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Versions: (draft-lodderstedt-oauth-security-topics) 00 01 02 03 04 05 06 07 08 09 10

Open Authentication Protocol                         T. Lodderstedt, Ed.
Internet-Draft                                                   yes.com
Intended status: Best Current Practice                        J. Bradley
Expires: May 23, 2019                                             Yubico
                                                             A. Labunets
                                                                Facebook
                                                                 D. Fett
                                                                 yes.com
                                                       November 19, 2018


                OAuth 2.0 Security Best Current Practice
                  draft-ietf-oauth-security-topics-10

Abstract

   This document describes best current security practice 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 covers 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 May 23, 2019.

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



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   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   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  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Recommendations . . . . . . . . . . . . . . . . . . . . . . .   4
     2.1.  Protecting Redirect-Based Flows . . . . . . . . . . . . .   4
       2.1.1.  Authorization Code Grant  . . . . . . . . . . . . . .   5
       2.1.2.  Implicit Grant  . . . . . . . . . . . . . . . . . . .   5
     2.2.  Token Replay Prevention . . . . . . . . . . . . . . . . .   6
     2.3.  Access Token Privilege Restriction  . . . . . . . . . . .   6
   3.  Attacks and Mitigations . . . . . . . . . . . . . . . . . . .   7
     3.1.  Insufficient Redirect URI Validation  . . . . . . . . . .   7
       3.1.1.  Attacks on Authorization Code Grant . . . . . . . . .   7
       3.1.2.  Attacks on Implicit Grant . . . . . . . . . . . . . .   8
       3.1.3.  Proposed Countermeasures  . . . . . . . . . . . . . .  10
     3.2.  Credential Leakage via Referrer Headers . . . . . . . . .  10
       3.2.1.  Leakage from the OAuth client . . . . . . . . . . . .  10
       3.2.2.  Leakage from the Authorization Server . . . . . . . .  11
       3.2.3.  Consequences  . . . . . . . . . . . . . . . . . . . .  11
       3.2.4.  Proposed Countermeasures  . . . . . . . . . . . . . .  11
     3.3.  Attacks through the Browser History . . . . . . . . . . .  12
       3.3.1.  Code in Browser History . . . . . . . . . . . . . . .  12
       3.3.2.  Access Token in Browser History . . . . . . . . . . .  12
     3.4.  Mix-Up  . . . . . . . . . . . . . . . . . . . . . . . . .  13
       3.4.1.  Attack Description  . . . . . . . . . . . . . . . . .  13
       3.4.2.  Countermeasures . . . . . . . . . . . . . . . . . . .  15
     3.5.  Authorization Code Injection  . . . . . . . . . . . . . .  16
       3.5.1.  Proposed Countermeasures  . . . . . . . . . . . . . .  18
     3.6.  Access Token Injection  . . . . . . . . . . . . . . . . .  19
       3.6.1.  Proposed Countermeasures  . . . . . . . . . . . . . .  20
     3.7.  Cross Site Request Forgery  . . . . . . . . . . . . . . .  20
       3.7.1.  Proposed Countermeasures  . . . . . . . . . . . . . .  20
     3.8.  Access Token Leakage at the Resource Server . . . . . . .  20
       3.8.1.  Access Token Phishing by Counterfeit Resource Server   20
         3.8.1.1.  Metadata  . . . . . . . . . . . . . . . . . . . .  21
         3.8.1.2.  Sender Constrained Access Tokens  . . . . . . . .  22
         3.8.1.3.  Audience Restricted Access Tokens . . . . . . . .  25
       3.8.2.  Compromised Resource Server . . . . . . . . . . . . .  26
     3.9.  Open Redirection  . . . . . . . . . . . . . . . . . . . .  27
       3.9.1.  Authorization Server as Open Redirector . . . . . . .  27
       3.9.2.  Clients as Open Redirector  . . . . . . . . . . . . .  27
     3.10. 307 Redirect  . . . . . . . . . . . . . . . . . . . . . .  27
     3.11. TLS Terminating Reverse Proxies . . . . . . . . . . . . .  28



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     3.12. Refresh Token Protection  . . . . . . . . . . . . . . . .  29
   4.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  31
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  31
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  31
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  31
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .  31
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  32
   Appendix A.  Document History . . . . . . . . . . . . . . . . . .  35
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  37

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
   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
   observed:

   o  OAuth implementations are being attacked through known
      implementation weaknesses and anti-patterns (CSRF, 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 relationship 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



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   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 [RFC8414] 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
   analysis of the threats and implementation issues which can be found
   in the wild today is given along with a discussion of potential
   countermeasures.

2.  Recommendations

   This section describes the set of security mechanisms the OAuth
   working group recommendeds to OAuth implementers.

2.1.  Protecting Redirect-Based Flows

   Authorization servers MUST 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
   attacks.

   Clients SHOULD avoid forwarding the user's browser to a URI obtained
   from a query parameter since such a function could be utilized to
   exfiltrate authorization codes and access tokens.  If there is a
   strong need for this kind of redirects, clients are advised to
   implement appropriate countermeasures against open redirection, e.g.,
   as described by the OWASP [owasp].

   Clients MUST prevent CSRF and ensure that each authorization response
   is only accepted once.  One-time use CSRF tokens carried in the
   "state" parameter, which are securely bound to the user agent, SHOULD
   be used for that purpose.

   In order to prevent mix-up attacks, clients MUST only process
   redirect responses of the OAuth authorization server they send the
   respective request to and from the same user agent this authorization
   request was initiated with.  Clients MUST memorize which
   authorization server they 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.  Clients SHOULD
   use AS-specific redirect URIs as a means to identify the AS a
   particular response came from.



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

2.1.1.  Authorization Code Grant

   Clients utilizing the authorization grant type MUST 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 for the same purpose.

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

   Authorization servers MUST bind authorization codes to a certain
   client and authenticate it using an appropriate mechanism (e.g.
   client credentials or PKCE).

   Authorization servers SHOULD furthermore consider the recommendations
   given in [RFC6819], Section 4.4.1.1, on authorization code replay
   prevention.

2.1.2.  Implicit Grant

   The implicit grant (response type "token") and other response types
   causing the authorization server to issue access tokens in the
   authorization response are vulnerable to access token leakage and
   access token replay as described in Section 3.1, Section 3.2,
   Section 3.3, and Section 3.6.

   Moreover, no viable mechanism exists to cryptographically bind access
   tokens issued in the authorization response to a certain client as it
   is recommended in Section 2.2.  This makes replay detection for such
   access tokens at resource servers impossible.

   In order to avoid these issues, Clients SHOULD NOT use the implicit
   grant or any other response type causing the authorization server to
   issue an access token in the authorization response.

   Clients SHOULD instead use the response type "code" (aka
   authorization code grant type) as specified in Section 2.1.1 or any
   other response type that causes the authorization server to issue



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   access tokens in the token response.  This allows the authorization
   server to detect replay attempts and generally reduces the attack
   surface since access tokens are not exposed in URLs.  It also allows
   the authorization server to sender-constrain the issued tokens.

2.2.  Token Replay Prevention

   Authorization servers SHOULD use TLS-based methods for sender
   constrained access tokens as described in Section 3.8.1.2, such as
   token binding [I-D.ietf-oauth-token-binding] or Mutual TLS for OAuth
   2.0 [I-D.ietf-oauth-mtls] in order to prevent token replay.  It is
   also recommended to use end-to-end TLS whenever possible.

2.3.  Access Token Privilege Restriction

   The privileges associated with an access token SHOULD be restricted
   to the minimum required for the particular application or use case.
   This prevents clients from exceeding the privileges authorized by the
   resource owner.  It also prevents users from exceeding their
   privileges authorized by the respective security policy.  Privilege
   restrictions also limit the impact of token leakage although more
   effective counter-measures are described in Section 2.2.

   In particular, access tokens SHOULD be restricted to certain resource
   servers, preferably to a single resource server.  To put this into
   effect, the authorization server associates the access token with
   certain resource servers and every resource server is obliged to
   verify for every request, whether the access token sent with that
   request was meant to be used for that particular resource server.  If
   not, the resource server MUST refuse to serve the respective request.
   Clients and authorization servers MAY utilize the parameters "scope"
   or "resource" as specified in [RFC6749] and
   [I-D.ietf-oauth-resource-indicators], respectively, to determine the
   resource server they want to access.

   Additionally, access tokens SHOULD be restricted to certain resources
   and actions on resource servers or resources.  To put this into
   effect, the authorization server associates the access token with the
   respective resource and actions and every resource server is obliged
   to verify for every request, whether the access token sent with that
   request was meant to be used for that particular action on the
   particular resource.  If not, the resource server must refuse to
   serve the respective request.  Clients and authorization servers MAY
   utilize the parameter "scope" as specified in [RFC6749] to determine
   those resources and/or actions.






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3.  Attacks and Mitigations

   This section gives a detailed description of attacks on OAuth
   implementations, along with potential countermeasures.  This section
   complements and enhances the description given in [RFC6819].

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://*.somesite.example/*"
   had been registered for the client "s6BhdRkqt3".  This pattern allows
   redirect URIs pointing to any host residing in the domain
   somesite.example.  So if an attacker manages to establish a host or
   subdomain in somesite.example he can impersonate the legitimate
   client.  Assume the attacker sets up the host
   "evil.somesite.example".

   (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.example".





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   (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.somesite.example%2Fcb HTTP/1.1
   Host: server.somesite.example

   (3)  The authorization server validates the redirect URI in order to
        identify the client.  Since the pattern allows arbitrary domains
        host names in "somesite.example", 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 [RFC6749]), the attack can be performed
        even easier.

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

   (5)  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 impersonate or utilize
   the legitimate client to redeem the code (e.g., by performing a code
   injection attack).  This kind of injections is covered in
   Section Authorization 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.

   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!).





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   Assume the pattern for client "s6BhdRkqt3" is
   "https://client.somesite.example/cb?*", i.e., any parameter is
   allowed for redirects to "https://client.somesite.example/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 an HTTP Location header redirect
   303.

   (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.example".

   (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.example" 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
  &redirect_uri=https%3A%2F%2Fclient.somesite.example%2Fcb%26redirect_to
  %253Dhttps%253A%252F%252Fclient.evil.example%252Fcb HTTP/1.1
Host: server.somesite.example

   (3)  Since the redirect URI matches the registered pattern, the
        authorization server allows the request and sends the resulting
        access token with a 303 redirect (some response parameters are
        omitted for better readability)

   HTTP/1.1 303 See Other
     Location: https://client.somesite.example/cb?
     redirect_to%3Dhttps%3A%2F%2Fclient.evil.example%2Fcb
     #access_token=2YotnFZFEjr1zCsicMWpAA&...

   (4)  At example.com, the request arrives at the open redirector.  It
        will read the redirect parameter and will issue an HTTP 303
        Location header redirect to the URL
        "https://client.evil.example/cb".

   HTTP/1.1 303 See Other
        Location: https://client.evil.example/cb

   (5)  Since the redirector at client.somesite.example 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:




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  https://client.evil.example/cb#access_token=2YotnFZFEjr1zCsicMWpAA&...

   (6)  The attacker's page at client.evil.example 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 must compare the two URIs using simple string comparison as
   defined in [RFC3986], Section 6.2.1..

   Additional recommendations:

   o  Servers on which callbacks are hosted must not expose open
      redirectors (see Section 3.9).

   o  Clients MAY drop fragments via intermediary URLs with "fix
      fragments" (see [fb_fragments]) to prevent the user agent from
      appending any unintended fragments.

   o  Clients SHOULD use the authorization code response type instead of
      response types causing access token issuance at the authorization
      endpoint.  This offers countermeasures against reuse of leaked
      credentials through the exchange process with the authorization
      server and token replay through certificate binding of the access
      tokens.

   As an alternative to exact redirect URI matching, the AS could also
   authenticate clients, e.g., using [I-D.ietf-oauth-jwsreq].

3.2.  Credential Leakage via Referrer Headers

   Authorization codes or values of "state" can unintentionally be
   disclosed to attackers through the referrer header, by leaking either
   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
   to access tokens conveyed in URI fragments due to browser
   implementation issues as illustrated by Chromium Issue 168213
   [bug.chromium].

3.2.1.  Leakage from the OAuth client

   This requires that the client, as a result of a successful
   authorization request, renders a page that




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   o  contains links to other pages under the attacker's control (ads,
      faq, ...) and a user clicks on such a link, or

   o  includes third-party content (iframes, images, etc.) for example
      if the page contains user-generated content (blog).

   As soon as the browser navigates to the attacker's page or loads the
   third-party content, the attacker receives the authorization response
   URL and can extract "code", "access token", or "state".

3.2.2.  Leakage from the Authorization Server

   In a similar way, an attacker can learn "state" if the authorization
   endpoint at the authorization server contains links or third-party
   content as above.

3.2.3.  Consequences

   An attacker that learns a valid code or access token through a
   referrer header can perform the attacks as described in
   Section 3.1.1, Section 3.5, and Section 3.6.  If the attacker learns
   "state", the CSRF protection achieved by using "state" is lost,
   resulting in CSRF attacks as described in [RFC6819],
   Section 4.4.1.8..

3.2.4.  Proposed Countermeasures

   The page rendered as a result of the OAuth authorization response and
   the authorization endpoint SHOULD not include third-party resources
   or links to external sites.

   The following measures further reduce the chances of a successful
   attack:

   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  Authorization codes SHOULD be invalidated by the AS after their
      first use at the token endpoint.  For example, if an AS
      invalidated the code after the legitimate client redeemed it, the
      attacker would fail exchanging this code later.  (This does not
      mitigate the attack if the attacker manages to exchange the code
      for a token before the legitimate client does so.)

   o  The "state" value SHOULD be invalidated by the client after its
      first use at the redirection endpoint.  If this is implemented,
      and an attacker receives a token through the referrer header from



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      the client's web site, the "state" was already used, invalidated
      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
      then the "state" has not been used at the redirection endpoint at
      the client yet.)

   o  Suppress the referrer header by adding the attribute
      "rel="noreferrer"" to HTML links or by applying an appropriate
      Referrer Policy [webappsec-referrer-policy] to the document
      (either as part of the "referrer" meta attribute or by setting a
      Referrer-Policy header).

   o  Use authorization code instead of response types causing access
      token issuance from the authorization endpoint.  This provides
      countermeasures against leakage on the OAuth protocol level
      through the code exchange process with the authorization server.

   o  Additionally, one might use the form post response mode instead of
      redirect for authorization response (see
      [oauth-v2-form-post-response-mode]).

3.3.  Attacks through the Browser History

   Authorization codes and access tokens can end up in the browser's
   history of visited URLs, enabling the attacks described in the
   following.

3.3.1.  Code in Browser History

   When a browser navigates to "client.example/
   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 4.4.1.1, 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.".



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   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.example/
   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.

   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 an attack on scenarios where an OAuth client interacts with
   multiple authorization servers, as is usually the case when dynamic
   registration is used.  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 instead of using them at
   the respective endpoint at the authorization/resource server.

3.4.1.  Attack Description

   For a detailed attack description, refer to [arXiv.1601.01229] and
   [I-D.ietf-oauth-mix-up-mitigation].  The description here closely
   follows [arXiv.1601.01229], with variants of the attack outlined
   below.

   Preconditions: For the attack to work, we assume that

   (1)  the implicit or authorization code grant are used with multiple
        AS of which one is considered "honest" (H-AS) and one is
        operated by the attacker (A-AS),

   (2)  the client stores the AS chosen by the user in a session bound
        to the user's browser and uses the same redirection endpoint URI
        for each AS, and

   (3)  the attacker can manipulate the first request/response pair from
        a user's browser to the client (in which the user selects a
        certain AS and is then redirected by the client to that AS).

   Some of the attack variants described below require different
   preconditions.



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   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://attacker.example", client id: 666RVZJTA).

   Attack on the authorization code grant:

   (1)  The user selects to start the grant using H-AS (e.g., by
        clicking on a button at the client's website).

   (2)  The attacker intercepts this request and changes the user's
        selection to "A-AS".

   (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
        by sending the following response:

HTTP/1.1 303 See Other
  Location: https://attacker.example/authorize?response_type=code&client_id=666RVZJTA

   (4)  Now the attacker intercepts this response and changes 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
        the client's id at H-AS, resulting in the following response
        being sent to the browser:

HTTP/1.1 303 See Other
  Location: https://honest.as.example/authorize?response_type=code&client_id=7ZGZldHQ

   (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
        the client.

   (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.

   (7)  The attacker therefore obtains code and can either exchange the
        code for an access token (for public clients) or perform a code
        injection attack as described in Section 3.5.

   Variants:

   Implicit Grant  In the implicit grant, the attacker receives an
           access token instead of the code; the rest of the attack
           works as 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):



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           Here, we assume that the user wants to start the grant using
           A-AS (and not H-AS).  After the client redirected the user to
           the authorization endpoint at A-AS, the attacker immediately
           redirects the user to H-AS (changing the client id
           "7ZGZldHQ").  (A vigilant user might at this point detect
           that she intended to use A-AS instead of H-AS.)  The attack
           now proceeds exactly as in step 1 of the attack description
           above.

   Per-AS Redirect URIs  If clients use different redirect URIs for
           different ASs, do not store the selected AS in the user's
           session, and ASs do not check the redirect URIs properly,
           attackers can mount an attack called "Cross-Social Network
           Request Forgery".  We refer to [oauth_security_jcs_14] for
           details.

   OpenID Connect  There are several variants that can be used to attack
           OpenID Connect.  They are described in detail in
           [arXiv.1704.08539], Appendix A, and [arXiv.1508.04324v2],
           Section 6 ("Malicious Endpoints Attacks").

3.4.2.  Countermeasures

   In scenarios where an OAuth client interacts with multiple
   authorization servers, clients MUST prevent mix-up attacks.

   Potential countermeasures:

   o  Configure authorization servers to return an AS identitifier
      ("iss") and the "client_id" for which a code or token was issued
      in the authorization response.  This enables clients to compare
      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
      discussed in detail in [I-D.ietf-oauth-mix-up-mitigation].  In
      OpenID Connect, if an ID token is returned in the authorization
      response, it carries client id and issuer.  It can be used for
      this mitigation.

   o  As it can be seen in the preconditions of the attacks above,
      clients can prevent mix-up attack by (1) using AS-specific
      redirect URIs with exact redirect URI matching, (2) storing, for
      each authorization request, the intended AS, and (3) comparing the
      intended AS with the actual redirect URI where the authorization
      response was received.







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3.5.  Authorization 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.

   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
        "redirect_uri".

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




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

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

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

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

   o  the client has bound this data to this particular instance.

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

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

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






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   This document therefore recommends to instead 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.

3.5.1.  Proposed Countermeasures

   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.  The assumption is that an
           attacker cannot get 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 what is described above.  This
           would require to add a further parameter "state" to the code
           exchange token endpoint request.  The authorization server
           would then compare 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 though it is used today to secure native apps,
           only.





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   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 is
           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 seems 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
   clients.

   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.  Access Token Injection

   In such an attack, the adversary attempts to inject a stolen access
   token into a legitimate client on a device under his control.  This
   will typically happen if the attacker wants to utilize a leaked
   access token to impersonate a user in a certain client.

   To conduct the attack, the adversary starts an OAuth flow with the
   client and modifies the authorization response by replacing the
   access token issued by the authorization server or directly makes up
   an authorization server response including the leaked access token.
   Since the response includes the state value generated by the client
   for this particular transaction, the client does not treat the
   response as a CSRF and will use the access token injected by the
   attacker.



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3.6.1.  Proposed Countermeasures

   There is no way to detect such an injection attack on the OAuth
   protocol level, since the token is issued without any binding to the
   transaction or the particular user agent.

   The recommendation is therefore to use the authorization code grant
   type instead of relying on response types issuing acess tokens at the
   authorization endpoint.  Code injection can be detected using one of
   the countermeasures discussed in Section 3.5.

3.7.  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.

3.7.1.  Proposed Countermeasures

   Standard CSRF defenses should be used to protect the redirection
   endpoint, for example:

   CSRF Tokens    Use of CSRF tokens which are bound to the user agent
                  and passed in the "state" parameter to the
                  authorization server.

   Origin Header  The Origin header can be used to detect and prevent
                  CSRF attacks.  Since this feature, at the time of
                  writing, is not consistently supported by all
                  browsers, CSRF tokens should be used in addition to
                  Origin header checking.

   For more details see [owasp_csrf].

3.8.  Access Token Leakage at the Resource Server

   Access tokens can leak from a resource server under certain
   circumstances.

3.8.1.  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.





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   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.

3.8.1.1.  Metadata

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

   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

   {
     "issuer":"https://server.somesite.example",
     "authorization_endpoint":
       "https://server.somesite.example/authorize",
     "resource_servers":[
       "email.somesite.example",
       "storage.somesite.example",
       "video.somesite.example"]
     ...
   }

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













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

   {
     "access_token":"2YotnFZFEjr1zCsicMWpAA",
     "access_token_resource_server":
       "https://hostedresource.somesite.example/path1",
   ...
   }

   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
   client has to contribute to the overall security.  But there are
   alternative countermeasures, as described in the next sections, which
   provide a better balance between the involved parties.

3.8.1.2.  Sender Constrained Access Tokens

   As the name suggests, sender constrained 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 binds 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



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       case it might automatically happens during the setup of a TLS
       connection.

   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
      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
      server).

   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.




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   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
      counter.

   [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
   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.








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3.8.1.3.  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 sent 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
   server.

   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.

   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.ietf-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.




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   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.

3.8.2.  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
   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 should 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 3.8.1.2
      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
      system.

   o  Audience restriction as described in Section 3.8.1.3 may be used
      to prevent replay of captured access tokens on other resource
      servers.




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3.9.  Open Redirection

   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-
   controlled web site.

3.9.1.  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.

   [RFC6749], Section 4.1.2.1, already prevents open redirects by
   stating the AS MUST NOT automatically redirect the user agent in case
   of an invalid combination of client_id and redirect_uri.

   However, as described in [I-D.ietf-oauth-closing-redirectors], an
   attacker could also utilize a correctly registered redirect URI to
   perform phishing attacks.  It could for example register a client via
   dynamic client [RFC7591] registration and intentionally send an
   erroneous authorization request, e.g., by using an invalid scope
   value, to cause the AS to automatically redirect the user agent to
   its phishing site.

   The AS MUST take precautions to prevent this threat.  Based on its
   risk assessment the AS needs to decide whether it can trust the
   redirect URI or not and SHOULD only automatically redirect the user
   agent, if it trusts the redirect URI.  If not, it MAY inform the user
   that it is about to redirect her to the another site and rely on the
   user to decide or MAY just inform the user about the error.

3.9.2.  Clients as Open Redirector

   Client MUST NOT expose URLs which could be utilized as open
   redirector.  Attackers may use an open redirector to produce URLs
   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.

3.10.  307 Redirect

   At the authorization endpoint, a typical protocol flow is that the AS
   prompts the user to enter her credentials in a form that is then
   submitted (using the HTTP POST method) back to the authorization



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   server.  The AS checks the credentials and, if successful, redirects
   the user agent to the client's redirection endpoint.

   In [RFC6749], the HTTP status code 302 is used for this purpose, but
   "any other method available via the user-agent to accomplish this
   redirection is allowed".  However, when the status code 307 is used
   for redirection, the user agent will send the form data (user
   credentials) via HTTP POST to the client since this status code does
   not require the user agent to rewrite the POST request to a GET
   request (and thereby dropping the form data in the POST request
   body).  If the relying party is malicious, it can use the credentials
   to impersonate the user at the AS.

   In the HTTP standard [RFC6749], only the status code 303
   unambigiously enforces rewriting the HTTP POST request to an HTTP GET
   request.  For all other status codes, including the popular 302, user
   agents can opt not to rewrite POST to GET requests and therefore to
   reveal the user credentials to the client.  (In practice, however,
   most user agents will only show this behaviour for 307 redirects.)

   AS which redirect a request that potentially contains user
   credentials therefore MUST not use the HTTP 307 status code for
   redirection.  If an HTTP redirection (and not, for example,
   JavaScript) is used for such a request, AS SHOULD use HTTP status
   code 303 "See Other".

3.11.  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).



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   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).

3.12.  Refresh Token Protection

   Refresh tokens are a convenient and UX-friendly way to obtain new
   access tokens after the expiration of older access tokens.  Refresh
   tokens also add to the security of OAuth since they allow the
   authorization server to issue access tokens with a short lifetime and
   reduced scope thus reducing the potential impact of access token
   leakage.

   Refresh tokens themself are an attractive target for attackers since
   they represent the overall grant a resource owner delegated to a
   certain client.  If an attacker is able to exfiltrate and
   successfully replay a refresh token, it will be able to mint access
   tokens and use them to access resource servers on behalf of the
   resource server.

   [RFC6749] already provides robust base protection by requiring

   o  confidentiality of the refresh tokens in transit and storage,

   o  the transmission of refresh tokens over TLS-protected connections
      between authorization server and client,

   o  the authorization server to maintain and check the binding of a
      refresh token to a certain client_id,

   o  authentication of this client_id during token refresh, if
      possible, and

   o  that refresh tokens cannot be generated, modified, or guessed.

   [RFC6749] also lays the foundation for further (implementation
   specific) security measures, such as refresh token expiration and



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   revocation as well as refresh token rotation by defining respective
   error codes and response behavior.

   This draft gives recommendations beyond the scope of [RFC6749] and
   clarifications.

   Authorization servers SHALL determine based on their risk assessment
   whether to issue refresh tokens to a certain client.  If the
   authorization server decides not to issue refresh tokens, the client
   may refresh access tokens by utilizing other grant types, such as the
   authorization code grant type.  In such a case, the authorization
   server may utilize cookies and persistents grants to optimize the
   user experience.

   If refresh tokens are issued, those refresh tokens MUST be bound to
   the scope and resource servers as consented by the resource owner.
   This is to prevent privilege escalation by the legit client and
   reduce the impact of refresh tokens leakage.

   Authorization server SHALL utilize one of the methods listed below to
   detect refresh token replay for public clients:

   o  Refresh token rotation: the authorization issues a new refresh
      token with every access token refresh response.  The previous
      refresh token is invalidated but information about the
      relationship is retained by the authorization server.  If a
      refresh token is compromised and subsequently used by both the
      attacker and the legitimate client, one of them will present an
      invalidated refresh token, which will inform the authorization
      server of the breach.  The authorization server cannot determine
      which party submitted the invalid refresh token, but it can revoke
      the active refresh token.  This stops the attack at the cost of
      forcing the legit client to obtain a fresh authorization grant.

   o  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].

   Authorization servers may revoke refresh tokens automatically in case
   of a security event, such as:

   o  password change

   o  logout at the authorization server

   Refresh tokens should expire if the client has been inactive for some
   time.



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4.  Acknowledgements

   We would like to thank Jim Manico, Phil Hunt, Nat Sakimura, Christian
   Mainka, Doug McDorman, Johan Peeters, Joseph Heenan, Brock Allen, 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

   [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>.




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   [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]
              Mladenov, V., Mainka, C., and J. Schwenk, "On the security
              of modern Single Sign-On Protocols: Second-Order
              Vulnerabilities in OpenID Connect", arXiv 1508.04324v2,
              January 2016, <http://arxiv.org/abs/1508.04324v2/>.

   [arXiv.1601.01229]
              Fett, D., Kuesters, R., and G. Schmitz, "A Comprehensive
              Formal Security Analysis of OAuth 2.0", arXiv 1601.01229,
              January 2016, <http://arxiv.org/abs/1601.01229/>.

   [arXiv.1704.08539]
              Fett, D., Kuesters, R., and G. Schmitz, "The Web SSO
              Standard OpenID Connect: In-Depth Formal Security Analysis
              and Security Guidelines", arXiv 1704.08539, April 2017,
              <http://arxiv.org/abs/1704.08539/>.

   [bug.chromium]
              "Referer header includes URL fragment when opening link
              using New Tab",
              <https://bugs.chromium.org/p/chromium/issues/
              detail?id=168213/>.

   [fb_fragments]
              "Facebook Developer Blog",
              <https://developers.facebook.com/blog/post/552/>.

   [I-D.bradley-oauth-jwt-encoded-state]
              Bradley, J., Lodderstedt, T., and H. Zandbelt, "Encoding
              claims in the OAuth 2 state parameter using a JWT", draft-
              bradley-oauth-jwt-encoded-state-09 (work in progress),
              November 2018.

   [I-D.ietf-oauth-closing-redirectors]
              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
              2016.






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   [I-D.ietf-oauth-jwsreq]
              Sakimura, N. and J. Bradley, "The OAuth 2.0 Authorization
              Framework: JWT Secured Authorization Request (JAR)",
              draft-ietf-oauth-jwsreq-17 (work in progress), October
              2018.

   [I-D.ietf-oauth-mix-up-mitigation]
              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.

   [I-D.ietf-oauth-mtls]
              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-12 (work in progress), October 2018.

   [I-D.ietf-oauth-pop-key-distribution]
              Bradley, J., Hunt, P., Jones, M., Tschofenig, H., and M.
              Mihaly, "OAuth 2.0 Proof-of-Possession: Authorization
              Server to Client Key Distribution", draft-ietf-oauth-pop-
              key-distribution-04 (work in progress), October 2018.

   [I-D.ietf-oauth-resource-indicators]
              Campbell, B., Bradley, J., and H. Tschofenig, "Resource
              Indicators for OAuth 2.0", draft-ietf-oauth-resource-
              indicators-01 (work in progress), October 2018.

   [I-D.ietf-oauth-signed-http-request]
              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.

   [I-D.ietf-oauth-token-binding]
              Jones, M., Campbell, B., Bradley, J., and W. Denniss,
              "OAuth 2.0 Token Binding", draft-ietf-oauth-token-
              binding-08 (work in progress), October 2018.

   [I-D.ietf-tokbind-https]
              Popov, A., Nystrom, M., Balfanz, D., Langley, A., Harper,
              N., and J. Hodges, "Token Binding over HTTP", draft-ietf-
              tokbind-https-18 (work in progress), June 2018.

   [I-D.sakimura-oauth-jpop]
              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.




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   [oauth-v2-form-post-response-mode]
              Microsoft and Ping Identity, "OAuth 2.0 Form Post Response
              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]
              Bansal, C., Bhargavan, K., Delignat-Lavaud, A., and S.
              Maffeis, "Discovering concrete attacks on website
              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 Connect Core 1.0 incorporating errata set 1", Nov
              2014,
              <http://openid.net/specs/openid-connect-core-1_0.html>.

   [owasp]    "Open Web Application Security Project Home Page",
              <https://www.owasp.org/>.

   [owasp_csrf]
              "Cross-Site Request Forgery (CSRF) Prevention Cheat
              Sheet", <https://www.owasp.org/index.php/
              Cross-Site_Request_Forgery_(CSRF)_Prevention_Cheat_Sheet>.

   [RFC2616]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
              Transfer Protocol -- HTTP/1.1", RFC 2616,
              DOI 10.17487/RFC2616, June 1999,
              <https://www.rfc-editor.org/info/rfc2616>.

   [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,
              <https://www.rfc-editor.org/info/rfc7636>.





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   [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,
              <https://www.rfc-editor.org/info/rfc7800>.

   [RFC8414]  Jones, M., Sakimura, N., and J. Bradley, "OAuth 2.0
              Authorization Server Metadata", RFC 8414,
              DOI 10.17487/RFC8414, June 2018,
              <https://www.rfc-editor.org/info/rfc8414>.

   [webappsec-referrer-policy]
              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 ]]

   -10

   o  incorporated feedback by Joseph Heenan

   o  changed occurrences of SHALL to MUST

   o  added text on lack of token/cert binding support tokens issued in
      the authorization response as justification to not recommend
      issuing tokens there at all

   o  added requirement to authenticate clients during code exchange
      (PKCE or client credential) to 2.1.1.

   o  added section on refresh tokens

   o  editorial enhancements to 2.1.2 based on feedback

   -09

   o  changed text to recommend not to use implicit but code

   o  added section on access token injection

   o  reworked sections 3.1 through 3.3 to be more specific on implicit
      grant issues

   -08

   o  added recommendations re implicit and token injection




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   o  uppercased key words in Section 2 according to RFC 2119

   -07

   o  incorporated findings of Doug McDorman

   o  added section on HTTP status codes for redirects

   o  added new section on access token privilege restriction based on
      comments from Johan Peeters

   -06

   o  reworked section 3.8.1

   o  incorporated Phil Hunt's feedback

   o  reworked section on mix-up

   o  extended section on code leakage via referrer header to also cover
      state leakage

   o  added Daniel Fett as author

   o  replaced text intended to inform WG discussion by recommendations
      to implementors

   o  modified example URLs to conform to RFC 2606

   -05

   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

   -04

   o  Restructured document for better readability



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   o  Added best practices on Token Leakage prevention

   -03

   o  Added section on Access Token Leakage at Resource Server

   o  incorporated Brian Campbell's findings

   -02

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

   o  reworked dynamic OAuth section

   -01

   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

   Email: torsten@lodderstedt.net


   John Bradley
   Yubico

   Email: ve7jtb@ve7jtb.com








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   Andrey Labunets
   Facebook

   Email: isciurus@fb.com


   Daniel Fett
   yes.com

   Email: mail@danielfett.de









































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