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.comPing 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.Introduction describes methods to bind (sender-constrain) access tokens
using mutual Transport Layer Security (TLS) authentication with X.509
certificates. provides mechanisms to
sender-constrain access tokens using HTTP token binding.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.This document outlines an application-level sender-constraining for
access and refresh tokens that can be used in cases where neither mTLS nor
OAuth Token Binding are available. It uses proof-of-possession based on
a public/private key pair and application-level signing.DPoP can be used with public clients and, in case of confidential
clients, can be combined with any client authentication method.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. A client uses a DPoP proof JWT to prove
the possession of a private key belonging to a certain public key.
Roughly speaking, a DPoP proof is a signature over some data of the
HTTP request to which it is attached to and a timestamp.Figure 1: Basic DPoP FlowThe basic steps of an OAuth flow with DPoP are shown in Figure 1:
(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
sender-constrained in the same way. For confidential clients,
refresh tokens are bound to the client_id, which is more
flexible than binding it 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).
(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 is public clients (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 uses so-called DPoP proof JWTs for binding public keys and proving
knowledge about private keys.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).
Figure 2 shows the JSON header and payload of a DPoP proof JWT.Figure 2: Example JWT content for DPoP proof header.Note: To keep DPoP simple to implement, only the HTTP method and URI
are signed in DPoP proofs. 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 Figure 3 illustrates the protocol for this (with extra line breaks
for display purposes only).Figure 3: Token Request for a DPoP sender-constrained token.The HTTP header DPoP MUST contain a valid DPoP proof.The authorization server, after checking the validity of the DPoP proof,
MUST associate the access token issued at the token endpoint with the
public key. It then sets token_type to DPoP in the token
response.A client typically cannot know whether a certain AS supports DPoP. It
therefore SHOULD use the value of the token_type parameter returned
from the AS to determine support for DPoP: If the token type returned
is Bearer or another value, the AS does not support DPoP. If it is
DPoP, DPoP is supported. Only then, the client needs to send
the DPoP header in subsequent requests and use the token type
DPoP in the Authorization header as described below.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.If a DPoP-bound refresh token is to be 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 token-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.The DPoP-bound access token must be sent in the Authorization header
with the prefix DPoP.If a resource server detects that an access token that is to be used
for resource access is bound to a public key using DPoP (via the
methods described in ) it MUST check that a header
DPoP was received in the HTTP request, and check the header's
contents according to the rules in .The resource server MUST NOT grant access to the resource unless all
checks are successful.Figure 4: Protected Resource Request with a DPoP sender-constrained access token.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 Figure 5.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.Figure 5: Example access token body with cnf claim.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.AcknowledgementsWe would like to thank David Waite, Filip Skokan, Mike Engan, and Justin Richer for
their valuable input and feedback.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).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 in the DPoP headers.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 registers the following access token type in the
OAuth Access Token Types registry defined in [RFC6749].
Specification document(s): [[ this specification ]]
JSON Web Signature and Encryption Type Values RegistrationThis specification registers the dpop+jwt type value in the IANA
JSON Web Signature and Encryption Type Values registry :
"typ" Header Parameter Value: "dpop+jwt"
Abbreviation for MIME Type: None
Change Controller: IETF
Specification Document(s): [[ this specification ]]
Normative ReferencesInformative ReferencesDocument History[[ To be removed from the final specification ]]-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