draft-lodderstedt-oauth-security-topics.xml

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<rfc category="bcp" docName="draft-lodderstedt-oauth-security-topics-01"
     ipr="trust200902">
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  <!-- ***** FRONT MATTER ***** -->

  <front>
    <!--
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    <title abbrev="Security Topics">OAuth Security Topics</title>

    <author fullname="Torsten Lodderstedt" initials="T." role="editor"
            surname="Lodderstedt">
      <organization>Deutsche Telekom AG</organization>

      <address>
        <email>torsten@lodderstedt.net</email>

        <!-- uri and facsimile elements may also be added -->
      </address>
	</author>
    <author fullname="John Bradley" initials="J." surname="Bradley">
	  <organization>Ping Identity</organization>
      <address>
        <email>ve7jtb@ve7jtb.com</email>
      </address>
    </author>

    <author fullname="Andrey Labunets" initials="A." surname="Labunets">
      <organization>Facebook</organization>
      <address>
        <email>isciurus@fb.com</email>
      </address>
    </author>

    <date day="16" month="November" year="2016" />

    <!-- Meta-data Declarations -->

    <area>Security Area</area>

    <workgroup>Open Authentication Protocol</workgroup>

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    <keyword>security</keyword>

    <keyword>oauth2</keyword>

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    <abstract>
      <t>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.</t>
    </abstract>
  </front>

  <middle>
    <section anchor="Introduction" title="Introduction">
      <t>It's been a while since OAuth has been published in 
	  <xref target="RFC6749">RFC 6749</xref> and <xref target="RFC6750">RFC 6750</xref>. 
	  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.
	  <list style="symbols">
	  <t>OAuth implementations are being attacked through known implementation 
	  weaknesses and anti-patterns (XSRF, referrer header). Although most of these
	  threats are discussed in <xref target="RFC6819">RFC 6819</xref>, continued exploitation 
 	  demonstrates there may be a need for more specific recommendations or that the existing 
	  mitigations are too difficult to deploy.</t>
	  <t>Technology has changed, e.g. the way browsers treat fragments in some situations, 
	  which may change the implicit grant's underlying security model.</t>
	  <t>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 <xref target="RFC6749">RFC 6749</xref> and <xref target="RFC6819">RFC 6819</xref>.</t>
	  </list>
	  </t>
	  <t>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.</t>
    </section>

    <section anchor="cred_leakage" title="OAuth Credentials Leakage">
	  <section title="Redirect URI validation of authorization requests">
	  <t>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
	  <list style="symbols">
	  <t>by directly sending the user agent to a URI under the attackers 
	  control or</t>
	  <t>usually via the client as open redirector in conjunction with 
	  fragment handling (implicit grant) carrying the response including 
	  the respective OAuth credentials.</t>
	  </list>
	  </t>
	  <section title="Authorization Code Grant">
	  <t>For a public client using the grant type code, an attack 
	  would look as follows: </t>
	  <t>Let's assume the pattern "https://*.example.com/*" had been registered 
	  for the client "s6BhdRkqt3". This pattern allows redirect URI 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".</t>
	  <t>
	   <list style="format (%d)" counter="my_count">
	    <t>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".</t>
	    <t>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):
	  </t>
	   </list>	   
	  </t>
	  <t>
	  	<figure>
          <artwork><![CDATA[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]]></artwork>
        </figure>	
	  </t>
	  <t>
	  <list style="format (%d)" counter="my_count">
	  <t>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.</t>
	  <t>If the user does not recognize the attack, the code is issued and directly 
	  sent to the attacker's client.</t>
	  <t>Since the attacker impersonated a public client, it can directly exchange 
	  the code for tokens at the respective token endpoint.</t>
	  </list>
	  </t>
	  <t>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 <xref format="title" target="cred_injection"/>.</t>
	   </section>
	   <section title="Implicit Grant">
	   <t>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.</t>
	   <t>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 <xref target="RFC7231"/>, section 9.5). In 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.</t>
	   <t>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.</t>
	  <t>
	   <list style="format (%d)" counter="my_count1">
	    <t>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".</t>
	    <t>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):</t>
	   </list>	   
	  </t>
	  <t>
	  	<figure>
          <artwork><![CDATA[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]]></artwork>
        </figure>	
	  </t>
	  <t>
	   <list style="format (%d)" counter="my_count1">
	    <t>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)</t>
	   </list>	   
	  </t>
	  <t>
	  	<figure>
          <artwork><![CDATA[HTTP/1.1 302 Found
  Location: https://client.example.com/cb?
  redirect_to%3Dhttps%3A%2F%2Fclient.evil.com%2Fcb
  #access_token=2YotnFZFEjr1zCsicMWpAA&...]]></artwork>
        </figure>	
	  </t>	
      <t>   	  
	   <list style="format (%d)" counter="my_count1">
	    <t>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".</t>
	   </list>	   
	  </t>
	  <t>
	  	<figure>
          <artwork><![CDATA[HTTP/1.1 302 Found
     Location: https://client.evil.com/cb]]></artwork>
        </figure>	
      </t>
	  <t>
	   <list style="format (%d)" counter="my_count1">
	    <t>Since the redirector at example.com does not include a fragment in the Location header, 
		the user agent will re-attach the original fragment <vspace/>"#access_token=2YotnFZFEjr1zCsicMWpAA&amp;..." 
		to the URL and will navigate to the following URL:</t>
	   </list>	  
	  </t>
	  <t>
  	  	<figure>
          <artwork><![CDATA[https://client.evil.com/cb#access_token=2YotnFZFEjr1zCsicMWpAA&...]]></artwork>
        </figure>
	  </t>
	  <t>
	   <list style="format (%d)" counter="my_count1">	  
      <t>The attacker's page at client.evil.com can access the fragment and obtain the access token.</t>
	   </list>	  
	  </t>	  
	   </section>
	   <section title="Countermeasure: exact redirect URI matching">
	   <t>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 <xref target="RFC3986"/>, Section 6.2.1..</t>
	   <t>This would cause the following impacts:
	    <list style="symbols">
		<t>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 whether this feature is used in deployed reality today.</t> 
		<t>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.</t>
		<t>Clients with multiple redirect URIs need to register all of them explicitly. 
		<vspace/>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?</t>
		<t>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.
		<vspace/>Note: Does redirect uri validation solve any problem for native apps? Effective against 
		impersonation when used in conjunction with claimed HTTPS redirect URIs only.</t>
		</list>
	   </t>
	   <t>Additional recommendations:
	   <list style="symbols">
	   <t>It is also advisable that the domains on which callbacks are hosted should not expose open redirectors (see respective section).</t>
	   <t>As a further recommendation, clients may drop fragments via intermediary URL with fix fragment 
	   (e.g. https://developers.facebook.com/blog/post/552/) to prevent the user agent from appending any unintended fragments.</t>
	   </list>
	   </t>
	   <t>Alternatives to exact redirect URI matching: authenticate client using digital 
	   signatures (JAR? https://tools.ietf.org/html/draft-ietf-oauth-jwsreq-09), ...</t>	   
	   </section>
	  </section> 
	   <section title="Authorization code leakage via referrer headers">
	   <t>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. </t>
	   <t>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.</t>
	   <t>It is also possible that an attacker injects cross-domain content somehow into the page, such 
	   as &lt;img&gt; (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.</t>
	   <section title="Countermeasures">
	   <t>There are some means to prevent leakage as described above:</t>
	   <t>
	   <list style="symbols">
	   <t>Use of the HTML link attribute rel="noreferrer" (Chrome 52.0.2743.116, FF 49.0.1, Edge 38.14393.0.0, IE/Win10)</t>
	   <t>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))</t>
	   <t>Redirect to intermediate page (sanitize history) before sending user agent to other pages<vspace/>Note: double 
	   check redirect/referrer header behavior</t>
	   <t>Use form post mode instead of redirect for authorization response</t>
	   </list>
	   </t>
	   <t>Note: There shouldn't be a referer header when loading HTTP content from a 
	   HTTPS -loaded page (e.g. help/faq pages)</t>
	   <t>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)</t>
	   </section>
	  </section>
	  <section title="Code in browser history (TBD)">
	  <t>When browser navigates to "client.com/redirection_endpoint?code=abcd" as a result 
	  of a redirect from a provider's authorization endpoint.</t>
	  <t>Proposal for counter-measures: code is one time use, has limited duration, is bound to client id/secret (confidential clients only)</t>
	  </section>
	  <section title="Access token in browser history (TBD)">
	  <t>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</t>
	  <t>When browser navigates to client.com/redirection_endpoint#access_token=abcef as a result of a 
	  redirect from a provider's authorization endpoint.</t>
	  <t>Proposal: replace implicit flow with postmessage communication</t>
	  </section>
	  <section title="Access token on bad resource servers (TBD)">
	  <t>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 <xref target="RFC6819"/>, Section 4.5.4).</t>
	  <t>As OAuth clients nowadays more and more bind dynamically at runtime to authorization and resource 
	  servers, there need to be alternative solutions to ensure clients do not deliver access tokens to
	  bad resource servers.</t>
	  <t>...</t>
	  <t>Potential mitigations:
	    <list style="symbols">
	     <t>PoP (https://tools.ietf.org/html/draft-ietf-oauth-pop-architecture-08)</t>
	     <t>Token Binding (https://tools.ietf.org/html/draft-jones-oauth-token-binding-00)</t>
	     <t>OAuth Response Metadata (https://tools.ietf.org/html/draft-sakimura-oauth-meta-07)</t>
		 <t>Resource Indicators (https://tools.ietf.org/html/draft-campbell-oauth-resource-indicators-01)</t>
	     <t>...</t>
	    </list>
	  </t>
	  </section>
	  <section title="Mix-Up (TBD)">
	  <t>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)</t>
	  <t>A detailed description of the attack and potential counter-measures is 
	  given in cf. https://tools.ietf.org/html/draft-ietf-oauth-mix-up-mitigation-01.</t>
	  <t>Potential mitigations:
	    <list style="symbols">
		 <t>AS returns client_id and its iss in the response. Client compares 
		  this data to AS it believed it sent the user agent to.</t>
		 <t>ID token (so requires OpenID Connect) carries client id and issuer</t>
	     <t>register AS-specific redirect URIs, bind transaction to AS</t>
	     <t>...</t>
	    </list>
	  </t>
	  </section>
    </section>

	<section anchor="cred_injection" title="OAuth Credentials Injection">
      <t>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). </t>
	  <section title="Code Injection">
	  <t>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:
	  <list style="symbols">
	   <t>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</t>
	   <t>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.</t>
	   <t>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.</t>
	   </list>
	   </t>
	   <t>How does an attack look like?</t>
	   <t>
	   <list style="format (%d)">
	    <t>The attacker obtains an authorization code by executing any of the attacks described 
		above (<xref format="title" target="cred_leakage"/>).</t>
		<t>It performs an OAuth authorization process with the legitimate client on his device.</t>
		<t>The attacker injects the stolen authorization code in the response of the 
		authorization server to the legitimate client.</t>
		<t>The client sends the code to the authorization server's token endpoint, 
		along with client id, client secret and actual redirect_uri.</t>
		<t>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.</t>
		<t>If all checks succeed, the authorization server issues access and other tokens 
		to the client. </t>
		<t>The attacker just impersonated the victim.</t>
	   </list>
	   </t>
	   <t>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.</t>
	   <t>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.</t>
	   <t><xref target="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.</t>
	   <t>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:
	   <list style="symbols">
	   <t>the redirect URI itself needs to contain a nonce or another kind of one-time use, secret data and</t>
	   <t>the client has bound this data to this particular instance</t>
	   </list>
	   </t>
	   <t>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.</t>
	   <t>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.</t>
       <t>It is also assumed that the requirements defined in <xref target="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.</t>
	   <t>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.</t>
	   <section title="Proposed Counter Measures">
	   <t>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:</t>
	    <t>
		<list hangIndent="8" style="hanging">
		 <t hangText="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.
		 <vspace/>
		 pro:
		 <vspace/>
		 - existing feature, used in the wild
		 <vspace/>
		 con:
		 <vspace/>
		 - OAuth does not have an ID Token - would need to push that down the stack</t>
		 
		 <t hangText="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).
		 <vspace/>
		 pro:
		 <vspace/>
		 - use existing concept
		 <vspace/>
		 con:
		 <vspace/>
		 - state needs to fulfil certain requirements (one time use, complexity)
         <vspace/>- new parameter means normative spec change</t>

		 <t hangText="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.
		 <vspace/>
		 pro:
		 <vspace/>
		 - existing and deployed OAuth feature
		 <vspace/>
		 con:
		 <vspace/>
		 - currently used and recommended for native apps, not web apps</t>
		 
		 <t hangText="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.
		 <vspace/>Note: token binding could be used in conjunction with 
		 PKCE as an option (https://tools.ietf.org/html/draft-campbell-oauth-tbpkce).
		 <vspace/>
		 pro:
		 <vspace/>
		 - highly secure
		 <vspace/>
		 con:
		 <vspace/>
		 - highly sophisticated, requires browser support, will it work for native apps?</t>

		 <t hangText="per instance client id/secret">...</t>		 
		</list>
		</t>
		<t>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. 
		<vspace/>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. 
		<vspace/>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)?</t>

	   </section>
	   
	   <section title="Access Token Injection (TBD)">
	   <t>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. 
	   <vspace/>(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?)</t>
	   <t>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.</t>
	   <t>Proposal: token binding, hybrid flow+nonce(OIDC), other cryptographical 
	   binding between access token and user agent instance</t>
	   </section>
	   <section title="XSRF (TBD)">
	    <t>injection of code or access token on a victim's device (e.g. to 
		cause client to access resources under the attacker's control)</t>
		<t>mitigation: XSRF tokens (one time use) w/ user agent binding 
		(cf. https://www.owasp.org/index.php/CrossSite_Request_Forgery_(CSRF)_Prevention_Cheat_Sheet)</t>
	   </section>
	  </section>
    </section>
	
	<section anchor="other" title="Other Attacks">
	<t>Using the AS as Open Redirector - error handling AS (redirects) (draft-ietf-oauth-closing-redirectors-00)</t>
	<t>Using the Client as Open Redirector</t>
	<t>redirect via status code 307 - use 302</t>
    </section>
	
	<section title="Other Topics">
	<t>why to rotate refresh tokens</t>
	<t>how to support multi AS per RS</t>
	<t>...</t>
	<t>differentiate native, JS and web clients</t>
	<t>federated login to apps (code flow to own AS in browser and federated login to 3rd party IDP in browser)</t>
	</section>
	
    <section anchor="Acknowledgements" title="Acknowledgements">
      <t>We would like to thank ... for their valuable feedback.</t>
    </section>

    <section anchor="IANA" title="IANA Considerations">
      <t>This draft includes no request to IANA.</t>
    </section>

    <section anchor="Security" title="Security Considerations">
      <t>All relevant security considerations have been given in the
      functional specification.</t>
    </section>
  </middle>

  <back>
    <references title="Normative References">
      <?rfc include="reference.RFC.3986"?>
      <?rfc include="reference.RFC.6749"?>
      <?rfc include="reference.RFC.6750"?>
      <?rfc include="reference.RFC.6819"?>
	  <?rfc include="reference.RFC.7231"?>
    </references>
<!--
    <references title="Informative References">
    </references> -->
    <section anchor="History" title="Document History">
      <t>[[ To be removed from the final specification ]]</t>

      <t>-01 <list style="symbols">
          <t>Added references to mitigation methods for token leakage</t>
          <t>Added reference to Token Binding for Authorization Code</t>
        </list></t>
    </section>		
  </back>
</rfc>