TLS Client Authentication
and Certificate-Bound Access Tokens", RFC 8705,
DOI 10.17487/RFC8705, February 2020,
<https://www.rfc-editor.org/info/rfc8705>.
[RFC8707] Campbell, B., Bradley, J., and H. Tschofenig, "Resource
Indicators for OAuth 2.0", RFC 8707, DOI 10.17487/RFC8707,
February 2020, <https://www.rfc-editor.org/info/rfc8707>.
[RFC9110] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "HTTP Semantics", STD 97, RFC 9110,
DOI 10.17487/RFC9110, June 2022,
<https://www.rfc-editor.org/info/rfc9110>.
[RFC9111] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "HTTP Caching", STD 98, RFC 9111,
DOI 10.17487/RFC9111, June 2022,
<https://www.rfc-editor.org/info/rfc9111>.
[RFC9396] Lodderstedt, T., Richer, J., and B. Campbell, "OAuth 2.0
Rich Authorization Requests", RFC 9396,
DOI 10.17487/RFC9396, May 2023,
<https://www.rfc-editor.org/info/rfc9396>.
[RFC9449] Fett, D., Campbell, B., Bradley, J., Lodderstedt, T.,
Jones, M., and D. Waite, "OAuth 2.0 Demonstrating Proof of
Possession (DPoP)", RFC 9449, DOI 10.17487/RFC9449,
September 2023, <https://www.rfc-editor.org/info/rfc9449>.
[RFC9525] Saint-Andre, P. and R. Salz, "Service Identity in TLS",
RFC 9525, DOI 10.17487/RFC9525, November 2023,
<https://www.rfc-editor.org/info/rfc9525>.
[UNICODE] The Unicode Consortium, "The Unicode Standard",
<https://www.unicode.org/versions/latest/>.
[USA15] Whistler, K., Ed., "Unicode Normalization Forms", Unicode
Standard Annex #15, 14 August 2024,
<https://www.unicode.org/reports/tr15/>.
9.2. Informative References
[FAPI.MessageSigning]
Tonge, D. and D. Fett, "FAPI 2.0 Message Signing (Draft)",
24 March 2023,
<https://openid.net/specs/fapi-2_0-message-signing.html>.
[IANA.JOSE]
IANA, "JSON Web Signature and Encryption Algorithms",
<https://www.iana.org/assignments/jose>.
[IANA.well-known]
IANA, "Well-Known URIs",
<https://www.iana.org/assignments/well-known-uris>.
[OpenID.Discovery]
Sakimura, N., Bradley, J., Jones, M., and E. Jay, "OpenID
Connect Discovery 1.0 incorporating errata set 2", 15
December 2023, <https://openid.net/specs/openid-connect-
discovery-1_0.html>.
[OWASP.SSRF]
OWASP Foundation, "OWASP Server-Side Request Forgery
Prevention Cheat Sheet",
<https://cheatsheetseries.owasp.org/cheatsheets/
Server_Side_Request_Forgery_Prevention_Cheat_Sheet.html>.
[RFC7033] Jones, P., Salgueiro, G., Jones, M., and J. Smarr,
"WebFinger", RFC 7033, DOI 10.17487/RFC7033, September
2013, <https://www.rfc-editor.org/info/rfc7033>.
[RFC8620] Jenkins, N. and C. Newman, "The JSON Meta Application
Protocol (JMAP)", RFC 8620, DOI 10.17487/RFC8620, July
2019, <https://www.rfc-editor.org/info/rfc8620>.
[RFC9470] Bertocci, V. and B. Campbell, "OAuth 2.0 Step Up
Authentication Challenge Protocol", RFC 9470,
DOI 10.17487/RFC9470, September 2023,
<https://www.rfc-editor.org/info/rfc9470>.
[RFC9700] Lodderstedt, T., Bradley, J., Labunets, A., and D. Fett,
"Best Current Practice for OAuth 2.0 Security", BCP 240,
RFC 9700, DOI 10.17487/RFC9700, January 2025,
<https://www.rfc-editor.org/info/rfc9700>.
Acknowledgements
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 also like to thank Amanda Baber,
Mike Bishop, Ralph Bragg, Brian Campbell, Deb Cooley, Gabriel Corona,
Roman Danyliw, Vladimir Dzhuvinov, George Fletcher, Arnt Gulbrandsen,
Pieter Kasselman, Murray Kucherawy, David Mandelberg, Tony Nadalin,
Francesca Palombini, John Scudder, Rifaat Shekh-Yusef, Filip Skokan,
Orie Steele, Atul Tulshibagwale, Éric Vyncke, Paul Wouters, and Bo Wu
for their contributions to the specification.
Authors' Addresses
Michael B. Jones
Self-Issued Consulting
Email: michael_b_jones@hotmail.com
URI: https://self-issued.info/
Phil Hunt
Independent Identity, Inc.
Email: phil.hunt@yahoo.com
Aaron Parecki
Okta
Email: aaron@parecki.com
URI: https://aaronparecki.com/