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Open Authentication Protocol                         T. Lodderstedt, Ed.
Internet-Draft                                             YES Europe AG
Intended status: Best Current Practice                        J. Bradley
Expires: September 29, 2017                                Ping Identity
                                                             A. Labunets
                                                                Facebook
                                                          March 30, 2017

                         OAuth Security Topics
                  draft-ietf-oauth-security-topics-02

Abstract

   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

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

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

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

   This Internet-Draft will expire on September 29, 2017.

Copyright Notice

   Copyright (c) 2017 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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







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   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 . . . . . . . . . . . . . . . . . . . . . . . . .  2
   2.  Recommended Best Practice  . . . . . . . . . . . . . . . . . .  3
     2.1.  Protecting redirect-based flows  . . . . . . . . . . . . .  4
     2.2.  TBD  . . . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Recommended modifications and extensions to OAuth  . . . . . .  4
   4.  OAuth Credentials Leakage  . . . . . . . . . . . . . . . . . .  5
     4.1.  Insufficient redirect URI validation . . . . . . . . . . .  5
       4.1.1.  Attacks on Authorization Code Grant  . . . . . . . . .  5
       4.1.2.  Attacks on Implicit Grant  . . . . . . . . . . . . . .  6
       4.1.3.  Proposed Countermeasures . . . . . . . . . . . . . . .  7
     4.2.  Authorization code leakage via referrer headers  . . . . .  9
       4.2.1.  Proposed Countermeasures . . . . . . . . . . . . . . .  9
     4.3.  Attacks in the Browser . . . . . . . . . . . . . . . . . .  9
       4.3.1.  Code in browser history (TBD)  . . . . . . . . . . . .  9
       4.3.2.  Access token in browser history (TBD)  . . . . . . . . 10
       4.3.3.  Javascript Code stealing Access Tokens (TBD) . . . . . 10
     4.4.  Dynamic OAuth Scenarios  . . . . . . . . . . . . . . . . . 10
       4.4.1.  Access Token Phishing by Counterfeit Resource Server . 10
       4.4.2.  Mix-Up . . . . . . . . . . . . . . . . . . . . . . . . 11
   5.  OAuth Credentials Injection  . . . . . . . . . . . . . . . . . 12
     5.1.  Code Injection . . . . . . . . . . . . . . . . . . . . . . 12
       5.1.1.  Proposed Countermeasures . . . . . . . . . . . . . . . 14
       5.1.2.  Access Token Injection (TBD) . . . . . . . . . . . . . 15
       5.1.3.  XSRF (TBD) . . . . . . . . . . . . . . . . . . . . . . 16
   6.  Other Attacks  . . . . . . . . . . . . . . . . . . . . . . . . 16
   7.  Other Topics . . . . . . . . . . . . . . . . . . . . . . . . . 16
   8.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 16
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 16
   10. Security Considerations  . . . . . . . . . . . . . . . . . . . 16
   11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 16
     11.1.  Normative References  . . . . . . . . . . . . . . . . . . 17
     11.2.  Informative References  . . . . . . . . . . . . . . . . . 17
   Appendix A. Document History . . . . . . . . . . . . . . . . . . . 17
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 18

1.  Introduction












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   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].

   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
   to OAuth intended to either simplify its usage and to strengten its
   security.

   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













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   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
   attacks.

   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 send 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.

   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

3.  Recommended modifications and extensions to OAuth






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   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

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
   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".



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

   (4 )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 easier.

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

   (6 )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!).






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

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

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

   GET /authorize?response_type=token&client_id=s6BhdRkqt3&state=xyz
     &redirect_uri=https%3A%2F%2Fclient.example.com%2Fcb%26redirect_to
     %253Dhttps%253A%252F%252Fclient.evil.com%252Fcb HTTP/1.1
   Host: server.example.com

   (5 )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?
     redirect_to%3Dhttps%3A%2F%2Fclient.evil.com%2Fcb
     #access_token=2YotnFZFEjr1zCsicMWpAA&...

   (6 )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

   (7 )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:

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

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





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4.1.3.  Proposed Countermeasures

   The complexitity 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 3.1.2.2
      allows for dynamic URI query parameters in the redirect URI. In
      order to assess the practical impact, the working group needs to
      collect data on whether this feature is realy used in deployments
      today.

   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
      explicitly.
   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 impotant as
   the redirect URI encodes the app identity.  For custom scheme
   redirects there is a question however it is probably a usfull 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.


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   Alternatives to exact redirect URI matching:

   o  authenticate client using digital signatures (JAR? https://
      tools.ietf.org/html/draft-ietf-oauth-jwsreq-09)

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
   request.

   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.  https:/
   /tools.ietf.org/html/rfc7231#section-5.5.2)

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.  Dynamic OAuth Scenarios

   OAuth initially assumed a static relationship between client,
   authorization server and resource servers.  The URLs of AS and RS
   were know 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 is 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 can 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 section.

4.4.1.  Access Token Phishing by Counterfeit Resource Server



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   An attacker may pretend to be a particular resource server and to
   accept tokens from a particular authorization server.  If the client
   sends a valid access token to this counterfeit resource server, the
   server in turn may use that token to access other services on behalf
   of the resource owner.

   Potential mitigation strategies:

   o  AS may publish information about its legitimate resource servers,
      clients must only send access tokens to this servers

   o  Clients indicate resource server they intend to use the access
      token for at AS, AS may refuse to issue tokens for ressource
      servers it does not know

   o  AS indicates resource servers a particular access token is good
      for to client - client enforced audience restriction - prevents
      disclosure (e.g.  OAuth Response Metadata (https://tools.ietf.org/
      html/draft-sakimura-oauth-meta-07)

   o  Access tokens are audience restricted - prevents replay if the
      audience is a URL determined by the client, reduces impact in case
      of legitimate resource server uses token at other resource server
      (e.g.  https://tools.ietf.org/html/draft-campbell-oauth-resource-
      indicators-01)

   o  Access Token is sender restricted - sender is cryptographically
      verified

      *  https://tools.ietf.org/html/draft-ietf-oauth-pop-
         architecture-08

      *  https://tools.ietf.org/html/draft-jones-oauth-token-binding-00

      *  https://datatracker.ietf.org/doc/draft-campbell-oauth-mtls

      *  https://datatracker.ietf.org/doc/html/draft-sakimura-oauth-jpop

4.4.2.  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-
   mitigation-01.

   Potential mitigations:


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   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
      required)

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
      redirect_uri.



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   (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.

   (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.


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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.

   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.
           pro:
           - existing feature, used in the wild
           con:
           - 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).
           pro:
           - use existing concept
           con:
           - state needs to fulfil certain requirements (one time use,
           complexity)
           - new parameter means normative spec change



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   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.
           pro:
           - existing and deployed OAuth feature
           con:
           - currently used and recommended for native apps, not web
           apps

   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-campbell-oauth-
           tbpkce).
           pro:
           - highly secure
           con:
           - 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)?

5.1.2.  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.
   (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.

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

5.1.3.  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.
   https://www.owasp.org/index.php/
   CrossSite_Request_Forgery_(CSRF)_Prevention_Cheat_Sheet)

6.  Other Attacks

   Using the AS as Open Redirector - error handling AS (redirects)
   (draft-ietf-oauth-closing-redirectors-00)

   Using the Client as Open Redirector

   redirect via status code 307 - use 302

7.  Other Topics

   why to rotate refresh tokens

   why audience restriction

   how to support multi AS per RS

   ...

   differentiate native, JS and web clients

   federated login to apps (code flow to own AS in browser and federated
   login to 3rd party IDP in browser)

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

8.  Acknowledgements

   We would like to thank Jim Manico and Phil Hunt 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

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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, <http://www.rfc-
              editor.org/info/rfc3986>.

   [RFC6749]  Hardt, D., Ed., "The OAuth 2.0 Authorization Framework",
              RFC 6749, DOI 10.17487/RFC6749, October 2012, <http://www
              .rfc-editor.org/info/rfc6749>.

   [RFC6750]  Jones, M. and D. Hardt, "The OAuth 2.0 Authorization
              Framework: Bearer Token Usage", RFC 6750, DOI 10.17487/
              RFC6750, October 2012, <http://www.rfc-editor.org/info/
              rfc6750>.

   [RFC6819]  Lodderstedt, T., Ed., McGloin, M. and P. Hunt, "OAuth 2.0
              Threat Model and Security Considerations", RFC 6819, DOI
              10.17487/RFC6819, January 2013, <http://www.rfc-editor.org
              /info/rfc6819>.

   [RFC7231]  Fielding, R.Ed.,  and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Semantics and Content", RFC 7231, DOI
              10.17487/RFC7231, June 2014, <http://www.rfc-editor.org/
              info/rfc7231>.

   [RFC7591]  Richer, J., Ed., Jones, M., Bradley, J., Machulak, M. and
              P. Hunt, "OAuth 2.0 Dynamic Client Registration Protocol",
              RFC 7591, DOI 10.17487/RFC7591, July 2015, <http://www
              .rfc-editor.org/info/rfc7591>.

11.2.  Informative References

   [I-D.bradley-oauth-jwt-encoded-state]
              Bradley, J., Lodderstedt, T. and H. Zandbelt, "Encoding
              claims in the OAuth 2 state parameter using a JWT",
              Internet-Draft draft-bradley-oauth-jwt-encoded-state-07,
              March 2017.

   [I-D.ietf-oauth-discovery]
              Jones, M., Sakimura, N. and J. Bradley, "OAuth 2.0
              Authorization Server Metadata", Internet-Draft draft-ietf-
              oauth-discovery-04, August 2016.

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

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

Appendix A.  Document History

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   [[ To be removed from the final specification ]]

   -01

   o  Added references to mitigation methods for token leakage

   o  Added reference to Token Binding for Authorization Code

   o  incorporated feedback of Phil Hunt

   o  fixed numbering issue in attack descriptions in section 2

   -00 (WG document)

   o  turned the ID into a WG document and a BCP

   o  Added federated app login as topic in Other Topics

Authors' Addresses

   Torsten Lodderstedt, editor
   YES Europe AG

   Email: torsten@lodderstedt.net


   John Bradley
   Ping Identity

   Email: ve7jtb@ve7jtb.com


   Andrey Labunets
   Facebook

   Email: isciurus@fb.com

















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