Internet Engineering Task Force (IETF) K. Meyer zu Selhausen
Request for Comments: 9207 Hackmanit
Category: Standards Track D. Fett
ISSN: 2070-1721 yes.com
March 2022
OAuth 2.0 Authorization Server Issuer Identification
Abstract
This document specifies a new parameter called iss. This parameter
is used to explicitly include the issuer identifier of the
authorization server in the authorization response of an OAuth
authorization flow. The iss parameter serves as an effective
countermeasure to "mix-up attacks".
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/rfc9207.
Copyright Notice
Copyright (c) 2022 IETF Trust and the persons identified as the
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Table of Contents
1. Introduction
1.1. Conventions and Terminology
2. Response Parameter iss
2.1. Example Authorization Response
2.2. Example Error Response
2.3. Providing the Issuer Identifier
2.4. Validating the Issuer Identifier
3. Authorization Server Metadata
4. Security Considerations
5. IANA Considerations
5.1. OAuth Authorization Server Metadata
5.2. OAuth Parameters Registration
6. References
6.1. Normative References
6.2. Informative References
Acknowledgements
Authors' Addresses
1. Introduction
The OAuth 2.0 Authorization Framework [RFC6749] allows clients to
interact with multiple independent authorization servers under the
control of separate entities. Some OAuth grant types utilize the
resource owner's user agent to deliver the authorization server's
response to the OAuth client. One example of this pattern is the
authorization response of the authorization code grant.
The authorization response as specified in Section 4.1.2 of [RFC6749]
does not contain any information about the identity of the
authorization server that issued the response. Therefore, clients
receiving a response from the resource owner's user agent cannot be
sure who initially issued the response and the secrets contained
therein. The lack of certainty about the origin of the response
enables a class of attacks called "mix-up attacks".
Mix-up attacks are a potential threat to all OAuth clients that
interact with multiple authorization servers. When at least one of
these authorization servers is under an attacker's control, the
attacker can launch a mix-up attack to acquire authorization codes or
access tokens issued by any one of the other authorization servers.
There are multiple ways in which an attacker can gain control over an
authorization server supported by the client; for instance, an
authorization server could become compromised, or the attacker could
register their own authorization server, for example, using dynamic
client registration [RFC7591].
OAuth clients that interact with only one authorization server are
not vulnerable to mix-up attacks. However, when such clients decide
to add support for a second authorization server in the future, they
become vulnerable and need to apply countermeasures to mix-up
attacks.
Mix-up attacks aim to steal an authorization code or access token by
tricking the client into sending the authorization code or access
token to the attacker instead of the honest authorization or resource
server. This marks a severe threat to the confidentiality and
integrity of resources whose access is managed with OAuth. A
detailed description and different variants of the mix-up attack
class can be found in Section 4.4 of "OAuth 2.0 Security Best Current
Practice" [OAUTH-SECURITY-TOPICS] as well as in the original research
first highlighting this attack class, "On the security of modern
Single Sign-On Protocols: Second-Order Vulnerabilities in OpenID
Connect" [arXiv.1508.04324] and "A Comprehensive Formal Security
Analysis of OAuth 2.0" [arXiv.1601.01229].
This document defines a new parameter in the authorization response
called iss. The iss parameter allows the authorization server to
include its identity in the authorization response explicitly. The
client can compare the value of the iss parameter to the issuer
identifier of the authorization server (e.g., retrieved from its
metadata) it believes it is interacting with. The iss parameter
gives the client certainty about the authorization server's identity
and enables it to send credentials such as authorization codes and
access tokens only to the intended recipients.
The effectiveness of the iss parameter against mix-up attacks was
analyzed and formally proven in "A Comprehensive Formal Security
Analysis of OAuth 2.0" [arXiv.1601.01229].
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
code", "authorization code grant", "authorization server", "resource
server", "authorization response", "grant type", and "client" defined
by the OAuth 2.0 Authorization Framework [RFC6749]. The term "issuer
identifier" is defined by OAuth 2.0 Authorization Server Metadata
[RFC8414].
2. Response Parameter iss
In authorization responses to the client, including error responses,
an authorization server supporting this specification MUST indicate
its identity by including the iss parameter in the response.
The iss parameter value is the issuer identifier of the authorization
server that created the authorization response, as defined in
[RFC8414]. Its value MUST be a URL that uses the "https" scheme
without any query or fragment components.
2.1. Example Authorization Response
The following example shows an authorization response from the
authorization server whose issuer identifier is
https://honest.as.example (extra line breaks and indentation are for
display purposes only):
HTTP/1.1 302 Found
Location: https://client.example/cb?
code=x1848ZT64p4IirMPT0R-X3141MFPTuBX-VFL_cvaplMH58
&state=ZWVlNDBlYzA1NjdkMDNhYjg3ZjUxZjAyNGQzMTM2NzI
&iss=https%3A%2F%2Fhonest.as.example
2.2. Example Error Response
The following example shows an error response from the same
authorization server (extra line breaks and indentation are for
display purposes only):
HTTP/1.1 302 Found
Location: https://client.example/cb?
error=access_denied
&state=N2JjNGJhY2JiZjRhYzA3MGJkMzNmMDE5OWJhZmJhZjA
&iss=https%3A%2F%2Fhonest.as.example
2.3. Providing the Issuer Identifier
Authorization servers supporting this specification MUST provide
their issuer identifier to enable clients to validate the iss
parameter effectively.
For authorization servers publishing metadata according to [RFC8414],
the following rules apply:
* The issuer identifier included in the server's metadata value
issuer MUST be identical to the iss parameter's value.
* The server MUST indicate its support for the iss parameter by
setting the metadata parameter
authorization_response_iss_parameter_supported, defined in
Section 3, to true.
Authorization servers MAY additionally provide the issuer identifier
to clients by any other mechanism, which is outside of the scope of
this specification.
2.4. Validating the Issuer Identifier
Clients that support this specification MUST extract the value of the
iss parameter from authorization responses they receive if the
parameter is present. Clients MUST then decode the value from its
"application/x-www-form-urlencoded" form according to Appendix B of
[RFC6749] and compare the result to the issuer identifier of the
authorization server where the authorization request was sent to.
This comparison MUST use simple string comparison as defined in
Section 6.2.1 of [RFC3986]. If the value does not match the expected
issuer identifier, clients MUST reject the authorization response and
MUST NOT proceed with the authorization grant. For error responses,
clients MUST NOT assume that the error originates from the intended
authorization server.
More precisely, clients that interact with authorization servers
supporting OAuth metadata [RFC8414] MUST compare the iss parameter
value to the issuer value in the server's metadata document. If
OAuth metadata is not used, clients MUST use deployment-specific ways
(for example, a static configuration) to decide if the returned iss
value is the expected value in the current flow (see also Section 4).
If clients interact with both authorization servers supporting this
specification and authorization servers not supporting this
specification, clients MUST retain state about whether each
authorization server supports the iss parameter. Clients MUST reject
authorization responses without the iss parameter from authorization
servers that do support the parameter according to the client's
configuration. Clients SHOULD discard authorization responses with
the iss parameter from authorization servers that do not indicate
their support for the parameter. However, there might be legitimate
authorization servers that provide the iss parameter without
indicating their support in their metadata. Local policy or
configuration can determine whether to accept such responses, and
specific guidance is out of scope for this specification.
In general, clients that support this specification MAY accept
authorization responses that do not contain the iss parameter or
reject them and exclusively support authorization servers that
provide the iss parameter in the authorization response. Local
policy or configuration can determine when to accept such responses,
and specific guidance is out of scope for this specification.
In OpenID Connect [OIDC.Core] flows where an ID Token is returned
from the authorization endpoint, the value in the iss parameter MUST
always be identical to the iss claim in the ID Token.
Section 4.1.2 of [RFC6749] already mandates that clients that do not
support this specification MUST ignore the unrecognized iss
parameter.
| Note: The "JWT Secured Authorization Response Mode for OAuth
| 2.0 (JARM)" [JARM] defines a mechanism that conveys all
| authorization response parameters in a JSON Web Token (JWT).
| This JWT contains an iss claim that provides the same
| protection if it is validated as described in Section 2.4.
| Therefore, an additional iss parameter outside the JWT is not
| necessary when JARM is used.
3. Authorization Server Metadata
The following parameter for the authorization server metadata
[RFC8414] is introduced to signal the authorization server's support
for this specification:
authorization_response_iss_parameter_supported: Boolean parameter
indicating whether the authorization server provides the iss
parameter in the authorization response as defined in Section 2.
If omitted, the default value is false.
4. Security Considerations
Clients MUST validate the iss parameter precisely as described in
Section 2.4 and MUST NOT allow multiple authorization servers to use
the same issuer identifier. In particular, when authorization server
details can be manually configured in the client, the client MUST
ensure that the accepted iss values are unique for each authorization
server.
The iss parameter enables a client to decide if an authorization
server "expects" to be used in an OAuth flow together with a certain
token endpoint and potentially other endpoints, like the userinfo
endpoint [OIDC.Core]. When OAuth metadata is used, the iss parameter
identifies the issuer and therefore the respective OAuth metadata
document that points to the other endpoints. When OAuth metadata is
not used, the client can use, for example, a statically configured
expected iss value for each configured authorization server.
The issuer identifier contained in the authorization response is not
cryptographically protected against tampering. In general,
mechanisms such as JWTs (as specified in [JARM]) could be used to
protect the integrity of the authorization response. However, in
mix-up attacks, the client generally receives the authorization
response from an uncompromised authorization server. If an attacker
can tamper with this authorization response before it is received by
the client, the attacker would also have direct access to the
authorization code. The attacker does not need to execute a mix-up
attack to steal the authorization code. Therefore, integrity
protection for the authorization response is not necessary to defend
against mix-up attacks.
There are also alternative countermeasures to mix-up attacks. When
an authorization response already includes an authorization server's
issuer identifier by other means and this identifier is checked as
laid out in Section 2.4, the use and verification of the iss
parameter is not necessary and MAY be omitted. For example, this is
the case when OpenID Connect response types that return an ID Token
from the authorization endpoint (e.g., response_type=code id_token)
or [JARM] are used. However, if a client receives an authorization
response that contains multiple issuer identifiers, the client MUST
reject the response if these issuer identifiers do not match. The
details of alternative countermeasures are outside of the scope of
this specification.
Mix-up attacks are only relevant to clients that interact with
multiple authorization servers. However, clients interacting with
only one authorization server might add support for a second
authorization server in the future. By supporting multiple
authorization servers, they become vulnerable to mix-up attacks and
need to apply countermeasures.
5. IANA Considerations
5.1. OAuth Authorization Server Metadata
IANA has registered the following value in the "OAuth Authorization
Server Metadata" registry of [IANA.OAuth.Parameters] established by
[RFC8414].
Metadata Name: authorization_response_iss_parameter_supported
Metadata Description: Boolean value indicating whether the
authorization server provides the iss parameter in the
authorization response.
Change Controller: IETF
Specification Document(s): Section 3 of RFC 9207
5.2. OAuth Parameters Registration
IANA has updated the iss entry to appear as follows in the "OAuth
Parameters" registry of [IANA.OAuth.Parameters] established by
[RFC6749].
Parameter name: iss
Parameter usage location: authorization request, authorization
response
Change Controller: IETF
Specification Document(s): Section 2 of RFC 9207, [RFC9101], and
Section 4.1.1 of [RFC7519].
6. References
6.1. 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>.
[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>.
[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>.
[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>.
[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>.
6.2. Informative References
[arXiv.1508.04324]
Mainka, C., Mladenov, V., and J. Schwenk, "On the security
of modern Single Sign-On Protocols: Second-Order
Vulnerabilities in OpenID Connect", August 2015,
<https://arxiv.org/abs/1508.04324>.
[arXiv.1601.01229]
Fett, D., Kuesters, R., and G. Schmitz, "A Comprehensive
Formal Security Analysis of OAuth 2.0",
DOI 10.1145/2976749.2978385, January 2016,
<https://arxiv.org/abs/1601.01229>.
[IANA.OAuth.Parameters]
IANA, "OAuth Parameters",
<https://www.iana.org/assignments/oauth-parameters>.
[JARM] Lodderstedt, T. and B. Campbell, "Financial-grade API: JWT
Secured Authorization Response Mode for OAuth 2.0 (JARM)",
October 2018,
<https://openid.net/specs/openid-financial-api-jarm.html>.
[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-19, 16 December 2021,
<https://datatracker.ietf.org/doc/html/draft-ietf-oauth-
security-topics-19>.
[OIDC.Core]
Sakimura, N., Bradley, J., Jones, M., de Medeiros, B., and
C. Mortimore, "OpenID Connect Core 1.0 incorporating
errata set 1", November 2014,
<https://openid.net/specs/openid-connect-core-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>.
[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>.
[RFC9101] Sakimura, N., Bradley, J., and M. Jones, "The OAuth 2.0
Authorization Framework: JWT-Secured Authorization Request
(JAR)", RFC 9101, DOI 10.17487/RFC9101, August 2021,
<https://www.rfc-editor.org/info/rfc9101>.
Acknowledgements
We would like to thank Brian Campbell, Roman Danyliw, Vladimir
Dzhuvinov, Joseph Heenan, Takahiko Kawasaki, Torsten Lodderstedt,
Christian Mainka, Vladislav Mladenov, Warren Parad, Aaron Parecki,
and Rifaat Shekh-Yusef for their valuable feedback on this document.
Authors' Addresses
Karsten Meyer zu Selhausen
Hackmanit
Email: karsten.meyerzuselhausen@hackmanit.de
Daniel Fett
yes.com
Email: mail@danielfett.de
