OAuth 2.0 Demonstrating Proof-of-Possession at the Application Layer (DPoP)
draft-ietf-oauth-dpop-13
| Document | Type | Active Internet-Draft (oauth WG) | |
|---|---|---|---|
| Authors | Daniel Fett , Brian Campbell , John Bradley , Torsten Lodderstedt , Michael Jones , David Waite | ||
| Last updated | 2023-01-20 | ||
| Replaces | draft-fett-oauth-dpop | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Intended RFC status | Proposed Standard | ||
| Formats | |||
| Reviews |
GENART Last Call Review
Incomplete, due 2023-01-20
OPSDIR Last Call Review
Incomplete, due 2023-01-20
ARTART Last Call Review
Incomplete, due 2023-01-20
|
||
| Additional resources | Mailing list discussion | ||
| Stream | WG state | Submitted to IESG for Publication | |
| Associated WG milestone |
|
||
| Document shepherd | Rifaat Shekh-Yusef | ||
| Shepherd write-up | Show Last changed 2022-11-17 | ||
| IESG | IESG state | Waiting for Writeup | |
| Action Holder | |||
| Consensus boilerplate | Yes | ||
| Telechat date | (None) | ||
| Responsible AD | Roman Danyliw | ||
| Send notices to | rifaat.s.ietf@gmail.com | ||
| IANA | IANA review state | Version Changed - Review Needed | |
| IANA expert review state | Reviews assigned |
draft-ietf-oauth-dpop-13
Web Authorization Protocol D. Fett
Internet-Draft yes.com
Intended status: Standards Track B. Campbell
Expires: 24 July 2023 Ping Identity
J. Bradley
Yubico
T. Lodderstedt
yes.com
M. Jones
Microsoft
D. Waite
Ping Identity
20 January 2023
OAuth 2.0 Demonstrating Proof-of-Possession at the Application Layer
(DPoP)
draft-ietf-oauth-dpop-13
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
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 24 July 2023.
Copyright Notice
Copyright (c) 2023 IETF Trust and the persons identified as the
document authors. All rights reserved.
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This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document. Code Components
extracted from this document must include Revised BSD License text as
described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Conventions and Terminology . . . . . . . . . . . . . . . 4
2. Objectives . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Concept . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
4. DPoP Proof JWTs . . . . . . . . . . . . . . . . . . . . . . . 7
4.1. The DPoP HTTP Header . . . . . . . . . . . . . . . . . . 8
4.2. DPoP Proof JWT Syntax . . . . . . . . . . . . . . . . . . 8
4.3. Checking DPoP Proofs . . . . . . . . . . . . . . . . . . 10
5. DPoP Access Token Request . . . . . . . . . . . . . . . . . . 11
5.1. Authorization Server Metadata . . . . . . . . . . . . . . 14
5.2. Client Registration Metadata . . . . . . . . . . . . . . 14
6. Public Key Confirmation . . . . . . . . . . . . . . . . . . . 15
6.1. JWK Thumbprint Confirmation Method . . . . . . . . . . . 15
6.2. JWK Thumbprint Confirmation Method in Token
Introspection . . . . . . . . . . . . . . . . . . . . . . 16
7. Protected Resource Access . . . . . . . . . . . . . . . . . . 17
7.1. The DPoP Authentication Scheme . . . . . . . . . . . . . 18
7.2. Compatibility with the Bearer Authentication Scheme . . . 21
7.3. Client Considerations . . . . . . . . . . . . . . . . . . 23
8. Authorization Server-Provided Nonce . . . . . . . . . . . . . 23
8.1. Providing a New Nonce Value . . . . . . . . . . . . . . . 25
9. Resource Server-Provided Nonce . . . . . . . . . . . . . . . 26
10. Authorization Code Binding to DPoP Key . . . . . . . . . . . 26
10.1. DPoP with Pushed Authorization Requests . . . . . . . . 27
11. Security Considerations . . . . . . . . . . . . . . . . . . . 28
11.1. DPoP Proof Replay . . . . . . . . . . . . . . . . . . . 28
11.2. DPoP Proof Pre-Generation . . . . . . . . . . . . . . . 29
11.3. DPoP Nonce Downgrade . . . . . . . . . . . . . . . . . . 30
11.4. Untrusted Code in the Client Context . . . . . . . . . . 30
11.5. Signed JWT Swapping . . . . . . . . . . . . . . . . . . 30
11.6. Signature Algorithms . . . . . . . . . . . . . . . . . . 31
11.7. Request Integrity . . . . . . . . . . . . . . . . . . . 31
11.8. Access Token and Public Key Binding . . . . . . . . . . 31
11.9. Authorization Code and Public Key Binding . . . . . . . 32
11.10. Hash Algorithm Agility . . . . . . . . . . . . . . . . . 32
12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 33
12.1. OAuth Access Token Type Registration . . . . . . . . . . 33
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12.2. OAuth Extensions Error Registration . . . . . . . . . . 33
12.3. OAuth Parameters Registration . . . . . . . . . . . . . 33
12.4. HTTP Authentication Scheme Registration . . . . . . . . 34
12.5. Media Type Registration . . . . . . . . . . . . . . . . 34
12.6. JWT Confirmation Methods Registration . . . . . . . . . 35
12.7. JSON Web Token Claims Registration . . . . . . . . . . . 35
12.7.1. "nonce" Registry Update . . . . . . . . . . . . . . 35
12.8. HTTP Message Header Field Names Registration . . . . . . 36
12.9. OAuth Authorization Server Metadata Registration . . . . 36
12.10. OAuth Dynamic Client Registration Metadata . . . . . . . 37
13. Normative References . . . . . . . . . . . . . . . . . . . . 37
14. Informative References . . . . . . . . . . . . . . . . . . . 38
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 41
Appendix B. Document History . . . . . . . . . . . . . . . . . . 42
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 46
1. Introduction
DPoP (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 JSON Web Token (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.
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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).
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 Augmented Backus-Naur Form (ABNF)
notation of [RFC5234].
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",
"client", "public client", and "confidential client" defined by The
OAuth 2.0 Authorization Framework [RFC6749].
The terms "request", "response", "header field", and "target URI" are
imported from [RFC9110].
The terms "JOSE" and "JOSE header" are imported from [RFC7515].
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
[CRIME], BREACH [BREACH], Heartbleed [Heartbleed], and the Cloudflare
parser bug [Cloudbleed] are some examples). There have also been
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numerous published token theft attacks on OAuth implementations
themselves ([GitHub.Tokens] as just one high profile example). 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
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.
XSS 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 pre-computed DPoP proofs is limited somewhat by the proof being
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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 11.
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.
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.
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* (B) The authorization server binds (sender-constrains) the access
token to the public key claimed by the client in the DPoP proof;
that is, the access token cannot be used without proving
possession of the respective private key. If a refresh token is
issued to a public client, it too is bound to the public key of
the DPoP proof.
* (C) To use the access token, the client has to prove possession of
the private key by, again, adding a header to the request that
carries a DPoP proof for that request. The resource server needs
to receive information about the public key to which the access
token is bound. This information may be encoded directly into the
access token (for JWT structured access tokens) or provided via
token introspection endpoint (not shown). The resource server
verifies that the public key to which the access token is bound
matches the public key of the DPoP proof. 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 target 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 11 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.
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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
request header field.
DPoP A JWT that adheres to the structure and syntax of Section 4.2.
Figure 2 shows an example DPoP HTTP header field (line breaks and
indentation 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 [RFC9110] 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 JSON Web
Signature (JWS) [RFC7515]) with a private key chosen by the client
(see below). The JOSE header of a DPoP JWT MUST contain at least the
following parameters:
* typ: with value dpop+jwt, which explicitly types the DPoP proof
JWT as recommended in Section 3.11 of [RFC8725].
* alg: an identifier for a JWS asymmetric digital signature
algorithm from [IANA.JOSE.ALGS]. MUST NOT be none or an
identifier for a symmetric algorithm (MAC).
* jwk: representing the public key chosen by the client, in JSON Web
Key (JWK) [RFC7517] format, as defined in Section 4.1.3 of
[RFC7515]. MUST NOT contain a private key.
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The payload of a DPoP proof MUST contain at least the following
claims:
* jti: Unique identifier for the DPoP proof JWT. 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 11.1.
* htm: The value of the HTTP method (Section 9.1 of [RFC9110]) of
the request to which the JWT is attached.
* htu: The HTTP target URI (Section 7.1 of [RFC9110]), without query
and fragment parts, of the request to which the JWT is attached.
* iat: Creation timestamp of the JWT ([RFC7519], section 4.1.6]).
When the DPoP proof is used in conjunction with the presentation of
an access token in protected resource access, see Section 7, the DPoP
proof MUST also contain the following claim:
* ath: hash of the access token. The value MUST be the result of a
base64url encoding (as defined in Section 2 of [RFC7515]) the
SHA-256 [SHS] hash of the ASCII encoding of the associated access
token's value.
When the authentication server or resource server provides a DPoP-
Nonce HTTP header in a response (see Section 8, Section 9), the DPoP
proof MUST also contain the following claim:
* nonce: A recent nonce provided via the DPoP-Nonce HTTP header.
A DPoP proof MAY contain other JOSE header parameters 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 JWT header and payload are
shown, but the signature part is omitted. As usual, line breaks and
extra spaces are included for formatting and readability.
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{
"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
Of the HTTP request, only the HTTP method and URI are included in the
DPoP JWT, and therefore only these two message parts are covered by
the DPoP proof. The idea is sign just enough of the HTTP data to
provide reasonable proof-of-possession with respect to the HTTP
request. This design approach of using only a minimal subset of the
HTTP header data is 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 11.7).
4.3. Checking DPoP Proofs
To validate a DPoP proof, the receiving server MUST ensure that
* there is not more than one DPoP HTTP request header field,
* the DPoP HTTP request header field value is a well-formed JWT,
* all required claims per Section 4.2 are contained in the JWT,
* the typ JOSE header parameter has the value dpop+jwt,
* the alg JOSE header parameter indicates a registered asymmetric
digital signature algorithm [IANA.JOSE.ALGS], is not none, is
supported by the application, and is acceptable per local policy,
* the JWT signature verifies with the public key contained in the
jwk JOSE header parameter,
* the jwk JOSE header parameter does not contain a private key,
* the htm claim matches the HTTP method of the current request,
* the htu claim matches the HTTP URI value for the HTTP request in
which the JWT was received, ignoring any query and fragment parts,
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* if the server provided a nonce value to the client, the nonce
claim matches the server-provided nonce value,
* the creation time of the JWT, as determined by either the iat
claim or a server managed timestamp via the nonce claim, is within
an acceptable window (see Section 11.1),
* if presented to a protected resource in conjunction with an access
token,
- ensure that the value of the ath claim equals the hash of that
access token,
- confirm that the public key to which the access token is bound
matches the public key from the DPoP proof.
To reduce the likelihood of false negatives, servers SHOULD employ
Syntax-Based Normalization (Section 6.2.2 of [RFC3986]) and Scheme-
Based Normalization (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 HTTP
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 indentation for display purposes only).
POST /token HTTP/1.1
Host: server.example.com
Content-Type: application/x-www-form-urlencoded
DPoP: eyJ0eXAiOiJkcG9wK2p3dCIsImFsZyI6IkVTMjU2IiwiandrIjp7Imt0eSI6Ik
VDIiwieCI6Imw4dEZyaHgtMzR0VjNoUklDUkRZOXpDa0RscEJoRjQyVVFVZldWQVdCR
nMiLCJ5IjoiOVZFNGpmX09rX282NHpiVFRsY3VOSmFqSG10NnY5VERWclUwQ2R2R1JE
QSIsImNydiI6IlAtMjU2In19.eyJqdGkiOiItQndDM0VTYzZhY2MybFRjIiwiaHRtIj
oiUE9TVCIsImh0dSI6Imh0dHBzOi8vc2VydmVyLmV4YW1wbGUuY29tL3Rva2VuIiwia
WF0IjoxNTYyMjYyNjE2fQ.2-GxA6T8lP4vfrg8v-FdWP0A0zdrj8igiMLvqRMUvwnQg
4PtFLbdLXiOSsX0x7NVY-FNyJK70nfbV37xRZT3Lg
grant_type=authorization_code
&client_id=s6BhdRkqt
&code=SplxlOBeZQQYbYS6WxSbIA
&redirect_uri=https%3A%2F%2Fclient%2Eexample%2Ecom%2Fcb
&code_verifier=bEaL42izcC-o-xBk0K2vuJ6U-y1p9r_wW2dFWIWgjz-
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Figure 4: Token Request for a DPoP sender-constrained token using an
authorization code
The DPoP HTTP header field 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.
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 indentation for display purposes
only).
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POST /token HTTP/1.1
Host: server.example.com
Content-Type: application/x-www-form-urlencoded
DPoP: eyJ0eXAiOiJkcG9wK2p3dCIsImFsZyI6IkVTMjU2IiwiandrIjp7Imt0eSI6Ik
VDIiwieCI6Imw4dEZyaHgtMzR0VjNoUklDUkRZOXpDa0RscEJoRjQyVVFVZldWQVdCR
nMiLCJ5IjoiOVZFNGpmX09rX282NHpiVFRsY3VOSmFqSG10NnY5VERWclUwQ2R2R1JE
QSIsImNydiI6IlAtMjU2In19.eyJqdGkiOiItQndDM0VTYzZhY2MybFRjIiwiaHRtIj
oiUE9TVCIsImh0dSI6Imh0dHBzOi8vc2VydmVyLmV4YW1wbGUuY29tL3Rva2VuIiwia
WF0IjoxNTYyMjY1Mjk2fQ.pAqut2IRDm_De6PR93SYmGBPXpwrAk90e8cP2hjiaG5Qs
GSuKDYW7_X620BxqhvYC8ynrrvZLTk41mSRroapUA
grant_type=refresh_token
&client_id=s6BhdRkqt
&refresh_token=Q..Zkm29lexi8VnWg2zPW1x-tgGad0Ibc3s3EwM_Ni4-g
Figure 6: Token Request for a DPoP-bound Token using a Refresh Token
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.
Since the authorization server both produces and validates its
refresh tokens, 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 the access token response contains a different token_type value
than DPoP, 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; otherwise,
it may continue as in a regular OAuth interaction.
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
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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
identifier 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 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.
dpop_signing_alg_values_supported A JSON array containing a list of
the JWS alg values (from the [IANA.JOSE.ALGS] registry) supported
by the authorization server for DPoP proof JWTs.
5.2. Client Registration Metadata
The Dynamic Client Registration Protocol [RFC7591] defines an API for
dynamically registering OAuth 2.0 client metadata with authorization
servers. The metadata defined by [RFC7591], and registered
extensions to it, also imply a general data model for clients that is
useful for authorization server implementations even when the Dynamic
Client Registration Protocol isn't in play. Such implementations
will typically have some sort of user interface available for
managing client configuration.
This document introduces the following client registration metadata
[RFC7591] parameter to indicate that the client always uses DPoP when
requesting tokens from the authorization server.
dpop_bound_access_tokens Boolean value specifying whether the client
always uses DPoP for token requests. If omitted, the default
value is false.
If true, the authorization server MUST reject token requests from
this client that do not contain the DPoP header.
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6. Public Key Confirmation
Resource servers MUST be able to reliably identify whether an access
token is DPoP-bound and ascertain sufficient information to verify
the binding to the public key of the 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 one bound to the access token.
6.1. JWK Thumbprint Confirmation Method
When access tokens are represented as JSON Web Tokens (JWT)
[RFC7519], the public key information is 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 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.
eyJhbGciOiJFUzI1NiIsImtpZCI6IkJlQUxrYiJ9.eyJzdWIiOiJzb21lb25lQGV4YW1
wbGUuY29tIiwiaXNzIjoiaHR0cHM6Ly9zZXJ2ZXIuZXhhbXBsZS5jb20iLCJuYmYiOjE
1NjIyNjI2MTEsImV4cCI6MTU2MjI2NjIxNiwiY25mIjp7ImprdCI6IjBaY09DT1JaTll
5LURXcHFxMzBqWnlKR0hUTjBkMkhnbEJWM3VpZ3VBNEkifX0.3Tyo8VTcn6u_PboUmAO
YUY1kfAavomW_YwYMkmRNizLJoQzWy2fCo79Zi5yObpIzjWb5xW4OGld7ESZrh0fsrA
Figure 7: JWT containing a JWK SHA-256 Thumbprint Confirmation
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{
"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.
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
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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
Protected resource requests with a DPoP-bound access token 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.
Binding the token value to the proof in this way prevents a proof to
be used with multiple different access token values across different
requests. For example, if a client holds tokens bound to two
different resource owners, AT1 and AT2, and uses the same key when
talking to the AS, it's possible that these tokens could be swapped.
Without the ath field to bind it, a captured signature applied to AT1
could be replayed with AT2 instead, changing the rights and access of
the intended request. This same substitution prevention remains for
rotated access tokens within the same combination of client and
resource owner -- a rotated token value would require the calculation
of a new proof. This binding additionally ensures that a proof
intended for use with the access token is not usable without an
access token, or vice-versa.
The resource server is required to calculate the hash of the token
value presented and verify that it is the same as the hash value in
the ath field as described in Section 4.3. Since the ath field value
is covered by the DPoP proof's signature, its inclusion binds the
access token value to the holder of the key used to generate the
signature.
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Note that the ath field alone does not prevent replay of the DPoP
proof or provide binding to the request in which the proof is
presented, and it is still important to check the time window of the
proof as well as the included message parameters such as htm and htu.
7.1. The DPoP Authentication Scheme
A DPoP-bound access token is sent using the Authorization request
header field per Section 11.6.2 of [RFC9110] using an authentication
scheme of DPoP. The syntax of the Authorization header field for the
DPoP scheme uses the token68 syntax defined in Section 11.2 of
[RFC9110] (repeated below for ease of reference) for credentials.
The ABNF notation syntax for DPoP authentication scheme credentials
is as follows:
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 JWT header
and payload are shown but the signature part is omitted. As usual,
line breaks and indentation are included for formatting and
readability in both examples.
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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
{
"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 to 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, the server can respond with a challenge to the
client to provide DPoP authentication information. Such a challenge
is made using the 401 (Unauthorized) response status code ([RFC9110],
Section 15.5.2) and the WWW-Authenticate header field ([RFC9110],
Section 11.6.1). The server MAY include the WWW-Authenticate header
in response to other conditions as well.
In such challenges:
* The scheme name is DPoP.
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* The authentication parameter realm MAY be included to indicate the
scope of protection in the manner described in [RFC9110],
Section 11.5.
* 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
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
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Note that browser-based client applications using CORS [WHATWG.Fetch]
only have access to CORS-safelisted response HTTP headers by default.
In order for the application to obtain and use the WWW-Authenticate
HTTP response header value, the server needs to make it available to
the application by including WWW-Authenticate in the Access-Control-
Expose-Headers response header list value.
This authentication scheme is for origin-server authentication only.
Therefore, this authentication scheme MUST NOT be used with the
Proxy-Authenticate or Proxy-Authorization header fields.
Note that the syntax of the Authorization header field for this
authentication scheme follows the usage of the Bearer scheme defined
in Section 2.1 of [RFC6750]. While not the preferred credential
syntax of [RFC9110], it is compatible with the general authentication
framework therein and was used for consistency and familiarity with
the Bearer scheme.
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 token.
Specifically, such a protected resource MUST reject a DPoP-bound
access token received as a bearer token per [RFC6750].
Section 11.6.1 of [RFC9110] 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 can 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.
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If a protected resource supporting both Bearer and DPoP schemes
elects to respond with multiple WWW-Authenticate challenges,
attention should be paid to which challenge(s) should deliver the
actual error information. It is RECOMMENDED that the following rules
be adhered to:
* If no authentication information has been included with the
request, then the challenges SHOULD NOT include an error code or
other error information, as per [RFC6750], Section 3.1
(Figure 16).
* If the mechanism used to attempt authentication could be
established unambiguously, then the corresponding challenge SHOULD
be used to deliver error information (Figure 17).
* Otherwise, both Bearer and DPoP challenged MAY be used to deliver
error information (Figure 18).
GET /protectedresource HTTP/1.1
Host: resource.example.org
HTTP/1.1 401 Unauthorized
WWW-Authenticate: Bearer, DPoP algs="ES256 PS256"
Figure 16: HTTP 401 Response to a Protected Resource Request without
Authentication
GET /protectedresource HTTP/1.1
Host: resource.example.org
Authorization: Bearer INVALID_TOKEN
HTTP/1.1 401 Unauthorized
WWW-Authenticate: Bearer error="invalid_token",
error_description="Invalid token", DPoP algs="ES256 PS256"
Figure 17: HTTP 401 Response to a Protected Resource Request with
Invalid Authentication
GET /protectedresource HTTP/1.1
Host: resource.example.org
Authorization: Bearer Kz~8mXK1EalYznwH-LC-1fBAo.4Ljp~zsPE_NeO.gxU
Authorization: DPoP Kz~8mXK1EalYznwH-LC-1fBAo.4Ljp~zsPE_NeO.gxU
HTTP/1.1 400 Bad Request
WWW-Authenticate: Bearer error="invalid_request",
error_description="Multiple methods used to include access token",
DPoP algs="ES256 PS256", error="invalid_request",
error_description="Multiple methods used to include access token"
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Figure 18: HTTP 400 Response to a Protected Resource Request with
Ambiguous Authentication
7.3. Client Considerations
Authorization including a DPoP proof may not be idempotent (depending
on server enforcement of jti, iat and nonce claims). Consequently,
all previously idempotent requests for protected resources that were
previously idempotent may no longer be idempotent. It is RECOMMENDED
that clients generate a unique DPoP proof even when retrying
idempotent requests in response to HTTP errors generally understood
as transient.
Clients that encounter frequent network errors may experience
additional challenges when interacting with servers with more strict
nonce validation implementations.
8. Authorization Server-Provided Nonce
This section specifies a mechanism using opaque nonces provided by
the server that can be used to limit the lifetime of 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.
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 determines when and if to issue
a new DPoP nonce challenge thereby requiring the use of the nonce
value in subsequent DPoP proofs. The logic through which the server
makes that determination is out of scope of this document.
An authorization server MAY supply a nonce value to be included by
the client in DPoP proofs sent. In this case, the authorization
server responds to requests not including a nonce with an HTTP 400
(Bad Request) error response per Section 5.2 of [RFC6749] using
use_dpop_nonce as the error code value. The authorization server
includes a DPoP-Nonce HTTP header in the response supplying a nonce
value to be used when sending the subsequent request. This same
error code is used when supplying a new nonce value when there was a
nonce mismatch. The client will typically retry the request with the
new nonce value supplied upon receiving a use_dpop_nonce error with
an accompanying nonce value.
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 19: 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 20: 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 21: Nonce ABNF
The nonce is opaque to the client.
If the nonce claim in the DPoP proof does not exactly match a nonce
recently 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.
The intent is that clients need to keep only one nonce value and
servers keep a window of recent nonces. That said, transient
circumstances may arise in which the server's and client's stored
nonce values differ. However, this situation is self-correcting;
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with any rejection message, the server can send the client the nonce
value that the server wants it to use and the client can store that
nonce value and retry the request with it. Even if the client and/or
server discard their stored nonce values, that situation is also
self-correcting because new nonce values can be communicated when
responding to or retrying failed requests.
Note that browser-based client applications using CORS [WHATWG.Fetch]
only have access to CORS-safelisted response HTTP headers by default.
In order for the application to obtain and use the DPoP-Nonce HTTP
response header value, the server needs to make it available to the
application by including DPoP-Nonce in the Access-Control-Expose-
Headers response header list value.
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 HTTP 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.
Responses that include the DPoP-Nonce HTTP header should be
uncacheable (e.g., using Cache-Control: no-store in response to a GET
request) to prevent the response being used to serve a subsequent
request and a stale nonce value being used as a result.
An example 200 OK response providing a new nonce value is:
HTTP/1.1 200 OK
Cache-Control: no-store
DPoP-Nonce: eyJ7S_zG.eyJbYu3.xQmBj-1
Figure 22: HTTP 200 Response Providing the Next Nonce Value
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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 the same way that authorization servers do
as described in Section 8 and Section 8.1. The error signaling is
performed as described in Section 7.1. Resource servers use an HTTP
401 (Unauthorized) error code with an accompanying WWW-Authenticate:
DPoP value and DPoP-Nonce value to accomplish this.
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 23: HTTP 401 Response to a Resource Request without a Nonce
Note that the nonces provided by an authorization server and a
resource server are different and should not be confused with one
another, since nonces will be only accepted by the server that issued
them. Likewise, should a client use multiple authorization servers
and/or resource servers, a nonce issued by any of them should be used
only at the issuing server. Developers should also take care to not
confuse DPoP nonces with the OpenID Connect [OpenID.Core] ID Token
nonce.
10. Authorization Code Binding to DPoP Key
Binding the authorization code issued to the client's proof-of-
possession key can enable end-to-end binding of the entire
authorization flow. This specification defines the dpop_jkt
authorization request parameter for this purpose. The value of the
dpop_jkt authorization request parameter is the JSON Web Key (JWK)
Thumbprint [RFC7638] of the proof-of-possession public key using the
SHA-256 hash function - the same value as used for the jkt
confirmation method defined in Section 6.1.
When a token request is received, the authorization server computes
the JWK thumbprint of the proof-of-possession public key in the DPoP
proof and verifies that it matches the dpop_jkt parameter value in
the authorization request. If they do not match, it MUST reject the
request.
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An example authorization request using the dpop_jkt authorization
request parameter is:
GET /authorize?response_type=code&client_id=s6BhdRkqt3&state=xyz
&redirect_uri=https%3A%2F%2Fclient%2Eexample%2Ecom%2Fcb
&code_challenge=E9Melhoa2OwvFrEMTJguCHaoeK1t8URWbuGJSstw-cM
&code_challenge_method=S256
&dpop_jkt=NzbLsXh8uDCcd-6MNwXF4W_7noWXFZAfHkxZsRGC9Xs HTTP/1.1
Host: server.example.com
Figure 24: Authorization Request using the dpop_jkt Parameter
Use of the dpop_jkt authorization request parameter is OPTIONAL.
Note that the dpop_jkt authorization request parameter MAY also be
used in combination with PKCE [RFC7636], which is recommended by
[I-D.ietf-oauth-security-topics] as a countermeasure to authorization
code injection. The dpop_jkt authorization request parameter only
provides similar protections when a unique DPoP key is used for each
authorization request.
10.1. DPoP with Pushed Authorization Requests
When Pushed Authorization Requests (PAR, [RFC9126]) are used in
conjunction with DPoP, there are two ways in which the DPoP key can
be communicated in the PAR request:
* The dpop_jkt parameter can be used as described in Section 10 to
bind the issued authorization code to a specific key. In this
case, dpop_jkt MUST be included alongside other authorization
request parameters in the POST body of the PAR request.
* Alternatively, the DPoP header can be added to the PAR request.
In this case, the authorization server MUST check the provided
DPoP proof JWT as defined in Section 4.3. It MUST further behave
as if the contained public key's thumbprint was provided using
dpop_jkt, i.e., reject the subsequent token request unless a DPoP
proof for the same key is provided. This can help to simplify the
implementation of the client, as it can "blindly" attach the DPoP
header to all requests to the authorization server regardless of
the type of request. Additionally, it provides a stronger
binding, as the DPoP header contains a proof of possession of the
private key.
Both mechanisms MUST be supported by an authorization server that
supports PAR and DPoP. If both mechanisms are used at the same time,
the authorization server MUST reject the request if the JWK
Thumbprint in dpop_jkt does not match the public key in the DPoP
header.
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Allowing both mechanisms ensures that clients that use dpop_jkt do
not need to distinguish between front-channel and pushed
authorization requests, and at the same time, clients that only have
one code path for protecting all calls to authorization server
endpoints do not need to distinguish between requests to the PAR
endpoint and the token endpoint.
11. Security Considerations
In DPoP, the prevention of token replay at a different endpoint (see
Section 2) is achieved through authentication of the server per
[RFC6125] and binding of the DPoP proof to a certain URI and HTTP
method. DPoP, however, has a somewhat different nature of protection
than TLS-based methods such as OAuth Mutual TLS [RFC8705] or OAuth
Token Binding [I-D.ietf-oauth-token-binding] (see also Section 11.1
and Section 11.7). TLS-based mechanisms can leverage a tight
integration between the TLS layer and the application layer to
achieve strong message integrity, authenticity, and replay
protection.
11.1. 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 limit this, servers MUST only accept DPoP proofs for a
limited time after their creation (preferably only for a relatively
brief period on the order of seconds or minutes).
To prevent multiple uses of the same DPoP proof, servers can store,
in the context of the target URI, the jti value of each DPoP proof
for the time window in which the respective DPoP proof JWT would be
accepted. HTTP requests to the same URI for which the jti value has
been seen before would be declined. Such a single-use check, when
strictly enforced, provides a very strong protection against DPoP
proof replay, but may not always be feasible in practice, e.g., when
multiple servers behind a single endpoint have no shared state.
In order to guard against memory exhaustion attacks, a server that is
tracking jti values should reject DPoP proof JWTs with unnecessarily
large jti values or store only a hash thereof.
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Note: To accommodate for clock offsets, the server MAY accept DPoP
proofs that carry an iat time in the reasonably near future (on the
order of seconds or minutes). Because clock skews between servers
and clients may be large, servers MAY limit DPoP proof lifetimes by
using server-provided nonce values containing the time at the server
rather than comparing the client-supplied iat time to the time at the
server. Nonces created in this way yield the same result even in the
face of arbitrarily large clock skews.
Server-provided nonces are an effective means for further reducing
the chances for successful DPoP proof replay. Unlike cryptographic
nonces, it is acceptable for clients to use the same nonce multiple
times, and for the server to accept the same nonce multiple times.
As long as the jti value is tracked and duplicates rejected for the
lifetime of the nonce, there is no additional risk of token replay.
11.2. DPoP Proof Pre-Generation
An attacker in control of the client can pre-generate DPoP proofs for
specific endpoints 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.
The ath claim limits the use of pre-generated DPoP proofs to the
lifetime of the access token. Deployments that do not utilize the
nonce mechanism SHOULD NOT issue long-lived DPoP constrained access
tokens, preferring instead to use short-lived access tokens and
refresh tokens. Whilst an attacker could pre-generate DPoP proofs to
use the refresh token to obtain a new access token, they would be
unable to realistically pre-generate DPoP proofs to use a newly
issued access token.
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11.3. 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.
11.4. 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.
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.
11.5. Signed JWT Swapping
Servers accepting signed DPoP proof JWTs MUST verify that the typ
field is dpop+jwt in the headers of the JWTs to ensure that
adversaries cannot use JWTs created for other purposes.
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11.6. Signature Algorithms
Implementers MUST ensure that only asymmetric digital signature
algorithms (such as ES256) that are deemed secure can be used for
signing DPoP proofs. In particular, the algorithm none MUST NOT be
allowed.
11.7. Request 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.
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.
11.8. 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.
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.
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11.9. Authorization Code and Public Key Binding
Cryptographic binding of the authorization code to the DPoP public
key, is specified in Section 10. This binding prevents attacks in
which the attacker captures the authorization code and creates a DPoP
proof using a proof-of-possession key other than that held by the
client and redeems the authorization code using that DPoP proof. By
ensuring end-to-end that only the client's DPoP key can be used, this
prevents captured authorization codes from being exfiltrated and used
at locations other than the one to which the authorization code was
issued.
Authorization codes can, for instance, be harvested by attackers from
places that the HTTP messages containing them are logged. Even when
efforts are made to make authorization codes one-time-use, in
practice, there is often a time window during which attackers can
replay them. For instance, when authorization servers are
implemented as scalable replicated services, some replicas may
temporarily not yet have the information needed to prevent replay.
DPoP binding of the authorization code solves these problems.
If an authorization server does not (or cannot) strictly enforce the
single-use limitation for authorization codes and an attacker can
access the authorization code (and if PKCE is used, the
code_verifier), the attacker can create a forged token request,
binding the resulting token to an attacker-controlled key. For
example, using cross-site scripting, attackers might obtain access to
the authorization code and PKCE parameters. Use of the dpop_jkt
parameter prevents this attack.
The binding of the authorization code to the DPoP public key uses a
JWK Thumbprint of the public key, just as the access token binding
does. The same JWK Thumbprint considerations apply.
11.10. Hash Algorithm Agility
The jkt confirmation method member, the ath JWT claim, and the
dpop_jkt authorization request parameter defined herein all use the
output of the SHA-256 hash function as their value. The use of a
single hash function by this specification was intentional and aimed
at simplicity and avoidance of potential security and
interoperability issues arising from common mistakes implementing and
deploying parameterized algorithm agility schemes. The use of a
different hash function is not precluded, however, if future
circumstances change making SHA-256 insufficient for the requirements
of this specification. Should that need arise, it is expected that a
short specification be produced that updates this one. That
specification will likely define, using the output of a then
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appropriate hash function as the value, a new confirmation method
member, a new JWT claim, and a new authorization request parameter.
These items will be used in place of, or alongside, their respective
counterparts in the same message structures and flows of the larger
protocol defined by this specification.
12. IANA Considerations
12.1. 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: IETF
* Specification document(s): [[ this specification ]]
12.2. 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 ]]
12.3. OAuth Parameters Registration
This specification requests registration of the following
authorization request parameter in the "OAuth Parameters" registry
[IANA.OAuth.Params] established by [RFC6749].
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* Name: dpop_jkt
* Parameter Usage Location: authorization request
* Change Controller: IETF
* Reference: [[ Section 10 of this specification ]]
12.4. HTTP Authentication Scheme Registration
This specification requests registration of the following scheme in
the "Hypertext Transfer Protocol (HTTP) Authentication Scheme
Registry" [RFC9110][IANA.HTTP.AuthSchemes]:
* Authentication Scheme Name: DPoP
* Reference: [[ Section 7.1 of this specification ]]
12.5. Media Type Registration
This section registers the application/dpop+jwt media type [RFC2046]
in the IANA "Media Types" registry [IANA.MediaTypes] in the manner
described in [RFC6838], which is used to indicate that the content is
a DPoP JWT.
* Type name: application
* Subtype name: dpop+jwt
* Required parameters: n/a
* Optional parameters: n/a
* Encoding considerations: binary; A DPoP JWT is a JWT; JWT values
are encoded as a series of base64url-encoded values (some of which
may be the empty string) separated by period ('.') characters.
* Security considerations: See Section 11 of [[ this specification
]]
* Interoperability considerations: n/a
* Published specification: [[ this specification ]]
* Applications that use this media type: Applications using [[ this
specification ]] for application-level proof of possession
* Fragment identifier considerations: n/a
* Additional information:
- File extension(s): n/a
- Macintosh file type code(s): n/a
* Person & email address to contact for further information: Michael
B. Jones, mbj@microsoft.com
* Intended usage: COMMON
* Restrictions on usage: none
* Author: Michael B. Jones, mbj@microsoft.com
* Change controller: IETF
* Provisional registration? No
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12.6. 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: IETF
* Specification Document(s): [[ Section 6 of this specification ]]
12.7. 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: IETF
* 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: IETF
* Specification Document(s): [[ Section 4.2 of this specification ]]
Access token hash:
* Claim Name: ath
* Claim Description: The base64url encoded SHA-256 hash of the ASCII
encoding of the associated access token's value
* Change Controller: IETF
* Specification Document(s): [[ Section 4.2 of this specification ]]
12.7.1. "nonce" Registry Update
The Internet Security Glossary [RFC4949] provides a useful definition
of nonce as a random or non-repeating value that is included in data
exchanged by a protocol, usually for the purpose of guaranteeing
liveness and thus detecting and protecting against replay attacks.
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However, the initial registration of the nonce claim by [OpenID.Core]
used language that was contextually specific to that application,
which was potentially limiting to its general applicability.
This specification therefore requests that the entry for nonce in the
IANA "JSON Web Token Claims" registry [IANA.JWT] be updated as
follows to reflect that the claim can be used appropriately in other
contexts.
* Claim Name: nonce
* Claim Description: Value used to associate a Client session with
an ID Token (MAY also be used for nonce values in other
applications of JWTs)
* Change Controller: OpenID Foundation Artifact Binding Working
Group - openid-specs-ab@lists.openid.net
* Specification Document(s): Section 2 of [OpenID.Core] and [[ this
specification ]]
12.8. HTTP Message Header Field Names Registration
This document specifies the following HTTP header fields,
registration of which is requested in the "Hypertext Transfer
Protocol (HTTP) Field Name Registry" registry
[RFC9110][IANA.HTTP.Fields]:
* Field name: DPoP
* Status: permanent
* Specification document: [[ this specification ]]
* Field name: DPoP-Nonce
* Status: permanent
* Specification document: [[ this specification ]]
12.9. OAuth Authorization Server Metadata Registration
This specification requests registration of the following value 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: IETF
* Specification Document(s): [[ Section 5.1 of this specification ]]
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12.10. OAuth Dynamic Client Registration Metadata
This specification requests registration of the following value in
the IANA "OAuth Dynamic Client Registration Metadata" registry
[IANA.OAuth.Parameters] established by [RFC7591].
* Metadata Name: dpop_bound_access_tokens
* Metadata Description: Boolean value specifying whether the client
always uses DPoP for token requests
* Change Controller: IETF
* Specification Document(s): [[ Section 5.2 of this specification ]]
13. Normative References
[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>.
[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>.
[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>.
[RFC6125] Saint-Andre, P. and J. Hodges, "Representation and
Verification of Domain-Based Application Service Identity
within Internet Public Key Infrastructure Using X.509
(PKIX) Certificates in the Context of Transport Layer
Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March
2011, <https://www.rfc-editor.org/info/rfc6125>.
[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>.
[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>.
[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>.
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[RFC7517] Jones, M., "JSON Web Key (JWK)", RFC 7517,
DOI 10.17487/RFC7517, May 2015,
<https://www.rfc-editor.org/info/rfc7517>.
[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>.
[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>.
[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>.
[SHS] National Institute of Standards and Technology, "Secure
Hash Standard (SHS)", FIPS PUB 180-4, August 2015,
<https://nvlpubs.nist.gov/nistpubs/FIPS/
NIST.FIPS.180-4.pdf>.
14. Informative References
[BREACH] "CVE-2013-3587", <https://cve.mitre.org/cgi-bin/
cvename.cgi?name=CVE-2013-3587>.
[CRIME] "CVE-2012-4929", <https://cve.mitre.org/cgi-bin/
cvename.cgi?name=cve-2012-4929>.
[Cloudbleed]
"Incident report on memory leak caused by Cloudflare
parser bug", <https://blog.cloudflare.com/incident-report-
on-memory-leak-caused-by-cloudflare-parser-bug/>.
[GitHub.Tokens]
"Security alert: Attack campaign involving stolen OAuth
user tokens issued to two third-party integrators",
<https://github.blog/2022-04-15-security-alert-stolen-
oauth-user-tokens/>.
[Heartbleed]
"CVE-2014-0160", <https://cve.mitre.org/cgi-bin/
cvename.cgi?name=cve-2014-0160>.
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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-21, 27 September 2022,
<https://www.ietf.org/archive/id/draft-ietf-oauth-
security-topics-21.txt>.
[I-D.ietf-oauth-token-binding]
Jones, M., 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://www.ietf.org/archive/id/draft-ietf-oauth-token-
binding-08.txt>.
[IANA.HTTP.AuthSchemes]
IANA, "Hypertext Transfer Protocol (HTTP) Authentication
Scheme Registry",
<https://www.iana.org/assignments/http-authschemes>.
[IANA.HTTP.Fields]
IANA, "Hypertext Transfer Protocol (HTTP) Field Name
Registry", <https://www.iana.org/assignments/http-fields/
http-fields.xhtml>.
[IANA.JOSE.ALGS]
IANA, "JSON Web Signature and Encryption Algorithms",
<https://www.iana.org/assignments/jose/jose.xhtml#web-
signature-encryption-algorithms>.
[IANA.JWT] IANA, "JSON Web Token Claims",
<http://www.iana.org/assignments/jwt>.
[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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[RFC2046] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part Two: Media Types", RFC 2046,
DOI 10.17487/RFC2046, November 1996,
<https://www.rfc-editor.org/info/rfc2046>.
[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>.
[RFC4949] Shirey, R., "Internet Security Glossary, Version 2",
FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007,
<https://www.rfc-editor.org/info/rfc4949>.
[RFC6838] Freed, N., Klensin, J., and T. Hansen, "Media Type
Specifications and Registration Procedures", BCP 13,
RFC 6838, DOI 10.17487/RFC6838, January 2013,
<https://www.rfc-editor.org/info/rfc6838>.
[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>.
[RFC7591] Richer, J., Ed., Jones, M., Bradley, J., Machulak, M., and
P. Hunt, "OAuth 2.0 Dynamic Client Registration Protocol",
RFC 7591, DOI 10.17487/RFC7591, July 2015,
<https://www.rfc-editor.org/info/rfc7591>.
[RFC7636] Sakimura, N., Ed., Bradley, J., and N. Agarwal, "Proof Key
for Code Exchange by OAuth Public Clients", RFC 7636,
DOI 10.17487/RFC7636, September 2015,
<https://www.rfc-editor.org/info/rfc7636>.
[RFC7662] Richer, J., Ed., "OAuth 2.0 Token Introspection",
RFC 7662, DOI 10.17487/RFC7662, October 2015,
<https://www.rfc-editor.org/info/rfc7662>.
[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>.
[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>.
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[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>.
[RFC9110] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "HTTP Semantics", STD 97, RFC 9110,
DOI 10.17487/RFC9110, June 2022,
<https://www.rfc-editor.org/info/rfc9110>.
[RFC9126] Lodderstedt, T., Campbell, B., Sakimura, N., Tonge, D.,
and F. Skokan, "OAuth 2.0 Pushed Authorization Requests",
RFC 9126, DOI 10.17487/RFC9126, September 2021,
<https://www.rfc-editor.org/info/rfc9126>.
[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>.
[WHATWG.Fetch]
WHATWG, "Fetch Living Standard", May 2022,
<https://fetch.spec.whatwg.org/>.
Appendix A. Acknowledgements
We would like to thank Annabelle Backman, Spencer Balogh, Dominick
Baier, Vittorio Bertocci, Jeff Corrigan, Domingos Creado, Andrii
Deinega, William Denniss, Vladimir Dzhuvinov, Mike Engan, Nikos
Fotiou, Mark Haine, Dick Hardt, Joseph Heenan, Bjorn Hjelm, Jacob
Ideskog, Jared Jennings, Benjamin Kaduk, Pieter Kasselman, Steinar
Noem, Neil Madden, Rohan Mahy, Karsten Meyer zu Selhausen, Nicolas
Mora, Rob Otto, Aaron Parecki, Michael Peck, Roberto Polli, Paul
Querna, Justin Richer, Rifaat Shekh-Yusef, 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 originated 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 ]]
-13
* Editorial updates/fixes
* Make sure RFC7519 is a normative reference
-12
* Updates from Roman Danyliw's AD review
* DPoP-Nonce now included in HTTP header field registration request
* Fixed section reference to URI Scheme-Based Normalization
* Attempt to better describe the rationale for SHA-256 only and
expectations for how hash algorithm agility would be achieved if
needed in the future
* Elaborate on the use of multiple WWW-Authenticate challenges by
protected resources
* Fix access token request examples that were missing a client_id
-11
* Updates addressing outstanding shepherd review comments per side
meeting discussions at IETF 114
* Added more explanation of the PAR considerations
* Added parenthetical remark "(such as ES256)" to Signature
Algorithms subsection
* Added more explanation for ath
* Added a reference to RFC8725 in mention of explicit JWT typing
-10
* Updates addressing some shepherd review comments
* Update HTTP references as RFCs 723x have been superseded by RFC
9110
* Editorial fixes
* Added some clarifications, etc. around nonce
* Added client considerations subsection
* Use bullets rather than numbers in Checking DPoP Proofs so as not
to imply specific order
* Added notes/reminders about browser-based client applications
using CORS needing access to response headers
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* Added a JWT claims registry update request for "nonce" to (better)
allow for more general use in other contexts
-09
* Add note/reminder about browser-based client applications using
CORS needing access to response headers.
* Fixed typo
-08
* Lots of editorial updates from WGLC feedback
* Further clarify that either iat or nonce can be used alone in
validating the timeliness of the proof and somewhat de-emphasize
jti tracking
-07
* Registered the application/dpop+jwt media type.
* Editorial updates/clarifications based on review feedback.
* Added "(on the order of seconds or minutes)" to somewhat qualify
"relatively brief period" and "reasonably near future" and give a
general idea of expected timeframe without being overly
prescriptive.
* Added a step to Section 4.3 to reiterate that the jwk header
cannot have a private key.
-06
* Editorial updates and fixes
* Changed name of client metadata parameter to
dpop_bound_access_tokens
-05
* Added Authorization Code binding via the dpop_jkt parameter.
* Described the authorization code reuse attack and how dpop_jkt
mitigates it.
* Enhanced description of DPoP proof expiration checking.
* Described nonce storage requirements and how nonce mismatches and
missing nonces are self-correcting.
* Specified the use of the use_dpop_nonce error for missing and
mismatched nonce values.
* Specified that authorization servers use 400 (Bad Request) errors
to supply nonces and resource servers use 401 (Unauthorized)
errors to do so.
* Added a bit more about ath and pre-generated proofs to the
security considerations.
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* Mentioned confirming the DPoP binding of the access token in the
list in Section 4.3.
* Added the always_uses_dpop client registration metadata parameter.
* Described the relationship between DPoP and Pushed Authorization
Requests (PAR).
* Updated references for drafts that are now RFCs.
-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
* 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
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* 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
* extracted and unified definition of DPoP JWTs
* improved description
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-00
* first draft
Authors' Addresses
Daniel Fett
yes.com
Email: mail@danielfett.de
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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