Open Authentication Protocol                         T. Lodderstedt, Ed.
Internet-Draft                                             YES Europe AG
Intended status: Best Current Practice                        J. Bradley
Expires: March 14, 2018                                           Yubico
                                                             A. Labunets
                                                      September 10, 2017

                         OAuth Security Topics


   This draft gives a comprehensive overview on open OAuth security
   topics.  It is intended to serve as a working document for the OAuth
   working group to systematically capture and discuss these security
   topics and respective mitigations and eventually recommend best
   current practice and also OAuth extensions needed to cope with the
   respective security threats.

Status of This Memo

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   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on March 14, 2018.

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   document authors.  All rights reserved.

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   carefully, as they describe your rights and restrictions with respect

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   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.  Recommended Best Practice . . . . . . . . . . . . . . . . . .   4
     2.1.  Protecting redirect-based flows . . . . . . . . . . . . .   4
     2.2.  TBD . . . . . . . . . . . . . . . . . . . . . . . . . . .   5
   3.  Recommended modifications and extensions to OAuth . . . . . .   5
   4.  OAuth Credentials Leakage . . . . . . . . . . . . . . . . . .   5
     4.1.  Insufficient redirect URI validation  . . . . . . . . . .   5
       4.1.1.  Attacks on Authorization Code Grant . . . . . . . . .   6
       4.1.2.  Attacks on Implicit Grant . . . . . . . . . . . . . .   7
       4.1.3.  Proposed Countermeasures  . . . . . . . . . . . . . .   8
     4.2.  Authorization code leakage via referrer headers . . . . .  10
       4.2.1.  Proposed Countermeasures  . . . . . . . . . . . . . .  10
     4.3.  Attacks in the Browser  . . . . . . . . . . . . . . . . .  10
       4.3.1.  Code in browser history (TBD) . . . . . . . . . . . .  11
       4.3.2.  Access token in browser history (TBD) . . . . . . . .  11
       4.3.3.  Javascript Code stealing Access Tokens (TBD)  . . . .  11
     4.4.  Access Token Leakage at the Resource Server . . . . . . .  11
       4.4.1.  Access Token Phishing by Counterfeit Resource Server   11  Metadata  . . . . . . . . . . . . . . . . . . . .  12  Sender Constrained Access Tokens  . . . . . . . .  13  Audience Restricted Access Tokens . . . . . . . .  15
       4.4.2.  Compromised Resource Server . . . . . . . . . . . . .  16
       4.4.3.  TLS Terminating Reverse Proxies . . . . . . . . . . .  17
     4.5.  Mix-Up  . . . . . . . . . . . . . . . . . . . . . . . . .  18
     4.6.  Refresh Token Leakage . . . . . . . . . . . . . . . . . .  18
   5.  OAuth Credentials Injection . . . . . . . . . . . . . . . . .  19
     5.1.  Code Injection  . . . . . . . . . . . . . . . . . . . . .  19
       5.1.1.  Proposed Countermeasures  . . . . . . . . . . . . . .  21
     5.2.  Access Token Injection (TBD)  . . . . . . . . . . . . . .  22
     5.3.  XSRF (TBD)  . . . . . . . . . . . . . . . . . . . . . . .  23
   6.  Other Attacks . . . . . . . . . . . . . . . . . . . . . . . .  23
   7.  Other Topics  . . . . . . . . . . . . . . . . . . . . . . . .  23
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  24
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  24
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  24
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  24
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  24
     11.2.  Informative References . . . . . . . . . . . . . . . . .  25
   Appendix A.  Document History . . . . . . . . . . . . . . . . . .  26
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  27

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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, 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 (XSRF, referrer
      header).  Although most of these threats are discussed in RFC 6819
      [RFC6819], continued exploitation demonstrates there may be a need
      for more specific recommendations or that the existing mitigations
      are too difficult to deploy.

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

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

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

   The remainder of the document is organized as follows: The next
   section gives a summary of the set of security mechanisms and
   practices, the working group shall consider to recommend to OAuth
   implementers.  This is followed by a section proposing modifications

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   to OAuth intended to either simplify its usage and to strengthen its

   The remainder of the draft gives a detailed analyses of the
   weaknesses and implementation issues, which can be found in the wild
   today along with a discussion of potential counter measures.  First,
   various scenarios how OAuth credentials (namely access tokens and
   authorization codes) may be disclosed to attackers and proposes
   countermeasures are discussed.  Afterwards, the document discusses
   attacks possible with captured credential and how they may be
   prevented.  The last sections discuss additional threats.

2.  Recommended Best Practice

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

2.1.  Protecting redirect-based flows

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

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

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

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

   Clients shall use PKCE [RFC7636] in order to (with the help of the
   authorization server) detect attempts to inject 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.

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

2.2.  TBD

   Add further topics:

   o  Access Token Leakage at resource servers

3.  Recommended modifications and extensions to OAuth

   This section describes the set of modifications and extensions the
   authors believe should be taken into consideration by the OAuth
   working group change and extend OAuth in order to strengthen its
   security and make it simpler to implement.  It also recommends some
   changes to the OAuth set of specs.

   Remove requirement to check actual redirect URI at token endpoint -
   seems to be complicated to implement properly and could be
   compromised.  The protection goal is achieved even more effective by
   utilizing PKCE as recommended in Section 2.1.

4.  OAuth Credentials Leakage

   This section describes a couple of different ways how OAuth
   credentials, namely authorization codes and access tokens, can be
   exposed to attackers.

4.1.  Insufficient redirect URI validation

   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

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   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.

4.1.1.  Attacks on Authorization Code Grant

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

   Let's assume the redirect URL pattern "https://*.example.com/*" had
   been registered for the client "s6BhdRkqt3".  This pattern allows
   redirect URIs from any host residing in the domain example.com.  So
   if an attacker manager to establish a host or subdomain in
   "example.com" he can impersonate the legitimate client.  Assume the
   attacker sets up the host "evil.example.com".

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

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

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

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

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

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

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

4.1.2.  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!).

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

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

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

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

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

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

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

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

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


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

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 shall compare the two URIs using simple string comparison as
   defined in [RFC3986], Section 6.2.1..

   This would cause the following impacts:

   o  This change will require all OAuth clients to maintain the
      transaction state (and XSRF tokens) in the "state" parameter.
      This is a normative change to RFC 6749 since section
      allows for dynamic URI query parameters in the redirect URI.  In

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      order to assess the practical impact, the working group needs to
      collect data on whether this feature is really used in deployments

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

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

   o  Exact redirect matching does not work for native apps utilizing a
      local web server due to dynamic port numbers - at least wild cards
      for port numbers are required.
      Question: Does redirect uri validation solve any problem for
      native apps?  Effective against impersonation when used in
      conjunction with claimed HTTPS redirect URIs only.
      For Windows token broker exact redirect URI matching is important
      as the redirect URI encodes the app identity.  For custom scheme
      redirects there is a question however it is probably a useful part
      of defense in depth.

   Additional recommendations:

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

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

   Alternatives to exact redirect URI matching:

   o  authenticate client using digital signatures (JAR?

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4.2.  Authorization code leakage via referrer headers

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

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

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

4.2.1.  Proposed Countermeasures

   There are some means to prevent leakage as described above:

   o  Use of the HTML link attribute rel="noreferrer" (Chrome
      52.0.2743.116, FF 49.0.1, Edge 38.14393.0.0, IE/Win10)

   o  Use of the "referrer" meta link attribute (possible values e.g.
      noreferrer, origin, ...) (cf. https://w3c.github.io/webappsec-
      referrer-policy/ - work in progress (seems Google, Chrome and Edge
      support it))

   o  Redirect to intermediate page (sanitize history) before sending
      user agent to other pages
      Note: double check redirect/referrer header behavior

   o  Use form post mode instead of redirect for authorization response
      (don't transport credentials via URL parameters and GET)

   Note: There shouldn't be a referer header when loading HTTP content
   from a HTTPS -loaded page (e.g. help/faq pages)

   Note: This kind of attack is not applicable to the implicit grant
   since fragments are not be included in referrer headers (cf.

4.3.  Attacks in the Browser

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4.3.1.  Code in browser history (TBD)

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

   Proposed countermeasures: code is one time use, has limited duration,
   is bound to client id/secret (confidential clients only)

4.3.2.  Access token in browser history (TBD)

   When 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

   When browser navigates to client.com/
   redirection_endpoint#access_token=abcef as a result of a redirect
   from a provider's authorization endpoint.

   Proposal: replace implicit flow with postmessage communication

4.3.3.  Javascript Code stealing Access Tokens (TBD)

   sandboxing using service workers

4.4.  Access Token Leakage at the Resource Server

4.4.1.  Access Token Phishing by Counterfeit Resource Server

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

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

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

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

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

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


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

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


   This mitigation strategy would rely on the client to enforce the
   security policy and to only send access tokens to legitimate
   destinations.  Results of OAuth related security research (see for
   example [oauth_security_ubc] and [oauth_security_cmu]) indicate a
   large portion of client implementations do not or fail to properly
   implement security controls, like state checks.  So relying on
   clients to detect and properly handle 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

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

   As the name suggests, sender constraint access token scope the
   applicability of an access token to a certain sender.  This sender is
   obliged to demonstrate knowledge of a certain secret as prerequisite
   for the acceptance of that token at a resource server.

   A typical flow looks like this:

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

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

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

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      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

   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 detection.

   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 detection is provided by building the signature
      over a server-provided nonce, client-provided nonce and a nonce

   [I-D.ietf-oauth-mtls] and [I-D.ietf-oauth-token-binding] are built on
   top of TLS and this way continue the successful OAuth 2.0 philosophy
   to leverage TLS to secure OAuth wherever possible.  Both mechanisms
   allow prevention of access token leakage in a fairly client developer
   friendly way.

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

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   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 detection).

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

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

   The audience can basically be expressed using logical names or
   physical addresses (like URLs).  In order to detect 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.

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   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.campbell-oauth-resource-indicators] or encode
   the information in the scope value.

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

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

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

4.4.2.  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.

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   Preventing and detecting 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 shall be taken into account by implementors in
   order to cope with access token replay:

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

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

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

4.4.3.  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

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   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).

4.5.  Mix-Up

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

   A detailed description of the attack and potential countermeasures is
   given in cf. https://tools.ietf.org/html/draft-ietf-oauth-mix-up-

   Potential mitigations:

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

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

   o  Clients use AS-specific redirect URIs, for every authorization
      request store intended AS and compare intention with actual
      redirect URI where the response was received (no change to OAuth

4.6.  Refresh Token Leakage

   mitm, log files on the device, ...

   refresh token rotation, mutual TLS authentication at the token

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5.  OAuth Credentials Injection

   Credential injection means an attacker somehow obtained a valid OAuth
   credential (code or token) and is able to utilize this to impersonate
   the legitimate resource owner or to cause a victim to access
   resources under the attacker's control (XSRF).

5.1.  Code Injection

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

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

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

   o  Another example could be that access to the authorization and
      resource servers is some how limited to networks, the attackers is
      unable to access directly.

   How does an attack look like?

   (1)  The attacker obtains an authorization code by executing any of
        the attacks described above (OAuth Credentials Leakage).

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

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

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

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

   (6)  If all checks succeed, the authorization server issues access
        and other tokens to the client.

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   (7)  The attacker just impersonated the victim.

   Obviously, the check in step (5) will fail, if the code was issued to
   another client id, e.g. a client set up by the attacker.

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

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

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

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

   o  the client has bound this data to this particular instance

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

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

   It is also assumed that the requirements defined in [RFC6749],
   Section 4.1.3, increase client implementation complexity as clients

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   need to memorize or re-construct the correct redirect URI for the
   call to the tokens endpoint.

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

5.1.1.  Proposed Countermeasures

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

   Nonce   OpenID Connect's existing "nonce" parameter is used for this
           purpose.  The nonce value is one time use and created by the
           client.  The client is supposed to bind it to the user agent
           session and sends it with the initial request to the OpenId
           Provider (OP).  The OP associates the nonce to the
           authorization code and attests this binding in the ID token,
           which is issued as part of the code exchange at the token
           endpoint.  If an attacker injected an authorization code in
           the authorization response, the nonce value in the client
           session and the nonce value in the ID token will not match
           and the attack is detected.  assumption: attacker cannot get
           hold of the user agent state on the victims device, where he
           has stolen the respective authorization code.
           - existing feature, used in the wild
           - OAuth does not have an ID Token - would need to push that
           down the stack

   Code-bound State  It has been discussed in the security workshop in
           December to use the OAuth state value much similar in the way
           as described above.  In the case of the state value, the idea
           is to add a further parameter state to the code exchange
           request.  The authorization server then compares the state
           value it associated with the code and the state value in the
           parameter.  If those values do not match, it is considered an
           attack and the request fails.  Note: a variant of this
           solution would be send a hash of the state (in order to
           prevent bulky requests and DoS).
           - use existing concept
           - state needs to fulfil certain requirements (one time use,

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           - new parameter means normative spec change

   PKCE    Basically, the PKCE challenge/verifier could be used in the
           same way as Nonce or State.  In contrast to its original
           intention, the verifier check would fail although the client
           uses its correct verifier but the code is associated with a
           challenge, which does not match.
           - existing and deployed OAuth feature
           - currently used and recommended for native apps, not web

   Token Binding  Code must be bind to UA-AS and UA-Client legs -
           requires further data (extension to response) to manifest
           binding id for particular code.
           Note: token binding could be used in conjunction with PKCE as
           an option (https://tools.ietf.org/html/draft-ietf-oauth-
           - highly secure
           - highly sophisticated, requires browser support, will it
           work for native apps?

   per instance client id/secret  ...

   Note on pre-warmed secrets: An attacker can circumvent the
   countermeasures described above if he is able to create or capture
   the respective secret or code_challenge on a device under his
   control, which is then used in the victim's authorization request.
   Exact redirect URI matching of authorization requests can prevent the
   attacker from using the pre-warmed secret in the faked authorization
   transaction on the victim's device.
   Unfortunately it does not work for all kinds of OAuth clients.  It is
   effective for web and JS apps and for native apps with claimed URLs.
   What about other native apps?  Treat nonce or PKCE challenge as
   replay detection tokens (needs to ensure cluster-wide one-time use)?

5.2.  Access Token Injection (TBD)

   Note: An attacker in possession of an access token can access any
   resources the access token gives him the permission to.  This kind of
   attacks simply illustrates the fact that bearer tokens utilized by
   OAuth are reusable similar to passwords unless they are protected by
   further means.

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   (where do we treat access token replay/use at the resource server?
   https://tools.ietf.org/html/rfc6819#section-4.6.4 has some text about
   it but is it sufficient?)

   The attack described in this section is about injecting a stolen
   access token into a legitimate client on a device under the
   adversaries control.  The attacker wants to impersonate a victim and
   cannot use his own client, since he wants to access certain functions
   in this particular client.

   Proposal: token binding, hybrid flow+nonce(OIDC), other
   cryptographical binding between access token and user agent instance

5.3.  XSRF (TBD)

   injection of code or access token on a victim's device (e.g. to cause
   client to access resources under the attacker's control)

   mitigation: XSRF tokens (one time use) w/ user agent binding (cf.

6.  Other Attacks

   Using the AS as Open Redirector - error handling AS (redirects)

   Using the Client as Open Redirector

   redirect via status code 307 - use 302

7.  Other Topics

   why to rotate refresh tokens

   how to support multi AS per RS

   differentiate native, JS and web clients

   do not put sensitive data in URL/GET parameters (Jim Manico)

   Incorporate Christian Mainka's feedback

   WPAD attack - https://www.blackhat.com/docs/us-16/materials/us-16-

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8.  Acknowledgements

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

9.  IANA Considerations

   This draft includes no request to IANA.

10.  Security Considerations

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

11.  References

11.1.  Normative References

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,

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

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

   [RFC6819]  Lodderstedt, T., Ed., McGloin, M., and P. Hunt, "OAuth 2.0
              Threat Model and Security Considerations", RFC 6819,
              DOI 10.17487/RFC6819, January 2013,

   [RFC7231]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
              DOI 10.17487/RFC7231, June 2014,

   [RFC7591]  Richer, J., Ed., Jones, M., Bradley, J., Machulak, M., and
              P. Hunt, "OAuth 2.0 Dynamic Client Registration Protocol",
              RFC 7591, DOI 10.17487/RFC7591, July 2015,

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11.2.  Informative References

              Bradley, J., Lodderstedt, T., and H. Zandbelt, "Encoding
              claims in the OAuth 2 state parameter using a JWT", draft-
              bradley-oauth-jwt-encoded-state-07 (work in progress),
              March 2017.

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

              Jones, M., Sakimura, N., and J. Bradley, "OAuth 2.0
              Authorization Server Metadata", draft-ietf-oauth-
              discovery-07 (work in progress), September 2017.

              Campbell, B., Bradley, J., Sakimura, N., and T.
              Lodderstedt, "Mutual TLS Profile for OAuth 2.0", draft-
              ietf-oauth-mtls-03 (work in progress), July 2017.

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

              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., Bradley, J., Campbell, B., and W. Denniss,
              "OAuth 2.0 Token Binding", draft-ietf-oauth-token-
              binding-04 (work in progress), July 2017.

              Popov, A., Nystrom, M., Balfanz, D., Langley, A., Harper,
              N., and J. Hodges, "Token Binding over HTTP", draft-ietf-
              tokbind-https-10 (work in progress), July 2017.

              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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              Carnegie Mellon University, Carnegie Mellon University,
              Microsoft Research, Carnegie Mellon University, Carnegie
              Mellon University, and Carnegie Mellon University, "OAuth
              Demystified for Mobile Application Developers", November

              University of British Columbia and University of British
              Columbia, "The Devil is in the (Implementation) Details:
              An Empirical Analysis of OAuth SSO Systems", October 2012,

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

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

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

Appendix A.  Document History

   [[ To be removed from the final specification ]]


   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

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   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 Europe AG

   Email: torsten@lodderstedt.net

   John Bradley

   Email: ve7jtb@ve7jtb.com

   Andrey Labunets

   Email: isciurus@fb.com

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