draft-ietf-oauth-security-topics-03.txt   draft-ietf-oauth-security-topics-04.txt 
Open Authentication Protocol T. Lodderstedt, Ed. Open Authentication Protocol T. Lodderstedt, Ed.
Internet-Draft YES Europe AG Internet-Draft YES.com AG
Intended status: Best Current Practice J. Bradley Intended status: Best Current Practice J. Bradley
Expires: March 14, 2018 Yubico Expires: May 17, 2018 Yubico
A. Labunets A. Labunets
Facebook Facebook
September 10, 2017 November 13, 2017
OAuth Security Topics OAuth Security Topics
draft-ietf-oauth-security-topics-03 draft-ietf-oauth-security-topics-04
Abstract Abstract
This draft gives a comprehensive overview on open OAuth security This draft gives a comprehensive overview on open OAuth security
topics. It is intended to serve as a working document for the OAuth topics. It is intended to serve as a working document for the OAuth
working group to systematically capture and discuss these security working group to systematically capture and discuss these security
topics and respective mitigations and eventually recommend best topics and respective mitigations and eventually recommend best
current practice and also OAuth extensions needed to cope with the current practice and also OAuth extensions needed to cope with the
respective security threats. respective security threats.
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Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
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working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
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Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on March 14, 2018. This Internet-Draft will expire on May 17, 2018.
Copyright Notice Copyright Notice
Copyright (c) 2017 IETF Trust and the persons identified as the Copyright (c) 2017 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Recommended Best Practice . . . . . . . . . . . . . . . . . . 4 2. Best Practices . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. Protecting redirect-based flows . . . . . . . . . . . . . 4 2.1. Protecting redirect-based flows . . . . . . . . . . . . . 4
2.2. TBD . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 2.2. Token Leakage Prevention . . . . . . . . . . . . . . . . 5
3. Recommended modifications and extensions to OAuth . . . . . . 5 3. Recommended Changes to OAuth . . . . . . . . . . . . . . . . 5
4. OAuth Credentials Leakage . . . . . . . . . . . . . . . . . . 5 4. Attacks and Mitigations . . . . . . . . . . . . . . . . . . . 5
4.1. Insufficient redirect URI validation . . . . . . . . . . 5 4.1. Insufficient redirect URI validation . . . . . . . . . . 5
4.1.1. Attacks on Authorization Code Grant . . . . . . . . . 6 4.1.1. Attacks on Authorization Code Grant . . . . . . . . . 6
4.1.2. Attacks on Implicit Grant . . . . . . . . . . . . . . 7 4.1.2. Attacks on Implicit Grant . . . . . . . . . . . . . . 7
4.1.3. Proposed Countermeasures . . . . . . . . . . . . . . 8 4.1.3. Proposed Countermeasures . . . . . . . . . . . . . . 8
4.2. Authorization code leakage via referrer headers . . . . . 10 4.2. Authorization code leakage via referrer headers . . . . . 9
4.2.1. Proposed Countermeasures . . . . . . . . . . . . . . 10 4.2.1. Proposed Countermeasures . . . . . . . . . . . . . . 10
4.3. Attacks in the Browser . . . . . . . . . . . . . . . . . 10 4.3. Attacks in the Browser . . . . . . . . . . . . . . . . . 10
4.3.1. Code in browser history (TBD) . . . . . . . . . . . . 11 4.3.1. Code in browser history (TBD) . . . . . . . . . . . . 10
4.3.2. Access token in browser history (TBD) . . . . . . . . 11 4.3.2. Access token in browser history (TBD) . . . . . . . . 10
4.3.3. Javascript Code stealing Access Tokens (TBD) . . . . 11 4.3.3. Javascript Code stealing Access Tokens (TBD) . . . . 11
4.4. Access Token Leakage at the Resource Server . . . . . . . 11 4.4. Mix-Up . . . . . . . . . . . . . . . . . . . . . . . . . 11
4.4.1. Access Token Phishing by Counterfeit Resource Server 11 4.5. Code Injection . . . . . . . . . . . . . . . . . . . . . 11
4.4.1.1. Metadata . . . . . . . . . . . . . . . . . . . . 12 4.5.1. Proposed Countermeasures . . . . . . . . . . . . . . 13
4.4.1.2. Sender Constrained Access Tokens . . . . . . . . 13 4.6. XSRF (TBD) . . . . . . . . . . . . . . . . . . . . . . . 15
4.4.1.3. Audience Restricted Access Tokens . . . . . . . . 15 4.7. Access Token Leakage at the Resource Server . . . . . . . 15
4.4.2. Compromised Resource Server . . . . . . . . . . . . . 16 4.7.1. Access Token Phishing by Counterfeit Resource Server 15
4.4.3. TLS Terminating Reverse Proxies . . . . . . . . . . . 17 4.7.1.1. Metadata . . . . . . . . . . . . . . . . . . . . 16
4.5. Mix-Up . . . . . . . . . . . . . . . . . . . . . . . . . 18 4.7.1.2. Sender Constrained Access Tokens . . . . . . . . 17
4.6. Refresh Token Leakage . . . . . . . . . . . . . . . . . . 18 4.7.1.3. Audience Restricted Access Tokens . . . . . . . . 19
5. OAuth Credentials Injection . . . . . . . . . . . . . . . . . 19 4.7.2. Compromised Resource Server . . . . . . . . . . . . . 20
5.1. Code Injection . . . . . . . . . . . . . . . . . . . . . 19 4.8. Refresh Token Leakage (TBD) . . . . . . . . . . . . . . . 21
5.1.1. Proposed Countermeasures . . . . . . . . . . . . . . 21 4.9. Open Redirection (TBD) . . . . . . . . . . . . . . . . . 21
5.2. Access Token Injection (TBD) . . . . . . . . . . . . . . 22 4.10. TLS Terminating Reverse Proxies . . . . . . . . . . . . . 22
5.3. XSRF (TBD) . . . . . . . . . . . . . . . . . . . . . . . 23 4.11. Other Topics . . . . . . . . . . . . . . . . . . . . . . 22
6. Other Attacks . . . . . . . . . . . . . . . . . . . . . . . . 23 5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 23
7. Other Topics . . . . . . . . . . . . . . . . . . . . . . . . 23 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 23
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 24 7. Security Considerations . . . . . . . . . . . . . . . . . . . 23
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 24 8. References . . . . . . . . . . . . . . . . . . . . . . . . . 23
10. Security Considerations . . . . . . . . . . . . . . . . . . . 24 8.1. Normative References . . . . . . . . . . . . . . . . . . 23
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 24 8.2. Informative References . . . . . . . . . . . . . . . . . 24
11.1. Normative References . . . . . . . . . . . . . . . . . . 24 Appendix A. Document History . . . . . . . . . . . . . . . . . . 25
11.2. Informative References . . . . . . . . . . . . . . . . . 25 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 26
Appendix A. Document History . . . . . . . . . . . . . . . . . . 26
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 27
1. Introduction 1. Introduction
It's been a while since OAuth has been published in RFC 6749 It's been a while since OAuth has been published in RFC 6749
[RFC6749] and RFC 6750 [RFC6750]. Since publication, OAuth 2.0 has [RFC6749] and RFC 6750 [RFC6750]. Since publication, OAuth 2.0 has
gotten massive traction in the market and became the standard for API gotten massive traction in the market and became the standard for API
protection and, as foundation of OpenID Connect, identity providing. protection and, as foundation of OpenID Connect, identity providing.
While OAuth was used in a variety of scenarios and different kinds of While OAuth was used in a variety of scenarios and different kinds of
deployments, the following challenges could be observed: deployments, the following challenges could be observed:
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The remainder of the document is organized as follows: The next The remainder of the document is organized as follows: The next
section gives a summary of the set of security mechanisms and section gives a summary of the set of security mechanisms and
practices, the working group shall consider to recommend to OAuth practices, the working group shall consider to recommend to OAuth
implementers. This is followed by a section proposing modifications implementers. This is followed by a section proposing modifications
to OAuth intended to either simplify its usage and to strengthen its to OAuth intended to either simplify its usage and to strengthen its
security. security.
The remainder of the draft gives a detailed analyses of the The remainder of the draft gives a detailed analyses of the
weaknesses and implementation issues, which can be found in the wild weaknesses and implementation issues, which can be found in the wild
today along with a discussion of potential counter measures. First, today, along with a discussion of potential counter measures.
various scenarios how OAuth credentials (namely access tokens and
authorization codes) may be disclosed to attackers and proposes
countermeasures are discussed. Afterwards, the document discusses
attacks possible with captured credential and how they may be
prevented. The last sections discuss additional threats.
2. Recommended Best Practice 2. Best Practices
This section describes the set of security mechanisms the authors This section describes the set of security mechanisms the authors
believe should be taken into consideration by the OAuth working group believe should be taken into consideration by the OAuth working group
to be recommended to OAuth implementers. to be recommended to OAuth implementers.
2.1. Protecting redirect-based flows 2.1. Protecting redirect-based flows
Authorization servers shall utilize exact matching of client redirect Authorization servers shall utilize exact matching of client redirect
URIs against pre-registered URIs. This measure contributes to the URIs against pre-registered URIs. This measure contributes to the
prevention of leakage of authorization codes and access tokens prevention of leakage of authorization codes and access tokens
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Clients shall use PKCE [RFC7636] in order to (with the help of the Clients shall use PKCE [RFC7636] in order to (with the help of the
authorization server) detect attempts to inject authorization codes authorization server) detect attempts to inject authorization codes
into the authorization response. The PKCE challenges must be into the authorization response. The PKCE challenges must be
transaction-specific and securely bound to the user agent, in which transaction-specific and securely bound to the user agent, in which
the transaction was started. the transaction was started.
Note: although PKCE so far was recommended as mechanism to protect Note: although PKCE so far was recommended as mechanism to protect
native apps, this advice applies to all kinds of OAuth clients, native apps, this advice applies to all kinds of OAuth clients,
including web applications. including web applications.
2.2. TBD 2.2. Token Leakage Prevention
Add further topics:
o Access Token Leakage at resource servers Authorization servers shall use TLS-based methods for sender
constraint access tokens as described in section Section 4.7.1.2,
such as token binding [I-D.ietf-oauth-token-binding] or Mutual TLS
for OAuth 2.0 [I-D.ietf-oauth-mtls]. It is also recommend to use
end-to-end TLS whenever possible.
3. Recommended modifications and extensions to OAuth 3. Recommended Changes to OAuth
This section describes the set of modifications and extensions the This section describes the set of modifications and extensions the
authors believe should be taken into consideration by the OAuth authors believe should be taken into consideration by the OAuth
working group change and extend OAuth in order to strengthen its working group change and extend OAuth in order to strengthen its
security and make it simpler to implement. It also recommends some security and make it simpler to implement. It also recommends some
changes to the OAuth set of specs. changes to the OAuth set of specs.
Remove requirement to check actual redirect URI at token endpoint - Remove requirement to check actual redirect URI at token endpoint -
seems to be complicated to implement properly and could be seems to be complicated to implement properly and could be
compromised. The protection goal is achieved even more effective by compromised. The protection goal is achieved even more effective by
utilizing PKCE as recommended in Section 2.1. utilizing PKCE as recommended in Section 2.1.
4. OAuth Credentials Leakage 4. Attacks and Mitigations
This section describes a couple of different ways how OAuth
credentials, namely authorization codes and access tokens, can be
exposed to attackers.
4.1. Insufficient redirect URI validation 4.1. Insufficient redirect URI validation
Some authorization servers allow clients to register redirect URI Some authorization servers allow clients to register redirect URI
patterns instead of complete redirect URIs. In those cases, the patterns instead of complete redirect URIs. In those cases, the
authorization server, at runtime, matches the actual redirect URI authorization server, at runtime, matches the actual redirect URI
parameter value at the authorization endpoint against this pattern. parameter value at the authorization endpoint against this pattern.
This approach allows clients to encode transaction state into This approach allows clients to encode transaction state into
additional redirect URI parameters or to register just a single additional redirect URI parameters or to register just a single
pattern for multiple redirect URIs. As a downside, it turned out to pattern for multiple redirect URIs. As a downside, it turned out to
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(2) If the user does not recognize the attack, the code is issued (2) If the user does not recognize the attack, the code is issued
and directly sent to the attacker's client. and directly sent to the attacker's client.
(3) Since the attacker impersonated a public client, it can directly (3) Since the attacker impersonated a public client, it can directly
exchange the code for tokens at the respective token endpoint. exchange the code for tokens at the respective token endpoint.
Note: This attack will not directly work for confidential clients, Note: This attack will not directly work for confidential clients,
since the code exchange requires authentication with the legitimate since the code exchange requires authentication with the legitimate
client's secret. The attacker will need to utilize the legitimate client's secret. The attacker will need to utilize the legitimate
client to redeem the code (e.g. by mounting a code injection attack). client to redeem the code (e.g. by mounting a code injection attack).
This and other kinds of injections are covered in This kind of injections is covered in Section Code Injection.
Section OAuth Credentials Injection.
4.1.2. Attacks on Implicit Grant 4.1.2. Attacks on Implicit Grant
The attack described above works for the implicit grant as well. If The attack described above works for the implicit grant as well. If
the attacker is able to send the authorization response to a URI the attacker is able to send the authorization response to a URI
under his control, he will directly get access to the fragment under his control, he will directly get access to the fragment
carrying the access token. carrying the access token.
Additionally, implicit clients can be subject to a further kind of Additionally, implicit clients can be subject to a further kind of
attacks. It utilizes the fact that user agents re-attach fragments attacks. It utilizes the fact that user agents re-attach fragments
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When browser navigates to client.com/ When browser navigates to client.com/
redirection_endpoint#access_token=abcef as a result of a redirect redirection_endpoint#access_token=abcef as a result of a redirect
from a provider's authorization endpoint. from a provider's authorization endpoint.
Proposal: replace implicit flow with postmessage communication Proposal: replace implicit flow with postmessage communication
4.3.3. Javascript Code stealing Access Tokens (TBD) 4.3.3. Javascript Code stealing Access Tokens (TBD)
sandboxing using service workers sandboxing using service workers
4.4. Access Token Leakage at the Resource Server 4.4. Mix-Up
4.4.1. Access Token Phishing by Counterfeit Resource Server Mix-up is another kind of attack on more dynamic OAuth scenarios (or
at least scenarios where a OAuth client interacts with multiple
authorization servers). The goal of the attack is to obtain an
authorization code or an access token by tricking the client into
sending those credentials to the attacker (which acts as MITM between
client and authorization server)
A detailed description of the attack and potential countermeasures is
given in cf. https://tools.ietf.org/html/draft-ietf-oauth-mix-up-
mitigation-01.
Potential mitigations:
o AS returns client_id and its iss in the response. Client compares
this data to AS it believed it sent the user agent to.
o ID token carries client id and issuer (requires OpenID Connect)
o Clients use AS-specific redirect URIs, for every authorization
request store intended AS and compare intention with actual
redirect URI where the response was received (no change to OAuth
required)
4.5. Code Injection
In such an attack, the adversary attempts to inject a stolen
authorization code into a legitimate client on a device under his
control. In the simplest case, the attacker would want to use the
code in his own client. But there are situations where this might
not be possible or intended. Example are:
o The code is bound to a particular confidential client and the
attacker is unable to obtain the required client credentials to
redeem the code himself and/or
o The attacker wants to access certain functions in this particular
client. As an example, the attacker potentially wants to
impersonate his victim in a certain app.
o Another example could be that access to the authorization and
resource servers is some how limited to networks, the attackers is
unable to access directly.
How does an attack look like?
(1) The attacker obtains an authorization code by executing any of
the attacks described above.
(2) It performs an OAuth authorization process with the legitimate
client on his device.
(3) The attacker injects the stolen authorization code in the
response of the authorization server to the legitimate client.
(4) The client sends the code to the authorization server's token
endpoint, along with client id, client secret and actual
redirect_uri.
(5) The authorization server checks the client secret, whether the
code was issued to the particular client and whether the actual
redirect URI matches the redirect_uri parameter.
(6) If all checks succeed, the authorization server issues access
and other tokens to the client.
(7) The attacker just impersonated the victim.
Obviously, the check in step (5) will fail, if the code was issued to
another client id, e.g. a client set up by the attacker.
An attempt to inject a code obtained via a malware pretending to be
the legitimate client should also be detected, if the authorization
server stored the complete redirect URI used in the authorization
request and compares it with the redirect_uri parameter.
[RFC6749], Section 4.1.3, requires the AS to ... "ensure that the
"redirect_uri" parameter is present if the "redirect_uri" parameter
was included in the initial authorization request as described in
Section 4.1.1, and if included ensure that their values are
identical." In the attack scenario described above, the legitimate
client would use the correct redirect URI it always uses for
authorization requests. But this URI would not match the tampered
redirect URI used by the attacker (otherwise, the redirect would not
land at the attackers page). So the authorization server would
detect the attack and refuse to exchange the code.
Note: this check could also detect attempt to inject a code, which
had been obtained from another instance of the same client on another
device, if certain conditions are fulfilled:
o the redirect URI itself needs to contain a nonce or another kind
of one-time use, secret data and
o the client has bound this data to this particular instance
But this approach conflicts with the idea to enforce exact redirect
URI matching at the authorization endpoint. Moreover, it has been
observed that providers very often ignore the redirect_uri check
requirement at this stage, maybe, because it doesn't seem to be
security-critical from reading the spec.
Other providers just pattern match the redirect_uri parameter against
the registered redirect URI pattern. This saves the authorization
server from storing the link between the actual redirect URI and the
respective authorization code for every transaction. But this kind
of check obviously does not fulfill the intent of the spec, since the
tampered redirect URI is not considered. So any attempt to inject a
code obtained using the client_id of a legitimate client or by
utilizing the legitimate client on another device won't be detected
in the respective deployments.
It is also assumed that the requirements defined in [RFC6749],
Section 4.1.3, increase client implementation complexity as clients
need to memorize or re-construct the correct redirect URI for the
call to the tokens endpoint.
The authors therefore propose to the working group to drop this
feature in favor of more effective and (hopefully) simpler approaches
to code injection prevention as described in the following section.
4.5.1. Proposed Countermeasures
The general proposal is to bind every particular authorization code
to a certain client on a certain device (or in a certain user agent)
in the context of a certain transaction. There are multiple
technical solutions to achieve this goal:
Nonce OpenID Connect's existing "nonce" parameter is used for this
purpose. The nonce value is one time use and created by the
client. The client is supposed to bind it to the user agent
session and sends it with the initial request to the OpenId
Provider (OP). The OP associates the nonce to the
authorization code and attests this binding in the ID token,
which is issued as part of the code exchange at the token
endpoint. If an attacker injected an authorization code in
the authorization response, the nonce value in the client
session and the nonce value in the ID token will not match
and the attack is detected. assumption: attacker cannot get
hold of the user agent state on the victims device, where he
has stolen the respective authorization code.
pro:
- existing feature, used in the wild
con:
- OAuth does not have an ID Token - would need to push that
down the stack
Code-bound State It has been discussed in the security workshop in
December to use the OAuth state value much similar in the way
as described above. In the case of the state value, the idea
is to add a further parameter state to the code exchange
request. The authorization server then compares the state
value it associated with the code and the state value in the
parameter. If those values do not match, it is considered an
attack and the request fails. Note: a variant of this
solution would be send a hash of the state (in order to
prevent bulky requests and DoS).
pro:
- use existing concept
con:
- state needs to fulfil certain requirements (one time use,
complexity)
- new parameter means normative spec change
PKCE Basically, the PKCE challenge/verifier could be used in the
same way as Nonce or State. In contrast to its original
intention, the verifier check would fail although the client
uses its correct verifier but the code is associated with a
challenge, which does not match.
pro:
- existing and deployed OAuth feature
con:
- currently used and recommended for native apps, not web
apps
Token Binding Code must be bind to UA-AS and UA-Client legs -
requires further data (extension to response) to manifest
binding id for particular code.
Note: token binding could be used in conjunction with PKCE as
an option (https://tools.ietf.org/html/draft-ietf-oauth-
token-binding-02#section-4).
pro:
- highly secure
con:
- highly sophisticated, requires browser support, will it
work for native apps?
per instance client id/secret ...
Note on pre-warmed secrets: An attacker can circumvent the
countermeasures described above if he is able to create or capture
the respective secret or code_challenge on a device under his
control, which is then used in the victim's authorization request.
Exact redirect URI matching of authorization requests can prevent the
attacker from using the pre-warmed secret in the faked authorization
transaction on the victim's device.
Unfortunately it does not work for all kinds of OAuth clients. It is
effective for web and JS apps and for native apps with claimed URLs.
What about other native apps? Treat nonce or PKCE challenge as
replay detection tokens (needs to ensure cluster-wide one-time use)?
4.6. XSRF (TBD)
injection of code or access token on a victim's device (e.g. to cause
client to access resources under the attacker's control)
mitigation: XSRF tokens (one time use) w/ user agent binding (cf.
https://www.owasp.org/index.php/
CrossSite_Request_Forgery_(CSRF)_Prevention_Cheat_Sheet)
4.7. Access Token Leakage at the Resource Server
4.7.1. Access Token Phishing by Counterfeit Resource Server
An attacker may setup his own resource server and trick a client into An attacker may setup his own resource server and trick a client into
sending access tokens to it, which are valid for other resource sending access tokens to it, which are valid for other resource
servers. If the client sends a valid access token to this servers. If the client sends a valid access token to this
counterfeit resource server, the attacker in turn may use that token counterfeit resource server, the attacker in turn may use that token
to access other services on behalf of the resource owner. to access other services on behalf of the resource owner.
This attack assumes the client is not bound to a certain resource This attack assumes the client is not bound to a certain resource
server (and the respective URL) at development time, but client server (and the respective URL) at development time, but client
instances are configured with an resource server's URL at runtime. instances are configured with an resource server's URL at runtime.
This kind of late binding is typical in situations, where the client This kind of late binding is typical in situations, where the client
uses a standard API, e.g. for e-Mail, calendar, health, or banking uses a standard API, e.g. for e-Mail, calendar, health, or banking
and is configured by an user or administrator for the standard-based and is configured by an user or administrator for the standard-based
service, this particular user or company uses. service, this particular user or company uses.
There are several potential mitigation strategies, which will be There are several potential mitigation strategies, which will be
discussed in the following sections. discussed in the following sections.
4.4.1.1. Metadata 4.7.1.1. Metadata
An authorization server could provide the client with additional An authorization server could provide the client with additional
information about the location where it is safe to use its access information about the location where it is safe to use its access
tokens. tokens.
In the simplest form, this would require the AS to publish a list of In the simplest form, this would require the AS to publish a list of
its known resource servers, illustrated in the following example its known resource servers, illustrated in the following example
using a metadata parameter "resource_servers": using a metadata parameter "resource_servers":
HTTP/1.1 200 OK HTTP/1.1 200 OK
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implement security controls, like state checks. So relying on implement security controls, like state checks. So relying on
clients to detect and properly handle access token phishing is likely clients to detect and properly handle access token phishing is likely
to fail as well. Moreover given the ratio of clients to to fail as well. Moreover given the ratio of clients to
authorization and resource servers, it is considered the more viable authorization and resource servers, it is considered the more viable
approach to move as much as possible security-related logic to those approach to move as much as possible security-related logic to those
entities. Clearly, the client has to contribute to the overall entities. Clearly, the client has to contribute to the overall
security. But there are alternative counter measures, as described security. But there are alternative counter measures, as described
in the next sections, which provide a better balance between the in the next sections, which provide a better balance between the
involved parties. involved parties.
4.4.1.2. Sender Constrained Access Tokens 4.7.1.2. Sender Constrained Access Tokens
As the name suggests, sender constraint access token scope the As the name suggests, sender constraint access token scope the
applicability of an access token to a certain sender. This sender is applicability of an access token to a certain sender. This sender is
obliged to demonstrate knowledge of a certain secret as prerequisite obliged to demonstrate knowledge of a certain secret as prerequisite
for the acceptance of that token at a resource server. for the acceptance of that token at a resource server.
A typical flow looks like this: A typical flow looks like this:
1. The authorization server associates data with the access token, 1. The authorization server associates data with the access token,
which bind this particular token to a certain client. The which bind this particular token to a certain client. The
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As one advantage, application-level signing allows for end-to-end As one advantage, application-level signing allows for end-to-end
protection including non-repudiation even if the TLS connection is protection including non-repudiation even if the TLS connection is
terminated between client and resource server. But deployment terminated between client and resource server. But deployment
experiences have revealed challenges regarding robustness (e.g. experiences have revealed challenges regarding robustness (e.g.
reproduction of the signature base string including correct URL) as reproduction of the signature base string including correct URL) as
well as state management (e.g. replay detection). well as state management (e.g. replay detection).
This document therefore recommends implementors to consider one of This document therefore recommends implementors to consider one of
TLS-based approaches wherever possible. TLS-based approaches wherever possible.
4.4.1.3. Audience Restricted Access Tokens 4.7.1.3. Audience Restricted Access Tokens
An audience restriction essentially restricts the resource server a An audience restriction essentially restricts the resource server a
particular access token can be used at. The authorization server particular access token can be used at. The authorization server
associates the access token with a certain resource server and every associates the access token with a certain resource server and every
resource server is obliged to verify for every request, whether the resource server is obliged to verify for every request, whether the
access token send with that request was meant to be used at the access token send with that request was meant to be used at the
particular resource server. If not, the resource server must refuse particular resource server. If not, the resource server must refuse
to serve the respective request. In the general case, audience to serve the respective request. In the general case, audience
restrictions limit the impact of a token leakage. In the case of a restrictions limit the impact of a token leakage. In the case of a
counterfeit resource server, it may (as described see below) also counterfeit resource server, it may (as described see below) also
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It shall be noted that audience restrictions, or generally speaking It shall be noted that audience restrictions, or generally speaking
an indication by the client to the authorization server where it an indication by the client to the authorization server where it
wants to use the access token, has additional benefits beyond the wants to use the access token, has additional benefits beyond the
scope of token leakage prevention. It allows the authorization scope of token leakage prevention. It allows the authorization
server to create different access token whose format and content is server to create different access token whose format and content is
specifically minted for the respective server. This has huge specifically minted for the respective server. This has huge
functional and privacy advantages in deployments using structured functional and privacy advantages in deployments using structured
access tokens. access tokens.
4.4.2. Compromised Resource Server 4.7.2. Compromised Resource Server
An attacker may compromise a resource server in order to get access An attacker may compromise a resource server in order to get access
to its resources and other resources of the respective deployment. to its resources and other resources of the respective deployment.
Such a compromise may range from partial access to the system, e.g. Such a compromise may range from partial access to the system, e.g.
its logfiles, to full control of the respective server. its logfiles, to full control of the respective server.
If the attacker was able to take over full control including shell If the attacker was able to take over full control including shell
access it will be able to circumvent all controls in place and access access it will be able to circumvent all controls in place and access
resources without access control. It will also get access to access resources without access control. It will also get access to access
tokens, which are sent to the compromised system and which tokens, which are sent to the compromised system and which
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compromised resource server and other resource servers of the compromised resource server and other resource servers of the
respective deployment. respective deployment.
The following measures shall be taken into account by implementors in The following measures shall be taken into account by implementors in
order to cope with access token replay: order to cope with access token replay:
o The resource server must treat access tokens like any other o The resource server must treat access tokens like any other
credentials. It is considered good practice to not log them and credentials. It is considered good practice to not log them and
not to store them in plain text. not to store them in plain text.
o Sender constraint access tokens as described in Section 4.4.1.2 o Sender constraint access tokens as described in Section 4.7.1.2
will prevent the attacker from replaying the access tokens on will prevent the attacker from replaying the access tokens on
other resource servers. Depending on the severity of the other resource servers. Depending on the severity of the
penetration, it will also prevent replay on the compromised penetration, it will also prevent replay on the compromised
system. system.
o Audience restriction as described in Section 4.4.1.3 may be used o Audience restriction as described in Section 4.7.1.3 may be used
to prevent replay of captured access tokens on other resource to prevent replay of captured access tokens on other resource
servers. servers.
4.4.3. TLS Terminating Reverse Proxies 4.8. Refresh Token Leakage (TBD)
mitm, log files on the device, ...
refresh token rotation, mutual TLS authentication at the token
endpoint
4.9. Open Redirection (TBD)
Using the AS as Open Redirector - error handling AS (redirects)
(draft-ietf-oauth-closing-redirectors-00)
Using the Client as Open Redirector
4.10. TLS Terminating Reverse Proxies
A common deployment architecture for HTTP applications is to have the A common deployment architecture for HTTP applications is to have the
application server sitting behind a reverse proxy, which terminates application server sitting behind a reverse proxy, which terminates
the TLS connection and dispatches the incoming requests to the the TLS connection and dispatches the incoming requests to the
respective application server nodes. respective application server nodes.
This section highlights some attack angles of this deployment This section highlights some attack angles of this deployment
architecture, which are relevant to OAuth, and give recommendations architecture, which are relevant to OAuth, and give recommendations
for security controls. for security controls.
skipping to change at page 18, line 18 skipping to change at page 22, line 42
application servers. application servers.
If an attacker would be able to get access to the internal network If an attacker would be able to get access to the internal network
between proxy and application server, it could also try to circumvent between proxy and application server, it could also try to circumvent
security controls in place. It is therefore important to ensure the security controls in place. It is therefore important to ensure the
authenticity of the communicating entities. Furthermore, the authenticity of the communicating entities. Furthermore, the
communication link between reverse proxy and application server must communication link between reverse proxy and application server must
therefore be protected against tapping and injection (including therefore be protected against tapping and injection (including
replay prevention). replay prevention).
4.5. Mix-Up 4.11. Other Topics
Mix-up is another kind of attack on more dynamic OAuth scenarios (or
at least scenarios where a OAuth client interacts with multiple
authorization servers). The goal of the attack is to obtain an
authorization code or an access token by tricking the client into
sending those credentials to the attacker (which acts as MITM between
client and authorization server)
A detailed description of the attack and potential countermeasures is
given in cf. https://tools.ietf.org/html/draft-ietf-oauth-mix-up-
mitigation-01.
Potential mitigations:
o AS returns client_id and its iss in the response. Client compares
this data to AS it believed it sent the user agent to.
o ID token carries client id and issuer (requires OpenID Connect)
o Clients use AS-specific redirect URIs, for every authorization
request store intended AS and compare intention with actual
redirect URI where the response was received (no change to OAuth
required)
4.6. Refresh Token Leakage
mitm, log files on the device, ...
refresh token rotation, mutual TLS authentication at the token
endpoint
5. OAuth Credentials Injection
Credential injection means an attacker somehow obtained a valid OAuth
credential (code or token) and is able to utilize this to impersonate
the legitimate resource owner or to cause a victim to access
resources under the attacker's control (XSRF).
5.1. Code Injection
In such an attack, the adversary attempts to inject a stolen
authorization code into a legitimate client on a device under his
control. In the simplest case, the attacker would want to use the
code in his own client. But there are situations where this might
not be possible or intended. Example are:
o The code is bound to a particular confidential client and the
attacker is unable to obtain the required client credentials to
redeem the code himself and/or
o The attacker wants to access certain functions in this particular
client. As an example, the attacker potentially wants to
impersonate his victim in a certain app.
o Another example could be that access to the authorization and
resource servers is some how limited to networks, the attackers is
unable to access directly.
How does an attack look like?
(1) The attacker obtains an authorization code by executing any of
the attacks described above (OAuth Credentials Leakage).
(2) It performs an OAuth authorization process with the legitimate
client on his device.
(3) The attacker injects the stolen authorization code in the
response of the authorization server to the legitimate client.
(4) The client sends the code to the authorization server's token
endpoint, along with client id, client secret and actual
redirect_uri.
(5) The authorization server checks the client secret, whether the
code was issued to the particular client and whether the actual
redirect URI matches the redirect_uri parameter.
(6) If all checks succeed, the authorization server issues access
and other tokens to the client.
(7) The attacker just impersonated the victim.
Obviously, the check in step (5) will fail, if the code was issued to
another client id, e.g. a client set up by the attacker.
An attempt to inject a code obtained via a malware pretending to be
the legitimate client should also be detected, if the authorization
server stored the complete redirect URI used in the authorization
request and compares it with the redirect_uri parameter.
[RFC6749], Section 4.1.3, requires the AS to ... "ensure that the
"redirect_uri" parameter is present if the "redirect_uri" parameter
was included in the initial authorization request as described in
Section 4.1.1, and if included ensure that their values are
identical." In the attack scenario described above, the legitimate
client would use the correct redirect URI it always uses for
authorization requests. But this URI would not match the tampered
redirect URI used by the attacker (otherwise, the redirect would not
land at the attackers page). So the authorization server would
detect the attack and refuse to exchange the code.
Note: this check could also detect attempt to inject a code, which
had been obtained from another instance of the same client on another
device, if certain conditions are fulfilled:
o the redirect URI itself needs to contain a nonce or another kind
of one-time use, secret data and
o the client has bound this data to this particular instance
But this approach conflicts with the idea to enforce exact redirect
URI matching at the authorization endpoint. Moreover, it has been
observed that providers very often ignore the redirect_uri check
requirement at this stage, maybe, because it doesn't seem to be
security-critical from reading the spec.
Other providers just pattern match the redirect_uri parameter against
the registered redirect URI pattern. This saves the authorization
server from storing the link between the actual redirect URI and the
respective authorization code for every transaction. But this kind
of check obviously does not fulfill the intent of the spec, since the
tampered redirect URI is not considered. So any attempt to inject a
code obtained using the client_id of a legitimate client or by
utilizing the legitimate client on another device won't be detected
in the respective deployments.
It is also assumed that the requirements defined in [RFC6749],
Section 4.1.3, increase client implementation complexity as clients
need to memorize or re-construct the correct redirect URI for the
call to the tokens endpoint.
The authors therefore propose to the working group to drop this
feature in favor of more effective and (hopefully) simpler approaches
to code injection prevention as described in the following section.
5.1.1. Proposed Countermeasures
The general proposal is to bind every particular authorization code
to a certain client on a certain device (or in a certain user agent)
in the context of a certain transaction. There are multiple
technical solutions to achieve this goal:
Nonce OpenID Connect's existing "nonce" parameter is used for this
purpose. The nonce value is one time use and created by the
client. The client is supposed to bind it to the user agent
session and sends it with the initial request to the OpenId
Provider (OP). The OP associates the nonce to the
authorization code and attests this binding in the ID token,
which is issued as part of the code exchange at the token
endpoint. If an attacker injected an authorization code in
the authorization response, the nonce value in the client
session and the nonce value in the ID token will not match
and the attack is detected. assumption: attacker cannot get
hold of the user agent state on the victims device, where he
has stolen the respective authorization code.
pro:
- existing feature, used in the wild
con:
- OAuth does not have an ID Token - would need to push that
down the stack
Code-bound State It has been discussed in the security workshop in
December to use the OAuth state value much similar in the way
as described above. In the case of the state value, the idea
is to add a further parameter state to the code exchange
request. The authorization server then compares the state
value it associated with the code and the state value in the
parameter. If those values do not match, it is considered an
attack and the request fails. Note: a variant of this
solution would be send a hash of the state (in order to
prevent bulky requests and DoS).
pro:
- use existing concept
con:
- state needs to fulfil certain requirements (one time use,
complexity)
- new parameter means normative spec change
PKCE Basically, the PKCE challenge/verifier could be used in the
same way as Nonce or State. In contrast to its original
intention, the verifier check would fail although the client
uses its correct verifier but the code is associated with a
challenge, which does not match.
pro:
- existing and deployed OAuth feature
con:
- currently used and recommended for native apps, not web
apps
Token Binding Code must be bind to UA-AS and UA-Client legs -
requires further data (extension to response) to manifest
binding id for particular code.
Note: token binding could be used in conjunction with PKCE as
an option (https://tools.ietf.org/html/draft-ietf-oauth-
token-binding-02#section-4).
pro:
- highly secure
con:
- highly sophisticated, requires browser support, will it
work for native apps?
per instance client id/secret ...
Note on pre-warmed secrets: An attacker can circumvent the
countermeasures described above if he is able to create or capture
the respective secret or code_challenge on a device under his
control, which is then used in the victim's authorization request.
Exact redirect URI matching of authorization requests can prevent the
attacker from using the pre-warmed secret in the faked authorization
transaction on the victim's device.
Unfortunately it does not work for all kinds of OAuth clients. It is
effective for web and JS apps and for native apps with claimed URLs.
What about other native apps? Treat nonce or PKCE challenge as
replay detection tokens (needs to ensure cluster-wide one-time use)?
5.2. Access Token Injection (TBD)
Note: An attacker in possession of an access token can access any
resources the access token gives him the permission to. This kind of
attacks simply illustrates the fact that bearer tokens utilized by
OAuth are reusable similar to passwords unless they are protected by
further means.
(where do we treat access token replay/use at the resource server?
https://tools.ietf.org/html/rfc6819#section-4.6.4 has some text about
it but is it sufficient?)
The attack described in this section is about injecting a stolen
access token into a legitimate client on a device under the
adversaries control. The attacker wants to impersonate a victim and
cannot use his own client, since he wants to access certain functions
in this particular client.
Proposal: token binding, hybrid flow+nonce(OIDC), other
cryptographical binding between access token and user agent instance
5.3. XSRF (TBD)
injection of code or access token on a victim's device (e.g. to cause
client to access resources under the attacker's control)
mitigation: XSRF tokens (one time use) w/ user agent binding (cf.
https://www.owasp.org/index.php/
CrossSite_Request_Forgery_(CSRF)_Prevention_Cheat_Sheet)
6. Other Attacks
Using the AS as Open Redirector - error handling AS (redirects)
(draft-ietf-oauth-closing-redirectors-00)
Using the Client as Open Redirector
redirect via status code 307 - use 302
7. Other Topics
why to rotate refresh tokens o redirect via status code 307 - use 302
how to support multi AS per RS o why to rotate refresh tokens
differentiate native, JS and web clients o how to support multi AS per RS
do not put sensitive data in URL/GET parameters (Jim Manico) o differentiate native, JS and web clients
Incorporate Christian Mainka's feedback o do not put sensitive data in URL/GET parameters (Jim Manico)
o Incorporate Christian Mainka's feedback
WPAD attack - https://www.blackhat.com/docs/us-16/materials/us-16- o WPAD attack - https://www.blackhat.com/docs/us-16/materials/us-16-
Kotler-Crippling-HTTPS-With-Unholy-PAC.pdf Kotler-Crippling-HTTPS-With-Unholy-PAC.pdf
8. Acknowledgements 5. Acknowledgements
We would like to thank Jim Manico, Phil Hunt, and Brian Campbell for We would like to thank Jim Manico, Phil Hunt, and Brian Campbell for
their valuable feedback. their valuable feedback.
9. IANA Considerations 6. IANA Considerations
This draft includes no request to IANA. This draft includes no request to IANA.
10. Security Considerations 7. Security Considerations
All relevant security considerations have been given in the All relevant security considerations have been given in the
functional specification. functional specification.
11. References 8. References
11.1. Normative References 8.1. Normative References
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform [RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66, Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, DOI 10.17487/RFC3986, January 2005, RFC 3986, DOI 10.17487/RFC3986, January 2005,
<https://www.rfc-editor.org/info/rfc3986>. <https://www.rfc-editor.org/info/rfc3986>.
[RFC6749] Hardt, D., Ed., "The OAuth 2.0 Authorization Framework", [RFC6749] Hardt, D., Ed., "The OAuth 2.0 Authorization Framework",
RFC 6749, DOI 10.17487/RFC6749, October 2012, RFC 6749, DOI 10.17487/RFC6749, October 2012,
<https://www.rfc-editor.org/info/rfc6749>. <https://www.rfc-editor.org/info/rfc6749>.
skipping to change at page 25, line 5 skipping to change at page 24, line 10
[RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer [RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Semantics and Content", RFC 7231, Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
DOI 10.17487/RFC7231, June 2014, DOI 10.17487/RFC7231, June 2014,
<https://www.rfc-editor.org/info/rfc7231>. <https://www.rfc-editor.org/info/rfc7231>.
[RFC7591] Richer, J., Ed., Jones, M., Bradley, J., Machulak, M., and [RFC7591] Richer, J., Ed., Jones, M., Bradley, J., Machulak, M., and
P. Hunt, "OAuth 2.0 Dynamic Client Registration Protocol", P. Hunt, "OAuth 2.0 Dynamic Client Registration Protocol",
RFC 7591, DOI 10.17487/RFC7591, July 2015, RFC 7591, DOI 10.17487/RFC7591, July 2015,
<https://www.rfc-editor.org/info/rfc7591>. <https://www.rfc-editor.org/info/rfc7591>.
11.2. Informative References 8.2. Informative References
[I-D.bradley-oauth-jwt-encoded-state] [I-D.bradley-oauth-jwt-encoded-state]
Bradley, J., Lodderstedt, T., and H. Zandbelt, "Encoding Bradley, J., Lodderstedt, T., and H. Zandbelt, "Encoding
claims in the OAuth 2 state parameter using a JWT", draft- claims in the OAuth 2 state parameter using a JWT", draft-
bradley-oauth-jwt-encoded-state-07 (work in progress), bradley-oauth-jwt-encoded-state-07 (work in progress),
March 2017. March 2017.
[I-D.campbell-oauth-resource-indicators] [I-D.campbell-oauth-resource-indicators]
Campbell, B., Bradley, J., and H. Tschofenig, "Resource Campbell, B., Bradley, J., and H. Tschofenig, "Resource
Indicators for OAuth 2.0", draft-campbell-oauth-resource- Indicators for OAuth 2.0", draft-campbell-oauth-resource-
indicators-02 (work in progress), November 2016. indicators-02 (work in progress), November 2016.
[I-D.ietf-oauth-discovery] [I-D.ietf-oauth-discovery]
Jones, M., Sakimura, N., and J. Bradley, "OAuth 2.0 Jones, M., Sakimura, N., and J. Bradley, "OAuth 2.0
Authorization Server Metadata", draft-ietf-oauth- Authorization Server Metadata", draft-ietf-oauth-
discovery-07 (work in progress), September 2017. discovery-07 (work in progress), September 2017.
[I-D.ietf-oauth-mtls] [I-D.ietf-oauth-mtls]
Campbell, B., Bradley, J., Sakimura, N., and T. Campbell, B., Bradley, J., Sakimura, N., and T.
Lodderstedt, "Mutual TLS Profile for OAuth 2.0", draft- Lodderstedt, "Mutual TLS Profile for OAuth 2.0", draft-
ietf-oauth-mtls-03 (work in progress), July 2017. ietf-oauth-mtls-05 (work in progress), November 2017.
[I-D.ietf-oauth-pop-key-distribution] [I-D.ietf-oauth-pop-key-distribution]
Bradley, J., Hunt, P., Jones, M., and H. Tschofenig, Bradley, J., Hunt, P., Jones, M., and H. Tschofenig,
"OAuth 2.0 Proof-of-Possession: Authorization Server to "OAuth 2.0 Proof-of-Possession: Authorization Server to
Client Key Distribution", draft-ietf-oauth-pop-key- Client Key Distribution", draft-ietf-oauth-pop-key-
distribution-03 (work in progress), February 2017. distribution-03 (work in progress), February 2017.
[I-D.ietf-oauth-signed-http-request] [I-D.ietf-oauth-signed-http-request]
Richer, J., Bradley, J., and H. Tschofenig, "A Method for Richer, J., Bradley, J., and H. Tschofenig, "A Method for
Signing HTTP Requests for OAuth", draft-ietf-oauth-signed- Signing HTTP Requests for OAuth", draft-ietf-oauth-signed-
http-request-03 (work in progress), August 2016. http-request-03 (work in progress), August 2016.
[I-D.ietf-oauth-token-binding] [I-D.ietf-oauth-token-binding]
Jones, M., Bradley, J., Campbell, B., and W. Denniss, Jones, M., Campbell, B., Bradley, J., and W. Denniss,
"OAuth 2.0 Token Binding", draft-ietf-oauth-token- "OAuth 2.0 Token Binding", draft-ietf-oauth-token-
binding-04 (work in progress), July 2017. binding-05 (work in progress), October 2017.
[I-D.ietf-tokbind-https] [I-D.ietf-tokbind-https]
Popov, A., Nystrom, M., Balfanz, D., Langley, A., Harper, Popov, A., Nystrom, M., Balfanz, D., Langley, A., Harper,
N., and J. Hodges, "Token Binding over HTTP", draft-ietf- N., and J. Hodges, "Token Binding over HTTP", draft-ietf-
tokbind-https-10 (work in progress), July 2017. tokbind-https-10 (work in progress), July 2017.
[I-D.sakimura-oauth-jpop] [I-D.sakimura-oauth-jpop]
Sakimura, N., Li, K., and J. Bradley, "The OAuth 2.0 Sakimura, N., Li, K., and J. Bradley, "The OAuth 2.0
Authorization Framework: JWT Pop Token Usage", draft- Authorization Framework: JWT Pop Token Usage", draft-
sakimura-oauth-jpop-04 (work in progress), March 2017. sakimura-oauth-jpop-04 (work in progress), March 2017.
skipping to change at page 26, line 35 skipping to change at page 25, line 40
[RFC7800] Jones, M., Bradley, J., and H. Tschofenig, "Proof-of- [RFC7800] Jones, M., Bradley, J., and H. Tschofenig, "Proof-of-
Possession Key Semantics for JSON Web Tokens (JWTs)", Possession Key Semantics for JSON Web Tokens (JWTs)",
RFC 7800, DOI 10.17487/RFC7800, April 2016, RFC 7800, DOI 10.17487/RFC7800, April 2016,
<https://www.rfc-editor.org/info/rfc7800>. <https://www.rfc-editor.org/info/rfc7800>.
Appendix A. Document History Appendix A. Document History
[[ To be removed from the final specification ]] [[ To be removed from the final specification ]]
-04
o Restructured document for better readability
o Added best practices on Token Leakage prevention
-03 -03
o Added section on Access Token Leakage at Resource Server o Added section on Access Token Leakage at Resource Server
o incorporated Brian Campbell's findings o incorporated Brian Campbell's findings
-02 -02
o Folded Mix up and Access Token leakage through a bad AS into new o Folded Mix up and Access Token leakage through a bad AS into new
section for dynamic OAuth threats section for dynamic OAuth threats
o reworked dynamic OAuth section o reworked dynamic OAuth section
-01 -01
o Added references to mitigation methods for token leakage o Added references to mitigation methods for token leakage
o Added reference to Token Binding for Authorization Code o Added reference to Token Binding for Authorization Code
skipping to change at page 27, line 17 skipping to change at page 26, line 28
-00 (WG document) -00 (WG document)
o turned the ID into a WG document and a BCP o turned the ID into a WG document and a BCP
o Added federated app login as topic in Other Topics o Added federated app login as topic in Other Topics
Authors' Addresses Authors' Addresses
Torsten Lodderstedt (editor) Torsten Lodderstedt (editor)
YES Europe AG YES.com AG
Email: torsten@lodderstedt.net Email: torsten@lodderstedt.net
John Bradley John Bradley
Yubico Yubico
Email: ve7jtb@ve7jtb.com Email: ve7jtb@ve7jtb.com
Andrey Labunets Andrey Labunets
Facebook Facebook
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