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  <title>Web User Interaction: Threat Trees</title>
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  <link rel="chapter" title="2 Threat Trees" href=
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    <h1><a href="#title" id="title" name="title" class="anchor">Web
    User Interaction: Threat Trees</a></h1>

    <h2><a href="#w3c-doctype" id="w3c-doctype" name="w3c-doctype"
    class="anchor">W3C Working Group Note 1 November 2007</a></h2>

    <dl>
      <dt>This version:</dt>

      <dd><a href=
      "http://www.w3.org/TR/2007/NOTE-wsc-threats-20071101/">http://www.w3.org/TR/2007/NOTE-wsc-threats-20071101/</a></dd>

      <dt>Latest version:</dt>

      <dd><a href=
      "http://www.w3.org/TR/wsc-threats/">http://www.w3.org/TR/wsc-threats/</a></dd>

      <dt>Editor:</dt>

      <dd>Thomas Roessler, <a href=
      "http://www.w3.org/">W3C</a></dd>
    </dl>

    <p class="copyright"><a href=
    "http://www.w3.org/Consortium/Legal/ipr-notice#Copyright">Copyright</a>&nbsp;©&nbsp;2007&nbsp;<a href="http://www.w3.org/"><acronym title="World Wide Web Consortium">W3C</acronym></a><sup>®</sup>
    (<a href="http://www.csail.mit.edu/"><acronym title=
    "Massachusetts Institute of Technology">MIT</acronym></a>,
    <a href="http://www.ercim.org/"><acronym title=
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  </div>
  <hr />

  <div>
    <h2><a href="#abstract" id="abstract" name="abstract" class=
    "anchor">Abstract</a></h2>This Note includes threat trees used
    to analyze the threats that the <a href=
    "#ref-wsc-xit">[WSC-XIT]</a> responds to. It is a companion
    document to <a href="#ref-wsc-usecases">[WSC-USECASES]</a>.
  </div>

  <div>
    <h2><a href="#status" id="status" name="status" class=
    "anchor">Status of this Document</a></h2>

    <p>This section describes the status of this document at the
    time of its publication. Other documents may supersede this
    document. A list of current W3C publications and the latest
    revision of this technical report can be found in the <a href=
    "http://www.w3.org/TR/">W3C technical reports index</a> at
    http://www.w3.org/TR/.</p>

    <p>This document is published as a companion document to
    <a href="#ref-wsc-usecases">[WSC-USECASES]</a>, to make some of
    the group's analysis available to a larger public.</p>

    <p>This document was developed by the <a href=
    "http://www.w3.org/2006/WSC/">Web Security Context Working
    Group</a>.</p>

    <p>The content of this document is mostly analytic. This
    document is published as a snapshot, and may be updated and
    changed as needed when the Working Group's analysis develops
    further.</p>

    <p>Please send comments about this document to
    public-usable-authentication@w3.org (with <a href=
    "http://lists.w3.org/Archives/Public/public-usable-authentication/">
    public archive</a>).</p>

    <p>Publication as a Working Group Note does not imply
    endorsement by the W3C Membership. This is a draft document and
    may be updated, replaced or obsoleted by other documents at any
    time. It is inappropriate to cite this document as other than
    work in progress.</p>

    <p>This document was produced by a group operating under the
    <a href=
    "http://www.w3.org/Consortium/Patent-Policy-20040205/">5
    February 2004 W3C Patent Policy</a>. W3C maintains a <a href=
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    accordance with <a href=
    "http://www.w3.org/Consortium/Patent-Policy-20040205/#sec-Disclosure">
    section 6 of the W3C Patent Policy</a>.</p>
  </div>

  <div class="toc">
    <h2><a href="#contents" id="contents" name="contents" class=
    "anchor">Table of Contents</a></h2>

    <p class="toc">1 <a href="#Overview">Overview</a><br />
    2 <a href="#Threat_Trees">Threat Trees</a><br />
    &nbsp;&nbsp;&nbsp;&nbsp;2.1 <a href="#Main_Tree">Main
    Tree</a><br />
    &nbsp;&nbsp;&nbsp;&nbsp;2.2 <a href=
    "#Branches_that_may_be_out_of_scope">Branches that may be out
    of scope</a><br />
    &nbsp;&nbsp;&nbsp;&nbsp;2.3 <a href=
    "#Uncategorized_attacks">Uncategorized attacks</a><br />
    &nbsp;&nbsp;&nbsp;&nbsp;2.4 <a href=
    "#HTTPS_Branch_of_threat_tree">HTTPS Branch of threat
    tree</a><br />
    3 <a href="#Acknowledgements">Acknowledgements</a><br />
    4 <a href="#References">References</a><br /></p>
  </div>
  <hr />

  <div class="body">
    <div class="div1">
      <h2><a href="#Overview" id="Overview" name="Overview" class=
      "anchor">1 Overview</a></h2>

      <p>This document includes a high-level analysis of threats
      faced in common Web usage scenarios.</p>

      <p>In the analysis, high-level threats are decomposed into
      the vulnerabilities that can be used by an attacker to
      realize that threat. These vulnerabilities can be met by
      countermeasures, which can in turn have vulnerabilities of
      their own, and so on.</p>

      <p>For example, to lure a user to a site that is controlled
      by an attacker, the attacker might use DNS spoofing (or
      similar techniques) to divert the user to a site of the
      attacker's choice. As a countermeasure, TLS could be
      deployed. If that countermeasure is in place, an attacker can
      try to obtain a certification authority to issue a
      certificate that can be used as part of an attack.</p>

      <p>For a more extensive introduction of the process, see
      chapter 4, Threat Modeling, of <a href=
      "#ref-securecode">[SECURECODE]</a>.</p>
    </div>

    <div class="div1">
      <h2><a href="#Threat_Trees" id="Threat_Trees" name=
      "Threat_Trees" class="anchor">2 Threat Trees</a></h2>

      <div class="div2">
        <h3><a href="#Main_Tree" id="Main_Tree" name="Main_Tree"
        class="anchor">2.1 Main Tree</a></h3>

        <ol class="enumar">
          <li>
            <p><em>Luring Attacks</em> - luring a user to the wrong
            site so that he connects to address not owned by party
            he believes it to be owned by</p>

            <ol class="enumla">
              <li>Attacker registers domain names similar to that
              of the legitimate domain and waits for users to
              mistype or mis-remember a URL.</li>

              <li>Attacker convinces user to bookmark the
              impersonated site's address with misleading
              information

                <ol class="enumlr">
                  <li>Attacker constructs misleading page title
                  which the browser will automatically copy into
                  the default bookmark name.</li>
                </ol>
              </li>

              <li>Attacker lures victim using a link in another
              application

                <ol class="enumlr">
                  <li>Email</li>

                  <li>IM</li>

                  <li>VOIP</li>

                  <li>Voice (e.g. calls purporting to be from
                  site's security department)</li>
                </ol>
              </li>

              <li>Attacker inserts or replaces links provided by
              other sites users trust (e.g. search engines)</li>

              <li>After user initiates a connection to the correct
              address, the attacker attacker intercepts
              communications to that address and forges responses
              from that address

                <ol class="enumlr">
                  <li>Compromise a DNS server on route from root to
                  requested domain</li>

                  <li>Intercept DNS request and replace query
                  response with forged response</li>

                  <li>Insert data into user's hosts file so that
                  DNS query is not required (out of
                  scope---attacker this powerful easily insert
                  spyware)</li>

                  <li>
                    <p>Countermeasure: HTTPS - See <a href=
                    "#HTTPS_Branch_of_threat_tree"><b>2.4 HTTPS
                    Branch of threat tree</b></a></p>
                  </li>
                </ol>
              </li>

              <li>Intercept and replace communications between
              client and the address of the legitimate site

                <ol class="enumlr">
                  <li>Take control of a system through which
                  communications is routed</li>

                  <li>Create a wifi access point and lure users to
                  it</li>

                  <li>Attack the infrastructure through which
                  routes are established (BGP-based attacks)</li>
                </ol>
              </li>
            </ol>
          </li>

          <li>
            <p><em>Site Impersonation Attacks</em> - an attack in
            which the attacker attempts to mimic someone else's
            website. Potential goals include credential theft (e.g.
            password theft), theft of other private information
            from user (bank account and routing numbers), or
            forging information sent to user (e.g. fake news story
            that will cause user to buy or sell stock).</p>

            <ol class="enumla">
              <li>Address spoofing

                <ol class="enumlr">
                  <li>Attacker registers confusing domain name

                    <ol class="enumua">
                      <li>semantic attacks
                      (e.g.,"ebay-security.com")</li>

                      <li>syntax attacks (e.g., "paypai.com", use
                      of non-ASCII characters)</li>
                    </ol>
                  </li>

                  <li>Attacker takes advantage of browser
                  vulnerabilities</li>
                </ol>
              </li>

              <li>Page Spoofing

                <ol class="enumlr">
                  <li>attacker copies content and indicators from
                  legitimate website content into the content of an
                  attack page (may add, remove or replace security
                  indicators)</li>

                  <li>attacker uses pop-up windows that mimic
                  legitimate site</li>

                  <li>attacker presents warning or error messages
                  (e.g., to confuse user, to justify why security
                  indicators/information is missing)</li>
                </ol>
              </li>

              <li>Chrome Spoofing

                <ol class="enumlr">
                  <li>attacker copies chrome elements (e.g., green
                  EV address bar), entire chrome window or dialog
                  boxes into the content of a website (also known
                  as Picture in Picture Attacks)</li>

                  <li>attacker mimics customized chrome content
                  (when customization is used as an anti-spoofing
                  technique, the attacker may use an educated guess
                  to replicate customization</li>

                  <li>attacker exploits flaws in GUI logic to
                  control what is displayed in chrome elements
                  (e.g. in the title bar, status bar or address
                  bar)</li>
                </ol>
              </li>
            </ol>
          </li>

          <li>
            <p><em>Cross-site request forgery</em> - (see <a href=
            "#ref-csrf">[CSRF]</a>)- causing a user to unwittingly
            send, to a legitimate site, a request containing data
            that he/she would not otherwise intend to send (e.g. to
            perform an action that he/she did not intend to
            take).</p>

            <ol class="enumla">
              <li>Attacker may first convince user to login to the
              target legitimate website (possibly in a separate
              window or tab).

                <ol class="enumlr">
                  <li>[Sequential first step] Attacker constructs a
                  link or form with field values that replicate
                  those that would be sent if user legitimately
                  wanted to perform this action.</li>

                  <li>[Sequential second step] Attacker causes the
                  browser to send this link to the legitimate
                  website.</li>
                </ol>
              </li>

              <li>Induce user to submit form data

                <ol class="enumlr">
                  <li>Lure user to click on link to cause GET
                  request with attacker-specified parameters</li>

                  <li>Lure user to click on form that will send
                  HTTP POST request with attacker-specified
                  parameters</li>

                  <li>Uses javascript to automatically send the
                  form (form.submit())</li>

                  <li>If user is not already logged in, attacker
                  may rely on the user login when reaching the
                  site. Many sites will then process the form data
                  from the initial request.</li>
                </ol>
              </li>
            </ol>
          </li>

          <li>
            <p><em>Cross-site scripting</em>- the injection of code
            into vulnerable web applications, which copy this code
            into web content in a form that would allow it to be
            executed when read by another user. An exploited
            cross-site scripting vulnerability can be used by
            attackers to bypass access controls such as the same
            origin policy. (This definition borrows heavily from
            that of <a href="#ref-xss">[XSS]</a>.) Potential goals
            include session hijacking (e.g. stealing a session
            cookie), credential theft (e.g. password theft), theft
            of other private information from user (bank account
            and routing numbers), or forging information sent to
            user (e.g. fake news story that will cause user to buy
            or sell stock).</p>

            <ul>
              <li>[First sequential step] Construct content to
              appear on legitimate site

                <ol class="enumla">
                  <li>Create an attack script to execute in other
                  users' browsers

                    <ol class="enumlr">
                      <li>Use the script to instruct other user's
                      browsers to send script-accessible
                      credentials (e.g. cookies) or other data to
                      the attacker</li>

                      <li>
                        <p>Use the script, which will execute in
                        other user's browsers within the site's
                        domain context, in order to execute actions
                        as that user or to gather additional data.
                        (For example, adding the attacker as a
                        trusted user/friend/administrator as was
                        the case with the <a href=
                        "#ref-mspworm">[MSPWORM]</a>.)</p>
                      </li>

                      <li>Use the content or script to exploit a
                      vulnerability in the browser</li>
                    </ol>
                  </li>

                  <li>Create content intended to appear to the user
                  as if is content generated entirely by the
                  legitimate site (and not a rendering of other
                  user's input).

                    <ol class="enumlr">
                      <li>Create HTML for a fake login form or
                      information request form which, when
                      submitted, directs data to attacker.</li>

                      <li>Create HTML content with other misleading
                      information that might affect user behavior
                      (e.g. a fake news story that could cause a
                      stock's price to rise or fall).</li>
                    </ol>
                  </li>
                </ol>
              </li>

              <li>[Second sequential step] Inject content into
              pages served by legitimate site

                <ol class="enumla">
                  <li>Use a cross-site request forgery attack (see
                  above)

                    <ul>
                      <li>Cross-site scripting attacks that use
                      this approach are known as type 1
                      attacks.</li>
                    </ul>
                  </li>

                  <li>Insert the script yourself into content that
                  the site will serve to other users (e.g. a
                  discussion group posting).

                    <ul>
                      <li>Cross-site scripting attacks that use
                      this approach are known as type 2
                      attacks</li>
                    </ul>
                  </li>
                </ol>
              </li>
            </ul>
          </li>

          <li>
            <p><em>Network-based eavesdropping-</em> a passive
            attack in which the attacker collects network traffic
            and reads the data sent between the client and the
            website. Potential goals include session hijacking
            (e.g. stealing a session cookie), credential theft
            (e.g. password theft), theft of other private
            information from user (bank account and routing
            numbers)</p>

            <ol class="enumla">
              <li>Attacker gains physical access to network and
              installs monitoring hardware/software</li>

              <li>
                <p>Attacker establishes rogue network (e.g. public
                WiFi access point) and waits for victims to join
                it</p>
              </li>
            </ol>
          </li>
        </ol>
      </div>

      <div class="div2">
        <h3><a href="#Branches_that_may_be_out_of_scope" id=
        "Branches_that_may_be_out_of_scope" name=
        "Branches_that_may_be_out_of_scope" class="anchor">2.2
        Branches that may be out of scope</a></h3>

        <ol class="enumar">
          <li>Browser Exploits</li>

          <li>Man in the Middle (MITM) attacks</li>

          <li>Malware/Spyware Attacks

            <ol class="enumla">
              <li>keyloggers</li>

              <li>screen loggers</li>

              <li>altering the users local DNS hostfile (sometimes
              referred to as "pharming")</li>

              <li>"transaction generator" - malware that hijacks a
              legitimate login session to send requests or
              transactions</li>
            </ol>
          </li>

          <li>Credential database compromise

            <ol class="enumla">
              <li>Browser credential store</li>

              <li>Server store

                <ol class="enumlr">
                  <li>insider attacks</li>

                  <li>external breach</li>
                </ol>
              </li>
            </ol>
          </li>

          <li>Masquerading attack

            <ol class="enumla">
              <li>reuse of credentials to masquerade as user</li>
            </ol>
          </li>

          <li>Dictionary Attacks

            <ol class="enumla">
              <li>online</li>

              <li>offline</li>
            </ol>
          </li>

          <li>Web timing attacks - using the time it takes a
          website to respond to determine information about the
          user (e.g., if they have an account at a particular
          website)</li>
        </ol>
      </div>

      <div class="div2">
        <h3><a href="#Uncategorized_attacks" id=
        "Uncategorized_attacks" name="Uncategorized_attacks" class=
        "anchor">2.3 Uncategorized attacks</a></h3>

        <p>This section serves as a repository for attacks that
        have not yet been folded into the structure above.</p>

        <ol class="enumar">
          <li>Iframe attacks- are these covered by cross site
          scripting above?

            <ol class="enumla">
              <li>attacker forces arbitrary URL to be displayed on
              a visible iframe at legitimate page</li>

              <li>attacker uses hidden iframe to inject content
              into legitimate page, capture user input, etc</li>
            </ol>
          </li>

          <li>DNS attacks

            <ol class="enumla">
              <li>cache poisoning</li>
            </ol>
          </li>

          <li>Cryptographic attacks

            <ol class="enumla">
              <li>Attacks on the protocol (SSL/TLS)</li>

              <li>Attacks on the encryption methods</li>
            </ol>
          </li>

          <li>system take-over attacks (install something on the
          system the user agent is running on which could be used
          later)</li>

          <li>Social Engineering Attacks (not covered above, but
          these are within scope)

            <ol class="enumla">
              <li>Re-enrollment attack- convince user to re-enroll
              their account or machine (e.g., to answer challenge
              questions in sitekey)</li>

              <li>Convince users they are already logged in</li>
            </ol>
          </li>
        </ol>
      </div>

      <div class="div2">
        <h3><a href="#HTTPS_Branch_of_threat_tree" id=
        "HTTPS_Branch_of_threat_tree" name=
        "HTTPS_Branch_of_threat_tree" class="anchor">2.4 HTTPS
        Branch of threat tree</a></h3>

        <ol class="enumar">
          <li>Get certification authority to issue you a cert</li>

          <li>Break public key in existing valid cert</li>

          <li>Break SSL/TLS protocol</li>

          <li>Prevent HTTPS from activating in the first place

            <ol class="enumla">
              <li>Intercept connections initiated via HTTP and
              prevent redirect to HTTPS

                <ul>
                  <li>Prevent protocol based redirect</li>

                  <li>Prevent any javascript based redirects</li>

                  <li>
                    <p>Rewrite any page contents containing
                    "<code>https://</code>" with
                    "<code>http://</code>"</p>
                  </li>
                </ul>
              </li>
            </ol>
          </li>
        </ol>
      </div>
    </div>

    <div class="div1">
      <h2><a href="#Acknowledgements" id="Acknowledgements" name=
      "Acknowledgements" class="anchor">3 Acknowledgements</a></h2>

      <p>The material in this note was created by Tyler Close,
      Rachna Dhamija, Johnathan Nightingale, and Stuart
      Schechter.</p>
    </div>

    <div class="div1">
      <h2><a href="#References" id="References" name="References"
      class="anchor">4 References</a></h2>

      <dl>
        <dt class="label"><a name="ref-csrf" id=
        "ref-csrf"></a>CSRF</dt>

        <dd><a href=
        "http://en.wikipedia.org/wiki/Cross-site_request_forgery"><cite>
        Cross-Site Request Forgery</cite></a>, Wikipedia entry,
        retrieved on 15 October 2007. Available at
        http://en.wikipedia.org/wiki/Cross-site_request_forgery.</dd>

        <dt class="label"><a name="ref-mspworm" id=
        "ref-mspworm"></a>MSPWORM</dt>

        <dd><a href=
        "http://namb.la/popular/tech.html"><cite>Technical
        explanation of the MySpace Worm</cite></a>, anonymous,
        retrieved on 15 October 2007. Available at
        http://namb.la/popular/tech.html .</dd>

        <dt class="label"><a name="ref-securecode" id=
        "ref-securecode"></a>SECURECODE</dt>

        <dd><cite>Writing Secure Code</cite>, M. Howard, D.
        LeBlanc. 2nd edition, Microsoft Press 2003.</dd>

        <dt class="label"><a name="ref-wsc-usecases" id=
        "ref-wsc-usecases"></a>WSC-USECASES</dt>

        <dd><a href=
        "http://www.w3.org/TR/2007/WD-wsc-usecases-20071101/"><cite>
        Web Security Experience, Indicators and Trust: Scope and
        Use Cases</cite></a>, T. Close, Editor, W3C Working Draft
        (work in progress), 1 November 2007. This version is
        http://www.w3.org/TR/2007/WD-wsc-usecases-20071101/. The
        <a href="http://www.w3.org/TR/wsc-usecases/">latest
        version</a> is available at
        http://www.w3.org/TR/wsc-usecases/ .</dd>

        <dt class="label"><a name="ref-wsc-xit" id=
        "ref-wsc-xit"></a>WSC-XIT</dt>

        <dd><a href=
        "http://www.w3.org/TR/2007/WD-wsc-xit-20071101/"><cite>Web
        Security Context: Experience, Indicators, and
        Trust</cite></a>, T. Roessler, A. Saldhana, Editors, W3C
        Working Draft (work in progress), 1 November 2007. This
        version is http://www.w3.org/TR/2007/WD-wsc-xit-20071101/.
        The <a href="http://www.w3.org/TR/wsc-xit/">latest
        version</a> is available at http://www.w3.org/TR/wsc-xit/
        .</dd>

        <dt class="label"><a name="ref-xss" id=
        "ref-xss"></a>XSS</dt>

        <dd><a href=
        "http://en.wikipedia.org/wiki/Cross_site_scripting"><cite>Cross
        Site Scripting</cite></a>, Wikipedia entry, retrieved on 15
        October 2007. Available at
        http://en.wikipedia.org/wiki/Cross_site_scripting.</dd>
      </dl>
    </div>
  </div>
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