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IETF RFC 9767
Last modified on Wednesday, April 23rd, 2025
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Internet Engineering Task Force (IETF) J. Richer, Ed.
Request for Comments: 9767 Bespoke Engineering
Category: Standards Track F. Imbault
ISSN: 2070-1721 acert.io
April 2025
Grant Negotiation and Authorization Protocol Resource Server Connections
 Abstract
The Grant Negotiation and Authorization Protocol (GNAP) defines a
mechanism for delegating authorization to a piece of software (the
client) and conveying the results and artifacts of that delegation to
the software. This extension defines methods for resource servers
(RSs) to connect with authorization servers (ASs) in an interoperable
fashion.
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/RFC 9767.
Copyright Notice
Copyright (c) 2025 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Revised BSD License text as described in Section 4.e of the
Trust Legal Provisions and are provided without warranty as described
in the Revised BSD License.
Table of Contents
1. Introduction
1.1. Terminology
2. Access Tokens
2.1. General-Purpose Access Token Model
2.1.1. Value
2.1.2. Issuer
2.1.3. Audience
2.1.4. Key Binding
2.1.5. Flags
2.1.6. Access Rights
2.1.7. Time Validity Window
2.1.8. Token Identifier
2.1.9. Authorizing Resource Owner
2.1.10. End User
2.1.11. Client Instance
2.1.12. Label
2.1.13. Parent Grant Request
2.1.14. AS-Specific Access Tokens
2.2. Access Token Formats
3. Resource-Server-Facing API
3.1. RS-Facing AS Discovery
3.2. Protecting RS Requests to the AS
3.3. Token Introspection
3.4. Registering a Resource Set
3.5. Error Responses
4. Deriving a Downstream Token
5. IANA Considerations
5.1. Well-Known URIs
5.2. GNAP Grant Request Parameters
5.3. GNAP Token Formats
5.3.1. Registry Template
5.3.2. Initial Registry Contents
5.4. GNAP Token Introspection Request
5.4.1. Registry Template
5.4.2. Initial Registry Contents
5.5. GNAP Token Introspection Response
5.5.1. Registry Template
5.5.2. Initial Registry Contents
5.6. GNAP Resource Set Registration Request Parameters
5.6.1. Registry Template
5.6.2. Initial Registry Contents
5.7. GNAP Resource Set Registration Response Parameters
5.7.1. Registry Template
5.7.2. Initial Registry Contents
5.8. GNAP RS-Facing Discovery Document Fields
5.8.1. Registry Template
5.8.2. Initial Registry Contents
5.9. GNAP RS-Facing Error Codes
5.9.1. Registration Template
5.9.2. Initial Contents
6. Security Considerations
6.1. TLS Protection in Transit
6.2. Token Validation
6.3. Caching Token Validation Result
6.4. Key Proof Validation
6.5. Token Exfiltration
6.6. Token Reuse by an RS
6.7. Token Format Considerations
6.8. Oversharing Token Contents
6.9. Resource References
6.10. Token Reissuance from an Untrusted AS
6.11. Introspection of Token Keys
6.12. RS Registration and Management
7. Privacy Considerations
7.1. Token Contents
7.2. Token Use Disclosure through Introspection
7.3. Mapping a User to an AS
8. References
8.1. Normative References
8.2. Informative References
Acknowledgements
Authors' Addresses
1. Introduction
The core GNAP specification [GNAP] defines distinct roles for the
authorization server (AS) and the resource server (RS). However, the
core specification does not define how the RS gets answers to
important questions, such as whether a given access token is still
valid or what set of access rights the access token is approved for.
While it's possible for the AS and RS to be tightly coupled, such as
a single deployed server with a shared storage system, GNAP does not
presume or require such a tight coupling. It is increasingly common
for the AS and RS to be run and managed separately, particularly in
cases where a single AS protects multiple RSs simultaneously.
This specification defines a set of RS-facing APIs that an AS can
make available for advanced loosely coupled deployments.
Additionally, this document defines a general-purpose model for
access tokens, which can be used in structured, formatted access
tokens or in token introspection responses. This specification also
defines a method for an RS to derive a downstream token for calling
another chained RS.
The means for the authorization server to issue the access token to
the client instance and the means for the client instance to present
the access token to the resource server are subjects of the core GNAP
specification [GNAP].
1.1. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC 2119] [RFC 8174] when, and only when, they appear in all
capitals, as shown here.
This document contains non-normative examples of partial and complete
HTTP messages, JSON structures, URLs, query components, keys, and
other elements. Some examples use a single trailing backslash \ to
indicate line wrapping for long values, as per [RFC 8792]. The \
character and leading spaces on wrapped lines are not part of the
value.
Terminology specific to GNAP is defined in the terminology section of
the core specification; see Section 1.1 of [GNAP]. The following
protocol roles are defined: authorization server, client, end user,
resource owner, and resource server. The following protocol elements
are defined: access token, attribute, grant, privilege, protected
resource, right, subject, and subject information. The same
definitions are used in this document.
2. Access Tokens
Access tokens are used as a mechanism for an AS to provide a client
instance limited access to an RS. These access tokens are artifacts
representing a particular set of access rights granted to the client
instance to act on behalf of the RO. While the format of access
tokens varies in different systems (see discussion in Section 2.2),
the concept of an access token is consistent across all GNAP systems.
2.1. General-Purpose Access Token Model
The core GNAP specification [GNAP] focuses on the relationship
between the client and the AS. Since the access token is opaque to
the client, the core specification does not define a token model.
However, the AS will need to create tokens, and the RS will need to
understand tokens. To facilitate a level of structural
interoperability, a common access token model is presented here.
Access tokens represent a common set of aspects across different GNAP
deployments. This list is not intended to be universal or
comprehensive but rather serves as guidance to implementers in
developing data structures and associated systems across a GNAP
deployment. These data structures are communicated between the AS
and RS by using either a structured token or an API-like mechanism
such as token introspection (see Section 3.3).
This general-purpose data model does not assume either approach; in
fact, both approaches can be used together to convey different pieces
of information. Where possible, mappings to the JSON Web Token (JWT)
[JWT] standard format are provided for each item in the model.
2.1.1. Value
All access tokens have a _value_, which is the string that is passed
on the wire between parties. In order for different access tokens to
be differentiated at runtime, the value of a token needs to be unique
within a security domain (such as all systems controlled by an AS).
Otherwise, two separate tokens would be confused for each other,
which would lead to security issues. The AS chooses the value, which
can be structured (see Section 2.2) or unstructured. When the token
is structured, the token value also has a _format_ known to the AS
and RS, and the other items in this token model are contained within
the token's value in some fashion. When the token is unstructured,
the values are usually retrieved by the RS using a service such as
token introspection described in Section 3.3.
The access token value is conveyed in the value field of an
access_token response; see Section 3.2 of [GNAP].
The format and content of the access token value is opaque to the
client software. While the client software needs to be able to carry
and present the access token value, the client software is never
expected nor intended to be able to understand the token value
itself.
If structured tokens like those in [JWT] are used, the value of the
token might not be stored by the AS. Instead, a token identifier can
be used along with protection by an AS-generated signature to
validate and identify an individual token.
2.1.2. Issuer
The access token is issued by the AS as defined in [GNAP]. The AS
will need to identify itself in order to allow an RS to recognize
tokens that the AS has issued, particularly in cases where tokens
from multiple different ASs could be presented to the same RS.
This information is not usually conveyed directly to the client
instance, since the client instance should know this information
based on where it receives the token from.
In the payload of a JSON Web Token [JWT] or a token introspection
response, this corresponds to the iss claim.
2.1.3. Audience
The access token is intended for use at one or more RSs. The AS can
list a token's intended RSs to allow each RS to ensure that the RS is
not receiving a token intended for someone else. The AS and RS have
to agree on the nature of any audience identifiers represented by the
token, but the URIs of the RS are a common pattern.
In the payload of a JSON Web Token [JWT] or token introspection
response, this corresponds to the aud claim.
In cases where more complex access is required, the location field of
objects in the access array can also convey audience information. In
such cases, the client instance might need to know the audience
information in order to differentiate between possible RSs to present
the token to.
2.1.4. Key Binding
Access tokens in GNAP are bound to the client instance's registered
or presented key, except in cases where the access token is a bearer
token. For all tokens bound to a key, the AS and RS need to be able
to identify which key the token is bound to; otherwise, an attacker
could substitute their own key during presentation of the token. In
the case of an asymmetric algorithm, the AS and RS need to know only
the public key, while the client instance will also need to know the
private key in order to present the token. In the case of a
symmetric algorithm, all parties will need to either know or be able
to derive the shared key.
The source of this key information can vary depending on deployment
decisions. For example, an AS could decide that all tokens issued to
a client instance are always bound to that client instance's current
key. When the key needs to be dereferenced, the AS looks up the
client instance to which the token was issued and finds the key
information there. Alternatively, the AS could bind each token to a
specific key that is managed separately from client instance
information. In such a case, the AS determines the key information
directly. This approach allows the client instance to use a
different key for each request or allows the AS to issue a key for
the client instance to use with the particular token.
In all cases, the key binding also includes a proofing mechanism,
along with any parameters needed for that mechanism such as a signing
or digest algorithm. If such information is not included with the
proofing key, an attacker could present a token with a seemingly
valid key using an insecure and incorrect proofing mechanism.
This value is conveyed to the client instance in the key field of the
access_token response in Section 3.2 of [GNAP]. Since the common
case is that the token is bound to the client instance's registered
key, this field can be omitted in this case since the client will be
aware of its own key.
In the payload of a JSON Web Token [JWT], this corresponds to the cnf
(confirmation) claim. In a token introspection response, this
corresponds to the key claim.
In the case of a bearer token, all parties need to know that a token
has no key bound to it and will therefore reject any attempts to use
the bearer token with a key in an undefined way.
2.1.5. Flags
GNAP access tokens can have multiple associated data flags that
indicate special processing or considerations for a token. For
example, the data flags can indicate whether a token is a bearer
token or should be expected to be durable across grant updates.
The client can request a set of flags using the flags field of the
access_token grant request parameter in Section 2.1.1 of [GNAP].
These flags are conveyed from the AS to the client in the flags field
of the access_token section of the grant response in Section 3.2 of
[GNAP].
For token introspection, flags are returned in the flags field of the
response.
2.1.6. Access Rights
Access tokens are tied to a limited set of access rights. These
rights specify in some detail what the token can be used for, how it
can be used, and where it can be used. The internal structure of
access rights is detailed in Section 8 of [GNAP].
The access rights associated with an access token are calculated from
the rights available to the client instance making the request, the
rights available to be approved by the RO, the rights actually
approved by the RO, and the rights corresponding to the RS in
question. The rights for a specific access token are a subset of the
overall rights in a grant request.
These rights are requested by the client instance in the access field
of the access_token request; see Section 2.1 of [GNAP].
The rights associated with an issued access token are conveyed to the
client instance in the access field of the access_token response in
Section 3.2 of [GNAP].
In token introspection responses, access rights correspond to the
access claim.
2.1.7. Time Validity Window
The access token can be limited to a certain time window outside of
which it is no longer valid for use at an RS. This window can be
explicitly bounded by an expiration time and a not-before time, or it
could be calculated based on the issuance time of the token. For
example, an RS could decide that it will accept tokens for most calls
within an hour of a token's issuance, but only within five minutes of
the token's issuance for certain high-value calls.
Since access tokens could be revoked at any time for any reason
outside of a client instance's control, the client instance often
does not know or concern itself with the validity time window of an
access token. However, this information can be made available to it
by using the expires_in field of an access token response; see
Section 3.2 of [GNAP].
The issuance time of the token is conveyed in the iat claim in the
payload of a JSON Web Token [JWT] or a token introspection response.
The expiration time of a token, after which it is to be rejected, is
conveyed in the exp claim in the payload of a JSON Web Token [JWT] or
a token introspection response.
The starting time of a token's validity window, before which it is to
be rejected, is conveyed in the nbf claim in the payload of a JSON
Web Token [JWT] or a token introspection response.
2.1.8. Token Identifier
Individual access tokens often need a unique internal identifier to
allow the AS to differentiate between multiple separate tokens. This
value of the token can often be used as the identifier, but in some
cases, a separate identifier is used.
This separate identifier can be conveyed in the jti claim in the
payload of a JSON Web Token [JWT] or a token introspection response.
This identifier is not usually exposed to the client instance using
the token, because the client instance only needs to use the token by
value.
2.1.9. Authorizing Resource Owner
Access tokens are approved on behalf of a resource owner (RO). The
identity of this RO can be used by the RS to determine exactly which
resource to access or which kinds of access to allow. For example,
an access token used to access identity information can hold a user
identifier to allow the RS to determine which profile information to
return. The nature of this information is subject to agreement by
the AS and RS.
This corresponds to the sub claim in the payload of a JSON Web Token
[JWT] or a token introspection response.
Detailed RO information is not returned to the client instance when
an access token is requested alone, and in many cases, returning this
information to the client instance would be a privacy violation on
the part of the AS. Since the access token represents a specific
delegated access, the client instance needs only to use the token at
its target RS. Following the profile example, the client instance
does not need to know the account identifier to get specific
attributes about the account represented by the token.
GNAP does allow for the return of subject information separately from
the access token, in the form of identifiers and assertions. These
values are returned directly to the client separately from any access
tokens that are requested, though it's common that they represent the
same party.
2.1.10. End User
The end user is the party operating the client software. The client
instance can facilitate the end user interacting with the AS in order
to determine the end user's identity, gather authorization, and
provide the results of that information back to the client instance.
In many instances, the end user is the same party as the resource
owner. However, in some cases, the two roles can be fulfilled by
different people, where the RO is consulted asynchronously. The
token model should be able to reflect these kinds of situations by
representing the end user and RO separately. For example, an end
user can request a financial payment, but the RO is the holder of the
account that the payment would be withdrawn from. The RO would be
consulted for approval by the AS outside of the flow of the GNAP
request. A token in such circumstances would need to show both the
RO and end user as separate entities.
2.1.11. Client Instance
Access tokens are issued to a specific client instance by the AS.
The identity of this instance can be used by the RS to allow specific
kinds of access or other attributes about the access token. For
example, an AS that binds all access tokens issued to a particular
client instance to that client instance's most recent key rotation
would need to be able to look up the client instance in order to find
the key binding detail.
This corresponds to the client_id claim in the payload of a JSON Web
Token [JWT] or the instance_id field of a token introspection
response.
The client is not normally informed of this information separately,
since a client instance can usually correctly assume that it is the
client instance to which a token that it receives was issued.
2.1.12. Label
When multiple access tokens are requested or a client instance uses
token labels, the parties will need to keep track of which labels
were applied to each individual token. Since labels can be reused
across different grant requests, the token label alone is not
sufficient to uniquely identify a given access token in a system.
However, within the context of a grant request, these labels are
required to be unique.
A client instance can request a specific label using the label field
of an access_token request; see Section 2.1 of [GNAP].
The AS can inform the client instance of a token's label using the
label field of an access_token response; see Section 3.2 of [GNAP].
This corresponds to the label field of a token introspection
response.
2.1.13. Parent Grant Request
All access tokens are issued in the context of a specific grant
request from a client instance. The grant request itself represents
a unique tuple of:
* The AS processing the grant request
* The client instance making the grant request
* The RO (or set of ROs) approving the grant request (or needing to
approve it)
* The access rights granted by the RO
* The current state of the grant request, as defined in Section 1.5
of [GNAP]
The AS can use this information to tie common information to a
specific token. For instance, instead of specifying a client
instance for every issued access token for a grant, the AS can store
the client information in the grant itself and look it up by
reference from the access token.
The AS can also use this information when a grant request is updated.
For example, if the client instance asks for a new access token from
an existing grant, the AS can use this link to revoke older non-
durable access tokens that had been previously issued under the
grant.
A client instance will have its own model of an ongoing grant
request, especially if that grant request can be continued using the
API defined in Section 5 of [GNAP] where several pieces of
statefulness need to be kept in hand. The client instance might need
to keep an association with the grant request that issued the token
in case the access token expires or does not have sufficient access
rights, so that the client instance can get a new access token
without having to restart the grant request process from scratch.
Since the grant itself does not need to be identified in any of the
protocol messages, GNAP does not define a specific grant identifier
to be conveyed between any parties in the protocol. Only the AS
needs to keep an explicit connection between an issued access token
and the parent grant that issued it.
2.1.14. AS-Specific Access Tokens
When an access token is used for the grant continuation API defined
in Section 5 of [GNAP] (the continuation access token), the token
management API defined in Section 6 of [GNAP] (the token management
access token), or the RS-facing API defined in Section 3 (the
resource server management access token), the AS MUST separate these
access tokens from other access tokens used at one or more RSs. The
AS can do this through the use of a flag on the access token data
structure, by using a special internal access right, or any other
means at its disposal. Just like other access tokens in GNAP, the
contents of these AS-specific access tokens are opaque to the
software presenting the token. Unlike other access tokens, the
contents of these AS-specific access tokens are also opaque to the
RS.
The client instance is given continuation access tokens only as part
of the continue field of the grant response in Section 3.1 of [GNAP].
The client instance is given token management access tokens only as
part of the manage field of the grant response in Section 3.2.1 of
[GNAP]. The means by which the RS is given resource server
management access tokens is out of scope of this specification, but
methods could include preconfiguration of the token value with the RS
software or granting the access token through a standard GNAP
process.
For continuation access tokens and token management access tokens, a
client instance MUST take steps to differentiate these special-
purpose access tokens from access tokens used at one or more RSs. To
facilitate this, a client instance can store AS-specific access
tokens separately from other access tokens in order to keep them from
being confused with each other and used at the wrong endpoints.
An RS should never see an AS-specific access token presented, so any
attempts to process one MUST fail. When introspection is used, the
AS MUST NOT return an active value of true for AS-specific access
tokens to the RS. If an AS implements its protected endpoints in
such a way that it uses token introspection internally, the AS MUST
differentiate these AS-specific access tokens from those issued for
use at an external RS.
2.2. Access Token Formats
When the AS issues an access token for use at an RS, the RS needs to
have some means of understanding what the access token is for in
order to determine how to respond to the request. The core GNAP
protocol makes neither assumptions nor demands on the format or
contents of the access token, and in fact, the token format and
contents are opaque to the client instance. However, such token
formats can be the topic of agreements between the AS and RS.
Self-contained structured token formats allow for the conveyance of
information between the AS and RS without requiring the RS to call
the AS at runtime as described in Section 3.3. Structured tokens can
also be used in combination with introspection, allowing the token
itself to carry one class of information and the introspection
response to carry another.
Some token formats, such as Macaroons [MACAROON] and Biscuits
[BISCUIT], allow for the RS to derive sub-tokens without having to
call the AS as described in Section 4.
The supported token formats can be communicated dynamically at
runtime between the AS and RS in several places:
* The AS can declare its supported token formats as part of RS-
facing discovery (Section 3.1).
* The RS can require a specific token format be used to access a
registered resource set (Section 3.4).
* The AS can return the token's format in an introspection response
(Section 3.3).
In all places where the token format is listed explicitly, it MUST be
one of the registered values in the "GNAP Token Formats" registry
Section 5.3.
3. Resource-Server-Facing API
To facilitate runtime and dynamic connections with an RS, the AS can
offer an RS-facing API consisting of one or more of the following
optional pieces:
* Discovery
* Introspection
* Token chaining
* Resource reference registration
3.1. RS-Facing AS Discovery
A GNAP AS offering RS-facing services can publish its features on a
well-known discovery document at the URL with the same schema and
authority as the grant request endpoint URL, at the path /.well-
known/gnap-as-rs.
The discovery response is a JSON document [RFC 8259] consisting of a
single JSON object with the following fields:
grant_request_endpoint (string): The location of the AS's grant
request endpoint defined by Section 9 of [GNAP]. This URL MUST be
the same URL used by client instances in support of GNAP requests.
The RS can use this to derive downstream access tokens, if
supported by the AS. The location MUST be a URL [RFC 3986] with a
scheme component that MUST be https, a host component, and
(optionally) port, path, and query components and no fragment
components. REQUIRED. See Section 4.
introspection_endpoint (string): The URL of the endpoint offering
introspection. The location MUST be a URL [RFC 3986] with a scheme
component that MUST be https, a host component, and (optionally)
port, path, and query components and no fragment components.
REQUIRED if the AS supports introspection. An absent value
indicates that the AS does not support introspection. See
Section 3.3.
token_formats_supported (array of strings): A list of token formats
supported by this AS. The values in this list MUST be registered
in the "GNAP Token Formats" registry per Section 5.3. OPTIONAL.
resource_registration_endpoint (string): The URL of the endpoint
offering resource registration. The location MUST be a URL
[RFC 3986] with a scheme component that MUST be https, a host
component, and (optionally) port, path, and query components and
no fragment components. REQUIRED if the AS supports dynamic
resource registration. An absent value indicates that the AS does
not support this feature. See Section 3.4.
key_proofs_supported (array of strings): A list of the AS's
supported key proofing mechanisms. The values of this list
correspond to possible values of the proof field of the key
section of the request. Values MUST be registered in the "GNAP
Key Proofing Methods" registry established by [GNAP]. OPTIONAL.
Additional fields are defined in the "GNAP RS-Facing Discovery
Document Fields" registry; see Section 5.8.
3.2. Protecting RS Requests to the AS
Unless otherwise specified, the RS MUST protect its calls to the AS
using any of the signature methods defined in Section 7 of [GNAP].
The RS MAY present its keys by reference or by value in a similar
fashion to a client instance calling the AS in the core protocol of
GNAP, as described in Section 7.1 of [GNAP]. In the protocols
defined here, this takes the form of the resource server identifying
itself by using a key field or by passing an instance identifier
directly.
POST /continue HTTP/1.1
Host: server.example.com
Authorization: GNAP 80UPRY5NM33OMUKMKSKU
Signature-Input: sig1=...
Signature: sig1=...
Content-Type: application/json
"resource_server": {
"key": {
"proof": "httpsig",
"jwk": {
"kty": "EC",
"crv": "secp256k1",
"kid": "2021-07-06T20:22:03Z",
"x": "-J9OJIZj4nmopZbQN7T8xv3sbeS5-f_vBNSy_EHnBZc",
"y": "sjrS51pLtu3P4LUTVvyAIxRfDV_be2RYpI5_f-Yjivw"
}
}
}
or by reference:
POST /continue HTTP/1.1
Host: server.example.com
Signature-Input: sig1=...
Signature: sig1=...
Content-Type: application/json
{
"resource_server": "7C7C4AZ9KHRS6X63AJAO"
}
The means by which an RS's keys are made known to the AS are out of
the scope of this specification. The AS MAY require an RS to
preregister its keys, or it could allow calls from arbitrary keys in
a trust-on-first-use model.
The AS MAY issue access tokens, called "resource server management
access tokens", to the RS to protect the RS-facing API endpoints. If
such tokens are issued, the RS MUST present them to the RS-facing API
endpoints along with the RS authentication.
POST /continue HTTP/1.1
Host: server.example.com
Authorization: GNAP 80UPRY5NM33OMUKMKSKU
Signature-Input: sig1=...
Signature: sig1=...
Content-Type: application/json
{
"resource_server": "7C7C4AZ9KHRS6X63AJAO"
}
3.3. Token Introspection
The AS issues access tokens representing a set of delegated access
rights to be used at one or more RSs. The AS can offer an
introspection service to allow an RS to validate that a given access
token:
* has been issued by the AS
* is valid at the current time
* has not been revoked
* is appropriate for the RS identified in the call
When the RS receives an access token, it can call the introspection
endpoint at the AS to get token information.
+--------+ +------+ +------+
| Client +--(1)->| RS | | AS |
|Instance| | +--(2)->| |
| | | | | |
| | | |<-(3)--+ |
| | | | +------+
| |<-(4)--+ |
+--------+ +------+
1. The client instance calls the RS with its access token.
2. The RS introspects the access token value at the AS. The RS
signs the request with its own key (not the client instance's key
or the token's key).
3. The AS validates the access token value and the RS's request and
returns the introspection response for the token.
4. The RS fulfills the request from the client instance.
The RS signs the request with its own key and sends the value of the
access token in the body of the request as a JSON object with the
following members:
access_token (string): The access token value presented to the RS by
the client instance. REQUIRED.
proof (string): The proofing method used by the client instance to
bind the token to the RS request. The value MUST be registered in
the "GNAP Key Proofing Methods" registry. RECOMMENDED.
resource_server (object/string): The identification used to
authenticate the resource server making this call, either by value
or by reference as described in Section 3.2. REQUIRED.
access (array of strings/objects): The minimum access rights
required to fulfill the request. This MUST be in the format
described in Section 8 of [GNAP]. OPTIONAL.
Additional fields are defined in the "GNAP Token Introspection
Request" registry (Section 5.4).
POST /introspect HTTP/1.1
Host: server.example.com
Content-Type: application/json
Signature-Input: sig1=...
Signature: sig1=...
Digest: sha256=...
{
"access_token": "OS9M2PMHKUR64TB8N6BW7OZB8CDFONP219RP1LT0",
"proof": "httpsig",
"resource_server": "7C7C4AZ9KHRS6X63AJAO"
}
The AS MUST validate the access token value and determine if the
token is active. The parameters of the request provide a context for
the AS to evaluate the access token, and the AS MUST take all
provided parameters into account when evaluating if the token is
active. If the AS is unable to process part of the request, such as
not understanding part of the access field presented, the AS MUST NOT
indicate the token as active.
An active access token is defined as a token that is all of the
following:
* was issued by the processing AS,
* has not been revoked,
* has not expired,
* is bound using the proof method indicated,
* is appropriate for presentation at the identified RS, and
* is appropriate for the access indicated (if present).
The AS responds with a data structure describing the token's current
state and any information the RS would need to validate the token's
presentation, such as its intended proofing mechanism and key
material.
active (boolean): If true, the access token presented is active, as
defined above. If any of the criteria for an active token are not
true, or if the AS is unable to make a determination (such as the
token is not found), the value is set to false and other fields
are omitted. REQUIRED.
If the access token is active, additional fields from the single
access token response structure defined in Section 3.2.1 of [GNAP]
are included. In particular, these include the following:
access (array of strings/objects): The access rights associated with
this access token. This MUST be in the format described in
Section 8 of [GNAP]. This array MAY be filtered or otherwise
limited for consumption by the identified RS, including being an
empty array, which indicates that the token has no explicit access
rights that can be disclosed to the RS. REQUIRED.
key (object/string): if the token is bound. The key bound to the
access token, to allow the RS to validate the signature of the
request from the client instance. If the access token is a bearer
token, this MUST NOT be included. REQUIRED
flags (array of strings): The set of flags associated with the
access token. OPTIONAL.
exp (integer): The timestamp after which this token is no longer
valid. Expressed as integer seconds from UNIX Epoch. OPTIONAL.
iat (integer): The timestamp at which this token was issued by the
AS. Expressed as integer seconds from UNIX Epoch. OPTIONAL.
nbf (integer): The timestamp before which this token is not valid.
Expressed as integer seconds from UNIX Epoch. OPTIONAL.
aud (string or array of strings): Identifiers for the resource
servers this token can be accepted at. OPTIONAL.
sub (string): Identifier of the resource owner who authorized this
token. OPTIONAL.
iss (string): Grant endpoint URL of the AS that issued this token.
REQUIRED.
instance_id (string): The instance identifier of the client instance
that the token was issued to. OPTIONAL.
Additional fields are defined in the "GNAP Token Introspection
Response" registry (Section 5.5).
The response MAY include any additional fields defined in an access
token response and MUST NOT include the access token value itself.
HTTP/1.1 200 OK
Content-Type: application/json
Cache-Control: no-store
{
"active": true,
"access": [
"dolphin-metadata", "some other thing"
],
"key": {
"proof": "httpsig",
"jwk": {
"kty": "RSA",
"e": "AQAB",
"kid": "xyz-1",
"alg": "RS256",
"n": "kOB5rR4Jv0GMeL...."
}
}
}
When processing the results of the introspection response, the RS
MUST determine the appropriate course of action. For instance, if
the RS determines that the access token's access rights are not
sufficient for the request to which the token was attached, the RS
can return an error or a public resource, as appropriate for the RS.
In all cases, the final determination of the response is at the
discretion of the RS.
3.4. Registering a Resource Set
If the RS needs to, it can post a set of resources, as described in
Section 8 ("Resource Access Rights") of [GNAP], to the AS's resource
registration endpoint along with information about what the RS will
need to validate the request.
access (array of objects/strings): The list of access rights
associated with the request in the format described in Section 8
("Resource Access Rights") of [GNAP]. REQUIRED.
resource_server (object/string): The identification used to
authenticate the resource server making this call, either by value
or by reference as described in Section 3.2. REQUIRED.
token_formats_supported (array of strings): The list of token
formats that the RS is able to process. The values in this array
MUST be registered in the "GNAP Token Formats" registry per
Section 5.3. If the field is omitted, the token format is at the
discretion of the AS. If the AS does not support any of the
requested token formats, the AS MUST return an error to the RS.
OPTIONAL.
token_introspection_required (boolean): If present and set to true,
the RS expects to make a token introspection request as described
in Section 3.3. If absent or set to false, the RS does not
anticipate needing to make an introspection request for tokens
relating to this resource set. If the AS does not support token
introspection for this RS, the AS MUST return an error to the RS.
OPTIONAL.
Additional fields are defined in the "GNAP Resource Set Registration
Request Parameters" registry (Section 5.6).
The RS MUST identify itself with its own key and sign the request.
POST /resource HTTP/1.1
Host: server.example.com
Content-Type: application/json
Signature-Input: sig1=...
Signature: sig1=...
Digest: ...
{
"access": [
{
"actions": [
"read",
"write",
"dolphin"
],
"locations": [
"https://server.example.net/",
"https://resource.local/other"
],
"datatypes": [
"metadata",
"images"
]
},
"dolphin-metadata"
],
"resource_server": "7C7C4AZ9KHRS6X63AJAO"
}
The AS responds with a reference appropriate to represent the
resources list that the RS presented in its request as well as any
additional information the RS might need in future requests.
resource_reference (string): A single string representing the list
of resources registered in the request. The RS MAY make this
handle available to a client instance as part of a discovery
response as described in Section 9.1 of [GNAP] or as documentation
to client software developers. REQUIRED.
instance_id (string): An instance identifier that the RS can use to
refer to itself in future calls to the AS, in lieu of sending its
key by value. See Section 3.2. OPTIONAL.
introspection_endpoint (string): The introspection endpoint of this
AS that is used to allow the RS to perform token introspection.
See Section 3.3. OPTIONAL.
Additional fields are defined in the "GNAP Resource Set Registration
Response Parameters" registry (Section 5.7).
HTTP/1.1 200 OK
Content-Type: application/json
Cache-Control: no-store
{
"resource_reference": "FWWIKYBQ6U56NL1"
}
If a resource was previously registered, the AS MAY return the same
resource reference value as in previous responses.
If the registration fails, the AS returns HTTP status code 400 (Bad
Request) to the RS, indicating that the registration was not
successful.
The client instance can then use the resource_reference value as a
string-type access reference as defined in Section 8.1 of [GNAP].
This value MAY be combined with any other additional access rights
requested by the client instance.
{
"access_token": {
"access": [
"FWWIKYBQ6U56NL1",
{
"type": "photo-api",
"actions": [
"read",
"write",
"dolphin"
],
"locations": [
"https://server.example.net/",
"https://resource.local/other"
],
"datatypes": [
"metadata",
"images"
]
},
"dolphin-metadata"
]
},
"client": "client-12351.bdxqf"
}
3.5. Error Responses
In the case of an error from the RS-facing API, the AS responds to
the RS with HTTP status code 400 (Bad Request) and a JSON object
consisting of a single error field, which is either an object or a
string.
When returned as a string, the error value is the error code:
{
error: "invalid_access"
}
When returned as an object, the error object contains the following
fields:
code (string): A single ASCII error code defining the error.
REQUIRED.
description (string): A human-readable string description of the
error intended for the developer of the client. OPTIONAL.
{
"error": {
"code": "invalid_access",
"description": "Access to 'foo' is not permitted for this RS."
}
}
This specification defines the following error code values:
"invalid_request": The request is missing a required parameter,
includes an invalid parameter value, or is otherwise malformed.
"invalid_resource_server": The request was made from an RS that was
not recognized or allowed by the AS, or the RS's signature
validation failed.
"invalid_access" The RS is not permitted to register or introspect
for the requested "access" value.
Additional error codes can be defined in the "GNAP RS-Facing Error
Codes" registry (Section 5.9).
4. Deriving a Downstream Token
Some architectures require an RS to act as a client instance and use
a derived access token for a secondary RS. Since the RS is not the
same entity that made the initial grant request, the RS is not
capable of referencing or modifying the existing grant. As such, the
RS needs to request or generate a new access token for its use at the
secondary RS. This internal secondary token is issued in the context
of the incoming access token.
While it is possible to use a token format (Section 2) that allows
for the RS to generate its own secondary token, the AS can allow the
RS to request this secondary access token using the same process used
by the original client instance to request the primary access token.
Since the RS is acting as its own client instance from the
perspective of GNAP, this process uses the same grant endpoint,
request structure, and response structure as a client instance's
request.
+--------+ +-------+ +------+ +-------+
| Client +--(1)->| RS1 | | AS | | RS2 |
|Instance| | +--(2)->| | | |
| | | |<-(3)--+ | | |
| | | | +------+ | |
| | | | | |
| | | +-----------(4)------->| |
| | | |<----------(5)--------+ |
| |<-(6)--+ | | |
+--------+ +-------+ +-------+
1. The client instance calls RS1 with an access token.
2. RS1 presents that token to the AS to get a derived token for use
at RS2. RS1 indicates that it has no ability to interact with
the RO. Note that RS1 signs its request with its own key, not
the token's key or the client instance's key.
3. The AS returns a derived token to RS1 for use at RS2.
4. RS1 calls RS2 with the token from (3).
5. RS2 fulfills the call from RS1.
6. RS1 fulfills the call from the original client instance.
If the RS needs to derive a token from one presented to it, it can
request one from the AS by making a token request as described in
[GNAP] and presenting the existing access token's value in the
"existing_access_token" field.
Since the RS is acting as a client instance, the RS MUST identify
itself with its own key in the client field and sign the request just
as any client instance would, as described in Section 3.2. The AS
MUST determine that the token being presented is appropriate for use
at the RS making the token chaining request.
POST /tx HTTP/1.1
Host: server.example.com
Content-Type: application/json
Detached-JWS: ejy0...
{
"access_token": {
"access": [
{
"actions": [
"read",
"write",
"dolphin"
],
"locations": [
"https://server.example.net/",
"https://resource.local/other"
],
"datatypes": [
"metadata",
"images"
]
},
"dolphin-metadata"
]
},
"client": "7C7C4AZ9KHRS6X63AJAO",
"existing_access_token": "OS9M2PMHKUR64TB8N6BW7OZB8CDFONP219RP1LT0"
}
The AS responds with a token for the downstream RS2 as described in
[GNAP]. The downstream RS2 could repeat this process as necessary
for calling further RSs.
5. IANA Considerations
IANA has added values to existing registries and created five
registries under the "Grant Negotiation and Authorization Protocol
(GNAP)" registry group.
5.1. Well-Known URIs
The "gnap-as-rs" URI suffix is registered in the "Well-Known URIs"
registry to support RS-facing discovery of the AS.
URI Suffix: gnap-as-rs
Change Controller: IETF
Specification Document: Section 3.1 of RFC 9767
Status: Permanent
5.2. GNAP Grant Request Parameters
The following parameter is registered in the "GNAP Grant Request
Parameters" registry:
Name: existing_access_token
Type: string
Reference: Section 4 of RFC 9767
5.3. GNAP Token Formats
This document defines a GNAP token format, for which IANA has created
and maintains a new registry titled "GNAP Token Formats". Initial
values for this registry are given in Section 5.3.2. Future
assignments and modifications to existing assignment are to be made
through the Specification Required registration policy [RFC 8126].
The designated expert (DE) is expected to ensure that:
* all registrations follow the template presented in Section 5.3.1.
* the format's definition is sufficiently unique from other formats
provided by existing parameters.
* the format's definition specifies the format of the access token
in sufficient detail to allow for the AS and RS to be able to
communicate the token information.
5.3.1. Registry Template
Name: The name of the format.
Status: Whether or not the format is in active use. Possible values
are Active and Deprecated.
Description: The human-readable description of the access token
format.
Reference: The specification that defines the token format.
5.3.2. Initial Registry Contents
+===============+========+====================+============+
| Name | Status | Description | Reference |
+===============+========+====================+============+
| jwt-signed | Active | JSON Web Token, | [JWT] |
| | | signed with JWS | |
+---------------+--------+--------------------+------------+
| jwt-encrypted | Active | JSON Web Token, | [JWT] |
| | | encrypted with JWE | |
+---------------+--------+--------------------+------------+
| macaroon | Active | Macaroon | [MACAROON] |
+---------------+--------+--------------------+------------+
| biscuit | Active | Biscuit | [BISCUIT] |
+---------------+--------+--------------------+------------+
| zcap | Active | ZCAP | [ZCAPLD] |
+---------------+--------+--------------------+------------+
Table 1: Initial Contents of the GNAP Token Formats Registry
5.4. GNAP Token Introspection Request
This document defines GNAP token introspection, for which IANA has
created and maintains a new registry titled "GNAP Token Introspection
Request". Initial values for this registry are given in
Section 5.4.2. Future assignments and modifications to existing
assignment are to be made through the Specification Required
registration policy [RFC 8126].
The DE is expected to ensure that:
* all registrations follow the template presented in Section 5.4.1.
* the claim's definition is sufficiently orthogonal to other claims
defined in the registry so as avoid overlapping functionality.
* the claim's definition specifies the syntax and semantics of the
claim in sufficient detail to allow for the AS and RS to be able
to communicate the token values.
5.4.1. Registry Template
Name: The name of the claim.
Type: The JSON data type of the claim value.
Reference: The specification that defines the claim.
5.4.2. Initial Registry Contents
The table below contains the initial contents of the "GNAP Token
Introspection Request" registry.
+=================+=================+=========================+
| Name | Type | Reference |
+=================+=================+=========================+
| access_token | string | Section 3.3 of RFC 9767 |
+-----------------+-----------------+-------------------------+
| proof | string | Section 3.3 of RFC 9767 |
+-----------------+-----------------+-------------------------+
| resource_server | object/string | Section 3.3 of RFC 9767 |
+-----------------+-----------------+-------------------------+
| access | array of | Section 3.3 of RFC 9767 |
| | strings/objects | |
+-----------------+-----------------+-------------------------+
Table 2: Initial Contents of the GNAP Token Introspection
Request Registry
5.5. GNAP Token Introspection Response
This document defines GNAP token introspection, for which IANA has
created and maintains a new registry titled "GNAP Token Introspection
Response". Initial values for this registry are given in
Section 5.5.2. Future assignments and modifications to existing
assignment are to be made through the Specification Required
registration policy [RFC 8126].
The DE is expected to ensure that:
* all registrations follow the template presented in Section 5.5.1.
* the claim's definition is sufficiently orthogonal to other claims
defined in the registry so as avoid overlapping functionality.
* the claim's definition specifies the syntax and semantics of the
claim in sufficient detail to allow for the AS and RS to be able
to communicate the token values.
5.5.1. Registry Template
Name: The name of the claim.
Type: The JSON data type of the claim value.
Reference: The specification that defines the claim.
5.5.2. Initial Registry Contents
The table below contains the initial contents of the "GNAP Token
Introspection Response" registry.
+=============+==========================+=========================+
| Name | Type | Reference |
+=============+==========================+=========================+
| active | boolean | Section 3.3 of RFC 9767 |
+-------------+--------------------------+-------------------------+
| access | array of strings/objects | Section 3.3 of RFC 9767 |
+-------------+--------------------------+-------------------------+
| key | object/string | Section 3.3 of RFC 9767 |
+-------------+--------------------------+-------------------------+
| flags | array of strings | Section 3.3 of RFC 9767 |
+-------------+--------------------------+-------------------------+
| exp | integer | Section 3.3 of RFC 9767 |
+-------------+--------------------------+-------------------------+
| iat | integer | Section 3.3 of RFC 9767 |
+-------------+--------------------------+-------------------------+
| nbf | integer | Section 3.3 of RFC 9767 |
+-------------+--------------------------+-------------------------+
| aud | string or array of | Section 3.3 of RFC 9767 |
| | strings | |
+-------------+--------------------------+-------------------------+
| sub | string | Section 3.3 of RFC 9767 |
+-------------+--------------------------+-------------------------+
| iss | string | Section 3.3 of RFC 9767 |
+-------------+--------------------------+-------------------------+
| instance_id | string | Section 3.3 of RFC 9767 |
+-------------+--------------------------+-------------------------+
Table 3: Initial Contents of the GNAP Token Introspection
Response Registry
5.6. GNAP Resource Set Registration Request Parameters
This document defines a means to register a resource set for a GNAP
AS, for which IANA has created and maintains a new registry titled
"GNAP Resource Set Registration Request Parameters". Initial values
for this registry are given in Section 5.6.2. Future assignments and
modifications to existing assignment are to be made through the
Expert Review registration policy [RFC 8126].
The DE is expected to ensure that:
* all registrations follow the template presented in Section 5.6.1.
* the parameter's definition is sufficiently orthogonal to other
parameters defined in the registry so as avoid overlapping
functionality.
* the parameter's definition specifies the syntax and semantics of
the parameter in sufficient detail to allow for the AS and RS to
be able to communicate the resource set.
5.6.1. Registry Template
Name: The name of the parameter.
Type: The JSON data type of the parameter value.
Reference: The specification that defines the token.
5.6.2. Initial Registry Contents
The table below contains the initial contents of the "GNAP Resource
Set Registration Request Parameters" registry.
+==============================+=================+=============+
| Name | Type | Reference |
+==============================+=================+=============+
| access | array of | Section 3.4 |
| | strings/objects | of RFC 9767 |
+------------------------------+-----------------+-------------+
| resource_server | object/string | Section 3.4 |
| | | of RFC 9767 |
+------------------------------+-----------------+-------------+
| token_formats_supported | array of | Section 3.4 |
| | strings | of RFC 9767 |
+------------------------------+-----------------+-------------+
| token_introspection_required | boolean | Section 3.4 |
| | | of RFC 9767 |
+------------------------------+-----------------+-------------+
Table 4: Initial Contents of the GNAP Resource Set
Registration Request Parameters Registry
5.7. GNAP Resource Set Registration Response Parameters
This document defines a means to register a resource set for a GNAP
AS, for which IANA has created and maintains a new registry titled
"GNAP Resource Set Registration Response Parameters". Initial values
for this registry are given in Section 5.7.2. Future assignments and
modifications to existing assignment are to be made through the
Expert Review registration policy [RFC 8126].
The DE is expected to ensure that:
* all registrations follow the template presented in Section 5.7.1.
* the parameter's definition is sufficiently orthogonal to other
claims defined in the registry so as avoid overlapping
functionality.
* the parameter's definition specifies the syntax and semantics of
the claim in sufficient detail to allow for the AS and RS to be
able to communicate the resource set.
5.7.1. Registry Template
Name: The name of the parameter.
Type: The JSON data type of the parameter value.
Reference: The specification that defines the parameter.
5.7.2. Initial Registry Contents
The table below contains the initial contents of the "GNAP Resource
Set Registration Response Parameters" registry.
+========================+========+=========================+
| Name | Type | Reference |
+========================+========+=========================+
| resource_reference | string | Section 3.4 of RFC 9767 |
+------------------------+--------+-------------------------+
| instance_id | string | Section 3.4 of RFC 9767 |
+------------------------+--------+-------------------------+
| introspection_endpoint | string | Section 3.4 of RFC 9767 |
+------------------------+--------+-------------------------+
Table 5: Initial Contents of the GNAP Resource Set
Registration Response Parameters Registry
5.8. GNAP RS-Facing Discovery Document Fields
This document defines a means to for a GNAP AS to be discovered by a
GNAP RS, for which IANA has created and maintains a new registry
titled "GNAP RS-Facing Discovery Document Fields". Initial values
for this registry are given in Section 5.8.2. Future assignments and
modifications to existing assignment are to be made through the
Expert Review registration policy [RFC 8126].
The DE is expected to ensure that:
* all registrations follow the template presented in Section 5.8.1.
* the field's definition is sufficiently orthogonal to other fields
defined in the registry so as avoid overlapping functionality.
* the field's definition specifies the syntax and semantics of the
fields in sufficient detail to allow for the RS to be able to
communicate with the AS.
5.8.1. Registry Template
Name: The name of the field.
Type: The JSON data type of the field value.
Reference: The specification that defines the field.
5.8.2. Initial Registry Contents
The table below contains the initial contents of the "GNAP RS-Facing
Discovery Document Fields" registry.
+================================+==========+=============+
| Name | Type | Reference |
+================================+==========+=============+
| introspection_endpoint | string | Section 3.1 |
| | | of RFC 9767 |
+--------------------------------+----------+-------------+
| token_formats_supported | array of | Section 3.1 |
| | strings | of RFC 9767 |
+--------------------------------+----------+-------------+
| resource_registration_endpoint | string | Section 3.1 |
| | | of RFC 9767 |
+--------------------------------+----------+-------------+
| grant_request_endpoint | string | Section 3.1 |
| | | of RFC 9767 |
+--------------------------------+----------+-------------+
| key_proofs_supported | array of | Section 3.1 |
| | strings | of RFC 9767 |
+--------------------------------+----------+-------------+
Table 6: Initial Contents of the GNAP RS-Facing
Discovery Document Fields Registry
5.9. GNAP RS-Facing Error Codes
This document defines a set of errors that the AS can return to the
RS, for which IANA has created and maintains a new registry titled
"GNAP RS-Facing Error Codes". Initial values for this registry are
given in Section 5.9.2. Future assignments and modifications to
existing assignments are to be made through the Specification
Required registration policy [RFC 8126].
The DE is expected to ensure that:
* all registrations follow the template presented in Section 5.9.1.
* the error response is sufficiently unique from other errors to
provide actionable information to the client instance.
* the definition of the error response specifies all conditions in
which the error response is returned and what the client
instance's expected action is.
5.9.1. Registration Template
Error: A unique string code for the error.
Reference: Reference to the document(s) that specifies the value,
preferably including a URI that can be used to retrieve a copy of
the document(s). An indication of the relevant sections may also
be included but is not required.
5.9.2. Initial Contents
+=========================+=========================+
| Error | Reference |
+=========================+=========================+
| invalid_request | Section 3.5 of RFC 9767 |
+-------------------------+-------------------------+
| invalid_resource_server | Section 3.5 of RFC 9767 |
+-------------------------+-------------------------+
| invalid_access | Section 3.5 of RFC 9767 |
+-------------------------+-------------------------+
Table 7: Initial Contents of the GNAP RS-Facing
Error Codes Registry
6. Security Considerations
In addition to the normative requirements in this document and in
[GNAP], implementers are strongly encouraged to consider the
following additional security considerations in implementations and
deployments of GNAP.
6.1. TLS Protection in Transit
All requests in GNAP made over untrusted network connections have to
be made over TLS as outlined in [BCP195] to protect the contents of
the request and response from manipulation and interception by an
attacker. This includes all requests from a client instance to the
RS and all requests from the RS to an AS.
6.2. Token Validation
The RS has a responsibility to validate the incoming access token in
a manner consistent with its deployment. For self-contained
stateless tokens such as those described in Section 2.2, this
consists of actions such as validating the token's signature and
ensuring the relevant fields and results are appropriate for the
request being made. For reference-style tokens or tokens that are
otherwise opaque to the RS, the token introspection RS-facing API can
be used to provide updated information about the state of the token,
as described in Section 3.3.
The RS needs to validate that a token:
* is intended for this RS (audience restriction)
* is presented using the appropriate key for the token (see also
Section 6.4)
* identifies an appropriate subject to access the resource (usually
this is the resource owner who authorized the token's issuance)
* is issued by a trusted AS for this resource
Even though key proofing mechanisms have to cover the value of the
token, validating the key proofing alone is not sufficient to protect
a request to an RS. If an RS validates only the presentation method
as described in Section 6.4 without validating the token itself, an
attacker could use a compromised key or a confused deputy to make
arbitrary calls to the RS beyond what has been authorized by the RO.
6.3. Caching Token Validation Result
Since token validation can be an expensive process, requiring either
cryptographic operations or network calls to an introspection service
as described in Section 3.3, an RS could cache the results of token
validation for a particular token. The trade-off for using a cached
validation for a token presents an important decision space for
implementers: relying on a cached validation result increases
performance and lowers processing overhead, but it comes at the
expense of the liveness and accuracy of the information in the cache.
While a cached value is in use at the RS, an attacker could present a
revoked token and have it accepted by the RS.
As with any cache, the consistency of this cache can be managed in a
variety of ways. One of the most simple methods is managing the
lifetime of the cache in order to balance the performance and
security properties. If the cache is too long, an attacker has a
larger window in which to use a revoked token. If the window is too
short, the benefits of using the cache are diminished. It is also
possible that an AS could send a proactive signal to the RS to
invalidate revoked access tokens, though such a mechanism is outside
the scope of this specification.
6.4. Key Proof Validation
For key-bound access tokens, the proofing method needs to be
validated alongside the value of the token itself, as described in
Section 6.2. The process of validation is defined by the key
proofing method, as described in Section 7.3 of [GNAP].
If the proofing method is not validated, an attacker could use a
compromised token without access to the token's bound key.
The RS also needs to ensure that the proofing method is appropriate
for the key associated with the token, including any choice of
algorithm or identifiers.
The proofing should be validated independently on each request to the
RS, particularly as aspects of the call could vary. As such, the RS
should never cache the results of a proof validation from one message
and apply it to a subsequent message.
6.5. Token Exfiltration
Since the RS sees the token value, it is possible for a compromised
RS to leak that value to an attacker. As such, the RS needs to
protect token values as sensitive information and protect them from
exfiltration.
This is especially problematic with bearer tokens and tokens bound to
a shared key, since an RS has access to all information necessary to
create a new, valid request using the token in question.
6.6. Token Reuse by an RS
If the access token is a bearer token, or the RS has access to the
key material needed to present the token, the RS could be tricked
into reusing an access token presented to it by a client. While it
is possible to build a system that makes use of this artifact as a
feature, it is safer to exchange the incoming access token for
another contextual token for use by the RS, as described in
Section 4. This access token can be bound to the RS's own keys and
limited to access needed by the RS, instead of the full set of rights
associated with the token issued to the client instance.
6.7. Token Format Considerations
With formatted tokens, the format of the token is likely to have its
own considerations, and the RS needs to follow any such
considerations during the token validation process. The application
and scope of these considerations is specific to the format and
outside the scope of this specification.
6.8. Oversharing Token Contents
The contents of the access token model divulge information about the
access token's context and rights to the RS. This is true whether
the contents are parsed from the token itself or sent in an
introspection response.
It's likely that every RS does not need to know all details of the
token model, especially in systems where a single access token is
usable across multiple RSs. An attacker could use this to gain
information about the larger system by compromising only one RS. By
limiting the information available to only that which is relevant to
a specific RS, such as using a limited introspection reply as defined
in Section 3.3, a system can follow the principle of least disclosure
to each RS.
6.9. Resource References
Resource references, as returned by the protocol in Section 3.4, are
intended to be opaque to both the RS and the client. However, since
they are under the control of the AS, the AS can put whatever content
it wants into the reference value. This value could unintentionally
disclose system structure or other internal details if it was
processed by an unintended party. Furthermore, such patterns could
lead to the client software and RS depending on certain structures
being present in the reference value, which diminishes the separation
of concerns of the different roles in a GNAP system.
To mitigate this, the AS should only use fully random or encrypted
values for resource references.
6.10. Token Reissuance from an Untrusted AS
It is possible for an attacker's client instance to issue its own
tokens to another client instance, acting as an AS that the second
client instance has chosen to trust. If the token is a bearer token
or the reissuance is bound using an AS-provided key, the target
client instance will not be able to tell that the token was
originally issued by the valid AS. This process allows an attacker
to insert their own session and rights into an unsuspecting client
instance in the guise of a valid token for the attacker that appears
to have been issued to the target client instance on behalf of its
own RO.
This attack is predicated on a misconfiguration with the targeted
client, as it has been configured to get tokens from the attacker's
AS and use those tokens with the target RS, which has no association
with the attacker's AS. However, since the token is ultimately
coming from the trusted AS and is being presented with a valid key,
the RS has no way of telling that the token was passed through an
intermediary.
To mitigate this, the RS can publish its association with the trusted
AS through either discovery or documentation. Therefore, a client
properly following this association would only go directly to the
trusted RS for access tokens for the RS.
Furthermore, limiting the use of bearer tokens and AS-provided keys
to only highly trusted ASs in certain circumstances prevents the
attacker from being able to willingly exfiltrate their token to an
unsuspecting client instance.
6.11. Introspection of Token Keys
The introspection response defined in Section 3.3 provides a means
for the AS to tell the RS what key material is needed to validate the
key proof of the request. Capture of the introspection response can
expose these security keys to an attacker. In the case of asymmetric
cryptography, only the public key is exposed, and the token cannot be
reused by the attacker based on this result alone. This could
potentially divulge information about the client instance that was
unknown otherwise.
If an access token is bound to a symmetric key, the RS will need
access to the full key value in order to validate the key proof of
the request, as described in Section 6.4. However, divulging the key
material to the RS also gives the RS the ability to create a new
request with the token. In this circumstance, the RS is under
similar risk of token exfiltration and reuse as a bearer token, as
described in Section 6.6. Consequently, symmetric keys should only
be used in systems where the RS can be fully trusted to not create a
new request with tokens presented to it.
6.12. RS Registration and Management
Most functions of the RS-facing API in Section 3 are protected by
requiring the RS to present proof of a signing key along with the
request, in order to identify the RS making the call, potentially
coupled with an AS-specific access token. This practice allows the
AS to differentially respond to API calls to different RSs, such as
answering introspection calls with only the access rights relevant to
a given RS instead of all access rights an access token could be good
for.
While the means by which an RS and its keys become known to the AS is
out of scope for this specification, it is anticipated that common
practice will be to statically register an RS, allowing it to protect
specific resources or certain classes of resources. Fundamentally,
the RS can only offer the resources that it serves. However, a rogue
AS could attempt to register a set of resources that mimics a
different RS in order to solicit an access token that is usable at
the target RS. If the access token is a bearer token or is bound to
a symmetric key that is known to the RS, then the attacker's RS gains
the ability and knowledge needed to use that token elsewhere.
In some ecosystems, dynamic registration of an RS and its associated
resources is feasible. In such systems, the identity of the RS could
be conveyed by a URI passed in the location field of an access rights
request, thereby allowing the AS to limit the view the RS has into
the larger system.
7. Privacy Considerations
7.1. Token Contents
The contents of the access token could potentially contain personal
information about the end user, RO, or other parties. This is true
whether the contents are parsed from the token itself or sent in an
introspection response.
While an RS will sometimes need this information for processing, it's
often the case that an RS is exposed to these details only in
passing, and not intentionally. For example, consider a client that
has been issued an access token that is usable for both medical and
non-medical APIs. If this access token contains a medical record
number to facilitate the RS serving the medical API, then any RS for
a non-medical API would also learn the user's medical record number
in the process, even though that API has no need to make such a
correlation.
To mitigate this, a formatted token could contain separate sections
targeted to different RSs to segregate data. Alternatively, token
introspection can be used to limit the data returned to each RS, as
defined in Section 3.3.
7.2. Token Use Disclosure through Introspection
When introspection is used by an RS, the AS is made aware of a
particular token being used at a particular RS. When the RS is a
separate system, the AS would not otherwise have insight into this
action. This can potentially lead to the AS learning about patterns
and actions of particular end users by watching which RSs are
accessed and when.
7.3. Mapping a User to an AS
When the client instance receives information about the protecting AS
from an RS, it can be used to derive information about the resources
being protected without releasing the resources themselves. For
example, if a medical record is protected by a personal AS, an
untrusted client could call an RS to discover the location of the AS
protecting the record. Since the AS is tied strongly to a single RO,
the untrusted and unauthorized client software can gain information
about the resource being protected without accessing the record
itself.
8. References
8.1. 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/RFC 8996, March 2021,
<https://www.rfc-editor.org/info/RFC 8996>.
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/RFC 9325, November
2022, <https://www.rfc-editor.org/info/RFC 9325>.
[GNAP] Richer, J., Ed. and F. Imbault, "Grant Negotiation and
Authorization Protocol (GNAP)", RFC 9635,
DOI 10.17487/RFC 9635, October 2024,
<https://www.rfc-editor.org/info/RFC 9635>.
[JWT] Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
(JWT)", RFC 7519, DOI 10.17487/RFC 7519, May 2015,
<https://www.rfc-editor.org/info/RFC 7519>.
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC 2119, March 1997,
<https://www.rfc-editor.org/info/RFC 2119>.
[RFC 3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, DOI 10.17487/RFC 3986, January 2005,
<https://www.rfc-editor.org/info/RFC 3986>.
[RFC 8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC 8174,
May 2017, <https://www.rfc-editor.org/info/RFC 8174>.
[RFC 8259] Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
Interchange Format", STD 90, RFC 8259,
DOI 10.17487/RFC 8259, December 2017,
<https://www.rfc-editor.org/info/RFC 8259>.
[RFC 8792] Watsen, K., Auerswald, E., Farrel, A., and Q. Wu,
"Handling Long Lines in Content of Internet-Drafts and
RFCs", RFC 8792, DOI 10.17487/RFC 8792, June 2020,
<https://www.rfc-editor.org/info/RFC 8792>.
8.2. Informative References
[BISCUIT] Biscuit, "Biscuit Authorization",
<https://www.biscuitsec.org/>.
[MACAROON] Birgisson, A., Politz, J. G., Erlingsson, U., Taly, A.,
Vrable, M., and M. Lentczner, "Macaroons: Cookies with
Contextual Caveats for Decentralized Authorization in the
Cloud", NDSS Symposium 2014, DOI 10.14722/ndss.2014.23212,
February 2014, <https://www.ndss-symposium.org/ndss2014/
ndss-2014-programme/macaroons-cookies-contextual-caveats-
decentralized-authorization-cloud/>.
[RFC 8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC 8126, June 2017,
<https://www.rfc-editor.org/info/RFC 8126>.
[ZCAPLD] Lemmer-Webber, C., Ed. and M. Sporny, Ed., "Authorization
Capabilities for Linked Data v0.3", W3C Draft Community
Group Report, January 2023,
<https://w3c-ccg.github.io/zcap-spec/>.
Acknowledgements
The editors would like to thank the following individuals for their
reviews, feedback, implementations, and contributions: Aaron Parecki,
Adrian Gropper, Andrii Deinega, Annabelle Backman, Dmitry Barinov,
Fabien Imbault, Florian Helmschmidt, George Fletcher, Justin Richer,
Kathleen Moriarty, Leif Johansson, Mike Varley, Nat Sakimura,
Takahiko Kawasaki, and Yaron Sheffer.
Additionally, the editors want to acknowledge the immense
contributions of Aaron Parecki to the content of this document. We
thank him for his insight, input, and hard work, without which GNAP
would not have grown to what it is.
Authors' Addresses
Justin Richer (editor)
Bespoke Engineering
Email: ietf@justin.richer.org
URI: https://bspk.io/
Fabien Imbault
acert.io
Email: fabien.imbault@acert.io
URI: https://acert.io/
RFC TOTAL SIZE: 84374 bytes
PUBLICATION DATE: Wednesday, April 23rd, 2025
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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