OAuth 2.0 Demonstrating Proof-of-Possession at the Application Layer (DPoP)
draft-ietf-oauth-dpop-04
Web Authorization Protocol D. Fett
Internet-Draft yes.com
Intended status: Standards Track B. Campbell
Expires: 7 April 2022 Ping Identity
J. Bradley
Yubico
T. Lodderstedt
yes.com
M. Jones
Microsoft
D. Waite
Ping Identity
4 October 2021
OAuth 2.0 Demonstrating Proof-of-Possession at the Application Layer
(DPoP)
draft-ietf-oauth-dpop-04
Abstract
This 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.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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and may be updated, replaced, or obsoleted by other documents at any
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This Internet-Draft will expire on 7 April 2022.
Copyright Notice
Copyright (c) 2021 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Conventions and Terminology . . . . . . . . . . . . . . . 4
2. Objectives . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Concept . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. DPoP Proof JWTs . . . . . . . . . . . . . . . . . . . . . . . 7
4.1. The DPoP HTTP Header . . . . . . . . . . . . . . . . . . 7
4.2. DPoP Proof JWT Syntax . . . . . . . . . . . . . . . . . . 8
4.3. Checking DPoP Proofs . . . . . . . . . . . . . . . . . . 10
5. DPoP Access Token Request . . . . . . . . . . . . . . . . . . 11
5.1. Authorization Server Metadata . . . . . . . . . . . . . . 13
6. Public Key Confirmation . . . . . . . . . . . . . . . . . . . 14
6.1. JWK Thumbprint Confirmation Method . . . . . . . . . . . 14
6.2. JWK Thumbprint Confirmation Method in Token
Introspection . . . . . . . . . . . . . . . . . . . . . . 15
7. Protected Resource Access . . . . . . . . . . . . . . . . . . 16
7.1. The DPoP Authentication Scheme . . . . . . . . . . . . . 16
7.2. Compatibility with the Bearer Authentication Scheme . . . 19
8. Authorization Server-Provided Nonce . . . . . . . . . . . . . 20
8.1. Providing a New Nonce Value . . . . . . . . . . . . . . . 22
9. Resource Server-Provided Nonce . . . . . . . . . . . . . . . 22
10. Security Considerations . . . . . . . . . . . . . . . . . . . 23
10.1. DPoP Proof Replay . . . . . . . . . . . . . . . . . . . 23
10.2. DPoP Proof Pre-Generation . . . . . . . . . . . . . . . 24
10.3. DPoP Nonce Downgrade . . . . . . . . . . . . . . . . . . 24
10.4. Untrusted Code in the Client Context . . . . . . . . . . 24
10.5. Signed JWT Swapping . . . . . . . . . . . . . . . . . . 25
10.6. Signature Algorithms . . . . . . . . . . . . . . . . . . 25
10.7. Message Integrity . . . . . . . . . . . . . . . . . . . 25
10.8. Access Token and Public Key Binding . . . . . . . . . . 26
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 26
11.1. OAuth Access Token Type Registration . . . . . . . . . . 26
11.2. OAuth Extensions Error Registration . . . . . . . . . . 27
11.3. HTTP Authentication Scheme Registration . . . . . . . . 27
11.4. Media Type Registration . . . . . . . . . . . . . . . . 27
11.5. JWT Confirmation Methods Registration . . . . . . . . . 28
11.6. JSON Web Token Claims Registration . . . . . . . . . . . 28
11.7. HTTP Message Header Field Names Registration . . . . . . 29
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11.8. Authorization Server Metadata Registration . . . . . . . 29
12. Normative References . . . . . . . . . . . . . . . . . . . . 29
13. Informative References . . . . . . . . . . . . . . . . . . . 30
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 33
Appendix B. Document History . . . . . . . . . . . . . . . . . . 34
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 36
1. Introduction
DPoP, 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
prove the possession of a public/private key pair by including a DPoP
header in an HTTP request. The value of the header is a JWT
[RFC7519] 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 [RFC8705] or
[I-D.ietf-oauth-token-binding], 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 are another example well
positioned to benefit from DPoP-bound tokens to guard against misuse
of tokens by a compromised or malicious resource. Such applications
often have dedicated protected storage for cryptographic keys.
DPoP can be used to sender-constrain access tokens regardless of the
client authentication method employed, but DPoP itself is not used
for client authentication. DPoP can also be used to sender-constrain
refresh tokens issued to public clients (those without authentication
credentials associated with the client_id).
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1.1. Conventions and 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 [RFC2119] [RFC8174] 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 [RFC6749].
2. Objectives
The 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 proves
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 discloses the access token to the
protected resources that it accesses. The attacker model in
[I-D.ietf-oauth-security-topics] describes cases where a protected
resource might be counterfeit, malicious or compromised and plays
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 for many deployments (even despite
extensions such as [RFC8707]). Sender-constraining access tokens is
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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 of 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 [W3C.WebCryptoAPI]), 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.
To prevent this, servers can optionally require clients to include a
server-chosen value into the proof that cannot be predicted by an
attacker (nonce). In the absence of the optional nonce, the impact
of 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 does not 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 Section 10.
3. Concept
The 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.
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Roughly speaking, a DPoP proof is a signature over some data of the
HTTP request to which it is attached, a timestamp, a unique
identifier, an optional server-provided nonce, and a hash of the
associated access token when an access token is present within the
request.
+--------+ +---------------+
| |--(A)-- Token Request ------------------->| |
| Client | (DPoP Proof) | Authorization |
| | | Server |
| |<-(B)-- DPoP-bound Access Token ----------| |
| | (token_type=DPoP) +---------------+
| |
| |
| | +---------------+
| |--(C)-- DPoP-bound Access Token --------->| |
| | (DPoP Proof) | Resource |
| | | Server |
| |<-(D)-- Protected Resource ---------------| |
| | +---------------+
+--------+
Figure 1: Basic DPoP Flow
The basic steps of an OAuth flow with DPoP (without the optional
nonce) are shown in Figure 1:
* (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
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matches the public key of the DPoP proof. It also verifies that
the access token hash in the DPoP proof matches the access token
presented in the request.
* (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 Section 10 for details.
4. DPoP Proof JWTs
DPoP 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 Section 5) and for resource
servers to verify the key-binding of tokens that it receives (see
Section 7.1), 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
Section 7.1, 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.
4.1. The DPoP HTTP Header
A 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 Section 4.2.
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Figure 2 shows an example DPoP HTTP header field (line breaks and
extra whitespace for display purposes only).
DPoP: eyJ0eXAiOiJkcG9wK2p3dCIsImFsZyI6IkVTMjU2IiwiandrIjp7Imt0eSI6Ik
VDIiwieCI6Imw4dEZyaHgtMzR0VjNoUklDUkRZOXpDa0RscEJoRjQyVVFVZldWQVdCR
nMiLCJ5IjoiOVZFNGpmX09rX282NHpiVFRsY3VOSmFqSG10NnY5VERWclUwQ2R2R1JE
QSIsImNydiI6IlAtMjU2In19.eyJqdGkiOiItQndDM0VTYzZhY2MybFRjIiwiaHRtIj
oiUE9TVCIsImh0dSI6Imh0dHBzOi8vc2VydmVyLmV4YW1wbGUuY29tL3Rva2VuIiwia
WF0IjoxNTYyMjYyNjE2fQ.2-GxA6T8lP4vfrg8v-FdWP0A0zdrj8igiMLvqRMUvwnQg
4PtFLbdLXiOSsX0x7NVY-FNyJK70nfbV37xRZT3Lg
Figure 2: Example DPoP header
Note that per [RFC7230] 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.
4.2. DPoP Proof JWT Syntax
A DPoP proof is a JWT ([RFC7519]) that is signed (using JWS,
[RFC7515]) 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 [RFC7518]
(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 [RFC7515] (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 [RFC4122]. The jti can
be used by the server for replay detection and prevention, see
Section 10.1.
* htm: The HTTP method for the request to which the JWT is attached,
as defined in [RFC7231] (REQUIRED).
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* 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 Section 7, the DPoP proof MUST also contain 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.
A DPoP proof MAY contain other headers or claims as defined by
extension, profile, or deployment specific requirements.
Figure 3 is a conceptual example showing the decoded content of the
DPoP proof in Figure 2. 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.
{
"typ":"dpop+jwt",
"alg":"ES256",
"jwk": {
"kty":"EC",
"x":"l8tFrhx-34tV3hRICRDY9zCkDlpBhF42UQUfWVAWBFs",
"y":"9VE4jf_Ok_o64zbTTlcuNJajHmt6v9TDVrU0CdvGRDA",
"crv":"P-256"
}
}
.
{
"jti":"-BwC3ESc6acc2lTc",
"htm":"POST",
"htu":"https://server.example.com/token",
"iat":1562262616
}
Figure 3: Example JWT content of a DPoP proof
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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 Section 10.7).
4.3. Checking DPoP Proofs
To check if a string that was received as part of an HTTP Request is
a valid DPoP proof, the receiving server MUST ensure that
1. that there is not more than one DPoP header in the request,
2. the string value of the header field is a well-formed JWT,
3. all required claims per Section 4.2 are contained in the JWT,
4. the typ field in the header has the value dpop+jwt,
5. 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,
6. the JWT signature verifies with the public key contained in the
jwk header of the JWT,
7. the htm claim matches the HTTP method value of the HTTP request
in which the JWT was received,
8. the htu claim matches the HTTPS URI value for the HTTP request
in which the JWT was received, ignoring any query and fragment
parts,
9. if the server provided a nonce value to the client, the nonce
claim matches the server-provided nonce value,
10. 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
Section 10.1).
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11. when presented to a protected resource in conjunction with an
access token, ensure that the value of the ath claim equals the
hash of the access token that has been presented alongside the
DPoP proof.
Servers SHOULD employ Syntax-Based Normalization and Scheme-Based
Normalization in accordance with Section 6.2.2. and Section 6.2.3. of
[RFC3986] before comparing the htu claim.
5. DPoP Access Token Request
To 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 [RFC7523]). The HTTPS
request shown in Figure 4 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).
POST /token HTTP/1.1
Host: server.example.com
Content-Type: application/x-www-form-urlencoded;charset=UTF-8
DPoP: eyJ0eXAiOiJkcG9wK2p3dCIsImFsZyI6IkVTMjU2IiwiandrIjp7Imt0eSI6Ik
VDIiwieCI6Imw4dEZyaHgtMzR0VjNoUklDUkRZOXpDa0RscEJoRjQyVVFVZldWQVdCR
nMiLCJ5IjoiOVZFNGpmX09rX282NHpiVFRsY3VOSmFqSG10NnY5VERWclUwQ2R2R1JE
QSIsImNydiI6IlAtMjU2In19.eyJqdGkiOiItQndDM0VTYzZhY2MybFRjIiwiaHRtIj
oiUE9TVCIsImh0dSI6Imh0dHBzOi8vc2VydmVyLmV4YW1wbGUuY29tL3Rva2VuIiwia
WF0IjoxNTYyMjYyNjE2fQ.2-GxA6T8lP4vfrg8v-FdWP0A0zdrj8igiMLvqRMUvwnQg
4PtFLbdLXiOSsX0x7NVY-FNyJK70nfbV37xRZT3Lg
grant_type=authorization_code
&code=SplxlOBeZQQYbYS6WxSbIA
&redirect_uri=https%3A%2F%2Fclient%2Eexample%2Ecom%2Fcb
&code_verifier=bEaL42izcC-o-xBk0K2vuJ6U-y1p9r_wW2dFWIWgjz-
Figure 4: Token Request for a DPoP sender-constrained token using an
authorization code
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 [RFC6749] with invalid_dpop_proof as the
value of the error parameter.
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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 Section 6. A token_type of DPoP MUST be
included in the access token response to signal to the client that
the access token was bound to its DPoP key and can be used as
described in Section 7.1. The example response shown in Figure 5
illustrates such a response.
HTTP/1.1 200 OK
Content-Type: application/json
Cache-Control: no-store
{
"access_token": "Kz~8mXK1EalYznwH-LC-1fBAo.4Ljp~zsPE_NeO.gxU",
"token_type": "DPoP",
"expires_in": 2677,
"refresh_token": "Q..Zkm29lexi8VnWg2zPW1x-tgGad0Ibc3s3EwM_Ni4-g"
}
Figure 5: Access Token Response
The example response in Figure 5 includes 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, as shown in the Figure 6
example (extra line breaks and whitespace for display purposes only).
POST /token HTTP/1.1
Host: server.example.com
Content-Type: application/x-www-form-urlencoded;charset=UTF-8
DPoP: eyJ0eXAiOiJkcG9wK2p3dCIsImFsZyI6IkVTMjU2IiwiandrIjp7Imt0eSI6Ik
VDIiwieCI6Imw4dEZyaHgtMzR0VjNoUklDUkRZOXpDa0RscEJoRjQyVVFVZldWQVdCR
nMiLCJ5IjoiOVZFNGpmX09rX282NHpiVFRsY3VOSmFqSG10NnY5VERWclUwQ2R2R1JE
QSIsImNydiI6IlAtMjU2In19.eyJqdGkiOiItQndDM0VTYzZhY2MybFRjIiwiaHRtIj
oiUE9TVCIsImh0dSI6Imh0dHBzOi8vc2VydmVyLmV4YW1wbGUuY29tL3Rva2VuIiwia
WF0IjoxNTYyMjY1Mjk2fQ.pAqut2IRDm_De6PR93SYmGBPXpwrAk90e8cP2hjiaG5Qs
GSuKDYW7_X620BxqhvYC8ynrrvZLTk41mSRroapUA
grant_type=refresh_token
&refresh_token=Q..Zkm29lexi8VnWg2zPW1x-tgGad0Ibc3s3EwM_Ni4-g
Figure 6: Token Request for a DPoP-bound Token using a Refresh Token
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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 is 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
[RFC6750]. 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.
If a client receives a different token_type value than DPoP in the
response, the access token protection provided by DPoP is not given.
The client MUST discard the response in this case if this protection
is deemed important for the security of the application and MAY
continue as in a regular OAuth interaction otherwise.
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.
5.1. Authorization Server Metadata
This document introduces the following new authorization server
metadata [RFC8414] parameter to signal support for DPoP in general
and the specific JWS alg values the authorization server supports for
DPoP proof JWTs.
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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.
6. Public Key Confirmation
Resource 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 Section 7.1).
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 Section 6.1, or through token introspection
as described in Section 6.2. 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.
6.1. JWK Thumbprint Confirmation Method
When access tokens are represented as JSON Web Tokens (JWT)
[RFC7519], 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 [RFC7800] 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
[RFC7515]) of the JWK SHA-256 Thumbprint (according to [RFC7638])
of the DPoP public key (in JWK format) to which the access token
is bound.
The following example JWT in Figure 7 with decoded JWT payload shown
in Figure 8 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
Section 5.
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eyJhbGciOiJFUzI1NiIsImtpZCI6IkJlQUxrYiJ9.eyJzdWIiOiJzb21lb25lQGV4YW1
wbGUuY29tIiwiaXNzIjoiaHR0cHM6Ly9zZXJ2ZXIuZXhhbXBsZS5jb20iLCJuYmYiOjE
1NjIyNjI2MTEsImV4cCI6MTU2MjI2NjIxNiwiY25mIjp7ImprdCI6IjBaY09DT1JaTll
5LURXcHFxMzBqWnlKR0hUTjBkMkhnbEJWM3VpZ3VBNEkifX0.3Tyo8VTcn6u_PboUmAO
YUY1kfAavomW_YwYMkmRNizLJoQzWy2fCo79Zi5yObpIzjWb5xW4OGld7ESZrh0fsrA
Figure 7: JWT containing a JWK SHA-256 Thumbprint Confirmation
{
"sub":"someone@example.com",
"iss":"https://server.example.com",
"nbf":1562262611,
"exp":1562266216,
"cnf":{"jkt":"0ZcOCORZNYy-DWpqq30jZyJGHTN0d2HglBV3uiguA4I"}
}
Figure 8: JWT Claims Set with a JWK SHA-256 Thumbprint Confirmation
6.2. JWK Thumbprint Confirmation Method in Token Introspection
OAuth 2.0 Token Introspection [RFC7662] 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 Section 6.1, 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.
If the token_type member is included in the introspection response,
it MUST contain the value DPoP.
The example introspection request in Figure 9 and corresponding
response in Figure 10 illustrate an introspection exchange for the
example DPoP-bound access token that was issued in Figure 5.
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POST /as/introspect.oauth2 HTTP/1.1
Host: server.example.com
Content-Type: application/x-www-form-urlencoded
Authorization: Basic cnM6cnM6TWt1LTZnX2xDektJZHo0ZnNON2tZY3lhK1Rp
token=Kz~8mXK1EalYznwH-LC-1fBAo.4Ljp~zsPE_NeO.gxU
Figure 9: Example Introspection Request
HTTP/1.1 200 OK
Content-Type: application/json
Cache-Control: no-store
{
"active": true,
"sub": "someone@example.com",
"iss": "https://server.example.com",
"nbf": 1562262611,
"exp": 1562266216,
"cnf": {"jkt": "0ZcOCORZNYy-DWpqq30jZyJGHTN0d2HglBV3uiguA4I"}
}
Figure 10: Example Introspection Response for a DPoP-Bound Access
Token
7. Protected Resource Access
To 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 Section 4 and the
access token as described in Section 7.1. The DPoP proof MUST
include the ath claim with a valid hash of the associated access
token.
7.1. The DPoP Authentication Scheme
A DPoP-bound access token is sent using the Authorization request
header field per Section 2 of [RFC7235] 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
[RFC7235] (repeated below for ease of reference) for credentials.
The Augmented Backus-Naur Form (ABNF) notation [RFC5234] syntax for
DPoP authentication scheme credentials is as follows:
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token68 = 1*( ALPHA / DIGIT /
"-" / "." / "_" / "~" / "+" / "/" ) *"="
credentials = "DPoP" 1*SP token68
Figure 11: DPoP Authentication Scheme ABNF
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 Section 4.3, and check
that the public key of the DPoP proof matches the public key to which
the access token is bound per Section 6.
The resource server MUST NOT grant access to the resource unless all
checks are successful.
Figure 12 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 Figure 13, 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.
GET /protectedresource HTTP/1.1
Host: resource.example.org
Authorization: DPoP Kz~8mXK1EalYznwH-LC-1fBAo.4Ljp~zsPE_NeO.gxU
DPoP: eyJ0eXAiOiJkcG9wK2p3dCIsImFsZyI6IkVTMjU2IiwiandrIjp7Imt0eSI6Ik
VDIiwieCI6Imw4dEZyaHgtMzR0VjNoUklDUkRZOXpDa0RscEJoRjQyVVFVZldWQVdCR
nMiLCJ5IjoiOVZFNGpmX09rX282NHpiVFRsY3VOSmFqSG10NnY5VERWclUwQ2R2R1JE
QSIsImNydiI6IlAtMjU2In19.eyJqdGkiOiJlMWozVl9iS2ljOC1MQUVCIiwiaHRtIj
oiR0VUIiwiaHR1IjoiaHR0cHM6Ly9yZXNvdXJjZS5leGFtcGxlLm9yZy9wcm90ZWN0Z
WRyZXNvdXJjZSIsImlhdCI6MTU2MjI2MjYxOCwiYXRoIjoiZlVIeU8ycjJaM0RaNTNF
c05yV0JiMHhXWG9hTnk1OUlpS0NBcWtzbVFFbyJ9.2oW9RP35yRqzhrtNP86L-Ey71E
OptxRimPPToA1plemAgR6pxHF8y6-yqyVnmcw6Fy1dqd-jfxSYoMxhAJpLjA
Figure 12: DPoP Protected Resource Request
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{
"typ":"dpop+jwt",
"alg":"ES256",
"jwk": {
"kty":"EC",
"x":"l8tFrhx-34tV3hRICRDY9zCkDlpBhF42UQUfWVAWBFs",
"y":"9VE4jf_Ok_o64zbTTlcuNJajHmt6v9TDVrU0CdvGRDA",
"crv":"P-256"
}
}
.
{
"jti":"e1j3V_bKic8-LAEB",
"htm":"GET",
"htu":"https://resource.example.org/protectedresource",
"iat":1562262618,
"ath":"fUHyO2r2Z3DZ53EsNrWBb0xWXoaNy59IiKCAqksmQEo"
}
Figure 13: Decoded Content of the DPoP Proof JWT in Figure 12
Upon receipt of a request for a URI of a protected resource within
the protection space requiring DPoP authentication, 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
([RFC7235], Section 3.1) and the WWW-Authenticate header field
([RFC7235], 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 [RFC7235],
Section 2.2.
* A scope authentication parameter MAY be included as defined in
[RFC6750], Section 3.
* An error parameter ([RFC6750], Section 3) SHOULD be included to
indicate the reason why the request was declined, if the request
included an access token but failed authentication. The error
parameter values described in Section 3.1 of [RFC6750] are
suitable as are any appropriate values defined by extension. The
value use_dpop_nonce can be used as described in Section 9 to
signal that a nonce is needed in the DPoP proof of subsequent
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request(s). And invalid_dpop_proof is used to indicate that the
DPoP proof itself was deemed invalid based on the criteria of
Section 4.3.
* An error_description parameter ([RFC6750], 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 ([RFC7515], 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:
HTTP/1.1 401 Unauthorized
WWW-Authenticate: DPoP algs="ES256 PS256"
Figure 14: HTTP 401 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:
HTTP/1.1 401 Unauthorized
WWW-Authenticate: DPoP error="invalid_token",
error_description="Invalid DPoP key binding", algs="ES256"
Figure 15: HTTP 401 Response to a Protected Resource Request with
an Invalid Token
7.2. Compatibility with the Bearer Authentication Scheme
Protected 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.
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Section 4.1 of [RFC7235] allows a protected resource to indicate
support for multiple authentication schemes (i.e., Bearer and DPoP)
with the WWW-Authenticate header field of a 401 (Unauthorized)
response.
A protected resource that supports only [RFC6750] and is unaware of
DPoP would most presumably accept a DPoP-bound access token as a
bearer token (JWT [RFC7519] says to ignore unrecognized claims,
Introspection [RFC7662] says that other parameters might be present
while placing no functional requirements on their presence, and
[RFC6750] 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.
8. Authorization Server-Provided Nonce
Including a nonce value contributed by the authorization server in
the DPoP proof MAY be used by authorization servers to limit the
lifetime of DPoP proofs. The server is in control of when to require
the use of a new nonce value in subsequent DPoP proofs.
Without employing such a mechanism, a malicious party controlling the
client (including potentially the end user) can create DPoP proofs
for use arbitrarily far in the future. This section specifies how
server-provided nonces are used with DPoP.
An authorization server MAY supply a nonce value to be included by
the client in DPoP proofs sent to it by responding to requests not
including a nonce with an error response per Section 5.2 of [RFC6749]
using use_dpop_nonce as the error code value and including a DPoP-
Nonce HTTP header in the response supplying a nonce value to be used
when sending the subsequent request.
For example, in response to a token request without a nonce when the
authorization server requires one, the authorization server can
respond with a DPoP-Nonce value such as the following to provide a
nonce value to include in the DPoP proof:
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HTTP/1.1 400 Bad Request
DPoP-Nonce: eyJ7S_zG.eyJH0-Z.HX4w-7v
{
"error": "use_dpop_nonce"
"error_description":
"Authorization server requires nonce in DPoP proof"
}
Figure 16: HTTP 400 Response to a Token Request without a Nonce
Other HTTP headers and JSON fields MAY also be included in the error
response, but there MUST NOT be more than one DPoP-Nonce header.
Upon receiving the nonce, the client is expected to retry its token
request using a DPoP proof including the supplied nonce value in the
nonce claim of the DPoP proof. An example unencoded JWT Payload of
such a DPoP proof including a nonce is:
{
"jti": "-BwC3ESc6acc2lTc",
"htm": "POST",
"htu": "https://server.example.com/token",
"iat": 1562262616,
"nonce": "eyJ7S_zG.eyJH0-Z.HX4w-7v"
}
Figure 17: DPoP Proof Payload Including a Nonce Value
The nonce syntax in ABNF as used by [RFC6749] (which is the same as
the scope-token syntax) is:
nonce = 1*NQCHAR
Figure 18: Nonce ABNF
The nonce is opaque to the client.
If the nonce claim in the DPoP proof of a token request does not
exactly match the nonce supplied by the authorization server to the
client, the authorization server MUST reject the request. The
rejection response MAY include a DPoP-Nonce HTTP header providing a
new nonce value to use for subsequent requests.
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8.1. Providing a New Nonce Value
It is up to the authorization server when to supply a new nonce value
for the client to use. The client is expected to use the existing
supplied nonce in DPoP proofs until the server supplies a new nonce
value.
The authorization server MAY supply the new nonce in the same way
that the initial one was supplied: by using a DPoP-Nonce HTTP header
in the response. Of course, each time this happens it requires an
extra protocol round trip.
A more efficient manner of supplying a new nonce value is also
defined -- by including a DPoP-Nonce HTTP header in the 200 OK
response from the previous request. The client MUST use the new
nonce value supplied for the next token request, and for all
subsequent token requests until the authorization server supplies a
new nonce.
An example 200 OK response providing a new nonce value is:
HTTP/1.1 200 OK
DPoP-Nonce: eyJ7S_zG.eyJbYu3.xQmBj-1
Figure 19: HTTP 200 Response Providing the Next Nonce Value
9. Resource Server-Provided Nonce
Resource servers can also choose to provide a nonce value to be
included in DPoP proofs sent to them. They provide the nonce using
the DPoP-Nonce header in same way that authorization servers do. The
error signaling is performed as described in Section 7.1.
For example, in response to a resource request without a nonce when
the resource server requires one, the resource server can respond
with a DPoP-Nonce value such as the following to provide a nonce
value to include in the DPoP proof:
HTTP/1.1 401 Unauthorized
WWW-Authenticate: DPoP error="use_dpop_nonce",
error_description="Resource server requires nonce in DPoP proof"
DPoP-Nonce: eyJ7S_zG.eyJH0-Z.HX4w-7v
Figure 20: HTTP 401 Response to a Resource Request without a Nonce
Note that the nonces provided by the two kinds of servers are
different and MUST not be confused with one another. In particular,
a nonce provided to the client by a particular server MUST only be
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used with that server and no other. Developers should also take care
to not confuse this nonce with the OpenID Connect [OpenID.Core] ID
Token nonce, should one also be present.
10. Security Considerations
In DPoP, the prevention of token replay at a different endpoint (see
Section 2) is achieved through the binding of the DPoP proof to a
certain URI and HTTP method plus the optional server-provided nonce.
DPoP, however, has a somewhat different nature of protection than
TLS-based methods such as OAuth Mutual TLS [RFC8705] or OAuth Token
Binding [I-D.ietf-oauth-token-binding] (see also Section 10.1 and
Section 10.7). 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.
10.1. DPoP Proof Replay
If 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). Because clock skews between servers and
clients may be large, servers may choose to limit DPoP proof
lifetimes by using server-provided nonce values rather than clock
times, yielding intended results even in the face of arbitrarily
large clock skews.
Server-provided nonces are an effective means of preventing DPoP
proof replay.
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10.2. DPoP Proof Pre-Generation
An attacker in control of the client can pre-generate DPoP proofs for
use arbitrarily far into the future by choosing the iat value in the
DPoP proof to be signed by the proof-of-possession key. Note that
one such attacker is the person who is the legitimate user of the
client. The user may pre-generate DPoP proofs to exfiltrate from the
machine possessing the proof-of-possession key upon which they were
generated and copy them to another machine that does not possess the
key. For instance, a bank employee might pre-generate DPoP proofs on
a bank computer and then copy them to another machine for use in the
future, thereby bypassing bank audit controls. When DPoP proofs can
be pre-generated and exfiltrated, all that is actually being proved
in DPoP protocol interactions is possession of a DPoP proof -- not of
the proof-of-possession key.
Use of server-provided nonce values that are not predictable by
attackers can prevent this attack. By providing new nonce values at
times of its choosing, the server can limit the lifetime of DPoP
proofs, preventing pre-generated DPoP proofs from being used. When
server-provided nonces are used, possession of the proof-of-
possession key is being demonstrated -- not just possession of a DPoP
proof.
10.3. DPoP Nonce Downgrade
A server MUST NOT accept any DPoP proofs without the nonce claim when
a DPoP nonce has been provided to the client.
10.4. Untrusted Code in the Client Context
If 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.
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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 [W3C.CSP] can be used as a second
layer of defense against cross-site scripting.
10.5. Signed JWT Swapping
Servers 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.
10.6. Signature Algorithms
Implementers 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.
10.7. Message Integrity
DPoP 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
[RFC8705] and OAuth Token Binding [I-D.ietf-oauth-token-binding].
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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.
10.8. Access Token and Public Key Binding
The binding of the access token to the DPoP public key, which is
specified in Section 6, 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 Section 4.2). 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.
11. IANA Considerations
11.1. OAuth Access Token Type Registration
This specification requests registration of the following access
token type in the "OAuth Access Token Types" registry
[IANA.OAuth.Params] established by [RFC6749].
* Type name: DPoP
* Additional Token Endpoint Response Parameters: (none)
* HTTP Authentication Scheme(s): DPoP
* Change controller: IESG
* Specification document(s): [[ this specification ]]
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11.2. OAuth Extensions Error Registration
This specification requests registration of the following error
values in the "OAuth Extensions Error" registry [IANA.OAuth.Params]
established by [RFC6749].
Invalid DPoP proof:
* Name: invalid_dpop_proof
* Usage Location: token error response, resource error response
* Protocol Extension: Demonstrating Proof of Possession (DPoP)
* Change controller: IETF
* Specification document(s): [[ this specification ]]
Use DPoP nonce:
* Name: use_dpop_nonce
* Usage Location: token error response, resource error response
* Protocol Extension: Demonstrating Proof of Possession (DPoP)
* Change controller: IETF
* Specification document(s): [[ this specification ]]
11.3. HTTP Authentication Scheme Registration
This specification requests registration of the following scheme in
the "Hypertext Transfer Protocol (HTTP) Authentication Scheme
Registry" [RFC7235][IANA.HTTP.AuthSchemes]:
* Authentication Scheme Name: DPoP
* Reference: [[ Section 7.1 of this specification ]]
11.4. Media Type Registration
[[ Is a media type registration at [IANA.MediaTypes] necessary for
application/dpop+jwt? There is a +jwt structured syntax suffix
registered already at [IANA.MediaType.StructuredSuffix] by
Section 7.2 of [RFC8417], 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
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the JWT per Section 3.11 of [RFC8725] 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 [IANA.MediaTypes]
registration with [I-D.ietf-oauth-access-token-jwt],
[I-D.ietf-oauth-jwsreq], [RFC8417], and of course [RFC7519]. But
precedence isn't always right. ]]
11.5. JWT Confirmation Methods Registration
This specification requests registration of the following value in
the IANA "JWT Confirmation Methods" registry [IANA.JWT] for JWT cnf
member values established by [RFC7800].
* Confirmation Method Value: jkt
* Confirmation Method Description: JWK SHA-256 Thumbprint
* Change Controller: IESG
* Specification Document(s): [[ Section 6 of this specification ]]
11.6. JSON Web Token Claims Registration
This specification requests registration of the following Claims in
the IANA "JSON Web Token Claims" registry [IANA.JWT] established by
[RFC7519].
HTTP method:
* Claim Name: htm
* Claim Description: The HTTP method of the request
* Change Controller: IESG
* Specification Document(s): [[ Section 4.2 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): [[ Section 4.2 of this specification ]]
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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): [[ Section 4.2 of this specification ]]
11.7. HTTP Message Header Field Names Registration
This document specifies the following new HTTP header fields,
registration of which is requested in the "Permanent Message Header
Field Names" registry [IANA.Headers] defined in [RFC3864].
* Header Field Name: DPoP
* Applicable protocol: HTTP
* Status: standard
* Author/change Controller: IETF
* Specification Document(s): [[ this specification ]]
11.8. Authorization Server Metadata Registration
This specification requests registration of the following values in
the IANA "OAuth Authorization Server Metadata" registry
[IANA.OAuth.Parameters] established by [RFC8414].
* 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): [[ Section 5.1 of this specification ]]
12. Normative References
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, DOI 10.17487/RFC3986, January 2005,
<https://www.rfc-editor.org/info/rfc3986>.
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[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
<https://www.rfc-editor.org/info/rfc5234>.
[RFC6749] Hardt, D., Ed., "The OAuth 2.0 Authorization Framework",
RFC 6749, DOI 10.17487/RFC6749, October 2012,
<https://www.rfc-editor.org/info/rfc6749>.
[RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
DOI 10.17487/RFC7231, June 2014,
<https://www.rfc-editor.org/info/rfc7231>.
[RFC7515] Jones, M., Bradley, J., and N. Sakimura, "JSON Web
Signature (JWS)", RFC 7515, DOI 10.17487/RFC7515, May
2015, <https://www.rfc-editor.org/info/rfc7515>.
[RFC7518] Jones, M., "JSON Web Algorithms (JWA)", RFC 7518,
DOI 10.17487/RFC7518, May 2015,
<https://www.rfc-editor.org/info/rfc7518>.
[RFC7638] Jones, M. and N. Sakimura, "JSON Web Key (JWK)
Thumbprint", RFC 7638, DOI 10.17487/RFC7638, September
2015, <https://www.rfc-editor.org/info/rfc7638>.
[RFC7800] Jones, M., Bradley, J., and H. Tschofenig, "Proof-of-
Possession Key Semantics for JSON Web Tokens (JWTs)",
RFC 7800, DOI 10.17487/RFC7800, April 2016,
<https://www.rfc-editor.org/info/rfc7800>.
13. Informative References
[I-D.ietf-oauth-access-token-jwt]
Bertocci, V., "JSON Web Token (JWT) Profile for OAuth 2.0
Access Tokens", Work in Progress, Internet-Draft, draft-
ietf-oauth-access-token-jwt-13, 25 May 2021,
<https://datatracker.ietf.org/doc/html/draft-ietf-oauth-
access-token-jwt-13>.
[I-D.ietf-oauth-jwsreq]
Sakimura, N., Bradley, J., and M. B. Jones, "The OAuth 2.0
Authorization Framework: JWT-Secured Authorization Request
(JAR)", Work in Progress, Internet-Draft, draft-ietf-
oauth-jwsreq-34, 8 April 2021,
<https://datatracker.ietf.org/doc/html/draft-ietf-oauth-
jwsreq-34>.
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[I-D.ietf-oauth-security-topics]
Lodderstedt, T., Bradley, J., Labunets, A., and D. Fett,
"OAuth 2.0 Security Best Current Practice", Work in
Progress, Internet-Draft, draft-ietf-oauth-security-
topics-18, 13 April 2021,
<https://datatracker.ietf.org/doc/html/draft-ietf-oauth-
security-topics-18>.
[I-D.ietf-oauth-token-binding]
Jones, M. B., Campbell, B., Bradley, J., and W. Denniss,
"OAuth 2.0 Token Binding", Work in Progress, Internet-
Draft, draft-ietf-oauth-token-binding-08, 19 October 2018,
<https://datatracker.ietf.org/doc/html/draft-ietf-oauth-
token-binding-08>.
[IANA.HTTP.AuthSchemes]
IANA, "Hypertext Transfer Protocol (HTTP) Authentication
Scheme Registry",
<https://www.iana.org/assignments/http-authschemes>.
[IANA.Headers]
IANA, "Message Headers",
<https://www.iana.org/assignments/message-headers>.
[IANA.JWT] IANA, "JSON Web Token Claims",
<http://www.iana.org/assignments/jwt>.
[IANA.MediaType.StructuredSuffix]
IANA, "Structured Syntax Suffix Registry",
<https://www.iana.org/assignments/media-type-structured-
suffix>.
[IANA.MediaTypes]
IANA, "Media Types",
<https://www.iana.org/assignments/media-types>.
[IANA.OAuth.Params]
IANA, "OAuth Parameters",
<https://www.iana.org/assignments/oauth-parameters>.
[OpenID.Core]
Sakimura, N., Bradley, J., Jones, M.B., Medeiros, B.d.,
and C. Mortimore, "OpenID Connect Core 1.0", November
2014,
<http://openid.net/specs/openid-connect-core-1_0.html>.
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[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC3864] Klyne, G., Nottingham, M., and J. Mogul, "Registration
Procedures for Message Header Fields", BCP 90, RFC 3864,
DOI 10.17487/RFC3864, September 2004,
<https://www.rfc-editor.org/info/rfc3864>.
[RFC4122] Leach, P., Mealling, M., and R. Salz, "A Universally
Unique IDentifier (UUID) URN Namespace", RFC 4122,
DOI 10.17487/RFC4122, July 2005,
<https://www.rfc-editor.org/info/rfc4122>.
[RFC6750] Jones, M. and D. Hardt, "The OAuth 2.0 Authorization
Framework: Bearer Token Usage", RFC 6750,
DOI 10.17487/RFC6750, October 2012,
<https://www.rfc-editor.org/info/rfc6750>.
[RFC7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Message Syntax and Routing",
RFC 7230, DOI 10.17487/RFC7230, June 2014,
<https://www.rfc-editor.org/info/rfc7230>.
[RFC7235] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Authentication", RFC 7235,
DOI 10.17487/RFC7235, June 2014,
<https://www.rfc-editor.org/info/rfc7235>.
[RFC7519] Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
(JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015,
<https://www.rfc-editor.org/info/rfc7519>.
[RFC7523] Jones, M., Campbell, B., and C. Mortimore, "JSON Web Token
(JWT) Profile for OAuth 2.0 Client Authentication and
Authorization Grants", RFC 7523, DOI 10.17487/RFC7523, May
2015, <https://www.rfc-editor.org/info/rfc7523>.
[RFC7662] Richer, J., Ed., "OAuth 2.0 Token Introspection",
RFC 7662, DOI 10.17487/RFC7662, October 2015,
<https://www.rfc-editor.org/info/rfc7662>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
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[RFC8414] Jones, M., Sakimura, N., and J. Bradley, "OAuth 2.0
Authorization Server Metadata", RFC 8414,
DOI 10.17487/RFC8414, June 2018,
<https://www.rfc-editor.org/info/rfc8414>.
[RFC8417] Hunt, P., Ed., Jones, M., Denniss, W., and M. Ansari,
"Security Event Token (SET)", RFC 8417,
DOI 10.17487/RFC8417, July 2018,
<https://www.rfc-editor.org/info/rfc8417>.
[RFC8705] Campbell, B., Bradley, J., Sakimura, N., and T.
Lodderstedt, "OAuth 2.0 Mutual-TLS Client Authentication
and Certificate-Bound Access Tokens", RFC 8705,
DOI 10.17487/RFC8705, February 2020,
<https://www.rfc-editor.org/info/rfc8705>.
[RFC8707] Campbell, B., Bradley, J., and H. Tschofenig, "Resource
Indicators for OAuth 2.0", RFC 8707, DOI 10.17487/RFC8707,
February 2020, <https://www.rfc-editor.org/info/rfc8707>.
[RFC8725] Sheffer, Y., Hardt, D., and M. Jones, "JSON Web Token Best
Current Practices", BCP 225, RFC 8725,
DOI 10.17487/RFC8725, February 2020,
<https://www.rfc-editor.org/info/rfc8725>.
[W3C.CSP] West, M., "Content Security Policy Level 3", World Wide
Web Consortium Working Draft WD-CSP3-20181015, 15 October
2018, <https://www.w3.org/TR/2018/WD-CSP3-20181015/>.
[W3C.WebCryptoAPI]
Watson, M., "Web Cryptography API", World Wide Web
Consortium Recommendation REC-WebCryptoAPI-20170126, 26
January 2017,
<https://www.w3.org/TR/2017/REC-WebCryptoAPI-20170126>.
Appendix A. Acknowledgements
We 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, Benjamin
Kaduk, Pieter Kasselman, Steinar Noem, Neil Madden, Rob Otto, Aaron
Parecki, Michael Peck, Paul Querna, Justin Richer, Filip Skokan,
Dmitry Telegin, 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.
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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).
Appendix B. Document History
[[ To be removed from the final specification ]]
-04
* Added the option for a server-provided nonce in the DPoP proof.
* Registered the invalid_dpop_proof and use_dpop_nonce error codes.
* Removed fictitious uses of realm from the examples, as they added
no value.
* State that if the introspection response has a token_type, it has
to be DPoP.
* Mention that RFC7235 allows multiple authentication schemes in
WWW-Authenticate with a 401.
* Editorial fixes.
-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
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* 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
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* 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
* extracted and unified definition of DPoP JWTs
* improved description
-00
* first draft
Authors' Addresses
Daniel Fett
yes.com
Email: mail@danielfett.de
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Brian Campbell
Ping Identity
Email: bcampbell@pingidentity.com
John Bradley
Yubico
Email: ve7jtb@ve7jtb.com
Torsten Lodderstedt
yes.com
Email: torsten@lodderstedt.net
Michael Jones
Microsoft
Email: mbj@microsoft.com
URI: https://self-issued.info/
David Waite
Ping Identity
Email: david@alkaline-solutions.com
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