Open Authentication Protocol                         T. Lodderstedt, Ed.
Internet-Draft                                       Deutsche Telekom AG
Intended status: Informational                                J. Bradley
Expires: May 16, 2017                                      Ping Identity
                                                             A. Labunets
                                                       November 12, 2016

                         OAuth Security Topics


   This draft gives a comprehensive overview on open OAuth security
   topics.  It is intended to serve as a tool for the OAuth working
   group to systematically address these open security topics,
   recommending mitigations, and potentially also defining OAuth
   extensions needed to cope with the respective security threats.  This
   draft will potentially become a BCP over time.

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
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   This Internet-Draft will expire on May 16, 2017.

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   to this document.  Code Components extracted from this document must
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  OAuth Credentials Leakage . . . . . . . . . . . . . . . . . .   3
     2.1.  Redirect URI validation of authorization requests . . . .   3
       2.1.1.  Authorization Code Grant  . . . . . . . . . . . . . .   3
       2.1.2.  Implicit Grant  . . . . . . . . . . . . . . . . . . .   4
       2.1.3.  Countermeasure: exact redirect URI matching . . . . .   6
     2.2.  Authorization code leakage via referrer headers . . . . .   7
       2.2.1.  Countermeasures . . . . . . . . . . . . . . . . . . .   7
     2.3.  Code in browser history (TBD) . . . . . . . . . . . . . .   8
     2.4.  Access token in browser history (TBD) . . . . . . . . . .   8
     2.5.  Access token on bad resource servers (TBD)  . . . . . . .   8
     2.6.  Mix-Up (TBD)  . . . . . . . . . . . . . . . . . . . . . .   9
   3.  OAuth Credentials Injection . . . . . . . . . . . . . . . . .   9
     3.1.  Code Injection  . . . . . . . . . . . . . . . . . . . . .   9
       3.1.1.  Proposed Counter Measures . . . . . . . . . . . . . .  11
       3.1.2.  Access Token Injection (TBD)  . . . . . . . . . . . .  13
       3.1.3.  XSRF (TBD)  . . . . . . . . . . . . . . . . . . . . .  13
   4.  Other Attacks . . . . . . . . . . . . . . . . . . . . . . . .  14
   5.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  14
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  14
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  14
   8.  Normative References  . . . . . . . . . . . . . . . . . . . .  14
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  15

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.

   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.

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   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 both RFC 6749
      [RFC6749] and RFC 6819 [RFC6819].

   This remainder of the document is organized as follows: The next
   section describes various scenarios how OAuth credentials (namely
   access tokens and authorization codes) may be disclosed to attackers
   and proposes countermeasures.  Afterwards, the document discusses
   attacks possible with captured credential and how they may be
   prevented.  The last sections discuss additional threats.

2.  OAuth Credentials Leakage

2.1.  Redirect URI validation of authorization requests

   The following implementation issue has been observed: Some
   authorization servers allow clients to register redirect URI patterns
   instead of complete redirect URIs.  In those cases, the authorization
   servers, at runtime, match 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  usually via the client as open redirector in conjunction with
      fragment handling (implicit grant) carrying the response including
      the respective OAuth credentials.

2.1.1.  Authorization Code Grant

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

   Let's assume the pattern "https://**" had been
   registered for the client "s6BhdRkqt3".  This pattern allows redirect
   URI from any host residing in the domain  So if an
   attacker manager to establish a host or subdomain in "" he

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   can impersonate the legitimate client.  Assume the attacker sets up
   the host "".

   (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 "".

   (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
     & HTTP/1.1

   (3)  The authorization validates the redirect URI in order to
        identify the client.  Since the pattern allows arbitrary domains
        host names in "", 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

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

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

   Note: This attack will not 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.  This and other kinds of injections are covered in
   Section OAuth Credentials Injection.

2.1.2.  Implicit Grant

   The attack described above for grant type authorization code works
   similarly for the implicit grant.  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, it is possible to conduct an attack utilizing the way
   user agents treat fragments in case of redirects.  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).  In

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   this attack this behavior is combined with the client as an open
   redirector in order to get access to access tokens.  This allows
   circumvention even of strict redirect URI patterns.

   Assume the pattern for client "s6BhdRkqt3" is
   "*", i.e. any parameter is allowed for
   redirects to "".  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 "".

   (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=" 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
     & HTTP/1.1

   (3)  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

   (4)  At the, the request arrives at the open redirector.
        It will read the redirect parameter and will issue a HTTP 302 to
        the URL "".

   HTTP/1.1 302 Found

   (5)  Since the redirector at does not include a fragment
        in the Location header, the user agent will re-attach the
        original fragment

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        "#access_token=2YotnFZFEjr1zCsicMWpAA&..." to the URL and will
        navigate to the following URL:

   (6)  The attacker's page at can access the fragment
        and obtain the access token.

2.1.3.  Countermeasure: exact redirect URI matching

   Since the cause of the implementation and management issues is the
   complexity of the pattern matching, this document proposes to
   recommend general use of exact redirect URI matching instead, i.e.
   the authorization server shall compare the two URIs using simple
   string comparison as defined in [RFC3986], Section 6.2.1..

   This would cause the following impacts:

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

   o  The working group might also consider this change as a step
      towards improved interoperability for OAuth implementations since
      RFC 6749 is somehow vague on redirect URI validation.  There is
      especially no rule for pattern matching.  So one may 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?

   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.
      Note: Does redirect uri validation solve any problem for native
      apps?  Effective against impersonation when used in conjunction
      with claimed HTTPS redirect URIs only.

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   Additional recommendations:

   o  It is also advisable that the domains on which callbacks are
      hosted should not expose open redirectors (see respective

   o  As a further recommendation, clients may drop fragments via
      intermediary URL with fix fragment (e.g. to prevent the
      user agent from appending any unintended fragments.

   Alternatives to exact redirect URI matching: authenticate client
   using digital signatures (JAR?
   ietf-oauth-jwsreq-09), ...

2.2.  Authorization code leakage via referrer headers

   The section above already discussed use of the referrer header for
   one kind of attack to obtain OAuth credentials.  It is also 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.

2.2.1.  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.
      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

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   o  Use form post mode instead of redirect for authorization response

   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.

2.3.  Code in browser history (TBD)

   When browser navigates to ""
   as a result of a redirect from a provider's authorization endpoint.

   Proposal for counter-measures: code is one time use, has limited
   duration, is bound to client id/secret (confidential clients only)

2.4.  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
   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
   redirection_endpoint#access_token=abcef as a result of a redirect
   from a provider's authorization endpoint.

   Proposal: replace implicit flow with postmessage communication

2.5.  Access token on bad resource servers (TBD)

   In the beginning, the basic assumption of OAuth 2.0 was that the
   OAuth client is implemented for and tightly bound to a certain
   deployment.  It therefore knows the URLs of the authorization and
   resource servers upfront, at development/deployment time.  So well-
   known URLs to resource servers serve as trust anchor.  The validation
   whether the client talks to a legitimate resource server is based on
   TLS server authentication (see [RFC6819], Section 4.5.4).

   As OAuth clients nowadays more and more bind dynamically at runtime
   to authorization and resource servers, there need to be alternative
   solutions to ensure, client do not deliver access tokens to bad
   resource servers.


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   Potential mitigations:

   o  PoP

   o  Token Binding

   o  AS-provided Meta Data

   o  ...

2.6.  Mix-Up (TBD)

   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 counter-measures
   is given in cf.

   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 (so requires OpenID Connect) carries client id and issuer

   o  register AS-specific redirect URIs, bind transaction to AS

   o  ...

3.  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).

3.1.  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:

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   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.

   (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

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   identical."  In the attack scenario described above, the legitimate
   client would use the correct redirect URI it always uses for
   authorization requests.  But this URI would not match the tampered
   redirect URI used by the attacker (otherwise, the redirect would not
   land at the attackers page).  So the authorization server would
   detect the attack and refuse to exchange the code.

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

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

   o  the client has bound this data to this particular instance

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

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

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

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

3.1.1.  Proposed Counter Measures

   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:

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   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

   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,
           - 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

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   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.
           - 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 counter-
   measures described above if he is able to create the respective
   secret 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, 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)?

3.1.2.  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? 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

3.1.3.  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)

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

4.  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

5.  Acknowledgements

   We would like to thank ... for their valuable feedback.

6.  IANA Considerations

   This draft includes no request to IANA.

7.  Security Considerations

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

8.  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,

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   [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,

Authors' Addresses

   Torsten Lodderstedt (editor)
   Deutsche Telekom AG


   John Bradley
   Ping Identity


   Andrey Labunets


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