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Open Authentication Protocol                         T. Lodderstedt, Ed.
Internet-Draft                                                YES.com AG
Intended status: Best Current Practice                        J. Bradley
Expires: September 19, 2018                                       Yubico
                                                             A. Labunets
                                                          March 18, 2018

                OAuth 2.0 Security Best Current Practice


   This document describes best current security practices for OAuth
   2.0..  It updates and extends the OAuth 2.0 Security Threat Model to
   incorporate practical experiences gathered since OAuth 2.0 was
   published and cover new threats relevant due to the broader
   application of OAuth 2.0.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on September 19, 2018.

Copyright Notice

   Copyright (c) 2018 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must

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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Recommendations . . . . . . . . . . . . . . . . . . . . . . .   4
     2.1.  Protecting redirect-based flows . . . . . . . . . . . . .   4
     2.2.  Token Replay Prevention . . . . . . . . . . . . . . . . .   5
   3.  Attacks and Mitigations . . . . . . . . . . . . . . . . . . .   5
     3.1.  Insufficient redirect URI validation  . . . . . . . . . .   5
       3.1.1.  Attacks on Authorization Code Grant . . . . . . . . .   5
       3.1.2.  Attacks on Implicit Grant . . . . . . . . . . . . . .   6
       3.1.3.  Proposed Countermeasures  . . . . . . . . . . . . . .   8
     3.2.  Authorization code leakage via referrer headers . . . . .   9
       3.2.1.  Proposed Countermeasures  . . . . . . . . . . . . . .   9
     3.3.  Attacks in the Browser  . . . . . . . . . . . . . . . . .  10
       3.3.1.  Code in browser history . . . . . . . . . . . . . . .  10
       3.3.2.  Access token in browser history . . . . . . . . . . .  10
     3.4.  Mix-Up  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     3.5.  Code Injection  . . . . . . . . . . . . . . . . . . . . .  11
       3.5.1.  Proposed Countermeasures  . . . . . . . . . . . . . .  13
     3.6.  Cross Site Request Forgery  . . . . . . . . . . . . . . .  15
     3.7.  Access Token Leakage at the Resource Server . . . . . . .  15
       3.7.1.  Access Token Phishing by Counterfeit Resource Server   15  Metadata  . . . . . . . . . . . . . . . . . . . .  16  Sender Constrained Access Tokens  . . . . . . . .  17  Audience Restricted Access Tokens . . . . . . . .  19
       3.7.2.  Compromised Resource Server . . . . . . . . . . . . .  20
     3.8.  Open Redirection  . . . . . . . . . . . . . . . . . . . .  21
       3.8.1.  Authorization Server as Open Redirector . . . . . . .  21
       3.8.2.  Clients as Open Redirector  . . . . . . . . . . . . .  21
     3.9.  TLS Terminating Reverse Proxies . . . . . . . . . . . . .  22
   4.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  23
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  23
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  23
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  23
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .  23
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  24
   Appendix A.  Document History . . . . . . . . . . . . . . . . . .  26
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  27

1.  Introduction

   It's been a while since OAuth has been published in RFC 6749
   [RFC6749] and RFC 6750 [RFC6750].  Since publication, OAuth 2.0 has
   gotten massive traction in the market and became the standard for API

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   protection and, as foundation of OpenID Connect [OpenID], identity
   providing.  While OAuth was used in a variety of scenarios and
   different kinds of deployments, the following challenges could be

   o  OAuth implementations are being attacked through known
      implementation weaknesses and anti-patterns (XSRF, referrer
      header).  Although most of these threats are discussed in the
      OAuth 2.0 Threat Model and Security Considerations [RFC6819],
      continued exploitation demonstrates there may be a need for more
      specific recommendations or that the existing mitigations are too
      difficult to deploy.

   o  Technology has changed, e.g. the way browsers treat fragments in
      some situations, which may change the implicit grant's underlying
      security model.

   o  OAuth is used in much more dynamic setups than originally
      anticipated, creating new challenges with respect to security.
      Those challenges go beyond the original scope of RFC 6749
      [RFC6749], RFC 6750 [RFC6749], and RFC 6819 [RFC6819].

   OAuth initially assumed a static relationship between client,
   authorization server and resource servers.  The URLs of AS and RS
   were known to the client at deployment time and built an anchor for
   the trust relationsship among those parties.  The validation whether
   the client talks to a legitimate server was based on TLS server
   authentication (see [RFC6819], Section 4.5.4).  With the increasing
   adoption of OAuth, this simple model dissolved and, in several
   scenarios, was replaced by a dynamic establishment of the
   relationship between clients on one side and the authorization and
   resource servers of a particular deployment on the other side.  This
   way the same client could be used to access services of different
   providers (in case of standard APIs, such as e-Mail or OpenID
   Connect) or serves as a frontend to a particular tenant in a multi-
   tenancy.  Extensions of OAuth, such as [RFC7591] and
   [I-D.ietf-oauth-discovery] were developed in order to support the
   usage of OAuth in dynamic scenarios.  As a challenge to the
   community, such usage scenarios open up new attack angles, which are
   discussed in this document.

   The remainder of the document is organized as follows: The next
   section summarizes the most important recommendations of the OAuth
   working group for every OAuth implementor.  Afterwards, a detailed
   analyses of the threats and implementation issues, which can be found
   in the wild today, is given along with a discussion of potential
   counter measures.

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2.  Recommendations

   This section describes the set of security mechanisms the authors
   believe should be taken into consideration by the OAuth working group
   to be recommended to OAuth implementers.

2.1.  Protecting redirect-based flows

   Authorization servers shall utilize exact matching of client redirect
   URIs against pre-registered URIs.  This measure contributes to the
   prevention of leakage of authorization codes and access tokens
   (depending on the grant type).  It also helps to detect mix up

   Clients shall avoid any redirects or forwards, which can be
   parameterized by URI query parameters, in order to provide a further
   layer of defence against token leakage.  If there is a need for this
   kind of redirects, clients are advised to implement appropriate
   counter measures against open redirection, e.g. as described by the
   OWASP [owasp].

   Clients shall ensure to only process redirect responses of the OAuth
   authorization server they send the respective request to and in the
   same user agent this request was initiated in.  In particular,
   clients shall implement appropriate XSRF prevention by utilizing one-
   time use XSRF tokens carried in the STATE parameter, which are
   securely bound to the user agent.  Moreover, the client shall
   memorize which authorization server it sent an authorization request
   to and bind this information to the user agent and ensure any sub-
   sequent messages are sent to the same authorization server.
   Furthermore, clients should use AS-specific redirect URIs as a means
   to identify the AS a particular response came from.  Matching this
   with the before mentioned information regarding the AS the client
   sent the request to helps to detect mix-up attacks.

   Note: [I-D.bradley-oauth-jwt-encoded-state] gives advice on how to
   implement XSRF prevention and AS matching using signed JWTs in the
   STATE parameter.

   Clients shall use PKCE [RFC7636] in order to (with the help of the
   authorization server) detect and prevent attempts to inject (replay)
   authorization codes into the authorization response.  The PKCE
   challenges must be transaction-specific and securely bound to the
   user agent, in which the transaction was started.  OpenID Connect
   clients may use the "nonce" parameter of the OpenID Connect
   authentication request as specified in [OpenID] in conjunction with
   the corresponding ID Token claim of the for the same purpose.

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   Note: although PKCE so far was recommended as mechanism to protect
   native apps, this advice applies to all kinds of OAuth clients,
   including web applications.

   Authorization servers shall consider the recommendations given in
   [RFC6819], section, on authorization code replay prevention.

2.2.  Token Replay Prevention

   Authorization servers shall use TLS-based methods for sender
   constraint access tokens as described in section Section,
   such as token binding [I-D.ietf-oauth-token-binding] or Mutual TLS
   for OAuth 2.0 [I-D.ietf-oauth-mtls].  It is also recommend to use
   end-to-end TLS whenever possible.

3.  Attacks and Mitigations

3.1.  Insufficient redirect URI validation

   Some authorization servers allow clients to register redirect URI
   patterns instead of complete redirect URIs.  In those cases, the
   authorization server, at runtime, matches the actual redirect URI
   parameter value at the authorization endpoint against this pattern.
   This approach allows clients to encode transaction state into
   additional redirect URI parameters or to register just a single
   pattern for multiple redirect URIs.  As a downside, it turned out to
   be more complex to implement and error prone to manage than exact
   redirect URI matching.  Several successful attacks have been observed
   in the wild, which utilized flaws in the pattern matching
   implementation or concrete configurations.  Such a flaw effectively
   breaks client identification or authentication (depending on grant
   and client type) and allows the attacker to obtain an authorization
   code or access token, either:

   o  by directly sending the user agent to a URI under the attackers
      control or

   o  by exposing the OAuth credentials to an attacker by utilizing an
      open redirector at the client in conjunction with the way user
      agents handle URL fragments.

3.1.1.  Attacks on Authorization Code Grant

   For a public client using the grant type code, an attack would look
   as follows:

   Let's assume the redirect URL pattern "https://*.example.com/*" had
   been registered for the client "s6BhdRkqt3".  This pattern allows

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   redirect URIs from any host residing in the domain example.com.  So
   if an attacker manages to establish a host or subdomain in
   "example.com" he can impersonate the legitimate client.  Assume the
   attacker sets up the host "evil.example.com".

   (1)  The attacker needs to trick the user into opening a tampered URL
        in his browser, which launches a page under the attacker's
        control, say "https://www.evil.com".

   (2)  This URL initiates an authorization request with the client id
        of a legitimate client to the authorization endpoint.  This is
        the example authorization request (line breaks are for display
        purposes only):

   GET /authorize?response_type=code&client_id=s6BhdRkqt3&state=xyz
     &redirect_uri=https%3A%2F%2Fevil.example.com%2Fcb HTTP/1.1
   Host: server.example.com

   (1)  The authorization validates the redirect URI in order to
        identify the client.  Since the pattern allows arbitrary domains
        host names in "example.com", the authorization request is
        processed under the legitimate client's identity.  This includes
        the way the request for user consent is presented to the user.
        If auto-approval is allowed (which is not recommended for public
        clients according to RFC 6749), the attack can be performed even

   (2)  If the user does not recognize the attack, the code is issued
        and directly sent to the attacker's client.

   (3)  Since the attacker impersonated a public client, it can directly
        exchange the code for tokens at the respective token endpoint.

   Note: This attack will not directly work for confidential clients,
   since the code exchange requires authentication with the legitimate
   client's secret.  The attacker will need to utilize the legitimate
   client to redeem the code (e.g. by performing a code injection
   attack).  This kind of injections is covered in
   Section Code Injection.

3.1.2.  Attacks on Implicit Grant

   The attack described above works for the implicit grant as well.  If
   the attacker is able to send the authorization response to a URI
   under his control, he will directly get access to the fragment
   carrying the access token.

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   Additionally, implicit clients can be subject to a further kind of
   attacks.  It utilizes the fact that user agents re-attach fragments
   to the destination URL of a redirect if the location header does not
   contain a fragment (see [RFC7231], section 9.5).  The attack
   described here combines this behavior with the client as an open
   redirector in order to get access to access tokens.  This allows
   circumvention even of strict redirect URI patterns (but not strict
   URL matching!).

   Assume the pattern for client "s6BhdRkqt3" is
   "https://client.example.com/cb?*", i.e. any parameter is allowed for
   redirects to "https://client.example.com/cb".  Unfortunately, the
   client exposes an open redirector.  This endpoint supports a
   parameter "redirect_to", which takes a target URL and will send the
   browser to this URL using a HTTP 302.

   (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
        attacker's control, say "https://www.evil.com".

   (2)  The URL initiates an authorization request, which is very
        similar to the attack on the code flow.  As differences, it
        utilizes the open redirector by encoding
        "redirect_to=https://client.evil.com" into the redirect URI and
        it uses the response type "token" (line breaks are for display
        purposes only):

   GET /authorize?response_type=token&client_id=s6BhdRkqt3&state=xyz
     %253Dhttps%253A%252F%252Fclient.evil.com%252Fcb HTTP/1.1
   Host: server.example.com

   (1)  Since the redirect URI matches the registered pattern, the
        authorization server allows the request and sends the resulting
        access token with a 302 redirect (some response parameters are
        omitted for better readability)

   HTTP/1.1 302 Found
     Location: https://client.example.com/cb?

   (2)  At the example.com, the request arrives at the open redirector.
        It will read the redirect parameter and will issue a HTTP 302 to
        the URL "https://evil.example.com/cb".

   HTTP/1.1 302 Found
        Location: https://client.evil.com/cb

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   (3)  Since the redirector at example.com does not include a fragment
        in the Location header, the user agent will re-attach the
        original fragment
        "#access_token=2YotnFZFEjr1zCsicMWpAA&..." to the URL and will
        navigate to the following URL:


   (4)  The attacker's page at client.evil.com can access the fragment
        and obtain the access token.

3.1.3.  Proposed Countermeasures

   The complexity of implementing and managing pattern matching
   correctly obviously causes security issues.  This document therefore
   proposes to simplify the required logic and configuration by using
   exact redirect URI matching only.  This means the authorization
   server shall compare the two URIs using simple string comparison as
   defined in [RFC3986], Section 6.2.1..

   This would cause the following impacts:

   o  This change will require all OAuth clients to maintain the
      transaction state (and XSRF tokens) in the "state" parameter.
      This is a normative change to RFC 6749 since section
      allows for dynamic URI query parameters in the redirect URI.  In
      order to assess the practical impact, the working group needs to
      collect data on whether this feature is really used in deployments

   o  The working group may also consider this change as a step towards
      improved interoperability for OAuth implementations since RFC 6749
      is somewhat vague on redirect URI validation.  Notably there are
      no rules for pattern matching.  One may therefore assume all
      clients utilizing pattern matching will do so in a deployment
      specific way.  On the other hand, RFC 6749 already recommends
      exact matching if the full URL had been registered.

   o  Clients with multiple redirect URIs need to register all of them
      Note: clients with just a single redirect URI would not even need
      to send a redirect URI with the authorization request.  Does it
      make sense to emphasize this option?  Would that further simplify
      use of the protocol and foster security?

   o  Exact redirect matching does not work for native apps utilizing a
      local web server due to dynamic port numbers - at least wild cards
      for port numbers are required.

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      Question: Does redirect uri validation solve any problem for
      native apps?  Effective against impersonation when used in
      conjunction with claimed HTTPS redirect URIs only.
      For Windows token broker exact redirect URI matching is important
      as the redirect URI encodes the app identity.  For custom scheme
      redirects there is a question however it is probably a useful part
      of defense in depth.

   Additional recommendations:

   o  Servers on which callbacks are hosted must not expose open
      redirectors (see Section 3.8).

   o  Clients may drop fragments via intermediary URLs with "fix
      fragments" (e.g. https://developers.facebook.com/blog/post/552/)
      to prevent the user agent from appending any unintended fragments.

   Alternatives to exact redirect URI matching:

   o  Authenticate clients using digital signatures (see

3.2.  Authorization code leakage via referrer headers

   It is possible authorization codes are unintentionally disclosed to
   attackers, if a OAuth client renders a page containing links to other
   pages (ads, faq, ...) as result of a successful authorization

   If the user clicks onto one of those links and the target is under
   the control of an attacker, it can get access to the response URL in
   the referrer header.

   It is also possible that an attacker injects cross-domain content
   somehow into the page, such as <img> (e.g if this is a blog web
   site).  The implication is obviously the same: loading this content
   by browser results in leaking referrer with a code.

3.2.1.  Proposed Countermeasures

   There are some means to prevent leakage as described above:

   o  Make authorization codes one-time use.  For example, if the
      legitimate client redeemed and invalidated the code in the above
      scenario, the attacker would fail exchanging this code later.

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   o  Bind authorization code to a confidential client or PKCE
      challenge.  In this case, the attacker lacks the secret to request
      the code exchange.

   o  Don't include links to external sites into the page rendered as
      result of a OAuth authorization response

   o  Use of the HTML link attribute rel="noreferrer" to suppress
      referrer header

   o  Use of the "referrer" meta link attribute to suppress referrer
      header (see [webappsec-referrer-policy])

   o  Use form post response mode instead of redirect for authorization
      response (see [oauth-v2-form-post-response-mode])

3.3.  Attacks in the Browser

3.3.1.  Code in browser history

   When a browser navigates to "client.com/
   redirection_endpoint?code=abcd" as a result of a redirect from a
   provider's authorization endpoint, the URL including the
   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.

   Proposed countermeasures:

   o  Authorization code replay prevention as described in [RFC6819],
      section, and Section 3.5

   o  Use form post response mode instead of redirect for authorization
      response (see [oauth-v2-form-post-response-mode])

3.3.2.  Access token in browser history

   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
   a page like "provider.com/get_user_profile?access_token=abcdef.".
   Actually [RFC6750]discourages this practice and asks to transfer
   tokens via a header, but in practice web sites often just pass access
   token in query parameters.

   In case of implicit grant, a URL like "client.com/
   redirection_endpoint#access_token=abcdef" may also end up in the
   browser history as a result of a redirect from a provider's
   authorization endpoint.

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   Proposed countermeasures:

   o  Replace implicit flow with postmessage communication or the
      authorization code grant

   o  Never pass access tokens in URL query parameters

3.4.  Mix-Up

   Mix-up is another kind of attack on more dynamic OAuth scenarios (or
   at least scenarios where a OAuth client interacts with multiple
   authorization servers).  The goal of the attack is to obtain an
   authorization code or an access token by tricking the client into
   sending those credentials to the attacker (which acts as MITM between
   client and authorization server)

   A detailed description of the attack and potential countermeasures is
   given in [I-D.ietf-oauth-mix-up-mitigation].

   Potential mitigations:

   o  Clients use AS-specific redirect URIs and, for every authorization
      request, store intended AS and compare intention with actual
      redirect URI where the response was received

   o  AS returns client_id and its iss in the response.  Client compares
      this data to AS it believed it sent the user agent to.

   o  ID token carries client id and issuer (OpenID Connect specific)

3.5.  Code Injection

   In such an attack, the adversary attempts to inject a stolen
   authorization code into a legitimate client on a device under his
   control.  In the simplest case, the attacker would want to use the
   code in his own client.  But there are situations where this might
   not be possible or intended.  Examples are:

   o  The attacker wants to access certain functions in this particular
      client.  As an example, the attacker wants to impersonate his
      victim in a certain app or on a certain web site.

   o  The code is bound to a particular confidential client and the
      attacker is unable to obtain the required client credentials to
      redeem the code himself.

   o  The authorization or resource servers are limited to certain
      networks, the attackers is unable to access directly.

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   How does an attack look like?

   (1)  The attacker obtains an authorization code by performing any of
        the attacks described above.

   (2)  It performs a regular OAuth authorization process with the
        legitimate client on his device.

   (3)  The attacker injects the stolen authorization code in the
        response of the authorization server to the legitimate client.

   (4)  The client sends the code to the authorization server's token
        endpoint, along with client id, client secret and actual

   (5)  The authorization server checks the client secret, whether the
        code was issued to the particular client and whether the actual
        redirect URI matches the redirect_uri parameter (see [RFC6749]).

   (6)  If all checks succeed, the authorization server issues access
        and other tokens to the client, so now the attacker is able to
        impersonate the legitimate user.

   Obviously, the check in step (5) will fail, if the code was issued to
   another client id, e.g. a client set up by the attacker.  The check
   will also fail if the authorization code was already redeemed by the
   legitimate user and was one-time use only.

   An attempt to inject a code obtained via a malware pretending to be
   the legitimate client should also be detected, if the authorization
   server stored the complete redirect URI used in the authorization
   request and compares it with the redirect_uri parameter.

   [RFC6749], Section 4.1.3, requires the AS to "... ensure that the
   "redirect_uri" parameter is present if the "redirect_uri" parameter
   was included in the initial authorization request as described in
   Section 4.1.1, and if included ensure that their values are
   identical.".  In the attack scenario described above, the legitimate
   client would use the correct redirect URI it always uses for
   authorization requests.  But this URI would not match the tampered
   redirect URI used by the attacker (otherwise, the redirect would not
   land at the attackers page).  So the authorization server would
   detect the attack and refuse to exchange the code.

   Note: this check could also detect attempt to inject a code, which
   had been obtained from another instance of the same client on another
   device, if certain conditions are fulfilled:

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   o  the redirect URI itself needs to contain a nonce or another kind
      of one-time use, secret data and

   o  the client has bound this data to this particular instance

   But this approach conflicts with the idea to enforce exact redirect
   URI matching at the authorization endpoint.  Moreover, it has been
   observed that providers very often ignore the redirect_uri check
   requirement at this stage, maybe, because it doesn't seem to be
   security-critical from reading the spec.

   Other providers just pattern match the redirect_uri parameter against
   the registered redirect URI pattern.  This saves the authorization
   server from storing the link between the actual redirect URI and the
   respective authorization code for every transaction.  But this kind
   of check obviously does not fulfill the intent of the spec, since the
   tampered redirect URI is not considered.  So any attempt to inject a
   code obtained using the client_id of a legitimate client or by
   utilizing the legitimate client on another device won't be detected
   in the respective deployments.

   It is also assumed that the requirements defined in [RFC6749],
   Section 4.1.3, increase client implementation complexity as clients
   need to memorize or re-construct the correct redirect URI for the
   call to the tokens endpoint.

   The authors therefore propose to the working group to drop this
   feature in favor of more effective and (hopefully) simpler approaches
   to code injection prevention as described in the following section.

3.5.1.  Proposed Countermeasures

   The general proposal is to bind every authorization code to a certain
   client instance on a certain device (or in a certain user agent) in
   the context of a certain transaction.  There are multiple technical
   solutions to achieve this goal:

   Nonce   OpenID Connect's existing "nonce" parameter could be used for
           this purpose.  The nonce value is one-time use and created by
           the client.  The client is supposed to bind it to the user
           agent session and sends it with the initial request to the
           OpenId Provider (OP).  The OP associates the nonce to the
           authorization code and attests this binding in the ID token,
           which is issued as part of the code exchange at the token
           endpoint.  If an attacker injected an authorization code in
           the authorization response, the nonce value in the client
           session and the nonce value in the ID token will not match
           and the attack is detected.  assumption: attacker cannot get

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           hold of the user agent state on the victims device, where he
           has stolen the respective authorization code.  The main
           advantage of this option is that Nonce is an existing feature
           used in the wild.  On the other hand, leveraging Nonce by the
           broader OAuth community would require AS and client to adopt
           ID Tokens.

   Code-bound State  The "state" parameter as specified in [RFC6749]
           could be used similarly to the way as described above.  This
           would require to add a further parameter "state" to the code
           exchange token endpoint request.  The authorization server
           would then compares the state value it associated with the
           code and the state value in the parameter.  If those values
           do not match, it is considered an attack and the request
           fails.  The advantage of this approach would be to utilize an
           existing OAuth parameter.  But it would also mean to re-
           interpret the purpose of state and to extend the token
           endpoint request.

   PKCE    The PKCE parameter "challenge" along with the corresponding
           "verifier" as specified in [RFC7636] could be used in the
           same way as "nonce" or "state".  In contrast to its original
           intention, the verifier check would fail although the client
           uses its correct verifier but the code is associated with a
           challenge, which does not match.  PKCE is a deployed OAuth
           feature, even so it is today used to secure native apps,

   Token Binding  Token binding [I-D.ietf-oauth-token-binding] 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
           the client.  This requires further data (extension to
           response) to manifest binding id for particular code.  Token
           binding is promising as a secure and convenient mechanism
           (due to its browser integration).  As a challenge, it
           requires broad browser support and use with native apps still
           under discussion.

   per instance client id/secret  One could use per instance client_id
           and secrets and bind the code to the respective client_id.
           Unfortunately, this does not fit into the web application
           programming model (would need to use per user client ids).

   PKCE seem to be the most obvious solution for OAuth clients as it
   available and effectively used today for similar purposes for OAuth
   native apps whereas "nonce" is appropriate for OpenId Connect

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   Note on pre-warmed secrets: An attacker can circumvent the
   countermeasures described above if he is able to create or capture
   the respective secret or code_challenge on a device under his
   control, which is then used in the victim's authorization request.
   Exact redirect URI matching of authorization requests can prevent the
   attacker from using the pre-warmed secret in the faked authorization
   transaction on the victim's device.
   Unfortunately it does not work for all kinds of OAuth clients.  It is
   effective for web and JS apps and for native apps with claimed URLs.
   Attacks on native apps using custom schemes or redirect URIs on
   localhost cannot be prevented this way, except if the AS enforces
   one-time use for PKCE verifier or Nonce values.

3.6.  Cross Site Request Forgery

   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
   client to access resources under the attacker's control.

   Proposed mitigation: use of XSRF tokens (one-time use), which are
   bound to the user agent and passed in the state parameter to the
   authorization server.  For more details see [owasp_csrf].

3.7.  Access Token Leakage at the Resource Server

3.7.1.  Access Token Phishing by Counterfeit Resource Server

   An attacker may setup his own resource server and trick a client into
   sending access tokens to it, which are valid for other resource
   servers.  If the client sends a valid access token to this
   counterfeit resource server, the attacker in turn may use that token
   to access other services on behalf of the resource owner.

   This attack assumes the client is not bound to a certain resource
   server (and the respective URL) at development time, but client
   instances are configured with an resource server's URL at runtime.
   This kind of late binding is typical in situations, where the client
   uses a standard API, e.g. for e-Mail, calendar, health, or banking
   and is configured by an user or administrator for the standard-based
   service, this particular user or company uses.

   There are several potential mitigation strategies, which will be
   discussed in the following sections.

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   An authorization server could provide the client with additional
   information about the location where it is safe to use its access

   In the simplest form, this would require the AS to publish a list of
   its known resource servers, illustrated in the following example
   using a metadata parameter "resource_servers":

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


   The AS could also return the URL(s) an access token is good for in
   the token response, illustrated by the example return parameter

HTTP/1.1 200 OK
Content-Type: application/json;charset=UTF-8
Cache-Control: no-store
Pragma: no-cache


   This mitigation strategy would rely on the client to enforce the
   security policy and to only send access tokens to legitimate
   destinations.  Results of OAuth related security research (see for
   example [oauth_security_ubc] and [oauth_security_cmu]) indicate a
   large portion of client implementations do not or fail to properly
   implement security controls, like state checks.  So relying on
   clients to prevent access token phishing is likely to fail as well.
   Moreover given the ratio of clients to authorization and resource
   servers, it is considered the more viable approach to move as much as
   possible security-related logic to those entities.  Clearly, the

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   client has to contribute to the overall security.  But there are
   alternative counter measures, as described in the next sections,
   which provide a better balance between the involved parties.  Sender Constrained Access Tokens

   As the name suggests, sender constraint access token scope 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 a resource server.

   A typical flow looks like this:

   1.  The authorization server associates data with the access token,
       which bind this particular token to a certain client.  The
       binding can utilize the client identity, but in most cases the AS
       utilizes key material (or data derived from the key material)
       known to the client.

   2.  This key material must be distributed somehow.  Either the key
       material already exists before the AS creates the binding or the
       AS creates ephemeral keys.  The way pre-existing key material is
       distributed varies among the different approaches.  For example,
       X.509 Certificates can be used in which case the distribution
       happens explicitly during the enrollment process.  Or the key
       material is created and distributed at the TLS layer, in which
       case it might automatically happens during the setup of a TLS

   3.  The RS must implement the actual proof of possession check.  This
       is typically done on the application level, it may utilize
       capabilities of the transport layer (e.g.  TLS).  Note: replay
       prevention is required as well!

   There exists several proposals to demonstrate the proof of possession
   in the scope of the OAuth working group:

   o  [I-D.ietf-oauth-token-binding]: In this approach, an access tokens
      is, via the so-called token binding id, bound to key material
      representing a long term association between a client and a
      certain TLS host.  Negotiation of the key material and proof of
      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
      of the target resource server and pass this data to the access
      token request.  The authorization server than associates the
      access token with this id.  The resource server checks on every
      invocation that the token binding id of the active TLS connection
      and the token binding id of associated with the access token

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      match.  Since all crypto-related functions are covered by the TLS
      stack, this approach is very client developer friendly.  As a
      prerequisite, token binding as described in
      [I-D.ietf-tokbind-https] (including federated token bindings) must
      be supported on all ends (client, authorization server, resource

   o  [I-D.ietf-oauth-mtls]: The approach as specified in this document
      allow use of mutual TLS for both client authentication and sender
      constraint access tokens.  For the purpose of sender constraint
      access tokens, the client is identified towards the resource
      server by the fingerprint of its public key.  During processing of
      an access token request, the authorization server obtains the
      client's public key from the TLS stack and associates its
      fingerprint with the respective access tokens.  The resource
      server in the same way obtains the public key from the TLS stack
      and compares its fingerprint with the fingerprint associated with
      the access token.

   o  [I-D.ietf-oauth-signed-http-request] specifies an approach to sign
      HTTP requests.  It utilizes [I-D.ietf-oauth-pop-key-distribution]
      and represents the elements of the signature in a JSON object.
      The signature is built using JWS.  The mechanism has built-in
      support for signing of HTTP method, query parameters and headers.
      It also incorporates a timestamp as basis for replay prevention.

   o  [I-D.sakimura-oauth-jpop]: this draft describes different ways to
      constrain access token usage, namely TLS or request signing.
      Note: Since the authors of this draft contributed the TLS-related
      proposal to [I-D.ietf-oauth-mtls], this document only considers
      the request signing part.  For request signing, the draft utilizes
      [I-D.ietf-oauth-pop-key-distribution] and RFC 7800 [RFC7800].  The
      signature data is represented in a JWT and JWS is used for
      signing.  Replay prevention is provided by building the signature
      over a server-provided nonce, client-provided nonce and a nonce

   [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
   to leverage TLS to secure OAuth wherever possible.  Both mechanisms
   allow prevention of access token leakage in a fairly client developer
   friendly way.

   There are some differences between both approaches: To start with, in
   [I-D.ietf-oauth-token-binding] all key material is automatically
   managed by the TLS stack whereas [I-D.ietf-oauth-mtls] requires the
   developer to create and maintain the key pairs and respective
   certificates.  Use of self-signed certificates, which is supported by

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   the draft, significantly reduce the complexity of this task.
   Furthermore, [I-D.ietf-oauth-token-binding] allows to use different
   key pairs for different resource servers, which is a privacy benefit.
   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
   short term.

   Application level signing approaches, like
   [I-D.ietf-oauth-signed-http-request] and [I-D.sakimura-oauth-jpop]
   have been debated for a long time in the OAuth working group without
   a clear outcome.

   As one advantage, application-level signing allows for end-to-end
   protection including non-repudiation even if the TLS connection is
   terminated between client and resource server.  But deployment
   experiences have revealed challenges regarding robustness (e.g.
   reproduction of the signature base string including correct URL) as
   well as state management (e.g. replay prevention).

   This document therefore recommends implementors to consider one of
   TLS-based approaches wherever possible.  Audience Restricted Access Tokens

   An audience restriction essentially restricts the resource server a
   particular access token can be used at.  The authorization server
   associates the access token with a certain resource server and every
   resource server is obliged to verify for every request, whether the
   access token send with that request was meant to be used at the
   particular resource server.  If not, the resource server must refuse
   to serve the respective request.  In the general case, audience
   restrictions limit the impact of a token leakage.  In the case of a
   counterfeit resource server, it may (as described see below) also
   prevent abuse of the phished access token at the legitimate resource

   The audience can basically be expressed using logical names or
   physical addresses (like URLs).  In order to prevent phishing, it is
   necessary to use the actual URL the client will send requests to.  In
   the phishing case, this URL will point to the counterfeit resource
   server.  If the attacker tries to use the access token at the
   legitimate resource server (which has a different URL), the resource
   server will detect the mismatch (wrong audience) and refuse to serve
   the request.

   In deployments where the authorization server knows the URLs of all
   resource servers, the authorization server may just refuse to issue
   access tokens for unknown resource server URLs.

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   The client needs to tell the authorization server, at which URL it
   will use the access token it is requesting.  It could use the
   mechanism proposed [I-D.campbell-oauth-resource-indicators] or encode
   the information in the scope value.

   Instead of the URL, it is also possible to utilize the fingerprint of
   the resource server's X.509 certificate as audience value.  This
   variant would also allow to detect an attempt to spoof the legit
   resource server's URL by using a valid TLS certificate obtained from
   a different CA.  It might also be considered a privacy benefit to
   hide the resource server URL from the authorization server.

   Audience restriction seems easy to use since it does not require any
   crypto on the client side.  But since every access token is bound to
   a certain resource server, the client also needs to obtain different
   RS-specific access tokens, if it wants to access several resource
   services.  [I-D.ietf-oauth-token-binding] has the same property,
   since different token binding ids must be associated with the access
   token.  [I-D.ietf-oauth-mtls] on the other hand allows a client to
   use the access token at multiple resource servers.

   It shall be noted that audience restrictions, or generally speaking
   an indication by the client to the authorization server where it
   wants to use the access token, has additional benefits beyond the
   scope of token leakage prevention.  It allows the authorization
   server to create different access token whose format and content is
   specifically minted for the respective server.  This has huge
   functional and privacy advantages in deployments using structured
   access tokens.

3.7.2.  Compromised Resource Server

   An attacker may compromise a resource server in order to get access
   to its resources and other resources of the respective deployment.
   Such a compromise may range from partial access to the system, e.g.
   its logfiles, to full control of the respective server.

   If the attacker was able to take over full control including shell
   access it will be able to circumvent all controls in place and access
   resources without access control.  It will also get access to access
   tokens, which are sent to the compromised system and which
   potentially are valid for access to other resource servers as well.
   Even if the attacker "only" is able to access logfiles or databases
   of the server system, it may get access to valid access tokens.

   Preventing server breaches by way of hardening and monitoring server
   systems is considered a standard operational procedure and therefore
   out of scope of this document.  This section will focus on the impact

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   of such breaches on OAuth-related parts of the ecosystem, which is
   the replay of captured access tokens on the compromised resource
   server and other resource servers of the respective deployment.

   The following measures shall be taken into account by implementors in
   order to cope with access token replay:

   o  The resource server must treat access tokens like any other
      credentials.  It is considered good practice to not log them and
      not to store them in plain text.

   o  Sender constraint access tokens as described in Section
      will prevent the attacker from replaying the access tokens on
      other resource servers.  Depending on the severity of the
      penetration, it will also prevent replay on the compromised

   o  Audience restriction as described in Section may be used
      to prevent replay of captured access tokens on other resource

3.8.  Open Redirection

3.8.1.  Authorization Server as Open Redirector

   Attackers could try to utilize a user's trust in the authorization
   server (and its URL in particular) for performing phishing attacks.
   The attacker could send an authorization request with an invalid
   combination of client_id and redirect_uri.  [RFC6749], section, already states that the AS MUST NOT automatically redirect
   the user agent in this case to prevent open redirection.

   But as described in [I-D.ietf-oauth-closing-redirectors], the
   attacker could also attempt to register a client and intentionally
   send an erroneous authorization request, e.g. by using an invalid
   scope value.  According to [RFC6749], the AS would send the user
   agent to the redirect_uri with an "invalid_request" error response.
   This is dangerous because the authorization server would serve as an
   open redirector.  Therefore this draft recommends that every invalid
   authorization request MUST NOT automatically redirect the user agent
   to the client's redirect URI.

3.8.2.  Clients as Open Redirector

   Client MUST not expose URLs which could be utilized as open
   redirector.  An open redirector is a way to cause the recipient of a
   HTTP request to issue a redirect to a target URL that is passed as a
   parameter.  Attackers may utilize such a mechanism to produce URLs,

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   which appear to point to the client, which might trick users to trust
   the URL and follow it in her browser.  Another abuse case is to
   produce URLs pointing to the client and utilize them to impersonate a
   client with an authorization server.

   In order to prevent open redirection, clients should only expose such
   a function, if the target URLs are whitelisted or if the origin of a
   request can be authenticated.

3.9.  TLS Terminating Reverse Proxies

   A common deployment architecture for HTTP applications is to have the
   application server sitting behind a reverse proxy, which terminates
   the TLS connection and dispatches the incoming requests to the
   respective application server nodes.

   This section highlights some attack angles of this deployment
   architecture, which are relevant to OAuth, and give recommendations
   for security controls.

   In some situations, the reverse proxy needs to pass security-related
   data to the upstream application servers for further processing.
   Examples include the IP address of the request originator, token
   binding ids and authenticated TLS client certificates.

   If the reverse proxy would pass through any header sent from the
   outside, an attacker could try to directly send the faked header
   values through the proxy to the application server in order to
   circumvent security controls that way.  For example, it is standard
   practice of reverse proxies to accept "forwarded_for" headers and
   just add the origin of the inbound request (making it a list).
   Depending on the logic performed in the application server, the
   attacker could simply add a whitelisted IP address to the header and
   render a IP whitelist useless.  A reverse proxy must therefore
   sanitize any inbound requests to ensure the authenticity and
   integrity of all header values relevant for the security of the
   application servers.

   If an attacker would be able to get access to the internal network
   between proxy and application server, it could also try to circumvent
   security controls in place.  It is therefore important to ensure the
   authenticity of the communicating entities.  Furthermore, the
   communication link between reverse proxy and application server must
   therefore be protected against tapping and injection (including
   replay prevention).

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4.  Acknowledgements

   We would like to thank Jim Manico, Phil Hunt, Nat Sakimura, Christian
   Mainka, and Brian Campbell for their valuable feedback.

5.  IANA Considerations

   This draft includes no request to IANA.

6.  Security Considerations

   All relevant security considerations have been given in the
   functional specification.

7.  References

7.1.  Normative References

   [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,

   [RFC6749]  Hardt, D., Ed., "The OAuth 2.0 Authorization Framework",
              RFC 6749, DOI 10.17487/RFC6749, October 2012,

   [RFC6750]  Jones, M. and D. Hardt, "The OAuth 2.0 Authorization
              Framework: Bearer Token Usage", RFC 6750,
              DOI 10.17487/RFC6750, October 2012,

   [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,

   [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,

   [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,

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7.2.  Informative References

              Bradley, J., Lodderstedt, T., and H. Zandbelt, "Encoding
              claims in the OAuth 2 state parameter using a JWT", draft-
              bradley-oauth-jwt-encoded-state-08 (work in progress),
              January 2018.

              Campbell, B., Bradley, J., and H. Tschofenig, "Resource
              Indicators for OAuth 2.0", draft-campbell-oauth-resource-
              indicators-02 (work in progress), November 2016.

              Bradley, J., Sanso, A., and H. Tschofenig, "OAuth 2.0
              Security: Closing Open Redirectors in OAuth", draft-ietf-
              oauth-closing-redirectors-00 (work in progress), February

              Jones, M., Sakimura, N., and J. Bradley, "OAuth 2.0
              Authorization Server Metadata", draft-ietf-oauth-
              discovery-10 (work in progress), March 2018.

              Sakimura, N. and J. Bradley, "The OAuth 2.0 Authorization
              Framework: JWT Secured Authorization Request (JAR)",
              draft-ietf-oauth-jwsreq-15 (work in progress), July 2017.

              Jones, M., Bradley, J., and N. Sakimura, "OAuth 2.0 Mix-Up
              Mitigation", draft-ietf-oauth-mix-up-mitigation-01 (work
              in progress), July 2016.

              Campbell, B., Bradley, J., Sakimura, N., and T.
              Lodderstedt, "OAuth 2.0 Mutual TLS Client Authentication
              and Certificate Bound Access Tokens", draft-ietf-oauth-
              mtls-07 (work in progress), January 2018.

              Bradley, J., Hunt, P., Jones, M., and H. Tschofenig,
              "OAuth 2.0 Proof-of-Possession: Authorization Server to
              Client Key Distribution", draft-ietf-oauth-pop-key-
              distribution-03 (work in progress), February 2017.

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              Richer, J., Bradley, J., and H. Tschofenig, "A Method for
              Signing HTTP Requests for OAuth", draft-ietf-oauth-signed-
              http-request-03 (work in progress), August 2016.

              Jones, M., Campbell, B., Bradley, J., and W. Denniss,
              "OAuth 2.0 Token Binding", draft-ietf-oauth-token-
              binding-06 (work in progress), March 2018.

              Popov, A., Nystrom, M., Balfanz, D., Langley, A., Harper,
              N., and J. Hodges, "Token Binding over HTTP", draft-ietf-
              tokbind-https-12 (work in progress), January 2018.

              Sakimura, N., Li, K., and J. Bradley, "The OAuth 2.0
              Authorization Framework: JWT Pop Token Usage", draft-
              sakimura-oauth-jpop-04 (work in progress), March 2017.

              Microsoft and Ping Identity, "OAuth 2.0 Form Post Response
              Mode", April 2015, <http://openid.net/specs/

              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

              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,

   [OpenID]   NRI, Ping Identity, Microsoft, Google, and Salesforce,
              "OpenID Connect Core 1.0 incorporating errata set 1", Nov

   [owasp]    "Open Web Application Security Project Home Page",

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              "Cross-Site Request Forgery (CSRF) Prevention Cheat
              Sheet", <https://www.owasp.org/index.php/

   [RFC7636]  Sakimura, N., Ed., Bradley, J., and N. Agarwal, "Proof Key
              for Code Exchange by OAuth Public Clients", RFC 7636,
              DOI 10.17487/RFC7636, September 2015,

   [RFC7800]  Jones, M., Bradley, J., and H. Tschofenig, "Proof-of-
              Possession Key Semantics for JSON Web Tokens (JWTs)",
              RFC 7800, DOI 10.17487/RFC7800, April 2016,

              Google Inc. and Google Inc., "Referrer Policy", April
              2017, <https://w3c.github.io/webappsec-referrer-policy>.

Appendix A.  Document History

   [[ To be removed from the final specification ]]


   o  Completed sections on code leakage via referrer header, attacks in
      browser, mix-up, and CSRF

   o  Reworked Code Injection Section

   o  Added reference to OpenID Connect spec

   o  removed refresh token leakage as respective considerations have
      been given in section 10.4 of RFC 6749

   o  first version on open redirection

   o  incorporated Christian Mainka's review feedback


   o  Restructured document for better readability

   o  Added best practices on Token Leakage prevention


   o  Added section on Access Token Leakage at Resource Server

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   o  incorporated Brian Campbell's findings


   o  Folded Mix up and Access Token leakage through a bad AS into new
      section for dynamic OAuth threats

   o  reworked dynamic OAuth section


   o  Added references to mitigation methods for token leakage

   o  Added reference to Token Binding for Authorization Code

   o  incorporated feedback of Phil Hunt

   o  fixed numbering issue in attack descriptions in section 2

   -00 (WG document)

   o  turned the ID into a WG document and a BCP

   o  Added federated app login as topic in Other Topics

Authors' Addresses

   Torsten Lodderstedt (editor)
   YES.com AG

   Email: torsten@lodderstedt.net

   John Bradley

   Email: ve7jtb@ve7jtb.com

   Andrey Labunets

   Email: isciurus@fb.com

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