Internet Engineering Task Force (IETF) V. Bertocci

Internet Engineering Task Force (IETF) V. Bertocci
Request for Comments: 9470 Auth0/Okta
Category: Standards Track B. Campbell
ISSN: 2070-1721 Ping Identity
September 2023

OAuth 2.0 Step Up Authentication Challenge Protocol

Abstract

It is not uncommon for resource servers to require different
authentication strengths or recentness according to the
characteristics of a request. This document introduces a mechanism
that resource servers can use to signal to a client that the
authentication event associated with the access token of the current
request does not meet its authentication requirements and, further,
how to meet them. This document also codifies a mechanism for a
client to request that an authorization server achieve a specific
authentication strength or recentness when processing an
authorization request.

Status of This Memo

This is an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9470.

Copyright Notice

Copyright (c) 2023 IETF Trust and the persons identified as the
document authors. All rights reserved.

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
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Trust Legal Provisions and are provided without warranty as described
in the Revised BSD License.

Table of Contents

1. Introduction
1.1. Conventions and Terminology
2. Protocol Overview
3. Authentication Requirements Challenge
4. Authorization Request
5. Authorization Response
6. Authentication Information Conveyed via Access Token
6.1. JWT Access Tokens
6.2. OAuth 2.0 Token Introspection
7. Authorization Server Metadata
8. Deployment Considerations
9. Security Considerations
10. IANA Considerations
10.1. OAuth Extensions Error Registration
10.2. OAuth Token Introspection Response Registration
11. References
11.1. Normative References
11.2. Informative References
Acknowledgements
Authors' Addresses

1. Introduction

In simple API authorization scenarios, an authorization server will
determine what authentication technique to use to handle a given
request on the basis of aspects such as the scopes requested, the
resource, the identity of the client, and other characteristics known
at provisioning time. Although that approach is viable in many
situations, it falls short in several important circumstances.
Consider, for instance, an eCommerce API requiring different
authentication strengths depending on whether the item being
purchased exceeds a certain threshold, dynamically estimated by the
API itself using a logic that is opaque to the authorization server.
An API might also determine that a more recent user authentication is
required based on its own risk evaluation of the API request.

This document extends the collection of error codes defined by
[RFC6750] with a new value, insufficient_user_authentication, which
can be used by resource servers to signal to the client that the
authentication event associated with the access token presented with
the request does not meet the authentication requirements of the
resource server. This document also introduces acr_values and
max_age parameters for the Bearer authentication scheme challenge
defined by [RFC6750]. The resource server can use these parameters
to explicitly communicate to the client the required authentication
strength or recentness.

The client can use that information to reach back to the
authorization server with an authorization request that specifies the
authentication requirements indicated by the protected resource.
This is accomplished by including the acr_values or max_age
authorization request parameters as defined in [OIDC].

Those extensions will make it possible to implement interoperable
step up authentication with minimal work from resource servers,
clients, and authorization servers.

1.1. Conventions and Terminology

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.

This specification uses the terms "access token", "authorization
server", "authorization endpoint", "authorization request", "client",
"protected resource", and "resource server" defined by "The OAuth 2.0
Authorization Framework" [RFC6749].

2. Protocol Overview

The following is an end-to-end sequence of a typical step up
authentication scenario implemented according to this specification.
The scenario assumes that, before the sequence described below takes
place, the client already obtained an access token for the protected
resource.

+----------+ +--------------+
| | | |
| |-----------(1) request ------------------>| |
| | | |
| |<---------(2) challenge ------------------| Resource |
| | | Server |
| Client | | |
| |-----------(5) request ------------------>| |
| | | |
| |<-----(6) protected resource -------------| |
| | +--------------+
| |
| |
| | +-------+ +---------------+
| |->| | | |
| | | |--(3) authorization request-->| |
| | | User | | |
| | | Agent |<-----------[...]------------>| Authorization |
| | | | | Server |
| |<-| | | |
| | +-------+ | |
| | | |
| |<-------- (4) access token --------------| |
| | | |
+----------+ +---------------+

Figure 1: Abstract Protocol Flow

1. The client requests a protected resource, presenting an access
token.

2. The resource server determines that the circumstances in which
the presented access token was obtained offer insufficient
authentication strength and/or recentness; hence, it denies the
request and returns a challenge describing (using a combination
of acr_values and max_age) what authentication requirements must
be met for the resource server to authorize a request.

3. The client directs the user agent to the authorization server
with an authorization request that includes the acr_values and/or
max_age indicated by the resource server in the previous step.

4. Whatever sequence required by the grant of choice plays out; this
will include the necessary steps to authenticate the user in
accordance with the acr_values and/or max_age values of the
authorization request. Then, the authorization server returns a
new access token to the client. The new access token contains or
references information about the authentication event.

5. The client repeats the request from step 1, presenting the newly
obtained access token.

6. The resource server finds that the user authentication performed
during the acquisition of the new access token complies with its
requirements and returns the representation of the requested
protected resource.

The validation operations mentioned in steps 2 and 6 imply that the
resource server has a way of evaluating the authentication that
occurred during the process by which the access token was obtained.
In the context of this document, the assessment by the resource
server of the specific authentication method used to obtain a token
for the requested resource is called an "authentication level". This
document will describe how the resource server can perform this
assessment of an authentication level when the access token is a JSON
Web Token (JWT) [RFC9068] or is validated via introspection
[RFC7662]. Other methods of determining the authentication level by
which the access token was obtained are possible, per agreement by
the authorization server and the protected resource, but they are
beyond the scope of this specification. Given an authentication
level of a token, the resource server determines whether it meets the
security criteria for the requested resource.

The terms "authentication level" and "step up" are metaphors in this
specification. These metaphors do not suggest that there is an
absolute hierarchy of authentication methods expressed in
interoperable fashion. The notion of a level emerges from the fact
that the resource server may only want to accept certain
authentication methods. When presented with a token derived from a
particular authentication method (i.e., a given authentication level)
that it does not want to accept (i.e., below the threshold or level
it will accept), the resource server seeks to step up (i.e.,
renegotiate) from the current authentication level to one that it may
accept. The "step up" metaphor is intended to convey a shift from
the original authentication level to one that is acceptable to the
resource server.

Although the case in which the new access token supersedes old tokens
by virtue of a higher authentication level is common, in line with
the connotation of the term "step up authentication", it is important
to keep in mind that this might not necessarily hold true in the
general case. For example, for a particular request, a resource
server might require a higher authentication level and a shorter
validity, resulting in a token suitable for one-off calls but leading
to frequent prompts: hence, offering a suboptimal user experience if
the token is reused for routine operations. In such a scenario, the
client would be better served by keeping both the old tokens, which
are associated with a lower authentication level, and the new one:
selecting the appropriate token for each API call. This is not a new
requirement for clients, as incremental consent and least-privilege
principles will require similar heuristics for managing access tokens
associated with different scopes and permission levels. This
document does not recommend any specific token-caching strategy: that
choice will be dependent on the characteristics of every particular
scenario and remains application-dependent as in the core OAuth
cases. Also recall that OAuth 2.0 [RFC6749] assumes access tokens
are treated as opaque by clients. The token format might be
unreadable to the client or might change at any time to become
unreadable. So, during the course of any token-caching strategy, a
client must not attempt to inspect the content of the access token to
determine the associated authentication information or other details
(see Section 6 of [RFC9068] for a more detailed discussion).

3. Authentication Requirements Challenge

This specification introduces a new error code value for the
challenge of the Bearer authentication scheme's error parameter (from
[RFC6750]) and other OAuth authentication schemes, such as those seen
in [RFC9449], which use the same error parameter:

insufficient_user_authentication: The authentication event
associated with the access token presented with the request does
not meet the authentication requirements of the protected
resource.

Note: the logic through which the resource server determines that the
current request does not meet the authentication requirements of the
protected resource, and associated functionality (such as expressing,
deploying and publishing such requirements), is out of scope for this
document.

Furthermore, this specification defines the following WWW-
Authenticate auth-param values for those OAuth authentication schemes
to convey the authentication requirements back to the client.

acr_values: A space-separated string listing the authentication
context class reference values in order of preference. The
protected resource requires one of these values for the
authentication event associated with the access token. As defined
in Section 1.2 of [OIDC], the authentication context conveys
information about how authentication takes place (e.g., what
authentication method(s) or assurance level to meet).

max_age: This value indicates the allowable elapsed time in seconds
since the last active authentication event associated with the
access token. An active authentication event entails a user
interacting with the authorization server in response to an
authentication prompt. Note that, while the auth-param value can
be conveyed as a token or quoted-string (see Section 11.2 of
[RFC9110]), it has to represent a non-negative integer.

Figure 2 is an example of a Bearer authentication scheme challenge
with the WWW-Authenticate header using:

* the insufficient_user_authentication error code value to inform
the client that the access token presented is not sufficient to
gain access to the protected resource, and

* the acr_values parameter to let the client know that the expected
authentication level corresponds to the authentication context
class reference identified by myACR.

Note that while this specification only defines usage of the above
auth-params with the insufficient_user_authentication error code, it
does not preclude future specifications or profiles from defining
their usage with other error codes.

HTTP/1.1 401 Unauthorized
WWW-Authenticate: Bearer error="insufficient_user_authentication",
error_description="A different authentication level is required",
acr_values="myACR"

Figure 2: Authentication Requirements Challenge Indicating acr_values

The example in Figure 3 shows a challenge informing the client that
the last active authentication event associated with the presented
access token is too old and a more recent authentication is needed.

HTTP/1.1 401 Unauthorized
WWW-Authenticate: Bearer error="insufficient_user_authentication",
error_description="More recent authentication is required",
max_age="5"

Figure 3: Authentication Requirements Challenge Indicating max_age

The auth-params max_age and acr_values MAY both occur in the same
challenge if the resource server needs to express requirements about
both recency and authentication level. If the resource server
determines that the request is also lacking the scopes required by
the requested resource, it MAY include the scope attribute with the
value necessary to access the protected resource, as described in
Section 3.1 of [RFC6750].

4. Authorization Request

A client receiving a challenge from the resource server carrying the
insufficient_user_authentication error code SHOULD parse the WWW-
Authenticate header for acr_values and max_age and use them, if
present, in constructing an authorization request. This request is
then conveyed to the authorization server's authorization endpoint
via the user agent in order to obtain a new access token complying
with the corresponding requirements. The acr_values and max_age
authorization request parameters are both OPTIONAL parameters defined
in Section 3.1.2.1. of [OIDC]. This document does not introduce any
changes in the authorization server behavior defined in [OIDC] for
processing those parameters; hence, any authorization server
implementing OpenID Connect will be able to participate in the flow
described here with little or no changes. See Section 5 for more
details.

The example authorization request URI below, which might be used
after receiving the challenge in Figure 2, indicates to the
authorization server that the client would like the authentication to
occur according to the authentication context class reference
identified by myACR.

https://as.example.net/authorize?client_id=s6BhdRkqt3
&response_type=code&scope=purchase&acr_values=myACR

Figure 4: Authorization Request Indicating acr_values

After the challenge in Figure 3, a client might direct the user agent
to the following example authorization request URI where the max_age
parameter indicates to the authorization server that the user-
authentication event needs to have occurred no more than five seconds
prior.

https://as.example.net/authorize?client_id=s6BhdRkqt3
&response_type=code&scope=purchase&max_age=5

Figure 5: Authorization Request Indicating max_age

5. Authorization Response

Section 5.5.1.1 of [OIDC] establishes that an authorization server
receiving a request containing the acr_values parameter MAY attempt
to authenticate the user in a manner that satisfies the requested
authentication context class reference and include the corresponding
value in the acr claim in the resulting ID Token. The same section
also establishes that, in case the desired authentication level
cannot be met, the authorization server SHOULD include a value
reflecting the authentication level of the current session (if any)
in the acr claim. Furthermore, Section 3.1.2.1 [OIDC] states that if
a request includes the max_age parameter, the authorization server
MUST include the auth_time claim in the issued ID Token. An
authorization server complying with this specification will react to
the presence of the acr_values and max_age parameters by including
acr and auth_time in the access token (see Section 6 for details).
Although [OIDC] leaves the authorization server free to decide how to
handle the inclusion of acr in the ID Token when requested via
acr_values, when it comes to access tokens in this specification, the
authorization server SHOULD consider the requested acr value as
necessary for successfully fulfilling the request. That is, the
requested acr value is included in the access token if the
authentication operation successfully met its requirements;
otherwise, the authorization request fails and returns an
unmet_authentication_requirements error as defined in [OIDCUAR]. The
recommended behavior will help prevent clients getting stuck in a
loop where the authorization server keeps returning tokens that the
resource server already identified as not meeting its requirements.

6. Authentication Information Conveyed via Access Token

To evaluate whether an access token meets the protected resource's
requirements, the resource server needs a way of accessing
information about the authentication event by which that access token
was obtained. This specification provides guidance on how to convey
that information in conjunction with two common access-token-
validation methods:

* the one described in [RFC9068], where the access token is encoded
in JWT format and verified via a set of validation rules, and

* the one described in [RFC7662], where the token is validated and
decoded by sending it to an introspection endpoint.

Authorization servers and resource servers MAY elect to use other
encoding and validation methods; however, those are out of scope for
this document.

6.1. JWT Access Tokens

When access tokens are represented as JSON Web Tokens (JWTs)
[RFC7519], the auth_time and acr claims (per Section 2.2.1 of
[RFC9068]) are used to convey the time and context of the user-
authentication event that the authentication server performed during
the course of obtaining the access token. It is useful to bear in
mind that the values of those two parameters are established at user-
authentication time and will not change in the event of access token
renewals. See the aforementioned Section 2.2.1 of [RFC9068] for
details. The following is a conceptual example showing the decoded
content of such a JWT access token.

Header:

{"typ":"at+JWT","alg":"ES256","kid":"LTacESbw"}

Claims:

{
"iss": "https://as.example.net",
"sub": "[email protected]",
"aud": "https://rs.example.com",
"exp": 1646343000,
"iat": 1646340200,
"jti" : "e1j3V_bKic8-LAEB_lccD0G",
"client_id": "s6BhdRkqt3",
"scope": "purchase",
"auth_time": 1646340198,
"acr": "myACR"
}

Figure 6: Decoded JWT Access Token

6.2. OAuth 2.0 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. The following two top-level introspection response
members are defined to convey information about the user-
authentication event that the authentication server performed during
the course of obtaining the access token.

acr: String specifying an authentication context class reference
value that identifies the authentication context class that was
satisfied by the user-authentication event performed.

auth_time: Time when the user authentication occurred. A JSON
numeric value representing the number of seconds from
1970-01-01T00:00:00Z UTC until the date/time of the authentication
event.

The following example shows an introspection response with
information about the user-authentication event by which the access
token was obtained.

HTTP/1.1 200 OK
Content-Type: application/json

{
"active": true,
"client_id": "s6BhdRkqt3",
"scope": "purchase",
"sub": "[email protected]",
"aud": "https://rs.example.com",
"iss": "https://as.example.net",
"exp": 1639528912,
"iat": 1618354090,
"auth_time": 1646340198,
"acr": "myACR"
}

Figure 7: Introspection Response

7. Authorization Server Metadata

Authorization servers can advertise their support of this
specification by including in their metadata document, as defined in
[RFC8414], the value acr_values_supported, as defined in Section 3 of
[OIDCDISC]. The presence of acr_values_supported in the
authorization server metadata document signals that the authorization
server will understand and honor the acr_values and max_age
parameters in incoming authorization requests.

8. Deployment Considerations

This specification facilitates the communication of requirements from
a resource server to a client, which, in turn, can enable a smooth
step up authentication experience. However, it is important to
realize that the user experience achievable in every specific
deployment is a function of the policies each resource server and
authorization server pair establishes. Imposing constraints on those
policies is out of scope for this specification; hence, it is
perfectly possible for resource servers and authorization servers to
impose requirements that are impossible for users to comply with or
that lead to an undesirable user-experience outcome. The
authentication prompts presented by the authorization server as a
result of the method of propagating authentication requirements
described here might require the user to perform some specific
actions such as using multiple devices, having access to devices
complying with specific security requirements, and so on. Those
extra requirements, that are more concerned with how to comply with a
particular requirement rather than indicating the identifier of the
requirement itself, are out of scope for this specification.

9. Security Considerations

This specification adds to previously defined OAuth mechanisms.
Their respective security considerations apply:

* OAuth 2.0 [RFC6749],

* JWT access tokens [RFC9068],

* Bearer WWW-Authenticate [RFC6750],

* token introspection [RFC7662], and

* authorization server metadata [RFC8414].

This document MUST NOT be used to position OAuth as an authentication
protocol. For the purposes of this specification, the way in which a
user authenticated with the authorization server to obtain an access
token is salient information, as a resource server might decide
whether to grant access on the basis of how that authentication
operation was performed. Nonetheless, this specification does not
attempt to define the mechanics by which authentication takes place,
relying on a separate authentication layer to take care of the
details. In line with other specifications of the OAuth family, this
document assumes the existence of a session without going into the
details of how it is established or maintained, what protocols are
used to implement that layer (e.g., OpenID Connect), and so forth.
Depending on the policies adopted by the resource server, the
acr_values parameter introduced in Section 3 might unintentionally
disclose information about the authenticated user, the resource
itself, the authorization server, and any other context-specific data
that an attacker might use to gain knowledge about their target. For
example, a resource server requesting an acr value corresponding to a
high level of assurance for some users but not others might identify
possible high-privilege users to target with spearhead phishing
attacks. Implementers should use care in determining what to
disclose in the challenge and in what circumstances. The logic
examining the incoming access token to determine whether or not a
challenge should be returned can be executed either before or after
the conventional token-validation logic, be it based on JWT
validation, introspection, or any other method. The resource server
MAY return a challenge without verifying the client presented a valid
token. However, this approach will leak the required properties of
an authorization token to an actor who has not proven they can obtain
a token for this resource server.

As this specification provides a mechanism for the resource server to
trigger user interaction, it's important for the authorization server
and clients to consider that a malicious resource server might abuse
that feature.

10. IANA Considerations

10.1. OAuth Extensions Error Registration

This specification registers the following error value in the "OAuth
Extensions Error Registry" [IANA.OAuth.Params] established by
[RFC6749].

Name: insufficient_user_authentication
Usage Location: resource access error response
Protocol Extension: OAuth 2.0 Step Up Authentication Challenge
Protocol
Change controller: IETF
Specification document(s): Section 3 of RFC 9470

10.2. OAuth Token Introspection Response Registration

This specification registers the following values in the "OAuth Token
Introspection Response" registry [IANA.OAuth.Params] established by
[RFC7662].

Authentication Context Class Reference:

Name: acr
Description: Authentication Context Class Reference
Change Controller: IETF
Specification Document(s): Section 6.2 of RFC 9470

Authentication Time:

Name: auth_time
Description: Time when the user authentication occurred
Change Controller: IETF
Specification Document(s): Section 6.2 of RFC 9470

11. References

11.1. 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>.

[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>.

[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>.

11.2. Informative References

[IANA.OAuth.Params]
IANA, "OAuth Parameters",
<https://www.iana.org/assignments/oauth-parameters>.

[OIDC] Sakimura, N., Bradley, J., Jones, M., de Medeiros, B., and
C. Mortimore, "OpenID Connect Core 1.0 incorporating
errata set 1", 8 November 2014,
<https://openid.net/specs/openid-connect-core-1_0.html>.

[OIDCDISC] Sakimura, N., Bradley, J., Jones, M., and E. Jay, "OpenID
Connect Discovery 1.0 incorporating errata set 1", 8
November 2014, <https://openid.net/specs/openid-connect-
discovery-1_0.html>.

[OIDCUAR] Lodderstedt, T., "OpenID Connect Core Error Code
unmet_authentication_requirements", 8 May 2019,
<https://openid.net/specs/openid-connect-unmet-
authentication-requirements-1_0.html>.

[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>.

[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>.

[RFC9068] Bertocci, V., "JSON Web Token (JWT) Profile for OAuth 2.0
Access Tokens", RFC 9068, DOI 10.17487/RFC9068, October
2021, <https://www.rfc-editor.org/info/rfc9068>.

[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>.

[RFC9449] Fett, D., Campbell, B., Bradley, J., Lodderstedt, T.,
Jones, M., and D. Waite, "OAuth 2.0 Demonstrating Proof of
Possession (DPoP)", RFC 9449, DOI 10.17487/RFC9449,
September 2023, <https://www.rfc-editor.org/info/rfc9449>.

Acknowledgements

I wanted to thank the Academy, the viewers at home, the shampoo
manufacturers, etc.

This specification was developed within the OAuth Working Group under
the chairpersonship of Rifaat Shekh-Yusef and Hannes Tschofenig with
Paul Wouters and Roman Danyliw serving as Security Area Directors.
Additionally, the following individuals contributed ideas, feedback,
corrections, and wording that helped shape this specification: Caleb
Baker, Ivan Kanakarakis, Pieter Kasselman, Aaron Parecki, Denis
Pinkas, Dima Postnikov, and Filip Skokan.

Some early discussion of the motivations and concepts that
precipitated the initial draft version of this document occurred at
the 2021 OAuth Security Workshop. The authors thank the organizers
of the workshop (Guido Schmitz, Steinar Noem, and Daniel Fett) for
hosting an event that is conducive to collaboration and community
input.

Authors' Addresses

Vittorio Bertocci
Auth0/Okta
Email: [email protected]

Brian Campbell
Ping Identity
Email: [email protected]