OAuth 2.0 Demonstration of 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.IntroductionThis document outlines a relatively simple 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
the DPoP header in an HTTP request. Using that header, an authorization
server is able to bind issued tokens to the public part of the 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 potentially stronger
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.DPoP can be used with public clients to sender-constrain access tokens and
refresh tokens. With confidential clients, DPoP can be used in conjunction
with any client authentication method to sender-constrain access tokens.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 .Main ObjectiveUnder the attacker model defined in ,
the mechanism defined by this specification aims to prevent token
replay at a different endpoint.More precisely, if an adversary is able to get hold of an access token
or refresh token because it set up a counterfeit authorization server
or resource server, the adversary is not able to replay the respective
token at another authorization or resource server.Secondary objectives are discussed in .ConceptThe main data structure introduced by this specification is a DPoP
proof JWT, described in detail below, 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 code
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 AS 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.
This is signaled to the client by using the token_type value
DPoP.
If a refresh token is issued to a public client, it is
bound to the public key of the DPoP proof in a similar way.
Note that for confidential clients, refresh tokens are required by
to bound to the client_id and associated authentication credentials,
which is a sender-constraining mechanism that is more flexible than binding
to a particular public key.
(C) If the client wants to use the access token, it has to prove
possession of the private key by, again, adding a header to the
request that carries the DPoP proof. The resource server needs to
receive information about the public key to which the access token is bound. This
information is either encoded directly into the access token (for
JWT structured access tokens), or provided at the token
introspection endpoint of the authorization server (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.
When a refresh token that is sender-constrained using DPoP is used
by the client, the client has to provide a DPoP proof just as in
the case of a resource access. The new access token will be bound
to the same public key.
The 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) 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 concept of a DPoP proof JWT, which is used for binding public
keys and proving knowledge about private keys. The DPoP proof JWT is sent with
an HTTP request using the DPoP header field.DPoP Proof JWT SyntaxA DPoP proof is a JWT () that is signed (using JWS,
) using 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 (REQUIRED)
The body 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 SHOULD 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).
shows the JSON header and payload of a DPoP proof JWT.Note: To keep DPoP simple to implement, only the HTTP method and URI
are signed in DPoP proofs. 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 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,
that the JWT is signed using 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 (case-insensitive),
the htu claims matches the HTTP 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 (see ), and
that, within a reasonable consideration of accuracy and resource utilization,
a JWT with the same jti value has not been received
previously (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.Token Request (Binding Tokens to a Public Key)To bind a token to a public key in the token request, the client MUST
provide a valid DPoP proof JWT in a DPoP header. The HTTPS request shown
in illustrates the protocol for this (with extra line breaks
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.The authorization server, after checking the validity of the DPoP proof,
associates the access token issued at the token endpoint with the
public key. It then sets token_type to DPoP in the token
response, which signals to the client that the access token was bound to
its DPoP key and can used as described in .If a refresh token is issued to a public client at the token endpoint
and a valid DPoP proof is presented, the refresh token MUST be bound
to the public key contained in the header of the DPoP proof JWT.When a DPoP-bound refresh token is used at the token endpoint by a
public client, the AS MUST ensure that the DPoP proof contains the
same public key as the one the refresh token is bound to. The access
token issued MUST be bound to the public key contained in the DPoP
proof.Resource Access (Proof of Possession for Access Tokens)To make use of an access token that is bound to a public key
using DPoP, a client MUST prove the possession of the corresponding
private key by providing a DPoP proof in the DPoP request header.A 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 header was received in the HTTP request,
check the header's contents 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.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 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:Public Key ConfirmationIt MUST be ensured that resource servers can reliably identify whether
a token is bound using DPoP and learn the public key to which the
token is bound.Access tokens that are represented as JSON Web Tokens (JWT)
MUST contain information about the DPoP public key (in JWK format) in
the member jkt of the cnf claim, as shown in .The value in jkt MUST be the base64url encoding of
the JWK SHA-256 Thumbprint (according to ) of the public
key to which the access token is bound.When access token introspection is used, the same cnf claim as above
MUST be contained in the introspection response.Resource servers MUST ensure that the fingerprint of the public key in
the DPoP proof JWT equals the value in the jkt claim in the access
token or introspection response.Authorization Server MetadataThis document introduces the following new authorization server metadata
parameter to signal the JWS alg values the authorization server
supports for DPoP proof JWTs:
dpop_signing_alg_values_supported
OPTIONAL. JSON array containing a list of the JWS alg values supported
by the authorization server for DPoP proof JWTs
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 does
not, however, achieve the same level of protection as 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 and replay protection. Therefore, it is
RECOMMENDED to prefer TLS-based methods over DPoP if such methods are
suitable for the scenario at hand.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.
Servers SHOULD store 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
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 near future (e.g., up to a few
seconds in the future).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 signature of DPoP proofs only contains the HTTP URI and
method, but not, for example, the message body or other request
headers.This is an intentional design decision 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 on (parts of) requests are out of the scope of
this specification, signatures or information to be signed can be
added into DPoP proofs.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 fullblown 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 ]]
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 ReferencesMedia TypesIANAJSON Web Token ClaimsIANAOAuth ParametersIANAStructured Syntax Suffix RegistryIANAMessage HeadersIANAHypertext Transfer Protocol (HTTP) Authentication Scheme RegistryIANAAcknowledgementsWe would like to thank
Filip Skokan,
Mike Engan,
Justin Richer,
Michael Peck,
Vladimir Dzhuvinov,
Rob Otto,
Dominick Baier,
Jim Willeke,
Annabelle Backman,
Bjorn Hjelm,
Steinar Noem,
Aaron Parecki,
Neil Madden,
Paul Querna,
Dick Hardt,
Dave Tonge,
Jared Jennings,
Mark Haine
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 ]]-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