Web Authorization Protocol                                T. Lodderstedt
Internet-Draft                                                   yes.com
Intended status: Best Current Practice                        J. Bradley
Expires: September 9, 2019                                        Yubico
                                                             A. Labunets
                                                                 D. Fett
                                                           March 8, 2019

                OAuth 2.0 Security Best Current Practice


   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
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   Internet-Drafts are draft documents valid for a maximum of six months
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   This Internet-Draft will expire on September 9, 2019.

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   Copyright (c) 2019 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
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   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
     1.1.  Structure . . . . . . . . . . . . . . . . . . . . . . . .   4
     1.2.  Conventions and Terminology . . . . . . . . . . . . . . .   4
   2.  The Updated OAuth 2.0 Attacker Model  . . . . . . . . . . . .   4
   3.  Recommendations . . . . . . . . . . . . . . . . . . . . . . .   6
     3.1.  Protecting Redirect-Based Flows . . . . . . . . . . . . .   6
       3.1.1.  Authorization Code Grant  . . . . . . . . . . . . . .   7
       3.1.2.  Implicit Grant  . . . . . . . . . . . . . . . . . . .   7
     3.2.  Token Replay Prevention . . . . . . . . . . . . . . . . .   8
     3.3.  Access Token Privilege Restriction  . . . . . . . . . . .   8
   4.  Attacks and Mitigations . . . . . . . . . . . . . . . . . . .   9
     4.1.  Insufficient Redirect URI Validation  . . . . . . . . . .   9
       4.1.1.  Redirect URI Validation Attacks on Authorization Code
               Grant . . . . . . . . . . . . . . . . . . . . . . . .   9
       4.1.2.  Redirect URI Validation Attacks on Implicit Grant . .  10
       4.1.3.  Proposed Countermeasures  . . . . . . . . . . . . . .  12
     4.2.  Credential Leakage via Referrer Headers . . . . . . . . .  12
       4.2.1.  Leakage from the OAuth Client . . . . . . . . . . . .  13
       4.2.2.  Leakage from the Authorization Server . . . . . . . .  13
       4.2.3.  Consequences  . . . . . . . . . . . . . . . . . . . .  13
       4.2.4.  Proposed Countermeasures  . . . . . . . . . . . . . .  13
     4.3.  Attacks through the Browser History . . . . . . . . . . .  14
       4.3.1.  Code in Browser History . . . . . . . . . . . . . . .  14
       4.3.2.  Access Token in Browser History . . . . . . . . . . .  15
     4.4.  Mix-Up  . . . . . . . . . . . . . . . . . . . . . . . . .  15
       4.4.1.  Attack Description  . . . . . . . . . . . . . . . . .  15
       4.4.2.  Countermeasures . . . . . . . . . . . . . . . . . . .  17
     4.5.  Authorization Code Injection  . . . . . . . . . . . . . .  18
       4.5.1.  Attack Description  . . . . . . . . . . . . . . . . .  18
       4.5.2.  Discussion  . . . . . . . . . . . . . . . . . . . . .  19
       4.5.3.  Proposed Countermeasures  . . . . . . . . . . . . . .  20
     4.6.  Access Token Injection  . . . . . . . . . . . . . . . . .  21
       4.6.1.  Proposed Countermeasures  . . . . . . . . . . . . . .  22
     4.7.  Cross Site Request Forgery  . . . . . . . . . . . . . . .  22
       4.7.1.  Proposed Countermeasures  . . . . . . . . . . . . . .  22
     4.8.  Access Token Leakage at the Resource Server . . . . . . .  22
       4.8.1.  Access Token Phishing by Counterfeit Resource Server   22
       4.8.2.  Compromised Resource Server . . . . . . . . . . . . .  28
     4.9.  Open Redirection  . . . . . . . . . . . . . . . . . . . .  28
       4.9.1.  Authorization Server as Open Redirector . . . . . . .  29

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       4.9.2.  Clients as Open Redirector  . . . . . . . . . . . . .  29
     4.10. 307 Redirect  . . . . . . . . . . . . . . . . . . . . . .  29
     4.11. TLS Terminating Reverse Proxies . . . . . . . . . . . . .  30
     4.12. Refresh Token Protection  . . . . . . . . . . . . . . . .  31
   5.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  33
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  33
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  33
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  33
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  33
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  34
   Appendix A.  Document History . . . . . . . . . . . . . . . . . .  37
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  39

1.  Introduction

   Since its publication in [RFC6749] and [RFC6750], 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

   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 [RFC6749],
      [RFC6749], and [RFC6819].

   Moreover, OAuth is being adopted in use cases with higher security
   requirements than considered initially, such as Open Banking,
   eHealth, eGovernment, and Electronic Signatures.  Those use cases
   call for stricter guidelines and additional protection.

   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

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   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 [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.

1.1.  Structure

   The remainder of the document is organized as follows: The next
   section updates the OAuth attacker model.  Afterwards, the most
   important recommendations of the OAuth working group for every OAuth
   implementor are summarized.  Subsequently, 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.

1.2.  Conventions and Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

2.  The Updated OAuth 2.0 Attacker Model

   In [RFC6819], an attacker model was laid out that described the
   capabilities of attackers against which OAuth deployments must
   defend.  In the following, this attacker model is updated to account
   for the potentially dynamic relationships involving multiple parties
   (as described above), to include new types of attackers, and to make
   it more clearly defined.

   OAuth 2.0 aims to ensure that the authorization of the resource owner
   (RO) (with a user agent) at an authorization server (AS) and the
   subsequent usage of the access token at the resource server (RS) is
   protected at least against the following attackers:

   o  (A1) Web Attackers that control an arbitrary number of network
      endpoints (except for RO, AS, and RS).  Web attackers may set up
      web sites that are visited by the RO, operate their own user

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      agents, participate in the protocol using their own user
      credentials, etc.
      Web attackers may, in particular, operate OAuth clients that are
      registered at AS, and operate their own authorization and resource
      servers that can be used (in parallel) by ROs.
      It must also be assumed that web attackers can lure the user to
      open arbitrary attacker-chosen URIs at any time.  This can be
      achieved through many ways, for example, by injecting malicious
      advertisements into advertisement networks, or by sending legit-
      looking emails.

   o  (A2) Network Attackers that additionally have full control over
      the network over which protocol participants communicate.  They
      can eavesdrop on, manipulate, and spoof messages, except when
      these are properly protected by cryptographic methods (e.g., TLS).
      Network attacker can also block specific messages.

   These attackers conform to the attacker model that was used in formal
   analysis efforts for OAuth [arXiv.1601.01229].  Previous attacks on
   OAuth have shown that OAuth deployments should protect against an
   even strong attacker model that is described as follows:

   o  (A3) Attackers that can read, but not modify, the contents of the
      authorization response (i.e., the authorization response can leak
      to an attacker).
      Examples for such attacks include open redirector attacks,
      problems existing on mobile operating systems (where different
      apps can register themselves on the same URI), so-called mix-up
      attacks, where the client is tricked into sending credentials to a
      attacker-controlled AS, and the fact that URLs are often stored/
      logged by browsers (history), proxy servers, and operating

   o  (A4) Attackers that can read, but not modify, the contents of the
      authorization request (i.e., the authorization request can leak,
      in the same manner as above, to an attacker).

   o  (A5) Attackers that control a resource server used by RO with an
      access token issued by AS.  For example, a resource server can be
      compromised by an attacker, an access token may be sent to an
      attacker-controlled resource server due to a misconfiguration, or
      an RO is social-engineered into using a attacker-controlled RS.

   Note that in this attacker model, an attacker can be a RO or act as
   one (see A1).  For example, an attacker can use his own browser to
   replay tokens or authorization codes obtained by any of the attacks
   described above at the client or RS.

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   This document discusses the additional threats resulting from these
   attackers in detail and recommends suitable mitigations.

   This is a minimal attacker model.  Implementers MUST take into
   account all possible attackers in the environment in which their
   OAuth implementations are expected to run.

3.  Recommendations

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

3.1.  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

   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 sent 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.

   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.

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   AS which redirect a request that potentially contains user
   credentials MUST avoid forwarding these user credentials accidentally
   (see Section 4.10).

3.1.1.  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 and the respective client.  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 SHOULD use client authentication if possible.

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

3.1.2.  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 4.1, Section 4.2,
   Section 4.3, and Section 4.6.

   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 3.2.  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 (response type "token") or any other response type issuing
   access tokens in the authorization response, such as "token id_token"
   and "code token id_token", unless the issued access tokens are
   sender-constrained and access token injection in the authorization
   response is prevented.

   A sender constrained access token scopes the applicability of an
   access token to a certain sender.  This sender is obliged to

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   demonstrate knowledge of a certain secret as prerequisite for the
   acceptance of that token at the recipient (e.g., a resource server).

   Clients SHOULD instead use the response type "code" (aka
   authorization code grant type) as specified in Section 3.1.1 or any
   other response type that causes the authorization server to issue
   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.

3.2.  Token Replay Prevention

   Authorization servers SHOULD use TLS-based methods for sender-
   constrained access tokens as described in Section, 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.  Refresh
   tokens MUST be sender-constrained or use refresh token rotation as
   described in Section 4.12.  It is also recommended to use end-to-end
   TLS whenever possible.

3.3.  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 3.2.

   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

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   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.

4.  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].

4.1.  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, utilizing flaws
   in the pattern matching implementation or concrete configurations,
   have been observed in the wild.  Insufficient validation of the
   redirect URI 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.

4.1.1.  Redirect URI Validation 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

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   The attack can then be conducted as follows:

   First, 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".  (See Attacker A1.)

   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=9ad67f13
        &redirect_uri=https%3A%2F%2Fevil.somesite.example%2Fcb HTTP/1.1
   Host: server.somesite.example

   Afterwards, 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.

   If the user does not recognize the attack, the code is issued and
   immediately sent to the attacker's client.

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

   Note: This attack will not work as easily 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 4.5.

4.1.2.  Redirect URI Validation 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
   attack.  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

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   circumvention even of very narrow redirect URI patterns (but not
   strict URL matching!).

   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

   The attack can now be conducted as follows:

   First, and 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".

   Afterwards, the website initiates an authorization request, which is
   very similar to the one in the attack on the code flow.  Different to
   above, 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&state=9ad67f13
        %252Fclient.evil.example%252Fcb HTTP/1.1
   Host: server.somesite.example

   Now, 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?

   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

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   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:


   The attacker's page at "client.evil.example" can now access the
   fragment and obtain the access token.

4.1.3.  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 4.9).

   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

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

4.2.  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 [RFC7231], Section 5.5.2, the same may happen
   to access tokens conveyed in URI fragments due to browser
   implementation issues as illustrated by Chromium Issue 168213

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4.2.1.  Leakage from the OAuth Client

   Leakage from the OAuth client requires that the client, as a result
   of a successful authorization request, renders a page that

   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".

4.2.2.  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.

4.2.3.  Consequences

   An attacker that learns a valid code or access token through a
   referrer header can perform the attacks as described in
   Section 4.1.1, Section 4.5, and Section 4.6.  If the attacker learns
   "state", the CSRF protection achieved by using "state" is lost,
   resulting in CSRF attacks as described in [RFC6819], Section

4.2.4.  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

   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  As described in [RFC6749], Section 4.1.2, authorization codes MUST
      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.

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      This does not mitigate the attack if the attacker manages to
      exchange the code for a token before the legitimate client does
      so.  Therefore, [RFC6749] further recommends that, when an attempt
      is made to redeem a code twice, the AS SHOULD revoke all tokens
      issued previously based on that code.

   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
      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

4.3.  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

4.3.1.  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, and Section 4.5

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

4.3.2.  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.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

4.4.  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.

4.4.1.  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

   Preconditions: For the attack to work, we assume that

   o  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),

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   o  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

   o  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), as in
      Attacker A2.

   Some of the attack variants described below require different

   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 response code "303 See Other" with a Location header
       containing the URL "https://attacker.example/

   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.  Therefore, the browser receives a
       redirection ("303 See Other") with a Location header pointing to

   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 4.5.

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

   o  *Mix-Up Without Interception*: A variant of the above attack works
      even if the first request/response pair cannot be intercepted (for
      example, because TLS is used to protect these messages): Here, we
      assume that the user wants to start the grant using A-AS (and not
      H-AS, see Attacker A1).  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 to "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
      Steps 3ff. of the attack description above.

   o  *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".
      Refer to [oauth_security_jcs_14] for details.

   o  *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").

4.4.2.  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

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      each authorization request, the intended AS, and (3) comparing the
      intended AS with the actual redirect URI where the authorization
      response was received.

4.5.  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 that the attacker is unable to access directly.

4.5.1.  Attack Description

   The attack works as follows:

   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

   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.

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4.5.2.  Discussion

   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 attempts 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 specification.

   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

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   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.

   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.

4.5.3.  Proposed Countermeasures

   There are multiple technical solutions to achieve this goal:

   o  *Nonce*: OpenID Connect's existing "nonce" parameter can be used
      for the purpose of detecting authorization code injection attacks.
      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
      binds "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 victim's 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 clients to adopt ID Tokens.

   o  *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.

   o  *PKCE*: The PKCE parameter "code_challenge" along with the
      corresponding "code_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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   o  *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.

   o  *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

   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

   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.

4.6.  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

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   response as a CSRF and will use the access token injected by the

4.6.1.  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 4.5.

4.7.  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.

4.7.1.  Proposed Countermeasures

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

   o  *CSRF Tokens*: Use of CSRF tokens which are bound to the user
      agent and passed in the "state" parameter to the authorization

   o  *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].

4.8.  Access Token Leakage at the Resource Server

   Access tokens can leak from a resource server under certain

4.8.1.  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 that are valid for other resource servers
   (see Attackers A1 and A5).  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 one specific resource
   server (and its URL) at development time, but client instances are
   provided with the resource server URL at runtime.  This kind of late
   binding is typical in situations where the client uses a service
   implementing a standardized API (e.g., for e-Mail, calendar, health,
   or banking) and where the client is configured by a user or
   administrator for a service which this user or company uses.

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

   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

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   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
   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.  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

   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  *OAuth Token Binding* ([I-D.ietf-oauth-token-binding]): In this
      approach, an access token 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 [RFC8473]
      (including federated token bindings) must be supported on all ends
      (client, authorization server, resource server).

   o  *OAuth Mutual TLS* ([I-D.ietf-oauth-mtls]): The approach as
      specified in this document allows the use of mutual TLS (mTLS) for
      both client authentication and sender-constrained access tokens.
      For the purpose of sender-constrained 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  *Signed HTTP Requests* ([I-D.ietf-oauth-signed-http-request]):
      This approach 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  *JWT Pop Tokens* ([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 [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

   Mutual TLS and 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,
   for OAuth Token Binding, all key material is automatically managed by
   the TLS stack whereas 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
   reduces the complexity of this task.  Furthermore, 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

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   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

   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.

   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

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   specifically minted for the respective server.  This has huge
   functional and privacy advantages in deployments using structured
   access tokens.

4.8.2.  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-constrained 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

4.9.  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.

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4.9.1.  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, 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 registration [RFC7591] 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.

4.9.2.  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.

4.10.  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
   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

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   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".

4.11.  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).
   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.

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   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).

4.12.  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

   Refresh tokens 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, the attacker will be able to mint access tokens and
   use them to access resource servers on behalf of the resource owner.

   [RFC6749] already provides a robust baseline 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 (i.e., "client_id"),

   o  authentication of this client during token refresh, if possible,

   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
   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

   Authorization servers MUST determine based on their risk assessment
   whether to issue refresh tokens to a certain client.  If the

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   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 persistent grants to optimize the user

   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 token leakage.

   Authorization server MUST utilize one of these methods to detect
   refresh token replay for public clients:

   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

   o  *Refresh token rotation:* the authorization server 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.
      Implementation note: refresh tokens belonging to the same grant
      may share a common id.  If any of those refresh tokens is used at
      the authorization server, the authorization server uses this
      common id to look up the currently active refresh token and can
      revoke it.

   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, i.e., the refresh token has not been used to obtain fresh
   access tokens for some time.  The expiration time is at the
   discretion of the authorization server.  It might be a global value

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   or determined based on the client policy or the grant associated with
   the refresh token (and its sensitivity).

5.  Acknowledgements

   We would like to thank Jim Manico, Phil Hunt, Nat Sakimura, Christian
   Mainka, Doug McDorman, Johan Peeters, Joseph Heenan, Brock Allen,
   Vittorio Bertocci, David Waite, Nov Matake, Tomek Stojecki, Dominick
   Baier, Neil Madden, William Dennis, Dick Hardt, Petteri Stenius,
   Annabelle Richard Backman, Aaron Parecki, George Fletscher, and Brian
   Campbell for their valuable feedback.

6.  IANA Considerations

   This draft includes no request to IANA.

7.  Security Considerations

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

8.  References

8.1.  Normative References

              Jones, M. and B. Campbell, "OAuth 2.0 Form Post Response
              Mode", April 2015, <http://openid.net/specs/

   [OpenID]   Sakimura, N., Bradley, J., Jones, M., de Medeiros, B., and
              C. Mortimore, "OpenID Connect Core 1.0 incorporating
              errata set 1", Nov 2014,

   [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,

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   [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,

   [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,

8.2.  Informative References

              Mladenov, V., Mainka, C., and J. Schwenk, "On the security
              of modern Single Sign-On Protocols: Second-Order
              Vulnerabilities in OpenID Connect", January 2016,

              Fett, D., Kuesters, R., and G. Schmitz, "A Comprehensive
              Formal Security Analysis of OAuth 2.0", January 2016,

              Fett, D., Kuesters, R., and G. Schmitz, "The Web SSO
              Standard OpenID Connect: In-Depth Formal Security Analysis
              and Security Guidelines", April 2017,

              "Referer header includes URL fragment when opening link
              using New Tab",

              "Facebook Developer Blog",

              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.

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              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

              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

              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-13 (work in progress), February 2019.

              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.

              Campbell, B., Bradley, J., and H. Tschofenig, "Resource
              Indicators for OAuth 2.0", draft-ietf-oauth-resource-
              indicators-02 (work in progress), January 2019.

              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-08 (work in progress), October 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.

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              Chen, E., Pei, Y., Chen, S., Tian, Y., Kotcher, R., and P.
              Tague, "OAuth Demystified for Mobile Application
              Developers", November 2014.

              Bansal, C., Bhargavan, K., Delignat-Lavaud, A., and S.
              Maffeis, "Discovering concrete attacks on website
              authorization by formal analysis", April 2014.

              Sun, S. and K. Beznosov, "The Devil is in the
              (Implementation) Details: An Empirical Analysis of OAuth
              SSO Systems", October 2012,

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

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

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [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,

   [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,

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

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   [RFC8414]  Jones, M., Sakimura, N., and J. Bradley, "OAuth 2.0
              Authorization Server Metadata", RFC 8414,
              DOI 10.17487/RFC8414, June 2018,

   [RFC8473]  Popov, A., Nystroem, M., Balfanz, D., Ed., Harper, N., and
              J. Hodges, "Token Binding over HTTP", RFC 8473,
              DOI 10.17487/RFC8473, October 2018,

              Eisinger, J. and E. Stark, "Referrer Policy", April 2017,

Appendix A.  Document History

   [[ To be removed from the final specification ]]


   o  Added updated attacker model


   o  Adapted section 2.1.2 to outcome of consensus call

   o  more text on refresh token inactivity and implementation note on
      refresh token replay detection via refresh token rotation


   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


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   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


   o  added recommendations re implicit and token injection

   o  uppercased key words in Section 2 according to RFC 2119


   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


   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


   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

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   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

   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

   Email: torsten@lodderstedt.net

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   John Bradley

   Email: ve7jtb@ve7jtb.com

   Andrey Labunets

   Email: isciurus@fb.com

   Daniel Fett

   Email: mail@danielfett.de

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