draft-ietf-oauth-security-topics-10.txt   draft-ietf-oauth-security-topics-11.txt 
Open Authentication Protocol T. Lodderstedt, Ed. Web Authorization Protocol T. Lodderstedt
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Intended status: Best Current Practice J. Bradley Intended status: Best Current Practice J. Bradley
Expires: May 23, 2019 Yubico Expires: 1 July 2019 Yubico
A. Labunets A. Labunets
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D. Fett D. Fett
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November 19, 2018 28 December 2018
OAuth 2.0 Security Best Current Practice OAuth 2.0 Security Best Current Practice
draft-ietf-oauth-security-topics-10 draft-ietf-oauth-security-topics-11
Abstract Abstract
This document describes best current security practice for OAuth 2.0. This document describes best current security practice for OAuth 2.0.
It updates and extends the OAuth 2.0 Security Threat Model to It updates and extends the OAuth 2.0 Security Threat Model to
incorporate practical experiences gathered since OAuth 2.0 was incorporate practical experiences gathered since OAuth 2.0 was
published and covers new threats relevant due to the broader published and covers new threats relevant due to the broader
application of OAuth 2.0. application of OAuth 2.0.
Status of This Memo Status of This Memo
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Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/. Drafts is at https://datatracker.ietf.org/drafts/current/.
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 May 23, 2019. This Internet-Draft will expire on 1 July 2019.
Copyright Notice Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the Copyright (c) 2018 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
2. Recommendations . . . . . . . . . . . . . . . . . . . . . . . 4 2. Recommendations
2.1. Protecting Redirect-Based Flows . . . . . . . . . . . . . 4 2.1. Protecting Redirect-Based Flows
2.1.1. Authorization Code Grant . . . . . . . . . . . . . . 5 2.1.1. Authorization Code Grant
2.1.2. Implicit Grant . . . . . . . . . . . . . . . . . . . 5 2.1.2. Implicit Grant
2.2. Token Replay Prevention . . . . . . . . . . . . . . . . . 6 2.2. Token Replay Prevention
2.3. Access Token Privilege Restriction . . . . . . . . . . . 6 2.3. Access Token Privilege Restriction
3. Attacks and Mitigations . . . . . . . . . . . . . . . . . . . 7 3. Attacks and Mitigations
3.1. Insufficient Redirect URI Validation . . . . . . . . . . 7 3.1. Insufficient Redirect URI Validation
3.1.1. Attacks on Authorization Code Grant . . . . . . . . . 7 3.1.1. Attacks on Authorization Code Grant
3.1.2. Attacks on Implicit Grant . . . . . . . . . . . . . . 8 3.1.2. Attacks on Implicit Grant
3.1.3. Proposed Countermeasures . . . . . . . . . . . . . . 10 3.1.3. Proposed Countermeasures
3.2. Credential Leakage via Referrer Headers . . . . . . . . . 10 3.2. Credential Leakage via Referrer Headers
3.2.1. Leakage from the OAuth client . . . . . . . . . . . . 10 3.2.1. Leakage from the OAuth client
3.2.2. Leakage from the Authorization Server . . . . . . . . 11 3.2.2. Leakage from the Authorization Server
3.2.3. Consequences . . . . . . . . . . . . . . . . . . . . 11 3.2.3. Consequences
3.2.4. Proposed Countermeasures . . . . . . . . . . . . . . 11 3.2.4. Proposed Countermeasures
3.3. Attacks through the Browser History . . . . . . . . . . . 12 3.3. Attacks through the Browser History
3.3.1. Code in Browser History . . . . . . . . . . . . . . . 12 3.3.1. Code in Browser History
3.3.2. Access Token in Browser History . . . . . . . . . . . 12 3.3.2. Access Token in Browser History
3.4. Mix-Up . . . . . . . . . . . . . . . . . . . . . . . . . 13 3.4. Mix-Up
3.4.1. Attack Description . . . . . . . . . . . . . . . . . 13 3.4.1. Attack Description
3.4.2. Countermeasures . . . . . . . . . . . . . . . . . . . 15 3.4.2. Countermeasures
3.5. Authorization Code Injection . . . . . . . . . . . . . . 16 3.5. Authorization Code Injection
3.5.1. Proposed Countermeasures . . . . . . . . . . . . . . 18 3.5.1. Proposed Countermeasures
3.6. Access Token Injection . . . . . . . . . . . . . . . . . 19 3.6. Access Token Injection
3.6.1. Proposed Countermeasures . . . . . . . . . . . . . . 20 3.6.1. Proposed Countermeasures
3.7. Cross Site Request Forgery . . . . . . . . . . . . . . . 20 3.7. Cross Site Request Forgery
3.7.1. Proposed Countermeasures . . . . . . . . . . . . . . 20 3.7.1. Proposed Countermeasures
3.8. Access Token Leakage at the Resource Server . . . . . . . 20 3.8. Access Token Leakage at the Resource Server
3.8.1. Access Token Phishing by Counterfeit Resource Server 20 3.8.1. Access Token Phishing by Counterfeit
3.8.1.1. Metadata . . . . . . . . . . . . . . . . . . . . 21 Resource Server
3.8.1.2. Sender Constrained Access Tokens . . . . . . . . 22 3.8.2. Compromised Resource Server
3.8.1.3. Audience Restricted Access Tokens . . . . . . . . 25 3.9. Open Redirection
3.8.2. Compromised Resource Server . . . . . . . . . . . . . 26 3.9.1. Authorization Server as Open Redirector
3.9. Open Redirection . . . . . . . . . . . . . . . . . . . . 27 3.9.2. Clients as Open Redirector
3.9.1. Authorization Server as Open Redirector . . . . . . . 27 3.10. 307 Redirect
3.9.2. Clients as Open Redirector . . . . . . . . . . . . . 27 3.11. TLS Terminating Reverse Proxies
3.10. 307 Redirect . . . . . . . . . . . . . . . . . . . . . . 27 3.12. Refresh Token Protection
3.11. TLS Terminating Reverse Proxies . . . . . . . . . . . . . 28 4. Acknowledgements
3.12. Refresh Token Protection . . . . . . . . . . . . . . . . 29 5. IANA Considerations
4. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 31 6. Security Considerations
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 31 7. Normative References
6. Security Considerations . . . . . . . . . . . . . . . . . . . 31 Appendix A. Document History
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 31 Authors' Addresses
7.1. Normative References . . . . . . . . . . . . . . . . . . 31
7.2. Informative References . . . . . . . . . . . . . . . . . 32
Appendix A. Document History . . . . . . . . . . . . . . . . . . 35
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 37
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 [RFC6749] and
[RFC6749] and RFC 6750 [RFC6750]. Since publication, OAuth 2.0 has [RFC6750]. Since publication, OAuth 2.0 has gotten massive traction
gotten massive traction in the market and became the standard for API in the market and became the standard for API protection and, as
protection and, as foundation of OpenID Connect [OpenID], identity foundation of OpenID Connect [OpenID], identity providing. While
providing. While OAuth was used in a variety of scenarios and OAuth was used in a variety of scenarios and different kinds of
different kinds of deployments, the following challenges could be deployments, the following challenges could be observed:
observed:
o OAuth implementations are being attacked through known * OAuth implementations are being attacked through known
implementation weaknesses and anti-patterns (CSRF, referrer implementation weaknesses and anti-patterns (CSRF, referrer
header). Although most of these threats are discussed in the header). Although most of these threats are discussed in the
OAuth 2.0 Threat Model and Security Considerations [RFC6819], OAuth 2.0 Threat Model and Security Considerations [RFC6819],
continued exploitation demonstrates there may be a need for more continued exploitation demonstrates there may be a need for more
specific recommendations or that the existing mitigations are too specific recommendations or that the existing mitigations are too
difficult to deploy. difficult to deploy.
o Technology has changed, e.g., the way browsers treat fragments in * Technology has changed, e.g., the way browsers treat fragments in
some situations, which may change the implicit grant's underlying some situations, which may change the implicit grant's underlying
security model. security model.
o OAuth is used in much more dynamic setups than originally * OAuth is used in much more dynamic setups than originally
anticipated, creating new challenges with respect to security. anticipated, creating new challenges with respect to security.
Those challenges go beyond the original scope of RFC 6749 Those challenges go beyond the original scope of [RFC6749],
[RFC6749], RFC 6750 [RFC6749], and RFC 6819 [RFC6819]. [RFC6749], and [RFC6819].
OAuth initially assumed a static relationship between client, OAuth initially assumed a static relationship between client,
authorization server and resource servers. The URLs of AS and RS authorization server and resource servers. The URLs of AS and RS
were known to the client at deployment time and built an anchor for were known to the client at deployment time and built an anchor for
the trust relationship among those parties. The validation whether the trust relationship among those parties. The validation whether
the client talks to a legitimate server was based on TLS server the client talks to a legitimate server was based on TLS server
authentication (see [RFC6819], Section 4.5.4). With the increasing authentication (see [RFC6819], Section 4.5.4). With the increasing
adoption of OAuth, this simple model dissolved and, in several adoption of OAuth, this simple model dissolved and, in several
scenarios, was replaced by a dynamic establishment of the scenarios, was replaced by a dynamic establishment of the
relationship between clients on one side and the authorization and relationship between clients on one side and the authorization and
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causing the authorization server to issue access tokens in the causing the authorization server to issue access tokens in the
authorization response are vulnerable to access token leakage and authorization response are vulnerable to access token leakage and
access token replay as described in Section 3.1, Section 3.2, access token replay as described in Section 3.1, Section 3.2,
Section 3.3, and Section 3.6. Section 3.3, and Section 3.6.
Moreover, no viable mechanism exists to cryptographically bind access Moreover, no viable mechanism exists to cryptographically bind access
tokens issued in the authorization response to a certain client as it tokens issued in the authorization response to a certain client as it
is recommended in Section 2.2. This makes replay detection for such is recommended in Section 2.2. This makes replay detection for such
access tokens at resource servers impossible. access tokens at resource servers impossible.
In order to avoid these issues, Clients SHOULD NOT use the implicit In order to avoid these issues, clients SHOULD NOT use the implicit
grant or any other response type causing the authorization server to grant (response type "token") or any other response type issuing
issue an access token in the authorization response. access tokens in the authorization response, such as "token id_token"
and "code token id_token", unless the issued access tokens are
sender-constrained and access token injection in the authorization
response is prevented.
A sender constrained access token scopes the applicability of an
access token to a certain sender. This sender is obliged to
demonstrate knowledge of a certain secret as prerequisite for the
acceptance of that token at the recipient (e.g., a resource server).
Clients SHOULD instead use the response type "code" (aka Clients SHOULD instead use the response type "code" (aka
authorization code grant type) as specified in Section 2.1.1 or any authorization code grant type) as specified in Section 2.1.1 or any
other response type that causes the authorization server to issue other response type that causes the authorization server to issue
access tokens in the token response. This allows the authorization access tokens in the token response. This allows the authorization
server to detect replay attempts and generally reduces the attack server to detect replay attempts and generally reduces the attack
surface since access tokens are not exposed in URLs. It also allows surface since access tokens are not exposed in URLs. It also allows
the authorization server to sender-constrain the issued tokens. the authorization server to sender-constrain the issued tokens.
2.2. Token Replay Prevention 2.2. Token Replay Prevention
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effective counter-measures are described in Section 2.2. effective counter-measures are described in Section 2.2.
In particular, access tokens SHOULD be restricted to certain resource In particular, access tokens SHOULD be restricted to certain resource
servers, preferably to a single resource server. To put this into servers, preferably to a single resource server. To put this into
effect, the authorization server associates the access token with effect, the authorization server associates the access token with
certain resource servers and every resource server is obliged to certain resource servers and every resource server is obliged to
verify for every request, whether the access token sent with that verify for every request, whether the access token sent with that
request was meant to be used for that particular resource server. If request was meant to be used for that particular resource server. If
not, the resource server MUST refuse to serve the respective request. not, the resource server MUST refuse to serve the respective request.
Clients and authorization servers MAY utilize the parameters "scope" Clients and authorization servers MAY utilize the parameters "scope"
or "resource" as specified in [RFC6749] and or "resource" as specified in [RFC6749] and [I-D.ietf-oauth-resource-
[I-D.ietf-oauth-resource-indicators], respectively, to determine the indicators], respectively, to determine the resource server they want
resource server they want to access. to access.
Additionally, access tokens SHOULD be restricted to certain resources Additionally, access tokens SHOULD be restricted to certain resources
and actions on resource servers or resources. To put this into and actions on resource servers or resources. To put this into
effect, the authorization server associates the access token with the effect, the authorization server associates the access token with the
respective resource and actions and every resource server is obliged respective resource and actions and every resource server is obliged
to verify for every request, whether the access token sent with that to verify for every request, whether the access token sent with that
request was meant to be used for that particular action on the request was meant to be used for that particular action on the
particular resource. If not, the resource server must refuse to particular resource. If not, the resource server must refuse to
serve the respective request. Clients and authorization servers MAY serve the respective request. Clients and authorization servers MAY
utilize the parameter "scope" as specified in [RFC6749] to determine utilize the parameter "scope" as specified in [RFC6749] to determine
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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
be more complex to implement and error prone to manage than exact be more complex to implement and error prone to manage than exact
redirect URI matching. Several successful attacks have been observed redirect URI matching. Several successful attacks have been observed
in the wild, which utilized flaws in the pattern matching in the wild, which utilized flaws in the pattern matching
implementation or concrete configurations. Such a flaw effectively implementation or concrete configurations. Such a flaw effectively
breaks client identification or authentication (depending on grant breaks client identification or authentication (depending on grant
and client type) and allows the attacker to obtain an authorization and client type) and allows the attacker to obtain an authorization
code or access token, either: code or access token, either:
o by directly sending the user agent to a URI under the attackers * by directly sending the user agent to a URI under the attackers
control or control or
o by exposing the OAuth credentials to an attacker by utilizing an * by exposing the OAuth credentials to an attacker by utilizing an
open redirector at the client in conjunction with the way user open redirector at the client in conjunction with the way user
agents handle URL fragments. agents handle URL fragments.
3.1.1. Attacks on Authorization Code Grant 3.1.1. Attacks on Authorization Code Grant
For a public client using the grant type code, an attack would look For a public client using the grant type code, an attack would look
as follows: as follows:
Let's assume the redirect URL pattern "https://*.somesite.example/*" Let's assume the redirect URL pattern "https://_.somesite.example/_"
had been registered for the client "s6BhdRkqt3". This pattern allows had been registered for the client "s6BhdRkqt3". This pattern allows
redirect URIs pointing to any host residing in the domain redirect URIs pointing to any host residing in the domain
somesite.example. So if an attacker manages to establish a host or somesite.example. So if an attacker manages to establish a host or
subdomain in somesite.example he can impersonate the legitimate subdomain in somesite.example he can impersonate the legitimate
client. Assume the attacker sets up the host client. Assume the attacker sets up the host
"evil.somesite.example". "evil.somesite.example".
(1) The attacker needs to trick the user into opening a tampered URL 1. The attacker needs to trick the user into opening a tampered URL
in his browser, which launches a page under the attacker's in his browser, which launches a page under the attacker's
control, say "https://www.evil.example". control, say "https://www.evil.example"
(https://www.evil.example").
(2) This URL initiates an authorization request with the client id This URL initiates an authorization request with the client id of
of a legitimate client to the authorization endpoint. This is a legitimate client to the authorization endpoint. This is the
the example authorization request (line breaks are for display example authorization request (line breaks are for display
purposes only): purposes only):
GET /authorize?response_type=code&client_id=s6BhdRkqt3&state=xyz GET /authorize?response_type=code&client_id=s6BhdRkqt3&state=xyz
&redirect_uri=https%3A%2F%2Fevil.somesite.example%2Fcb HTTP/1.1 &redirect_uri=https%3A%2F%2Fevil.somesite.example%2Fcb HTTP/1.1
Host: server.somesite.example Host: server.somesite.example
(3) The authorization server validates the redirect URI in order to 2. The authorization server validates the redirect URI in order to
identify the client. Since the pattern allows arbitrary domains identify the client. Since the pattern allows arbitrary domains
host names in "somesite.example", the authorization request is host names in "somesite.example", the authorization request is
processed under the legitimate client's identity. This includes processed under the legitimate client's identity. This includes
the way the request for user consent is presented to the user. the way the request for user consent is presented to the user.
If auto-approval is allowed (which is not recommended for public If auto-approval is allowed (which is not recommended for public
clients according to [RFC6749]), the attack can be performed clients according to [RFC6749]), the attack can be performed even
even easier. easier.
(4) If the user does not recognize the attack, the code is issued If the user does not recognize the attack, the code is issued and
and directly sent to the attacker's client. directly sent to the attacker's client.
(5) Since the attacker impersonated a public client, it can directly 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 impersonate or utilize client's secret. The attacker will need to impersonate or utilize
the legitimate client to redeem the code (e.g., by performing a code the legitimate client to redeem the code (e.g., by performing a code
injection attack). This kind of injections is covered in injection attack). This kind of injections is covered in
Section Authorization Code Injection. Section 3.5.
3.1.2. Attacks on Implicit Grant 3.1.2. 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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URL matching!). URL matching!).
Assume the pattern for client "s6BhdRkqt3" is Assume the pattern for client "s6BhdRkqt3" is
"https://client.somesite.example/cb?*", i.e., any parameter is "https://client.somesite.example/cb?*", i.e., any parameter is
allowed for redirects to "https://client.somesite.example/cb". allowed for redirects to "https://client.somesite.example/cb".
Unfortunately, the client exposes an open redirector. This endpoint Unfortunately, the client exposes an open redirector. This endpoint
supports a parameter "redirect_to" which takes a target URL and will supports a parameter "redirect_to" which takes a target URL and will
send the browser to this URL using an HTTP Location header redirect send the browser to this URL using an HTTP Location header redirect
303. 303.
(1) Same as above, the attacker needs to trick the user into opening 1. Same as above, the attacker needs to trick the user into opening
a tampered URL in his browser, which launches a page under the a tampered URL in his browser, which launches a page under the
attacker's control, say "https://www.evil.example". attacker's control, say "https://www.evil.example".
(2) The URL initiates an authorization request, which is very 2. The URL initiates an authorization request, which is very similar
similar to the attack on the code flow. As differences, it to the attack on the code flow. As differences, it utilizes the
utilizes the open redirector by encoding open redirector by encoding
"redirect_to=https://client.evil.example" into the redirect URI "redirect_to=https://client.evil.example" into the redirect URI
and it uses the response type "token" (line breaks are for and it uses the response type "token" (line breaks are for
display purposes only): display purposes only):
GET /authorize?response_type=token&client_id=s6BhdRkqt3&state=xyz GET /authorize?response_type=token&client_id=s6BhdRkqt3&state=xyz
&redirect_uri=https%3A%2F%2Fclient.somesite.example%2Fcb%26redirect_to &redirect_uri=https%3A%2F%2Fclient.somesite.example%2Fcb%26redirect_to
%253Dhttps%253A%252F%252Fclient.evil.example%252Fcb HTTP/1.1 %253Dhttps%253A%252F%252Fclient.evil.example%252Fcb HTTP/1.1
Host: server.somesite.example Host: server.somesite.example
(3) Since the redirect URI matches the registered pattern, the 3. Since the redirect URI matches the registered pattern, the
authorization server allows the request and sends the resulting authorization server allows the request and sends the resulting
access token with a 303 redirect (some response parameters are access token with a 303 redirect (some response parameters are
omitted for better readability) omitted for better readability)
HTTP/1.1 303 See Other HTTP/1.1 303 See Other
Location: https://client.somesite.example/cb? Location: https://client.somesite.example/cb?
redirect_to%3Dhttps%3A%2F%2Fclient.evil.example%2Fcb redirect_to%3Dhttps%3A%2F%2Fclient.evil.example%2Fcb
#access_token=2YotnFZFEjr1zCsicMWpAA&... #access_token=2YotnFZFEjr1zCsicMWpAA&...
(4) At example.com, the request arrives at the open redirector. It 4. At example.com, the request arrives at the open redirector. It
will read the redirect parameter and will issue an HTTP 303 will read the redirect parameter and will issue an HTTP 303
Location header redirect to the URL Location header redirect to the URL "https://client.evil.example/
"https://client.evil.example/cb". cb".
HTTP/1.1 303 See Other HTTP/1.1 303 See Other
Location: https://client.evil.example/cb Location: https://client.evil.example/cb
(5) Since the redirector at client.somesite.example does not include 5. Since the redirector at client.somesite.example does not include
a fragment in the Location header, the user agent will re-attach a fragment in the Location header, the user agent will re-attach
the original fragment the original fragment
"#access_token=2YotnFZFEjr1zCsicMWpAA&..." to the URL and will "#access_token=2YotnFZFEjr1zCsicMWpAA&..." to the URL and
navigate to the following URL: will navigate to the following URL:
https://client.evil.example/cb#access_token=2YotnFZFEjr1zCsicMWpAA&... https://client.evil.example/cb#access_token=2YotnFZFEjr1zCsicMWpAA&...
(6) The attacker's page at client.evil.example can access the 6. The attacker's page at client.evil.example can access the
fragment and obtain the access token. fragment and obtain the access token.
3.1.3. Proposed Countermeasures 3.1.3. Proposed Countermeasures
The complexity of implementing and managing pattern matching The complexity of implementing and managing pattern matching
correctly obviously causes security issues. This document therefore correctly obviously causes security issues. This document therefore
proposes to simplify the required logic and configuration by using proposes to simplify the required logic and configuration by using
exact redirect URI matching only. This means the authorization exact redirect URI matching only. This means the authorization
server must compare the two URIs using simple string comparison as server must compare the two URIs using simple string comparison as
defined in [RFC3986], Section 6.2.1.. defined in [RFC3986], Section 6.2.1..
Additional recommendations: Additional recommendations:
o Servers on which callbacks are hosted must not expose open * Servers on which callbacks are hosted must not expose open
redirectors (see Section 3.9). redirectors (see (#Open.Redirection)).
o Clients MAY drop fragments via intermediary URLs with "fix * Clients MAY drop fragments via intermediary URLs with "fix
fragments" (see [fb_fragments]) to prevent the user agent from fragments" (see [fb_fragments]) to prevent the user agent from
appending any unintended fragments. appending any unintended fragments.
o Clients SHOULD use the authorization code response type instead of * Clients SHOULD use the authorization code response type instead of
response types causing access token issuance at the authorization response types causing access token issuance at the authorization
endpoint. This offers countermeasures against reuse of leaked endpoint. This offers countermeasures against reuse of leaked
credentials through the exchange process with the authorization credentials through the exchange process with the authorization
server and token replay through certificate binding of the access server and token replay through certificate binding of the access
tokens. tokens.
As an alternative to exact redirect URI matching, the AS could also As an alternative to exact redirect URI matching, the AS could also
authenticate clients, e.g., using [I-D.ietf-oauth-jwsreq]. authenticate clients, e.g., using [I-D.ietf-oauth-jwsreq].
3.2. Credential Leakage via Referrer Headers 3.2. Credential Leakage via Referrer Headers
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from a client's web site or from an AS's web site. Note: even if from a client's web site or from an AS's web site. Note: even if
specified otherwise in [RFC2616], section 14.36, the same may happen specified otherwise in [RFC2616], section 14.36, the same may happen
to access tokens conveyed in URI fragments due to browser to access tokens conveyed in URI fragments due to browser
implementation issues as illustrated by Chromium Issue 168213 implementation issues as illustrated by Chromium Issue 168213
[bug.chromium]. [bug.chromium].
3.2.1. Leakage from the OAuth client 3.2.1. Leakage from the OAuth client
This requires that the client, as a result of a successful This requires that the client, as a result of a successful
authorization request, renders a page that authorization request, renders a page that
o contains links to other pages under the attacker's control (ads,
* contains links to other pages under the attacker's control (ads,
faq, ...) and a user clicks on such a link, or faq, ...) and a user clicks on such a link, or
o includes third-party content (iframes, images, etc.) for example * includes third-party content (iframes, images, etc.) for example
if the page contains user-generated content (blog). if the page contains user-generated content (blog).
As soon as the browser navigates to the attacker's page or loads the As soon as the browser navigates to the attacker's page or loads the
third-party content, the attacker receives the authorization response third-party content, the attacker receives the authorization response
URL and can extract "code", "access token", or "state". URL and can extract "code", "access token", or "state".
3.2.2. Leakage from the Authorization Server 3.2.2. Leakage from the Authorization Server
In a similar way, an attacker can learn "state" if the authorization In a similar way, an attacker can learn "state" if the authorization
endpoint at the authorization server contains links or third-party endpoint at the authorization server contains links or third-party
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3.2.4. Proposed Countermeasures 3.2.4. Proposed Countermeasures
The page rendered as a result of the OAuth authorization response and The page rendered as a result of the OAuth authorization response and
the authorization endpoint SHOULD not include third-party resources the authorization endpoint SHOULD not include third-party resources
or links to external sites. or links to external sites.
The following measures further reduce the chances of a successful The following measures further reduce the chances of a successful
attack: attack:
o Bind authorization code to a confidential client or PKCE * Bind authorization code to a confidential client or PKCE
challenge. In this case, the attacker lacks the secret to request challenge. In this case, the attacker lacks the secret to request
the code exchange. the code exchange.
o Authorization codes SHOULD be invalidated by the AS after their * Authorization codes SHOULD be invalidated by the AS after their
first use at the token endpoint. For example, if an AS first use at the token endpoint. For example, if an AS
invalidated the code after the legitimate client redeemed it, the invalidated the code after the legitimate client redeemed it, the
attacker would fail exchanging this code later. (This does not attacker would fail exchanging this code later. (This does not
mitigate the attack if the attacker manages to exchange the code mitigate the attack if the attacker manages to exchange the code
for a token before the legitimate client does so.) for a token before the legitimate client does so.)
o The "state" value SHOULD be invalidated by the client after its * The "state" value SHOULD be invalidated by the client after its
first use at the redirection endpoint. If this is implemented, first use at the redirection endpoint. If this is implemented,
and an attacker receives a token through the referrer header from and an attacker receives a token through the referrer header from
the client's web site, the "state" was already used, invalidated the client's web site, the "state" was already used, invalidated
by the client and cannot be used again by the attacker. (This by the client and cannot be used again by the attacker. (This
does not help if the "state" leaks from the AS's web site, since does not help if the <spanx style="verb">state</spanx> leaks from
then the "state" has not been used at the redirection endpoint at the AS's web site, since then the <spanx
the client yet.) style="verb">state</spanx> has not been used at the redirection
endpoint at the client yet.)
o Suppress the referrer header by adding the attribute * Suppress the referrer header by adding the attribute
"rel="noreferrer"" to HTML links or by applying an appropriate "rel="noreferrer"" to HTML links or by applying an appropriate
Referrer Policy [webappsec-referrer-policy] to the document Referrer Policy [webappsec-referrer-policy] to the document
(either as part of the "referrer" meta attribute or by setting a (either as part of the "referrer" meta attribute or by setting a
Referrer-Policy header). Referrer-Policy header).
o Use authorization code instead of response types causing access * Use authorization code instead of response types causing access
token issuance from the authorization endpoint. This provides token issuance from the authorization endpoint. This provides
countermeasures against leakage on the OAuth protocol level countermeasures against leakage on the OAuth protocol level
through the code exchange process with the authorization server. through the code exchange process with the authorization server.
o Additionally, one might use the form post response mode instead of * Additionally, one might use the form post response mode instead of
redirect for authorization response (see redirect for authorization response (see [oauth-v2-form-post-
[oauth-v2-form-post-response-mode]). response-mode]).
3.3. Attacks through the Browser History 3.3. Attacks through the Browser History
Authorization codes and access tokens can end up in the browser's Authorization codes and access tokens can end up in the browser's
history of visited URLs, enabling the attacks described in the history of visited URLs, enabling the attacks described in the
following. following.
3.3.1. Code in Browser History 3.3.1. Code in Browser History
When a browser navigates to "client.example/ When a browser navigates to "client.example/
redirection_endpoint?code=abcd" as a result of a redirect from a redirection_endpoint?code=abcd" as a result of a redirect from a
provider's authorization endpoint, the URL including the provider's authorization endpoint, the URL including the
authorization code may end up in the browser's history. An attacker authorization code may end up in the browser's history. An attacker
with access to the device could obtain the code and try to replay it. with access to the device could obtain the code and try to replay it.
Proposed countermeasures: Proposed countermeasures:
o Authorization code replay prevention as described in [RFC6819], * Authorization code replay prevention as described in [RFC6819],
Section 4.4.1.1, and Section 3.5 Section 4.4.1.1, and Section 3.5
o Use form post response mode instead of redirect for authorization * Use form post response mode instead of redirect for authorization
response (see [oauth-v2-form-post-response-mode]) response (see [oauth-v2-form-post-response-mode])
3.3.2. Access Token in Browser History 3.3.2. Access Token in Browser History
An access token may end up in the browser history if a a client or An access token may end up in the browser history if a a client or
just a web site, which already has a token, deliberately navigates to just a web site, which already has a token, deliberately navigates to
a page like "provider.com/get_user_profile?access_token=abcdef.". a page like "provider.com/get_user_profile?access_token=abcdef.".
Actually [RFC6750] discourages this practice and asks to transfer Actually [RFC6750] discourages this practice and asks to transfer
tokens via a header, but in practice web sites often just pass access tokens via a header, but in practice web sites often just pass access
token in query parameters. token in query parameters.
In case of implicit grant, a URL like "client.example/ In case of implicit grant, a URL like "client.example/
redirection_endpoint#access_token=abcdef" may also end up in the redirection_endpoint#access_token=abcdef" may also end up in the
browser history as a result of a redirect from a provider's browser history as a result of a redirect from a provider's
authorization endpoint. authorization endpoint.
Proposed countermeasures: Proposed countermeasures:
skipping to change at page 13, line 16 skipping to change at line 589
tokens via a header, but in practice web sites often just pass access tokens via a header, but in practice web sites often just pass access
token in query parameters. token in query parameters.
In case of implicit grant, a URL like "client.example/ In case of implicit grant, a URL like "client.example/
redirection_endpoint#access_token=abcdef" may also end up in the redirection_endpoint#access_token=abcdef" may also end up in the
browser history as a result of a redirect from a provider's browser history as a result of a redirect from a provider's
authorization endpoint. authorization endpoint.
Proposed countermeasures: Proposed countermeasures:
o Replace implicit flow with postmessage communication or the * Replace implicit flow with postmessage communication or the
authorization code grant authorization code grant
o Never pass access tokens in URL query parameters * Never pass access tokens in URL query parameters
3.4. Mix-Up 3.4. Mix-Up
Mix-up is an attack on scenarios where an OAuth client interacts with Mix-up is an attack on scenarios where an OAuth client interacts with
multiple authorization servers, as is usually the case when dynamic multiple authorization servers, as is usually the case when dynamic
registration is used. The goal of the attack is to obtain an registration is used. The goal of the attack is to obtain an
authorization code or an access token by tricking the client into authorization code or an access token by tricking the client into
sending those credentials to the attacker instead of using them at sending those credentials to the attacker instead of using them at
the respective endpoint at the authorization/resource server. the respective endpoint at the authorization/resource server.
3.4.1. Attack Description 3.4.1. Attack Description
For a detailed attack description, refer to [arXiv.1601.01229] and For a detailed attack description, refer to [arXiv.1601.01229] and
[I-D.ietf-oauth-mix-up-mitigation]. The description here closely [I-D.ietf-oauth-mix-up-mitigation]. The description here closely
follows [arXiv.1601.01229], with variants of the attack outlined follows [arXiv.1601.01229], with variants of the attack outlined
below. below.
Preconditions: For the attack to work, we assume that Preconditions: For the attack to work, we assume that
(1) the implicit or authorization code grant are used with multiple * the implicit or authorization code grant are used with multiple AS
AS of which one is considered "honest" (H-AS) and one is of which one is considered "honest" (H-AS) and one is operated by
operated by the attacker (A-AS), the attacker (A-AS),
(2) the client stores the AS chosen by the user in a session bound * the client stores the AS chosen by the user in a session bound to
to the user's browser and uses the same redirection endpoint URI the user's browser and uses the same redirection endpoint URI for
for each AS, and each AS, and
(3) the attacker can manipulate the first request/response pair from * the attacker can manipulate the first request/response pair from a
a user's browser to the client (in which the user selects a user's browser to the client (in which the user selects a certain
certain AS and is then redirected by the client to that AS). AS and is then redirected by the client to that AS).
Some of the attack variants described below require different Some of the attack variants described below require different
preconditions. preconditions.
In the following, we assume that the client is registered with H-AS In the following, we assume that the client is registered with H-AS
(URI: "https://honest.as.example", client id: 7ZGZldHQ) and with A-AS (URI: "https://honest.as.example" (https://honest.as.example"),
(URI: "https://attacker.example", client id: 666RVZJTA). client id: 7ZGZldHQ) and with A-AS (URI: "https://attacker.example"
(https://attacker.example"), client id: 666RVZJTA).
Attack on the authorization code grant: Attack on the authorization code grant:
(1) The user selects to start the grant using H-AS (e.g., by 1. The user selects to start the grant using H-AS (e.g., by clicking
clicking on a button at the client's website). on a button at the client's website).
(2) The attacker intercepts this request and changes the user's 2. The attacker intercepts this request and changes the user's
selection to "A-AS". selection to "A-AS".
(3) The client stores in the user's session that the user selected 3. The client stores in the user's session that the user selected
"A-AS" and redirects the user to A-AS's authorization endpoint "A-AS" and redirects the user to A-AS's authorization endpoint by
by sending the following response: sending the following response:
HTTP/1.1 303 See Other HTTP/1.1 303 See Other
Location: https://attacker.example/authorize?response_type=code&client_id=666RVZJTA Location: https://attacker.example/authorize?response_type=code&client_id=666RVZJTA
(4) Now the attacker intercepts this response and changes the 4. Now the attacker intercepts this response and changes the
redirection such that the user is being redirected to H-AS. The redirection such that the user is being redirected to H-AS. The
attacker also replaces the client id of the client at A-AS with attacker also replaces the client id of the client at A-AS with
the client's id at H-AS, resulting in the following response the client's id at H-AS, resulting in the following response
being sent to the browser: being sent to the browser:
HTTP/1.1 303 See Other HTTP/1.1 303 See Other
Location: https://honest.as.example/authorize?response_type=code&client_id=7ZGZldHQ Location: https://honest.as.example/authorize?response_type=code&client_id=7ZGZldHQ
(5) Now, the user authorizes the client to access her resources at 5. Now, the user authorizes the client to access her resources at
H-AS. H-AS issues a code and sends it (via the browser) back to H-AS. H-AS issues a code and sends it (via the browser) back to
the client. the client.
(6) Since the client still assumes that the code was issued by A-AS, 6. Since the client still assumes that the code was issued by A-AS,
it will try to redeem the code at A-AS's token endpoint. it will try to redeem the code at A-AS's token endpoint.
(7) The attacker therefore obtains code and can either exchange the 7. The attacker therefore obtains code and can either exchange the
code for an access token (for public clients) or perform a code code for an access token (for public clients) or perform a code
injection attack as described in Section 3.5. injection attack as described in Section 3.5.
Variants: Variants:
Implicit Grant In the implicit grant, the attacker receives an * *Implicit Grant*: In the implicit grant, the attacker receives an
access token instead of the code; the rest of the attack access token instead of the code; the rest of the attack works as
works as above. above.
Mix-Up Without Interception A variant of the above attack works even
if the first request/response pair cannot be intercepted (for
example, because TLS is used to protect these messages):
Here, we assume that the user wants to start the grant using * *Mix-Up Without Interception*: A variant of the above attack works
A-AS (and not H-AS). After the client redirected the user to even if the first request/response pair cannot be intercepted (for
the authorization endpoint at A-AS, the attacker immediately example, because TLS is used to protect these messages): Here, we
redirects the user to H-AS (changing the client id assume that the user wants to start the grant using A-AS (and not
"7ZGZldHQ"). (A vigilant user might at this point detect H-AS). After the client redirected the user to the authorization
that she intended to use A-AS instead of H-AS.) The attack endpoint at A-AS, the attacker immediately redirects the user to
now proceeds exactly as in step 1 of the attack description H-AS (changing the client id "7ZGZldHQ"). (A vigilant user might
above. at this point detect that she intended to use A-AS instead of
H-AS.) The attack now proceeds exactly as in step <xref
format="counter" target="list_mixup_acg_after_authep"/> of the
attack description above. <!-- I think this counter is not working
properly! -->
Per-AS Redirect URIs If clients use different redirect URIs for * *Per-AS Redirect URIs*: If clients use different redirect URIs for
different ASs, do not store the selected AS in the user's different ASs, do not store the selected AS in the user's session,
session, and ASs do not check the redirect URIs properly, and ASs do not check the redirect URIs properly, attackers can
attackers can mount an attack called "Cross-Social Network mount an attack called "Cross-Social Network Request Forgery".
Request Forgery". We refer to [oauth_security_jcs_14] for Refer to [oauth_security_jcs_14] for details.
details.
OpenID Connect There are several variants that can be used to attack * *OpenID Connect*: There are several variants that can be used to
OpenID Connect. They are described in detail in attack OpenID Connect. They are described in detail in
[arXiv.1704.08539], Appendix A, and [arXiv.1508.04324v2], [arXiv.1704.08539], Appendix A, and [arXiv.1508.04324v2],
Section 6 ("Malicious Endpoints Attacks"). Section 6 ("Malicious Endpoints Attacks").
3.4.2. Countermeasures 3.4.2. Countermeasures
In scenarios where an OAuth client interacts with multiple In scenarios where an OAuth client interacts with multiple
authorization servers, clients MUST prevent mix-up attacks. authorization servers, clients MUST prevent mix-up attacks.
Potential countermeasures: Potential countermeasures:
o Configure authorization servers to return an AS identitifier * Configure authorization servers to return an AS identitifier
("iss") and the "client_id" for which a code or token was issued ("iss") and the "client_id" for which a code or token was issued
in the authorization response. This enables clients to compare in the authorization response. This enables clients to compare
this data to their own client id and the "iss" identifier of the this data to their own client id and the "iss" identifier of the
AS it believed it sent the user agent to. This mitigation is AS it believed it sent the user agent to. This mitigation is
discussed in detail in [I-D.ietf-oauth-mix-up-mitigation]. In discussed in detail in [I-D.ietf-oauth-mix-up-mitigation]. In
OpenID Connect, if an ID token is returned in the authorization OpenID Connect, if an ID token is returned in the authorization
response, it carries client id and issuer. It can be used for response, it carries client id and issuer. It can be used for
this mitigation. this mitigation.
o As it can be seen in the preconditions of the attacks above, * As it can be seen in the preconditions of the attacks above,
clients can prevent mix-up attack by (1) using AS-specific clients can prevent mix-up attack by (1) using AS-specific
redirect URIs with exact redirect URI matching, (2) storing, for redirect URIs with exact redirect URI matching, (2) storing, for
each authorization request, the intended AS, and (3) comparing the each authorization request, the intended AS, and (3) comparing the
intended AS with the actual redirect URI where the authorization intended AS with the actual redirect URI where the authorization
response was received. response was received.
3.5. Authorization Code Injection 3.5. Authorization Code Injection
In such an attack, the adversary attempts to inject a stolen In such an attack, the adversary attempts to inject a stolen
authorization code into a legitimate client on a device under his authorization code into a legitimate client on a device under his
control. In the simplest case, the attacker would want to use the control. In the simplest case, the attacker would want to use the
code in his own client. But there are situations where this might code in his own client. But there are situations where this might
not be possible or intended. Examples are: not be possible or intended. Examples are:
o The attacker wants to access certain functions in this particular * The attacker wants to access certain functions in this particular
client. As an example, the attacker wants to impersonate his client. As an example, the attacker wants to impersonate his
victim in a certain app or on a certain web site. victim in a certain app or on a certain web site.
o The code is bound to a particular confidential client and the * The code is bound to a particular confidential client and the
attacker is unable to obtain the required client credentials to attacker is unable to obtain the required client credentials to
redeem the code himself. redeem the code himself.
o The authorization or resource servers are limited to certain * The authorization or resource servers are limited to certain
networks, the attackers is unable to access directly. networks, the attackers is unable to access directly.
How does an attack look like? How does an attack look like?
(1) The attacker obtains an authorization code by performing any of 1. The attacker obtains an authorization code by performing any of
the attacks described above. the attacks described above.
(2) It performs a regular OAuth authorization process with the 2. It performs a regular OAuth authorization process with the
legitimate client on his device. legitimate client on his device.
(3) The attacker injects the stolen authorization code in the 3. The attacker injects the stolen authorization code in the
response of the authorization server to the legitimate client. response of the authorization server to the legitimate client.
(4) The client sends the code to the authorization server's token 4. The client sends the code to the authorization server's token
endpoint, along with client id, client secret and actual endpoint, along with client id, client secret and actual
"redirect_uri". "redirect_uri".
(5) The authorization server checks the client secret, whether the 5. The authorization server checks the client secret, whether the
code was issued to the particular client and whether the actual code was issued to the particular client and whether the actual
redirect URI matches the "redirect_uri" parameter (see redirect URI matches the "redirect_uri" parameter (see
[RFC6749]). [RFC6749]).
(6) If all checks succeed, the authorization server issues access 6. If all checks succeed, the authorization server issues access and
and other tokens to the client, so now the attacker is able to other tokens to the client, so now the attacker is able to
impersonate the legitimate user. impersonate the legitimate user.
Obviously, the check in step (5) will fail, if the code was issued to Obviously, the check in step (5.) will fail, if the code was issued
another client id, e.g., a client set up by the attacker. The check to another client id, e.g., a client set up by the attacker. The
will also fail if the authorization code was already redeemed by the check will also fail if the authorization code was already redeemed
legitimate user and was one-time use only. by the legitimate user and was one-time use only.
An attempt to inject a code obtained via a malware pretending to be An attempt to inject a code obtained via a malware pretending to be
the legitimate client should also be detected, if the authorization the legitimate client should also be detected, if the authorization
server stored the complete redirect URI used in the authorization server stored the complete redirect URI used in the authorization
request and compares it with the redirect_uri parameter. request and compares it with the redirect_uri parameter.
[RFC6749], Section 4.1.3, requires the AS to "... ensure that the [RFC6749], Section 4.1.3, requires the AS to "... ensure that the
"redirect_uri" parameter is present if the "redirect_uri" parameter "redirect_uri" parameter is present if the "redirect_uri" parameter
was included in the initial authorization request as described in was included in the initial authorization request as described in
Section 4.1.1, and if included ensure that their values are Section 4.1.1, and if included ensure that their values are
skipping to change at page 17, line 25 skipping to change at line 789
client would use the correct redirect URI it always uses for client would use the correct redirect URI it always uses for
authorization requests. But this URI would not match the tampered authorization requests. But this URI would not match the tampered
redirect URI used by the attacker (otherwise, the redirect would not redirect URI used by the attacker (otherwise, the redirect would not
land at the attackers page). So the authorization server would land at the attackers page). So the authorization server would
detect the attack and refuse to exchange the code. detect the attack and refuse to exchange the code.
Note: this check could also detect attempt to inject a code, which Note: this check could also detect attempt to inject a code, which
had been obtained from another instance of the same client on another had been obtained from another instance of the same client on another
device, if certain conditions are fulfilled: device, if certain conditions are fulfilled:
o the redirect URI itself needs to contain a nonce or another kind * the redirect URI itself needs to contain a nonce or another kind
of one-time use, secret data and of one-time use, secret data and
o the client has bound this data to this particular instance. * the client has bound this data to this particular instance.
But this approach conflicts with the idea to enforce exact redirect But this approach conflicts with the idea to enforce exact redirect
URI matching at the authorization endpoint. Moreover, it has been URI matching at the authorization endpoint. Moreover, it has been
observed that providers very often ignore the redirect_uri check observed that providers very often ignore the redirect_uri check
requirement at this stage, maybe because it doesn't seem to be requirement at this stage, maybe because it doesn't seem to be
security-critical from reading the spec. security-critical from reading the spec.
Other providers just pattern match the redirect_uri parameter against Other providers just pattern match the redirect_uri parameter against
the registered redirect URI pattern. This saves the authorization the registered redirect URI pattern. This saves the authorization
server from storing the link between the actual redirect URI and the server from storing the link between the actual redirect URI and the
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call to the tokens endpoint. call to the tokens endpoint.
This document therefore recommends to instead bind every This document therefore recommends to instead bind every
authorization code to a certain client instance on a certain device authorization code to a certain client instance on a certain device
(or in a certain user agent) in the context of a certain transaction. (or in a certain user agent) in the context of a certain transaction.
3.5.1. Proposed Countermeasures 3.5.1. Proposed Countermeasures
There are multiple technical solutions to achieve this goal: There are multiple technical solutions to achieve this goal:
Nonce OpenID Connect's existing "nonce" parameter could be used for * *Nonce*: OpenID Connect's existing "nonce" parameter could be used
this purpose. The nonce value is one-time use and created by 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 the client. The client is supposed to bind it to the user agent
agent session and sends it with the initial request to the session and sends it with the initial request to the OpenId
OpenId Provider (OP). The OP associates the nonce to the Provider (OP). The OP associates the nonce to the authorization
authorization code and attests this binding in the ID token, code and attests this binding in the ID token, which is issued as
which is issued as part of the code exchange at the token part of the code exchange at the token endpoint. If an attacker
endpoint. If an attacker injected an authorization code in injected an authorization code in the authorization response, the
the authorization response, the nonce value in the client nonce value in the client session and the nonce value in the ID
session and the nonce value in the ID token will not match token will not match and the attack is detected. The assumption
and the attack is detected. The assumption is that an is that an attacker cannot get hold of the user agent state on the
attacker cannot get hold of the user agent state on the victims device, where he has stolen the respective authorization
victims device, where he has stolen the respective code. The main advantage of this option is that Nonce is an
authorization code. The main advantage of this option is existing feature used in the wild. On the other hand, leveraging
that Nonce is an existing feature used in the wild. On the Nonce by the broader OAuth community would require AS and client
other hand, leveraging Nonce by the broader OAuth community to adopt ID Tokens.
would require AS and client to adopt ID Tokens.
Code-bound State The "state" parameter as specified in [RFC6749] * *Code-bound State*: The "state" parameter as specified in
could be used similarly to what is described above. This [RFC6749] could be used similarly to what is described above.
would require to add a further parameter "state" to the code This would require to add a further parameter "state" to the code
exchange token endpoint request. The authorization server exchange token endpoint request. The authorization server would
would then compare the "state" value it associated with the then compare the "state" value it associated with the code and the
code and the "state" value in the parameter. If those values "state" value in the parameter. If those values do not match, it
do not match, it is considered an attack and the request is considered an attack and the request fails. The advantage of
fails. The advantage of this approach would be to utilize an this approach would be to utilize an existing OAuth parameter.
existing OAuth parameter. But it would also mean to re- But it would also mean to re-interpret the purpose of "state" and
interpret the purpose of "state" and to extend the token to extend the token endpoint request.
endpoint request.
PKCE The PKCE parameter "challenge" along with the corresponding * *PKCE*: The PKCE parameter "challenge" along with the
"verifier" as specified in [RFC7636] could be used in the corresponding "verifier" as specified in [RFC7636] could be used
same way as "nonce" or "state". In contrast to its original in the same way as "nonce" or "state". In contrast to its
intention, the verifier check would fail although the client original intention, the verifier check would fail although the
uses its correct verifier but the code is associated with a client uses its correct verifier but the code is associated with a
challenge, which does not match. PKCE is a deployed OAuth challenge, which does not match. PKCE is a deployed OAuth
feature, even though it is used today to secure native apps, feature, even though it is used today to secure native apps, only.
only.
Token Binding Token binding [I-D.ietf-oauth-token-binding] could * *Token Binding*: Token binding [I-D.ietf-oauth-token-binding]
also be used. In this case, the code would need to be bound could also be used. In this case, the code would need to be bound
to two legs, between user agent and AS and the user agent and to two legs, between user agent and AS and the user agent and the
the client. This requires further data (extension to client. This requires further data (extension to response) to
response) to manifest binding id for particular code. Token manifest binding id for particular code. Token binding is
binding is promising as a secure and convenient mechanism promising as a secure and convenient mechanism (due to its browser
(due to its browser integration). As a challenge, it integration). As a challenge, it requires broad browser support
requires broad browser support and use with native apps is and use with native apps is still under discussion.
still under discussion.
per instance client id/secret One could use per instance "client_id" * *per instance client id/secret*: One could use per instance
and secrets and bind the code to the respective "client_id". "client_id" and secrets and bind the code to the respective
Unfortunately, this does not fit into the web application "client_id". Unfortunately, this does not fit into the web
programming model (would need to use per user client ids). application programming model (would need to use per user client
ids). </list>
PKCE seems to be the most obvious solution for OAuth clients as it PKCE seems to be the most obvious solution for OAuth clients as it
available and effectively used today for similar purposes for OAuth available and effectively used today for similar purposes for OAuth
native apps whereas "nonce" is appropriate for OpenId Connect native apps whereas "nonce" is appropriate for OpenId Connect
clients. clients.
Note on pre-warmed secrets: An attacker can circumvent the Note on pre-warmed secrets: An attacker can circumvent the
countermeasures described above if he is able to create or capture countermeasures described above if he is able to create or capture
the respective secret or code_challenge on a device under his the respective secret or code_challenge on a device under his
control, which is then used in the victim's authorization request. control, which is then used in the victim's authorization request.
skipping to change at page 20, line 27 skipping to change at line 932
An attacker might attempt to inject a request to the redirect URI of An attacker might attempt to inject a request to the redirect URI of
the legitimate client on the victim's device, e.g., to cause the the legitimate client on the victim's device, e.g., to cause the
client to access resources under the attacker's control. client to access resources under the attacker's control.
3.7.1. Proposed Countermeasures 3.7.1. Proposed Countermeasures
Standard CSRF defenses should be used to protect the redirection Standard CSRF defenses should be used to protect the redirection
endpoint, for example: endpoint, for example:
CSRF Tokens Use of CSRF tokens which are bound to the user agent * *CSRF Tokens*: Use of CSRF tokens which are bound to the user
and passed in the "state" parameter to the agent and passed in the "state" parameter to the authorization
authorization server. server.
Origin Header The Origin header can be used to detect and prevent * *Origin Header*: The Origin header can be used to detect and
CSRF attacks. Since this feature, at the time of prevent CSRF attacks. Since this feature, at the time of writing,
writing, is not consistently supported by all is not consistently supported by all browsers, CSRF tokens should
browsers, CSRF tokens should be used in addition to be used in addition to Origin header checking.
Origin header checking.
For more details see [owasp_csrf]. For more details see [owasp_csrf].
3.8. Access Token Leakage at the Resource Server 3.8. Access Token Leakage at the Resource Server
Access tokens can leak from a resource server under certain Access tokens can leak from a resource server under certain
circumstances. circumstances.
3.8.1. Access Token Phishing by Counterfeit Resource Server 3.8.1. Access Token Phishing by Counterfeit Resource Server
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3.8.1.1. Metadata 3.8.1.1. 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 Content-Type: application/json
Content-Type: application/json
{ {
"issuer":"https://server.somesite.example", "issuer":"https://server.somesite.example",
"authorization_endpoint": "authorization_endpoint":
"https://server.somesite.example/authorize", "https://server.somesite.example/authorize",
"resource_servers":[ “resource_servers”:[
"email.somesite.example", “email.somesite.example”,
"storage.somesite.example", ”storage.somesite.example”,
"video.somesite.example"] ”video.somesite.example”]
... ...
} }
The AS could also return the URL(s) an access token is good for in The AS could also return the URL(s) an access token is good for in
the token response, illustrated by the example return parameter the token response, illustrated by the example return parameter
"access_token_resource_server": "access_token_resource_server":
HTTP/1.1 200 OK HTTP/1.1 200 OK
Content-Type: application/json;charset=UTF-8 Content-Type: application/json;charset=UTF-8
Cache-Control: no-store Cache-Control: no-store
Pragma: no-cache Pragma: no-cache
{ {
"access_token":"2YotnFZFEjr1zCsicMWpAA", "access_token":"2YotnFZFEjr1zCsicMWpAA",
"access_token_resource_server": “access_token_resource_server”:
"https://hostedresource.somesite.example/path1", "https://hostedresource.somesite.example/path1",
... ...
} }
This mitigation strategy would rely on the client to enforce the This mitigation strategy would rely on the client to enforce the
security policy and to only send access tokens to legitimate security policy and to only send access tokens to legitimate
destinations. Results of OAuth related security research (see for destinations. Results of OAuth related security research (see for
example [oauth_security_ubc] and [oauth_security_cmu]) indicate a example [#@!oauth_security_ubc] and [#!oauth_security_cmu]) indicate
large portion of client implementations do not or fail to properly a large portion of client implementations do not or fail to properly
implement security controls, like "state" checks. So relying on implement security controls, like "state" checks. So relying on
clients to prevent access token phishing is likely to fail as well. clients to prevent access token phishing is likely to fail as well.
Moreover given the ratio of clients to authorization and resource Moreover given the ratio of clients to authorization and resource
servers, it is considered the more viable approach to move as much as servers, it is considered the more viable approach to move as much as
possible security-related logic to those entities. Clearly, the possible security-related logic to those entities. Clearly, the
client has to contribute to the overall security. But there are client has to contribute to the overall security. But there are
alternative countermeasures, as described in the next sections, which alternative countermeasures, as described in the next sections, which
provide a better balance between the involved parties. provide a better balance between the involved parties.
3.8.1.2. Sender Constrained Access Tokens 3.8.1.2. Sender Constrained Access Tokens
skipping to change at page 23, line 15 skipping to change at line 1053
connection. connection.
3. The RS must implement the actual proof of possession check. This 3. The RS must implement the actual proof of possession check. This
is typically done on the application level, it may utilize is typically done on the application level, it may utilize
capabilities of the transport layer (e.g., TLS). Note: replay capabilities of the transport layer (e.g., TLS). Note: replay
prevention is required as well! prevention is required as well!
There exists several proposals to demonstrate the proof of possession There exists several proposals to demonstrate the proof of possession
in the scope of the OAuth working group: in the scope of the OAuth working group:
o [I-D.ietf-oauth-token-binding]: In this approach, an access tokens * [I-D.ietf-oauth-token-binding]: In this approach, an access tokens
is, via the so-called token binding id, bound to key material is, via the so-called token binding id, bound to key material
representing a long term association between a client and a representing a long term association between a client and a
certain TLS host. Negotiation of the key material and proof of certain TLS host. Negotiation of the key material and proof of
possession in the context of a TLS handshake is taken care of by possession in the context of a TLS handshake is taken care of by
the TLS stack. The client needs to determine the token binding id the TLS stack. The client needs to determine the token binding id
of the target resource server and pass this data to the access of the target resource server and pass this data to the access
token request. The authorization server than associates the token request. The authorization server than associates the
access token with this id. The resource server checks on every access token with this id. The resource server checks on every
invocation that the token binding id of the active TLS connection invocation that the token binding id of the active TLS connection
and the token binding id of associated with the access token and the token binding id of associated with the access token
match. Since all crypto-related functions are covered by the TLS match. Since all crypto-related functions are covered by the TLS
stack, this approach is very client developer friendly. As a stack, this approach is very client developer friendly. As a
prerequisite, token binding as described in prerequisite, token binding as described in [I-D.ietf-tokbind-
[I-D.ietf-tokbind-https] (including federated token bindings) must https] (including federated token bindings) must be supported on
be supported on all ends (client, authorization server, resource all ends (client, authorization server, resource server).
server).
o [I-D.ietf-oauth-mtls]: The approach as specified in this document * [I-D.ietf-oauth-mtls]: The approach as specified in this document
allow use of mutual TLS for both client authentication and sender allow use of mutual TLS for both client authentication and sender
constraint access tokens. For the purpose of sender constraint constraint access tokens. For the purpose of sender constraint
access tokens, the client is identified towards the resource access tokens, the client is identified towards the resource
server by the fingerprint of its public key. During processing of server by the fingerprint of its public key. During processing of
an access token request, the authorization server obtains the an access token request, the authorization server obtains the
client's public key from the TLS stack and associates its client's public key from the TLS stack and associates its
fingerprint with the respective access tokens. The resource fingerprint with the respective access tokens. The resource
server in the same way obtains the public key from the TLS stack server in the same way obtains the public key from the TLS stack
and compares its fingerprint with the fingerprint associated with and compares its fingerprint with the fingerprint associated with
the access token. the access token.
o [I-D.ietf-oauth-signed-http-request] specifies an approach to sign * [I-D.ietf-oauth-signed-http-request] specifies an approach to sign
HTTP requests. It utilizes [I-D.ietf-oauth-pop-key-distribution] HTTP requests. It utilizes [I-D.ietf-oauth-pop-key-distribution]
and represents the elements of the signature in a JSON object. and represents the elements of the signature in a JSON object.
The signature is built using JWS. The mechanism has built-in The signature is built using JWS. The mechanism has built-in
support for signing of HTTP method, query parameters and headers. support for signing of HTTP method, query parameters and headers.
It also incorporates a timestamp as basis for replay prevention. It also incorporates a timestamp as basis for replay prevention.
o [I-D.sakimura-oauth-jpop]: this draft describes different ways to * [I-D.sakimura-oauth-jpop]: this draft describes different ways to
constrain access token usage, namely TLS or request signing. constrain access token usage, namely TLS or request signing.
Note: Since the authors of this draft contributed the TLS-related Note: Since the authors of this draft contributed the TLS-related
proposal to [I-D.ietf-oauth-mtls], this document only considers proposal to [I-D.ietf-oauth-mtls], this document only considers
the request signing part. For request signing, the draft utilizes the request signing part. For request signing, the draft utilizes
[I-D.ietf-oauth-pop-key-distribution] and RFC 7800 [RFC7800]. The [I-D.ietf-oauth-pop-key-distribution] and [RFC7800]. The
signature data is represented in a JWT and JWS is used for signature data is represented in a JWT and JWS is used for
signing. Replay prevention is provided by building the signature signing. Replay prevention is provided by building the signature
over a server-provided nonce, client-provided nonce and a nonce over a server-provided nonce, client-provided nonce and a nonce
counter. counter.
[I-D.ietf-oauth-mtls] and [I-D.ietf-oauth-token-binding] are built on [I-D.ietf-oauth-mtls] and [I-D.ietf-oauth-token-binding] are built on
top of TLS and this way continue the successful OAuth 2.0 philosophy top of TLS and this way continue the successful OAuth 2.0 philosophy
to leverage TLS to secure OAuth wherever possible. Both mechanisms to leverage TLS to secure OAuth wherever possible. Both mechanisms
allow prevention of access token leakage in a fairly client developer allow prevention of access token leakage in a fairly client developer
friendly way. friendly way.
skipping to change at page 24, line 34 skipping to change at line 1118
managed by the TLS stack whereas [I-D.ietf-oauth-mtls] requires the managed by the TLS stack whereas [I-D.ietf-oauth-mtls] requires the
developer to create and maintain the key pairs and respective developer to create and maintain the key pairs and respective
certificates. Use of self-signed certificates, which is supported by certificates. Use of self-signed certificates, which is supported by
the draft, significantly reduce the complexity of this task. the draft, significantly reduce the complexity of this task.
Furthermore, [I-D.ietf-oauth-token-binding] allows to use different Furthermore, [I-D.ietf-oauth-token-binding] allows to use different
key pairs for different resource servers, which is a privacy benefit. key pairs for different resource servers, which is a privacy benefit.
On the other hand, [I-D.ietf-oauth-mtls] only requires widely On the other hand, [I-D.ietf-oauth-mtls] only requires widely
deployed TLS features, which means it might be easier to adopt in the deployed TLS features, which means it might be easier to adopt in the
short term. short term.
Application level signing approaches, like Application level signing approaches, like [I-D.ietf-oauth-signed-
[I-D.ietf-oauth-signed-http-request] and [I-D.sakimura-oauth-jpop] http-request] and [I-D.sakimura-oauth-jpop] have been debated for a
have been debated for a long time in the OAuth working group without long time in the OAuth working group without a clear outcome.
a clear outcome.
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 prevention). well as state management (e.g., replay prevention).
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.
skipping to change at page 26, line 39 skipping to change at line 1214
Preventing server breaches by way of hardening and monitoring server Preventing server breaches by way of hardening and monitoring server
systems is considered a standard operational procedure and therefore systems is considered a standard operational procedure and therefore
out of scope of this document. This section will focus on the impact out of scope of this document. This section will focus on the impact
of such breaches on OAuth-related parts of the ecosystem, which is of such breaches on OAuth-related parts of the ecosystem, which is
the replay of captured access tokens on the compromised resource the replay of captured access tokens on the compromised resource
server and other resource servers of the respective deployment. server and other resource servers of the respective deployment.
The following measures should be taken into account by implementors The following measures should be taken into account by implementors
in order to cope with access token replay: in order to cope with access token replay:
o The resource server must treat access tokens like any other * 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 3.8.1.2 * Sender constraint access tokens as described in Section 3.8.1.2
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 3.8.1.3 may be used * Audience restriction as described in Section 3.8.1.3 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.
3.9. Open Redirection 3.9. Open Redirection
The following attacks can occur when an AS or client has an open The following attacks can occur when an AS or client has an open
redirector, i.e., a URL which causes an HTTP redirect to an attacker- redirector, i.e., a URL which causes an HTTP redirect to an attacker-
controlled web site. controlled web site.
3.9.1. Authorization Server as Open Redirector 3.9.1. Authorization Server as Open Redirector
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Refresh tokens themself are an attractive target for attackers since Refresh tokens themself are an attractive target for attackers since
they represent the overall grant a resource owner delegated to a they represent the overall grant a resource owner delegated to a
certain client. If an attacker is able to exfiltrate and certain client. If an attacker is able to exfiltrate and
successfully replay a refresh token, it will be able to mint access successfully replay a refresh token, it will be able to mint access
tokens and use them to access resource servers on behalf of the tokens and use them to access resource servers on behalf of the
resource server. resource server.
[RFC6749] already provides robust base protection by requiring [RFC6749] already provides robust base protection by requiring
o confidentiality of the refresh tokens in transit and storage, * confidentiality of the refresh tokens in transit and storage,
o the transmission of refresh tokens over TLS-protected connections * the transmission of refresh tokens over TLS-protected connections
between authorization server and client, between authorization server and client,
o the authorization server to maintain and check the binding of a * the authorization server to maintain and check the binding of a
refresh token to a certain client_id, refresh token to a certain client_id,
o authentication of this client_id during token refresh, if * authentication of this client_id during token refresh, if
possible, and possible, and
o that refresh tokens cannot be generated, modified, or guessed. * that refresh tokens cannot be generated, modified, or guessed.
[RFC6749] also lays the foundation for further (implementation [RFC6749] also lays the foundation for further (implementation
specific) security measures, such as refresh token expiration and specific) security measures, such as refresh token expiration and
revocation as well as refresh token rotation by defining respective revocation as well as refresh token rotation by defining respective
error codes and response behavior. error codes and response behavior.
This draft gives recommendations beyond the scope of [RFC6749] and This draft gives recommendations beyond the scope of [RFC6749] and
clarifications. clarifications.
Authorization servers SHALL determine based on their risk assessment Authorization servers MUST determine based on their risk assessment
whether to issue refresh tokens to a certain client. If the whether to issue refresh tokens to a certain client. If the
authorization server decides not to issue refresh tokens, the client authorization server decides not to issue refresh tokens, the client
may refresh access tokens by utilizing other grant types, such as the may refresh access tokens by utilizing other grant types, such as the
authorization code grant type. In such a case, the authorization authorization code grant type. In such a case, the authorization
server may utilize cookies and persistents grants to optimize the server may utilize cookies and persistents grants to optimize the
user experience. user experience.
If refresh tokens are issued, those refresh tokens MUST be bound to If refresh tokens are issued, those refresh tokens MUST be bound to
the scope and resource servers as consented by the resource owner. the scope and resource servers as consented by the resource owner.
This is to prevent privilege escalation by the legit client and This is to prevent privilege escalation by the legit client and
reduce the impact of refresh tokens leakage. reduce the impact of refresh tokens leakage.
Authorization server SHALL utilize one of the methods listed below to Authorization server MUST utilize one of the methods listed below to
detect refresh token replay for public clients: detect refresh token replay for public clients:
o Refresh token rotation: the authorization issues a new refresh * Sender constrained refresh tokens: the authorization server
cryptographically binds the refresh token to a certain client
instance by utilizing [I-D.ietf-oauth-token-binding] or [I-D.ietf-
oauth-mtls].
* Refresh token rotation: the authorization issues a new refresh
token with every access token refresh response. The previous token with every access token refresh response. The previous
refresh token is invalidated but information about the refresh token is invalidated but information about the
relationship is retained by the authorization server. If a relationship is retained by the authorization server. If a
refresh token is compromised and subsequently used by both the refresh token is compromised and subsequently used by both the
attacker and the legitimate client, one of them will present an attacker and the legitimate client, one of them will present an
invalidated refresh token, which will inform the authorization invalidated refresh token, which will inform the authorization
server of the breach. The authorization server cannot determine server of the breach. The authorization server cannot determine
which party submitted the invalid refresh token, but it can revoke which party submitted the invalid refresh token, but it can revoke
the active refresh token. This stops the attack at the cost of the active refresh token. This stops the attack at the cost of
forcing the legit client to obtain a fresh authorization grant. forcing the legit client to obtain a fresh authorization grant.
o Sender constrained refresh tokens: the authorization server Implementation note: refresh tokens belonging to the same grant
cryptographically binds the refresh token to a certain client may share a common id. If any of those refresh tokens is used at
instance by utilizing [I-D.ietf-oauth-token-binding] or the authorization server, the authorization server uses this
[I-D.ietf-oauth-mtls]. common id to look up the currently active refresh token and can
revoke it.
Authorization servers may revoke refresh tokens automatically in case Authorization servers may revoke refresh tokens automatically in case
of a security event, such as: of a security event, such as:
o password change * password change
o logout at the authorization server * logout at the authorization server
Refresh tokens should expire if the client has been inactive for some Refresh tokens SHOULD expire if the client has been inactive for some
time. time,i.e. the refresh token has not been used to obtain fresh access
tokens for some time. The expiration time is at the discretion of
the authorization server. It might be a global value or determined
based on the client policy or the grant associated with the refresh
token (and its sensitivity).
4. Acknowledgements 4. Acknowledgements
We would like to thank Jim Manico, Phil Hunt, Nat Sakimura, Christian We would like to thank Jim Manico, Phil Hunt, Nat Sakimura, Christian
Mainka, Doug McDorman, Johan Peeters, Joseph Heenan, Brock Allen, and Mainka, Doug McDorman, Johan Peeters, Joseph Heenan, Brock Allen,
Brian Campbell for their valuable feedback. Vittorio Bertocci, David Waite, Nov Matake, Tomek Stojecki, Dominick
Baier, Neil Madden, William Dennis, Dick Hardt, Petteri Stenius,
Annabelle Richard Backman, Aaron Parecki, George Fletscher, and Brian
Campbell for their valuable feedback.
5. IANA Considerations 5. IANA Considerations
This draft includes no request to IANA. This draft includes no request to IANA.
6. Security Considerations 6. Security Considerations
All relevant security considerations have been given in the All relevant security considerations have been given in the
functional specification. functional specification.
7. References 7. Normative References
7.1. 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>.
[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>.
[RFC6819] Lodderstedt, T., Ed., McGloin, M., and P. Hunt, "OAuth 2.0
Threat Model and Security Considerations", RFC 6819,
DOI 10.17487/RFC6819, January 2013,
<https://www.rfc-editor.org/info/rfc6819>.
[RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
DOI 10.17487/RFC7231, June 2014,
<https://www.rfc-editor.org/info/rfc7231>.
[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>.
7.2. Informative References
[arXiv.1508.04324v2] [arXiv.1508.04324v2]
Mladenov, V., Mainka, C., and J. Schwenk, "On the security Schwenk, J., "On the security of modern Single Sign-On
of modern Single Sign-On Protocols: Second-Order Protocols: Second-Order Vulnerabilities in OpenID
Vulnerabilities in OpenID Connect", arXiv 1508.04324v2, Connect", 7 January 2016.
January 2016, <http://arxiv.org/abs/1508.04324v2/>.
[arXiv.1601.01229] [arXiv.1601.01229]
Fett, D., Kuesters, R., and G. Schmitz, "A Comprehensive Schmitz, G., "A Comprehensive Formal Security Analysis of
Formal Security Analysis of OAuth 2.0", arXiv 1601.01229, OAuth 2.0", 6 January 2016.
January 2016, <http://arxiv.org/abs/1601.01229/>.
[arXiv.1704.08539] [arXiv.1704.08539]
Fett, D., Kuesters, R., and G. Schmitz, "The Web SSO Schmitz, G., "The Web SSO Standard OpenID Connect: In-
Standard OpenID Connect: In-Depth Formal Security Analysis Depth Formal Security Analysis and Security Guidelines",
and Security Guidelines", arXiv 1704.08539, April 2017, 27 April 2017.
<http://arxiv.org/abs/1704.08539/>.
[bug.chromium] [bug.chromium]
"Referer header includes URL fragment when opening link "Referer header includes URL fragment when opening link
using New Tab", using New Tab", December 2018.
<https://bugs.chromium.org/p/chromium/issues/
detail?id=168213/>.
[fb_fragments] [fb_fragments]
"Facebook Developer Blog", "Facebook Developer Blog", December 2018.
<https://developers.facebook.com/blog/post/552/>.
[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-09 (work in progress), bradley-oauth-jwt-encoded-state-09 (work in progress), 4
November 2018. November 2018,
<https://www.ietf.org/archive/id/draft-bradley-oauth-jwt-
encoded-state-09>.
[I-D.ietf-oauth-closing-redirectors] [I-D.ietf-oauth-closing-redirectors]
Bradley, J., Sanso, A., and H. Tschofenig, "OAuth 2.0 Bradley, J., Sanso, A., and H. Tschofenig, "OAuth 2.0
Security: Closing Open Redirectors in OAuth", draft-ietf- Security: Closing Open Redirectors in OAuth", draft-ietf-
oauth-closing-redirectors-00 (work in progress), February oauth-closing-redirectors-00 (work in progress), 4
2016. February 2016,
<https://www.ietf.org/archive/id/draft-ietf-oauth-closing-
redirectors-00>.
[I-D.ietf-oauth-jwsreq] [I-D.ietf-oauth-jwsreq]
Sakimura, N. and J. Bradley, "The OAuth 2.0 Authorization Sakimura, N. and J. Bradley, "The OAuth 2.0 Authorization
Framework: JWT Secured Authorization Request (JAR)", Framework: JWT Secured Authorization Request (JAR)",
draft-ietf-oauth-jwsreq-17 (work in progress), October draft-ietf-oauth-jwsreq-17 (work in progress), 21 October
2018. 2018,
<https://www.ietf.org/archive/id/draft-ietf-oauth-jwsreq-
17>.
[I-D.ietf-oauth-mix-up-mitigation] [I-D.ietf-oauth-mix-up-mitigation]
Jones, M., Bradley, J., and N. Sakimura, "OAuth 2.0 Mix-Up Jones, M., Bradley, J., and N. Sakimura, "OAuth 2.0 Mix-Up
Mitigation", draft-ietf-oauth-mix-up-mitigation-01 (work Mitigation", draft-ietf-oauth-mix-up-mitigation-01 (work
in progress), July 2016. in progress), 7 July 2016,
<https://www.ietf.org/archive/id/draft-ietf-oauth-mix-up-
mitigation-01>.
[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, "OAuth 2.0 Mutual TLS Client Authentication Lodderstedt, "OAuth 2.0 Mutual TLS Client Authentication
and Certificate Bound Access Tokens", draft-ietf-oauth- and Certificate Bound Access Tokens", draft-ietf-oauth-
mtls-12 (work in progress), October 2018. mtls-12 (work in progress), 18 October 2018,
<https://www.ietf.org/archive/id/
draft-ietf-oauth-mtls-12>.
[I-D.ietf-oauth-pop-key-distribution] [I-D.ietf-oauth-pop-key-distribution]
Bradley, J., Hunt, P., Jones, M., Tschofenig, H., and M. Bradley, J., Hunt, P., Jones, M., Tschofenig, H., and M.
Mihaly, "OAuth 2.0 Proof-of-Possession: Authorization Mihaly, "OAuth 2.0 Proof-of-Possession: Authorization
Server to Client Key Distribution", draft-ietf-oauth-pop- Server to Client Key Distribution", draft-ietf-oauth-pop-
key-distribution-04 (work in progress), October 2018. key-distribution-04 (work in progress), 23 October 2018,
<https://www.ietf.org/archive/id/draft-ietf-oauth-pop-key-
distribution-04>.
[I-D.ietf-oauth-resource-indicators] [I-D.ietf-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-ietf-oauth-resource- Indicators for OAuth 2.0", draft-ietf-oauth-resource-
indicators-01 (work in progress), October 2018. indicators-01 (work in progress), 19 October 2018,
<https://www.ietf.org/archive/id/draft-ietf-oauth-
resource-indicators-01>.
[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), 8 August 2016,
<https://www.ietf.org/archive/id/draft-ietf-oauth-signed-
http-request-03>.
[I-D.ietf-oauth-token-binding] [I-D.ietf-oauth-token-binding]
Jones, M., Campbell, B., Bradley, J., 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-08 (work in progress), October 2018. binding-08 (work in progress), 19 October 2018,
<https://www.ietf.org/archive/id/draft-ietf-oauth-token-
binding-08>.
[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-18 (work in progress), June 2018. tokbind-https-18 (work in progress), 26 June 2018,
<https://www.ietf.org/archive/id/draft-ietf-tokbind-https-
18>.
[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), 27 March 2017,
<https://www.ietf.org/archive/id/draft-sakimura-oauth-
jpop-04>.
[oauth-v2-form-post-response-mode] [oauth-v2-form-post-response-mode]
Microsoft and Ping Identity, "OAuth 2.0 Form Post Response "OAuth 2.0 Form Post Response Mode", 27 April 2015.
Mode", April 2015, <http://openid.net/specs/
oauth-v2-form-post-response-mode-1_0.html>.
[oauth_security_cmu]
Carnegie Mellon University, Carnegie Mellon University,
Microsoft Research, Carnegie Mellon University, Carnegie
Mellon University, and Carnegie Mellon University, "OAuth
Demystified for Mobile Application Developers", November
2014.
[oauth_security_jcs_14] [oauth_security_jcs_14]
Bansal, C., Bhargavan, K., Delignat-Lavaud, A., and S. Maffeis, S., "Discovering concrete attacks on website
Maffeis, "Discovering concrete attacks on website authorization by formal analysis", 23 April 2014.
authorization by formal analysis", April 2014.
[oauth_security_ubc]
University of British Columbia and University of British
Columbia, "The Devil is in the (Implementation) Details:
An Empirical Analysis of OAuth SSO Systems", October 2012,
<http://passwordresearch.com/papers/paper267.html>.
[OpenID] NRI, Ping Identity, Microsoft, Google, and Salesforce, [OpenID] "OpenID Connect Core 1.0 incorporating errata set 1", 8
"OpenID Connect Core 1.0 incorporating errata set 1", Nov November 2014.
2014,
<http://openid.net/specs/openid-connect-core-1_0.html>.
[owasp] "Open Web Application Security Project Home Page", [owasp] "Open Web Application Security Project Home Page",
<https://www.owasp.org/>. December 2018.
[owasp_csrf] [owasp_csrf]
"Cross-Site Request Forgery (CSRF) Prevention Cheat "Cross-Site Request Forgery (CSRF) Prevention Cheat
Sheet", <https://www.owasp.org/index.php/ Sheet", December 2018.
Cross-Site_Request_Forgery_(CSRF)_Prevention_Cheat_Sheet>.
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., [RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616, Transfer Protocol -- HTTP/1.1", RFC 2616,
DOI 10.17487/RFC2616, June 1999, DOI 10.17487/RFC2616, June 1999,
<https://www.rfc-editor.org/info/rfc2616>. <https://www.rfc-editor.org/info/rfc2616>.
[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>.
[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>.
[RFC6819] Lodderstedt, T., Ed., McGloin, M., and P. Hunt, "OAuth 2.0
Threat Model and Security Considerations", RFC 6819,
DOI 10.17487/RFC6819, January 2013,
<https://www.rfc-editor.org/info/rfc6819>.
[RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
DOI 10.17487/RFC7231, June 2014,
<https://www.rfc-editor.org/info/rfc7231>.
[RFC7591] Richer, J., Ed., Jones, M., Bradley, J., Machulak, M., and
P. Hunt, "OAuth 2.0 Dynamic Client Registration Protocol",
RFC 7591, DOI 10.17487/RFC7591, July 2015,
<https://www.rfc-editor.org/info/rfc7591>.
[RFC7636] Sakimura, N., Ed., Bradley, J., and N. Agarwal, "Proof Key [RFC7636] Sakimura, N., Ed., Bradley, J., and N. Agarwal, "Proof Key
for Code Exchange by OAuth Public Clients", RFC 7636, for Code Exchange by OAuth Public Clients", RFC 7636,
DOI 10.17487/RFC7636, September 2015, DOI 10.17487/RFC7636, September 2015,
<https://www.rfc-editor.org/info/rfc7636>. <https://www.rfc-editor.org/info/rfc7636>.
[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>.
[RFC8414] Jones, M., Sakimura, N., and J. Bradley, "OAuth 2.0 [RFC8414] Jones, M., Sakimura, N., and J. Bradley, "OAuth 2.0
Authorization Server Metadata", RFC 8414, Authorization Server Metadata", RFC 8414,
DOI 10.17487/RFC8414, June 2018, DOI 10.17487/RFC8414, June 2018,
<https://www.rfc-editor.org/info/rfc8414>. <https://www.rfc-editor.org/info/rfc8414>.
[webappsec-referrer-policy] [webappsec-referrer-policy]
Google Inc. and Google Inc., "Referrer Policy", April "Referrer Policy", 20 April 2017.
2017, <https://w3c.github.io/webappsec-referrer-policy>.
Appendix A. Document History Appendix A. Document History
[[ To be removed from the final specification ]] [[ To be removed from the final specification ]]
-11
* Adapted section 2.1.2 to outcome of consensus call
* more text on refresh token inactivity and implementation note on
refres token replay detection via refresh token rotation
-10 -10
o incorporated feedback by Joseph Heenan * incorporated feedback by Joseph Heenan
o changed occurrences of SHALL to MUST * changed occurrences of SHALL to MUST
o added text on lack of token/cert binding support tokens issued in * added text on lack of token/cert binding support tokens issued in
the authorization response as justification to not recommend the authorization response as justification to not recommend
issuing tokens there at all issuing tokens there at all
o added requirement to authenticate clients during code exchange * added requirement to authenticate clients during code exchange
(PKCE or client credential) to 2.1.1. (PKCE or client credential) to 2.1.1.
o added section on refresh tokens * added section on refresh tokens
o editorial enhancements to 2.1.2 based on feedback * editorial enhancements to 2.1.2 based on feedback
-09 -09
o changed text to recommend not to use implicit but code * changed text to recommend not to use implicit but code
o added section on access token injection * added section on access token injection
o reworked sections 3.1 through 3.3 to be more specific on implicit * reworked sections 3.1 through 3.3 to be more specific on implicit
grant issues grant issues
-08 -08
o added recommendations re implicit and token injection * added recommendations re implicit and token injection
o uppercased key words in Section 2 according to RFC 2119
* uppercased key words in Section 2 according to RFC 2119
-07 -07
o incorporated findings of Doug McDorman * incorporated findings of Doug McDorman
o added section on HTTP status codes for redirects * added section on HTTP status codes for redirects
o added new section on access token privilege restriction based on * added new section on access token privilege restriction based on
comments from Johan Peeters comments from Johan Peeters
-06 -06
o reworked section 3.8.1 * reworked section 3.8.1
o incorporated Phil Hunt's feedback * incorporated Phil Hunt's feedback
o reworked section on mix-up * reworked section on mix-up
o extended section on code leakage via referrer header to also cover * extended section on code leakage via referrer header to also cover
state leakage state leakage
o added Daniel Fett as author * added Daniel Fett as author
o replaced text intended to inform WG discussion by recommendations * replaced text intended to inform WG discussion by recommendations
to implementors to implementors
o modified example URLs to conform to RFC 2606 * modified example URLs to conform to RFC 2606
-05 -05
o Completed sections on code leakage via referrer header, attacks in * Completed sections on code leakage via referrer header, attacks in
browser, mix-up, and CSRF browser, mix-up, and CSRF
o Reworked Code Injection Section * Reworked Code Injection Section
o Added reference to OpenID Connect spec * Added reference to OpenID Connect spec
o removed refresh token leakage as respective considerations have * removed refresh token leakage as respective considerations have
been given in section 10.4 of RFC 6749 been given in section 10.4 of RFC 6749
o first version on open redirection * first version on open redirection
o incorporated Christian Mainka's review feedback * incorporated Christian Mainka's review feedback
-04 -04
o Restructured document for better readability * Restructured document for better readability
o Added best practices on Token Leakage prevention
* Added best practices on Token Leakage prevention
-03 -03
o Added section on Access Token Leakage at Resource Server * Added section on Access Token Leakage at Resource Server
o incorporated Brian Campbell's findings * incorporated Brian Campbell's findings
-02 -02
o Folded Mix up and Access Token leakage through a bad AS into new * 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 * reworked dynamic OAuth section
-01 -01
o Added references to mitigation methods for token leakage * Added references to mitigation methods for token leakage
o Added reference to Token Binding for Authorization Code * Added reference to Token Binding for Authorization Code
o incorporated feedback of Phil Hunt * incorporated feedback of Phil Hunt
o fixed numbering issue in attack descriptions in section 2 * fixed numbering issue in attack descriptions in section 2
-00 (WG document) -00 (WG document)
o turned the ID into a WG document and a BCP * turned the ID into a WG document and a BCP
o Added federated app login as topic in Other Topics * Added federated app login as topic in Other Topics
Authors' Addresses Authors' Addresses
Torsten Lodderstedt (editor) Torsten Lodderstedt
yes.com yes.com
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
Email: isciurus@fb.com Email: isciurus@fb.com
Daniel Fett Daniel Fett
yes.com yes.com
Email: mail@danielfett.de Email: mail@danielfett.de
 End of changes. 180 change blocks. 
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