OAuth 2.0 Demonstrating Proof-of-Possession at the Application Layer (DPoP)yes.commail@danielfett.dePing Identitybcampbell@pingidentity.comYubicove7jtb@ve7jtb.comyes.comtorsten@lodderstedt.netMicrosoftmbj@microsoft.comhttps://self-issued.info/Ping Identitydavid@alkaline-solutions.com
Security
Web Authorization Protocolsecurityoauth2This document describes a mechanism for sender-constraining OAuth 2.0
tokens via a proof-of-possession mechanism on the application level.
This mechanism allows for the detection of replay attacks with access and refresh
tokens.IntroductionDPoP, an abbreviation for Demonstrating Proof-of-Possession at the Application Layer,
is an application-level mechanism for
sender-constraining OAuth access and refresh tokens. It enables a client to
demonstrate proof-of-possession of a public/private key pair by including
a DPoP header in an HTTP request. The value of the header is a JWT that
enables the authorization
server to bind issued tokens to the public part of a client's
key pair. Recipients of such tokens are then able to verify the binding of the
token to the key pair that the client has demonstrated that it holds via
the DPoP header, thereby providing some assurance that the client presenting
the token also possesses the private key.
In other words, the legitimate presenter of the token is constrained to be
the sender that holds and can prove possession of the private part of the
key pair.The mechanism described herein can be used in cases where other
methods of sender-constraining tokens that utilize elements of the underlying
secure transport layer, such as or ,
are not available or desirable. For example, due to a sub-par user experience
of TLS client authentication in user agents and a lack of support for HTTP token
binding, neither mechanism can be used if an OAuth client is a Single Page
Application (SPA) running in a web browser. Native applications installed
and run on a user's device, which often have dedicated protected storage
for cryptographic keys are another example well positioned to benefit
from DPoP-bound tokens to guard against misuse of tokens by a compromised
or malicious resource.DPoP can be used to sender-constrain access tokens regardless of the
client authentication method employed. Furthermore, DPoP can
also be used to sender-constrain refresh tokens issued to public clients
(those without authentication credentials associated with the client_id).Conventions and TerminologyThe key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL
NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED",
"MAY", and "OPTIONAL" in this document are to be interpreted as
described in BCP 14 when, and only when, they
appear in all capitals, as shown here.This specification uses the terms "access token", "refresh token",
"authorization server", "resource server", "authorization endpoint",
"authorization request", "authorization response", "token endpoint",
"grant type", "access token request", "access token response", and
"client" defined by The OAuth 2.0 Authorization Framework .ObjectivesThe primary aim of DPoP is to prevent unauthorized or illegitimate
parties from using leaked or stolen access tokens by binding a token
to a public key upon issuance and requiring that the client demonstrate
possession of the corresponding private key when using the token.
This constrains the legitimate sender of the token to only the party with
access to the private key and gives the server receiving the token added
assurances that the sender is legitimately authorized to use it.Access tokens that are sender-constrained via DPoP thus stand in
contrast to the typical bearer token, which can be used by any party in
possession of such a token. Although protections generally exist to
prevent unintended disclosure of bearer tokens, unforeseen vectors for
leakage have occurred due to vulnerabilities and implementation issues
in other layers in the protocol or software stack (CRIME, BREACH,
Heartbleed, and the Cloudflare parser bug are some examples).
There have also been numerous published token theft attacks on OAuth
implementations themselves. DPoP provides a general defense in depth
against the impact of unanticipated token leakage. DPoP is not, however,
a substitute for a secure transport and MUST always be used in
conjunction with HTTPS.The very nature of the typical OAuth protocol interaction
necessitates that the client disclose the access token to the
protected resources that it accesses. The attacker model
in describes cases where a
protected resource might be counterfeit, malicious or compromised
and play received tokens against other protected resources to gain
unauthorized access. Properly audience restricting access tokens can
prevent such misuse, however, doing so in practice has proven to be
prohibitively cumbersome (even despite extensions such as )
for many deployments.
Sender-constraining access tokens is a more robust and straightforward
mechanism to prevent such token replay at a different endpoint and DPoP
is an accessible application layer means of doing so.Due to the potential for cross-site scripting (XSS), browser-based
OAuth clients bring to bear added considerations with respect to protecting
tokens. The most straightforward XSS-based attack is for an attacker to
exfiltrate a token and use it themselves completely independent from the
legitimate client. A stolen access token is used for protected
resource access and a stolen refresh token for obtaining new access tokens.
If the private key is non-extractable (as is possible with ),
DPoP renders exfiltrated tokens alone unusable.XXS vulnerabilities also allow an attacker to execute code in the context of
the browser-based client application and maliciously use a token indirectly
through the client. That execution context has access to utilize the signing
key and thus can produce DPoP proofs to use in conjunction with the token.
At this application layer there is most likely no feasible defense against
this threat except generally preventing XSS, therefore it is considered
out of scope for DPoP.Malicious XSS code executed in the context of the browser-based client application
is also in a position to create DPoP proofs with timestamp values in the future
and exfiltrate them in conjunction with a token. These stolen artifacts
can later be used together independent of the client application to access
protected resources. The impact of such precomputed DPoP proofs is limited
somewhat by the proof being bound to an access token on protected resource access.
Because a proof covering an access token that don't yet exist cannot feasibly be created,
access tokens obtained with an exfiltrated refresh token and pre-computed proofs will be
unusable.Additional security considerations are discussed in .ConceptThe main data structure introduced by this specification is a DPoP
proof JWT, described in detail below, which is sent as a header in an
HTTP request. A client uses a DPoP proof JWT to prove
the possession of a private key corresponding to a certain public key.
Roughly speaking, a DPoP proof is a signature over a timestamp and some
data of the HTTP request to which it is attached.The basic steps of an OAuth flow with DPoP are shown in :
(A) In the Token Request, the client sends an authorization grant
(e.g., an authorization code, refresh token, etc.)
to the authorization server in order to obtain an access token
(and potentially a refresh token). The client attaches a DPoP
proof to the request in an HTTP header.
(B) The authorization server binds (sender-constrains) the access token to the
public key claimed by the client in the DPoP proof; that is, the access token cannot
be used without proving possession of the respective private key.
If a refresh token is issued to a public client, it too is
bound to the public key of the DPoP proof.
(C) To use the access token the client has to prove
possession of the private key by, again, adding a header to the
request that carries a DPoP proof for that request. The resource server needs to
receive information about the public key to which the access token is bound. This
information may be encoded directly into the access token (for
JWT structured access tokens) or provided via token
introspection endpoint (not shown).
The resource server verifies that the public key to which the
access token is bound matches the public key of the DPoP proof.
(D) The resource server refuses to serve the request if the
signature check fails or the data in the DPoP proof is wrong,
e.g., the request URI does not match the URI claim in the DPoP
proof JWT. The access token itself, of course, must also be
valid in all other respects.
The DPoP mechanism presented herein is not a client authentication method.
In fact, a primary use case of DPoP is for public clients (e.g., single page
applications and native applications) that do not use client authentication. Nonetheless, DPoP
is designed such that it is compatible with private_key_jwt and all
other client authentication methods.DPoP does not directly ensure message integrity but relies on the TLS
layer for that purpose. See for details.DPoP Proof JWTsDPoP introduces the concept of a DPoP proof, which is a JWT created by
the client and sent with an HTTP request using the DPoP header field.
Each HTTP request requires a unique DPoP proof.
A valid DPoP proof demonstrates to the server that the client holds the private
key that was used to sign the DPoP proof JWT. This enables authorization servers to bind
issued tokens to the corresponding public key (as described in )
and for resource servers to verify the key-binding of tokens that
it receives (see ), which prevents said tokens from
being used by any entity that does not have access to the private key.The DPoP proof demonstrates possession of a key and, by itself, is not
an authentication or access control mechanism. When presented
in conjunction with a key-bound access token as described in ,
the DPoP proof provides additional assurance about the legitimacy of the client
to present the access token. However, a valid DPoP proof JWT is not sufficient alone
to make access control decisions.The DPoP HTTP HeaderA DPoP proof is included in an HTTP request using the following message header field.
DPoP
A JWT that adheres to the structure and syntax of .
shows an example DPoP HTTP header field (line breaks
and extra whitespace for display purposes only).Note that per header field names are case-insensitive;
so DPoP, DPOP, dpop, etc., are all valid and equivalent header
field names. Case is significant in the header field value, however.DPoP Proof JWT SyntaxA DPoP proof is a JWT () that is signed (using JWS,
) with a private key chosen by the client (see below). The
header of a DPoP JWT contains at least the following parameters:
typ: type header, value dpop+jwt (REQUIRED).
alg: a digital signature algorithm identifier as per
(REQUIRED). MUST NOT be none or an identifier for a symmetric
algorithm (MAC).
jwk: representing the public key chosen by the client, in JWK
format, as defined in Section 4.1.3 of (REQUIRED).
MUST NOT contain the private key.
The payload of a DPoP proof contains at least the following claims:
jti: Unique identifier for the DPoP proof JWT (REQUIRED).
The value MUST be assigned such that there is a negligible
probability that the same value will be assigned to any
other DPoP proof used in the same context during the time window of validity.
Such uniqueness can be accomplished by encoding (base64url or any other
suitable encoding) at least 96 bits of
pseudorandom data or by using a version 4 UUID string according to .
The jti can be used by the server for replay
detection and prevention, see .
htm: The HTTP method for the request to which the JWT is
attached, as defined in (REQUIRED).
htu: The HTTP URI used for the request, without query and
fragment parts (REQUIRED).
iat: Time at which the JWT was created (REQUIRED).
When the DPoP proof is used in conjunction with the presentation of an access token, see
, the DPoP proof also contains the following claim:
ath: hash of the access token (REQUIRED).
The value MUST be the result of a base64url encoding (with no padding) the SHA-256
hash of the ASCII encoding of the associated access token's value.
is a conceptual example showing the decoded content of the DPoP
proof in . The JSON of the JOSE header and payload are shown
but the signature part is omitted. As usual, line breaks and extra whitespace
are included for formatting and readability.Of the HTTP content in the request, only the HTTP method and URI are
included in the DPoP JWT, and therefore only these 2 headers of the request
are covered by the DPoP proof and its signature.
The idea is sign just enough of the HTTP data to
provide reasonable proof-of-possession with respect to the HTTP request. But
that it be a minimal subset of the HTTP data so as to avoid the substantial
difficulties inherent in attempting to normalize HTTP messages.
Nonetheless, DPoP proofs can be extended to contain other information of the
HTTP request (see also ).Checking DPoP ProofsTo check if a string that was received as part of an HTTP Request is a
valid DPoP proof, the receiving server MUST ensure that
the string value is a well-formed JWT,
all required claims per are contained in the JWT,
the typ field in the header has the value dpop+jwt,
the algorithm in the header of the JWT indicates an asymmetric digital
signature algorithm, is not none, is supported by the
application, and is deemed secure,
the JWT signature verifies with the public key contained in the jwk
header of the JWT,
the htm claim matches the HTTP method value of the HTTP
request in which the JWT was received,
the htu claims matches the HTTPS URI value for the HTTP
request in which the JWT was received, ignoring any query and
fragment parts,
the token was issued within an acceptable timeframe and,
within a reasonable consideration of accuracy and resource utilization,
a proof JWT with the same jti value has not previously been received at the same resource
during that time period (see ).
Servers SHOULD employ Syntax-Based Normalization and Scheme-Based
Normalization in accordance with Section 6.2.2. and Section 6.2.3. of
before comparing the htu claim.If presented with an access token to a protected resource, the server MUST ensure
that the value of the ath claim equals the hash of the access token that has been
presented along side the DPoP proof.DPoP Access Token RequestTo request an access token that is bound to a public key using DPoP, the client MUST
provide a valid DPoP proof JWT in a DPoP header when making an access token
request to the authorization server's token endpoint. This is applicable for all
access token requests regardless of grant type (including, for example,
the common authorization_code and refresh_token grant types but also extension grants
such as the JWT authorization grant ). The HTTPS request shown in
illustrates such an access
token request using an authorization code grant with a DPoP proof JWT
in the DPoP header (extra line breaks and whitespace for display purposes only).The DPoP HTTP header MUST contain a valid DPoP proof JWT.
If the DPoP proof is invalid, the authorization server issues an error
response per Section 5.2 of with invalid_dpop_proof as the
value of the error parameter.To sender-constrain the access token, after checking the validity of the
DPoP proof, the authorization server associates the issued access token with the
public key from the DPoP proof, which can be accomplished as described in .
A token_type of DPoP in the access token
response signals to the client that the access token was bound to
its DPoP key and can used as described in .
The example response shown in illustrates such a
response.The example response in included a refresh token, which the
client can use to obtain a new access token when the previous one expires.
Refreshing an access token is a token request using the refresh_token
grant type made to the authorization server's token endpoint. As with
all access token requests, the client makes it a DPoP request by including
a DPoP proof, which is shown in the example
(extra line breaks and whitespace for display purposes only).When an authorization server supporting DPoP issues a
refresh token to a public client that presents a valid DPoP proof at the
token endpoint, the refresh token MUST be bound
to the respective public key. The binding MUST be validated when the refresh
token is later presented to get new access tokens. As a result, such a client
MUST present a DPoP proof for the same key that was used to obtain the refresh
token each time that refresh token is used to obtain a new access token.
The implementation details of the binding of the refresh token are at the discretion of
the authorization server. The server both produces and
validates the refresh tokens that it issues so there's no interoperability
consideration in the specific details of the binding.An authorization server MAY elect to issue access tokens which are not DPoP bound,
which is signaled to the client with a value of Bearer in the token_type parameter
of the access token response per . For a public client that is
also issued a refresh token, this has the effect of DPoP-binding the refresh token
alone, which can improve the security posture even when protected resources are not
updated to support DPoP.A client expecting a DPoP-bound access token MAY discard the response, if
a Bearer token type is received.Refresh tokens issued to confidential clients (those having
established authentication credentials with the authorization server)
are not bound to the DPoP proof public key because they are already
sender-constrained with a different existing mechanism. The OAuth 2.0 Authorization
Framework [RFC6749] already requires that an authorization server bind
refresh tokens to the client to which they were issued and that
confidential clients authenticate to the authorization server when
presenting a refresh token. As a result, such refresh tokens
are sender-constrained by way of the client ID and the associated
authentication requirement. This existing sender-constraining mechanism
is more flexible (e.g., it allows credential rotation for the client
without invalidating refresh tokens) than binding directly to a particular public key.Authorization Server MetadataThis document introduces the following new authorization server metadata
parameter to signal support for DPoP in general and the specific
JWS alg values the authorization server supports for DPoP proof JWTs.
dpop_signing_alg_values_supported
A JSON array containing a list of the JWS alg values supported
by the authorization server for DPoP proof JWTs.
Public Key ConfirmationResource servers MUST be able to reliably identify whether
an access token is bound using DPoP and ascertain sufficient information
about the public key to which the token is bound in order to verify the
binding with respect to the presented DPoP proof (see ).
Such a binding is accomplished by associating the public key
with the token in a way that can be
accessed by the protected resource, such as embedding the JWK
hash in the issued access token directly, using the syntax described
in , or through token introspection as described in
. Other methods of associating a
public key with an access token are possible, per agreement by the
authorization server and the protected resource, but are beyond the
scope of this specification.Resource servers supporting DPoP MUST ensure that the public key from
the DPoP proof matches the public key to which the access token is bound.JWK Thumbprint Confirmation MethodWhen access tokens are represented as JSON Web Tokens (JWT) ,
the public key information SHOULD be represented
using the jkt confirmation method member defined herein.
To convey the hash of a public key in a JWT, this specification
introduces the following new JWT Confirmation Method member for
use under the cnf claim.
jkt
JWK SHA-256 Thumbprint Confirmation Method. The value of the jkt member
MUST be the base64url encoding (as defined in )
of the JWK SHA-256 Thumbprint (according to ) of the DPoP public key
(in JWK format) to which the access token is bound.
The following example JWT in with decoded JWT payload shown in
contains a cnf claim with the jkt JWK thumbprint confirmation
method member. The jkt value in these examples is the hash of the public key
from the DPoP proofs in the examples in .JWK Thumbprint Confirmation Method in Token IntrospectionOAuth 2.0 Token Introspection defines a method for a
protected resource to query an authorization server about the active
state of an access token as well as to determine metainformation
about the token.For a DPoP-bound access token, the hash of the public key to which the token
is bound is conveyed to the protected resource as metainformation in a token
introspection response. The hash is conveyed using the same cnf content with
jkt member structure as the JWK thumbprint confirmation method, described in
, as a top-level member of the
introspection response JSON. Note that the resource server
does not send a DPoP proof with the introspection request and the authorization
server does not validate an access token's DPoP binding at the introspection
endpoint. Rather the resource server uses the data of the introspection response
to validate the access token binding itself locally.The example introspection request in and corresponding response in
illustrate an introspection exchange for the example DPoP-bound
access token that was issued in .Protected Resource AccessTo make use of an access token that is bound to a public key
using DPoP, a client MUST prove possession of the corresponding
private key by providing a DPoP proof in the DPoP request header.
As such, protected resource requests with a DPoP-bound access token
necessarily must include both a DPoP proof as per and
the access token as described in .
The DPoP proof MUST include the ath claim with a valid hash of the
associated access token.The DPoP Authorization Request Header SchemeA DPoP-bound access token is sent using the Authorization request
header field per Section 2 of using an
authentication scheme of DPoP. The syntax of the Authorization
header field for the DPoP scheme
uses the token68 syntax defined in Section 2.1 of
(repeated below for ease of reference) for credentials.
The Augmented Backus-Naur Form (ABNF) notation syntax
for DPoP Authorization scheme credentials is as follows:For such an access token, a resource server MUST check that a DPoP proof
was also received in the DPoP header field of the HTTP request,
check the DPoP proof according to the rules in ,
and check that the public key of the DPoP proof matches the public
key to which the access token is bound per .The resource server MUST NOT grant access to the resource unless all
checks are successful. shows an example request to a protected
resource with a DPoP-bound access token in the Authorization header
and the DPoP proof in the DPoP header.
Following that is , which shows the decoded content of that DPoP
proof. The JSON of the JOSE header and payload are shown
but the signature part is omitted. As usual, line breaks and extra whitespace
are included for formatting and readability in both examples.Upon receipt of a request for a URI of a protected resource within
the protection space requiring DPoP authorization, if the request does
not include valid credentials or does not contain an access
token sufficient for access to the protected resource, the server
can reply with a challenge using the 401 (Unauthorized) status code
(, Section 3.1) and the WWW-Authenticate header field
(, Section 4.1). The server MAY include the
WWW-Authenticate header in response to other conditions as well.In such challenges:
The scheme name is DPoP.
The authentication parameter realm MAY be included to indicate the
scope of protection in the manner described in , Section 2.2.
A scope authentication parameter MAY be included as defined in
, Section 3.
An error parameter (, Section 3) SHOULD be included
to indicate the reason why the request was declined,
if the request included an access token but failed authorization.
Parameter values are described in Section 3.1 of .
An error_description parameter (, Section 3) MAY be included
along with the error parameter to provide developers a human-readable
explanation that is not meant to be displayed to end-users.
An algs parameter SHOULD be included to signal to the client the
JWS algorithms that are acceptable for the DPoP proof JWT.
The value of the parameter is a space-delimited list of JWS alg (Algorithm)
header values (, Section 4.1.1).
Additional authentication parameters MAY be used and unknown parameters
MUST be ignored by recipients
For example, in response to a protected resource request without
authentication:And in response to a protected resource request that was rejected
because the confirmation of the DPoP binding in the access token failed:The Bearer Authorization Request Header SchemeProtected resources simultaneously supporting both the DPoP and Bearer
schemes need to update how evaluation of bearer tokens is performed to prevent
downgraded usage of a DPoP-bound access tokens.
Specifically, such a protected resource MUST reject an access
token received as a bearer token per [!@RFC6750], if that token is
determined to be DPoP-bound.A protected resource that supports only and is unaware of DPoP
would most presumably accept a DPoP-bound access token as a bearer token
(JWT says to ignore unrecognized claims, Introspection
says that other parameters might be present while placing no functional
requirements on their presence, and is effectively silent on
the content of the access token as it relates to validity). As such, a
client MAY send a DPoP-bound access token using the Bearer scheme upon
receipt of a WWW-Authenticate: Bearer challenge from a protected resource
(or if it has prior such knowledge about the capabilities of the protected
resource). The effect of this likely simplifies the logistics of phased
upgrades to protected resources in their support DPoP or even
prolonged deployments of protected resources with mixed token type support.Security ConsiderationsIn DPoP, the prevention of token replay at a different endpoint (see
) is achieved through the
binding of the DPoP proof to a certain URI and HTTP method. DPoP, however,
has a somewhat different nature of protection than TLS-based
methods such as OAuth Mutual TLS or OAuth Token
Binding (see also and ).
TLS-based mechanisms can leverage a tight integration
between the TLS layer and the application layer to achieve a very high
level of message integrity with respect to the transport layer to which the token is bound
and replay protection in general.DPoP Proof ReplayIf an adversary is able to get hold of a DPoP proof JWT, the adversary
could replay that token at the same endpoint (the HTTP endpoint
and method are enforced via the respective claims in the JWTs). To
prevent this, servers MUST only accept DPoP proofs for a limited time
window after their iat time, preferably only for a relatively brief period
(on the order of a few seconds).
Servers SHOULD store, in the context of the request URI, the jti value of
each DPoP proof for the time window in which the respective DPoP proof JWT
would be accepted and decline HTTP requests to the same URI
for which the jti value has been seen before. In order to guard against
memory exhaustion attacks a server SHOULD reject DPoP proof JWTs with unnecessarily
large jti values or store only a hash thereof.Note: To accommodate for clock offsets, the server MAY accept DPoP
proofs that carry an iat time in the reasonably near future (e.g., a few
seconds in the future).Untrusted Code in the Client ContextIf an adversary is able to run code in the client's execution context,
the security of DPoP is no longer guaranteed. Common issues in web
applications leading to the execution of untrusted code are cross-site
scripting and remote code inclusion attacks.If the private key used for DPoP is stored in such a way that it
cannot be exported, e.g., in a hardware or software security module,
the adversary cannot exfiltrate the key and use it to create arbitrary
DPoP proofs. The adversary can, however, create new DPoP proofs as
long as the client is online, and use these proofs (together with the
respective tokens) either on the victim's device or on a device under
the attacker's control to send arbitrary requests that will be
accepted by servers.To send requests even when the client is offline, an adversary can try
to pre-compute DPoP proofs using timestamps in the future and
exfiltrate these together with the access or refresh token.An adversary might further try to associate tokens issued from the
token endpoint with a key pair under the adversary's control. One way
to achieve this is to modify existing code, e.g., by replacing
cryptographic APIs. Another way is to launch a new authorization grant
between the client and the authorization server in an iframe. This
grant needs to be "silent", i.e., not require interaction with the
user. With code running in the client's origin, the adversary has
access to the resulting authorization code and can use it to associate
their own DPoP keys with the tokens returned from the token endpoint.
The adversary is then able to use the resulting tokens on their own
device even if the client is offline.Therefore, protecting clients against the execution of untrusted code
is extremely important even if DPoP is used. Besides secure coding
practices, Content Security Policy can be used as a second
layer of defense against cross-site scripting.Signed JWT SwappingServers accepting signed DPoP proof JWTs MUST check the typ field in the
headers of the JWTs to ensure that adversaries cannot use JWTs created
for other purposes.Signature AlgorithmsImplementers MUST ensure that only asymmetric digital signature algorithms that
are deemed secure can be used for signing DPoP proofs. In particular,
the algorithm none MUST NOT be allowed.Message IntegrityDPoP does not ensure the integrity of the payload or headers of
requests. The DPoP proof only contains claims for the HTTP URI and
method, but not, for example, the message body or general request
headers.This is an intentional design decision intended to keep DPoP simple to use, but
as described, makes DPoP potentially susceptible to replay attacks
where an attacker is able to modify message contents and headers. In
many setups, the message integrity and confidentiality provided by TLS
is sufficient to provide a good level of protection.Implementers that have stronger requirements on the integrity of
messages are encouraged to either use TLS-based mechanisms or signed
requests. TLS-based mechanisms are in particular OAuth Mutual TLS
and OAuth Token Binding
.Note: While signatures covering other parts of requests are out of the scope of
this specification, additional information to be signed can be
added into DPoP proofs.Access Token and Public Key BindingThe binding of the access token to the DPoP public key, which is
specified in , uses a cryptographic hash of the JWK
representation of the public key. It relies
on the hash function having sufficient second-preimage resistance so
as to make it computationally infeasible to find or create another
key that produces to the same hash output value. The SHA-256
hash function was used because it meets the aforementioned
requirement while being widely available. If, in the future,
JWK thumbprints need to be computed using hash function(s)
other than SHA-256, it is suggested that an additional related JWT
confirmation method member be defined for that purpose,
registered in the respective IANA registry, and used in place of the
jkt confirmation method defined herein.Similarly, the binding of the DPoP proof to the access token uses a
hash of that access token as the value of the ath claim
in the DPoP proof (see ). This relies on the value
of the hash being sufficiently unique so as to reliably identify the
access token. The collision resistance of SHA-256 meets that requirement.
If, in the future, access token digests need be computed using hash function(s)
other than SHA-256, it is suggested that an additional related JWT
claim be defined for that purpose, registered in the respective IANA registry,
and used in place of the ath claim defined herein.IANA ConsiderationsOAuth Access Token Type RegistrationThis specification requests registration of the following access token
type in the "OAuth Access Token Types" registry
established by .
Specification document(s): [[ this specification ]]
HTTP Authentication Scheme RegistrationThis specification requests registration of the following scheme in the
"Hypertext Transfer Protocol (HTTP) Authentication Scheme Registry" :
Authentication Scheme Name: DPoP
Reference: [[ of this specification ]]
Media Type Registration[[
Is a media type registration at necessary for application/dpop+jwt?
There is a +jwt structured syntax suffix registered already at
by Section 7.2 of , which is maybe sufficient? A full-blown registration
of application/dpop+jwt seems like it'd be overkill.
The dpop+jwt is used in the JWS/JWT typ header for explicit typing of the JWT per
Section 3.11 of but it is not used anywhere else (such as the Content-Type of HTTP messages).Note that there does seem to be some precedence for registration with
, , , and of course .
But precedence isn't always right.
]]JWT Confirmation Methods RegistrationThis specification requests registration of the following value
in the IANA "JWT Confirmation Methods" registry
for JWT cnf member values established by .
Specification Document(s): [[ of this specification ]]
JSON Web Token Claims RegistrationThis specification requests registration of the following Claims in the
IANA "JSON Web Token Claims" registry established by .HTTP method:
Claim Name: htm
Claim Description: The HTTP method of the request
Change Controller: IESG
Specification Document(s): [[ of this specification ]]
HTTP URI:
Claim Name: htu
Claim Description: The HTTP URI of the request (without query and fragment parts)
Change Controller: IESG
Specification Document(s): [[ of this specification ]]
Access token hash:
Claim Name: ath
Claim Description: The base64url encoded SHA-256 hash of the ASCII encoding of the associated access token's value
Change Controller: IESG
Specification Document(s): [[ of this specification ]]
HTTP Message Header Field Names RegistrationThis document specifies the following new HTTP header fields,
registration of which is requested in the "Permanent Message Header
Field Names" registry defined in .
Header Field Name: DPoP
Applicable protocol: HTTP
Status: standard
Author/change Controller: IETF
Specification Document(s): [[ this specification ]]
Authorization Server Metadata RegistrationThis specification requests registration of the following values
in the IANA "OAuth Authorization Server Metadata" registry [IANA.OAuth.Parameters]
established by .
Metadata Name: dpop_signing_alg_values_supported
Metadata Description: JSON array containing a list of the JWS algorithms supported for DPoP proof JWTs
Change Controller: IESG
Specification Document(s): [[ of this specification ]]
Normative ReferencesInformative ReferencesWeb Cryptography APIContent Security Policy Level 3Hypertext Transfer Protocol (HTTP) Authentication Scheme RegistryIANAMedia TypesIANAOAuth ParametersIANAStructured Syntax Suffix RegistryIANAJSON Web Token ClaimsIANAMessage HeadersIANAAcknowledgementsWe would like to thank
Annabelle Backman,
Dominick Baier,
Andrii Deinega,
William Denniss,
Vladimir Dzhuvinov,
Mike Engan,
Nikos Fotiou,
Mark Haine,
Dick Hardt,
Bjorn Hjelm,
Jared Jennings,
Steinar Noem,
Neil Madden,
Rob Otto,
Aaron Parecki,
Michael Peck,
Paul Querna,
Justin Richer,
Filip Skokan,
Dave Tonge,
Jim Willeke,
Philippe De Ryck,
and others (please let us know, if you've been mistakenly omitted)
for their valuable input, feedback and general support of this work.This document resulted from discussions at the 4th OAuth Security
Workshop in Stuttgart, Germany. We thank the organizers of this
workshop (Ralf Kusters, Guido Schmitz).Document History[[ To be removed from the final specification ]]-03
Add an access token hash (ath) claim to the DPoP proof when used in conjunction with the presentation of an access token for protected resource access
add Untrusted Code in the Client Context section to security considerations
Editorial updates and fixes
-02
Lots of editorial updates and additions including expanding on the objectives,
better defining the key confirmation representations, example updates and additions,
better describing mixed bearer/dpop token type deployments, clarify RT binding only being
done for public clients and why, more clearly allow for a bound RT but with bearer AT,
explain/justify the choice of SHA-256 for key binding, and more
Require that a protected resource supporting bearer and DPoP at the same time
must reject an access token received as bearer, if that token is DPoP-bound
Remove the case-insensitive qualification on the htm claim check
Relax the jti tracking requirements a bit and qualify it by URI
-01
Editorial updates
Attempt to more formally define the DPoP Authorization header scheme
Define the 401/WWW-Authenticate challenge
Added invalid_dpop_proof error code for DPoP errors in token request
Fixed up and added to the IANA section
Added dpop_signing_alg_values_supported authorization server metadata
Moved the Acknowledgements into an Appendix and added a bunch of names (best effort)
-00 [[ Working Group Draft ]]
Working group draft
-04
Update OAuth MTLS reference to RFC 8705
Use the newish RFC v3 XML and HTML format
-03
rework the text around uniqueness requirements on the jti claim in the DPoP proof JWT
make tokens a bit smaller by using htm, htu, and jkt rather than http_method, http_uri, and jkt#S256 respectively
more explicit recommendation to use mTLS if that is available
added David Waite as co-author
editorial updates
-02
added normalization rules for URIs
removed distinction between proof and binding
"jwk" header again used instead of "cnf" claim in DPoP proof
renamed "Bearer-DPoP" token type to "DPoP"
removed ability for key rotation
added security considerations on request integrity
explicit advice on extending DPoP proofs to sign other parts of the HTTP messages
only use the jkt#S256 in ATs
iat instead of exp in DPoP proof JWTs
updated guidance on token_type evaluation
-01
fixed inconsistencies
moved binding and proof messages to headers instead of parameters