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html5/webstorage/publish LCWD-webstorage-20111025.html,NONE,1.1

From: Art Barstow via cvs-syncmail <cvsmail@w3.org>
Date: Fri, 21 Oct 2011 12:14:50 +0000
To: public-html-commits@w3.org
Message-Id: <E1RHDzu-0003nx-H0@lionel-hutz.w3.org>
Update of /sources/public/html5/webstorage/publish
In directory hutz:/tmp/cvs-serv14602

Added Files:
	LCWD-webstorage-20111025.html 
Log Message:
25-Sep-2011 LCWD based on rev1.184

--- NEW FILE: LCWD-webstorage-20111025.html ---
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   <p><a href="http://www.w3.org/"><img alt="W3C" height="48" src="http://www.w3.org/Icons/w3c_home" width="72"></a></p>

   <h1>Web Storage</h1>
   <h2 class="no-num no-toc" id="working-draft-25-october-2011">W3C Working Draft 25 October 2011</h2>
   <dl>
    <dt>This Version:</dt>
    <dd><a href="http://www.w3.org/TR/2011/WD-webstorage-20111025/">http://www.w3.org/TR/2011/WD-webstorage-20111025/</a></dd>
    <dt>Latest Published Version:</dt>
    <dd><a href="http://www.w3.org/TR/webstorage/">http://www.w3.org/TR/webstorage/</a></dd>
    <dt>Latest Editor's Draft:</dt>
    <dd><a class="latest-link" href="http://dev.w3.org/html5/webstorage/">http://dev.w3.org/html5/webstorage/</a></dd>

    <dt>Previous Versions:</dt>
    <dd><a href="http://www.w3.org/TR/2011/WD-webstorage-20110901/">http://www.w3.org/TR/2011/WD-webstorage-20110901/</a></dd>
    <dd><a href="http://www.w3.org/TR/2011/WD-webstorage-20110208/">http://www.w3.org/TR/2011/WD-webstorage-20110208/</a></dd>
    <dd><a href="http://www.w3.org/TR/2009/WD-webstorage-20091222/">http://www.w3.org/TR/2009/WD-webstorage-20091222/</a></dd>
    <dd><a href="http://www.w3.org/TR/2009/WD-webstorage-20091029/">http://www.w3.org/TR/2009/WD-webstorage-20091029/</a></dd>
    <dd><a href="http://www.w3.org/TR/2009/WD-webstorage-20090423/">http://www.w3.org/TR/2009/WD-webstorage-20090423/</a></dd>
<!-- :ZZZ -->
    <dt>Editor:</dt>
    <dd><a href="mailto:ian@hixie.ch">Ian Hickson</a>, Google, Inc.</dd>
   </dl><p class="copyright"><a href="http://www.w3.org/Consortium/Legal/ipr-notice#Copyright">Copyright</a>
   &copy; 2011 <a href="http://www.w3.org/"><abbr title="World Wide
   Web Consortium">W3C</abbr></a><sup>&reg;</sup> (<a href="http://www.csail.mit.edu/"><abbr title="Massachusetts
   Institute of Technology">MIT</abbr></a>, <a href="http://www.ercim.eu/"><abbr title="European Research
   Consortium for Informatics and Mathematics">ERCIM</abbr></a>, <a href="http://www.keio.ac.jp/">Keio</a>), All Rights Reserved. W3C
   <a href="http://www.w3.org/Consortium/Legal/ipr-notice#Legal_Disclaimer">liability</a>,
   <a href="http://www.w3.org/Consortium/Legal/ipr-notice#W3C_Trademarks">trademark</a>
   and <a href="http://www.w3.org/Consortium/Legal/copyright-documents">document
   use</a> rules apply.</p>


   <!-- UNDER NO CIRCUMSTANCES IS THE FOLLOWING PARAGRAPH TO BE REMOVED OR EDITED WITHOUT TALKING TO IAN FIRST -->
   <p class="alt">The bulk of the text of this specification is also
   available in the WHATWG <a href="http://www.whatwg.org/specs/web-apps/current-work/complete.html#webstorage">Web Applications 1.0</a> specification, under a license that permits
   reuse of the specification text.</p>
   <!-- UNDER NO CIRCUMSTANCES IS THE PRECEDING PARAGRAPH TO BE REMOVED OR EDITED WITHOUT TALKING TO IAN FIRST -->


  </div><hr class="top"><h2 class="no-num no-toc" id="abstract">Abstract</h2><p>This specification defines an API for persistent data storage of
  key-value pair data in Web clients.<h2 class="no-num no-toc" id="status-of-this-document">Status of This document</h2><p><em>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 <!-- DO NOT CHANGE THIS BACK TO THE STANDARD BOILERPLATE, AS IT IS INACCURATE -->

  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/.</em></p><p>If you wish to make comments regarding this document in a manner
  that is tracked by the W3C, please submit them via using <a href="http://www.w3.org/Bugs/Public/enter_bug.cgi?product=HTML%20WG">our
  public bug database</a>. If you do not have an account then you can
  enter feedback using this form:<form action="http://www.whatwg.org/specs/web-apps/current-work/file-spam.cgi" method="post">
   <fieldset><legend>Feedback Comments</legend>
    <input name="id" type="hidden" value="top"><input name="component" type="hidden" value="Web Storage (editor: Ian Hickson)"><input name="response" type="hidden" value="html"><p><label for="feedbackBox">Please enter your feedback, carefully
    indicating the title of the section for which you are submitting
    feedback, quoting the text that's wrong today if appropriate. If
    you're suggesting a new feature, it's really important to say
    <em>what</em> the problem you're trying to solve is. That's more
    important than the solution, in fact.</label></p>
    <p><textarea cols="79" id="feedbackBox" name="text" rows="10"></textarea></p>
    <p class="note">Please don't use section numbers as these tend to
    change rapidly and make your feedback harder to understand.</p>
    <script type="text/javascript">
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      } else if (form.elements.text.value.match(/ [^ ]+ [^ ]+ [^ ]+ [^ ]+ /)) {
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          alert('Please enter your feedback, explaining what is wrong, and without repeating the instructions. Thanks!');
          return true;
        } else if (form.elements.text.value.match(/ [^ ]+ [^ ]+ [^ ]+ [^ ]+ /)) {
          form.action = "http://www.whatwg.org/specs/web-apps/current-work/file-bug.cgi";
          return true;
        } else {
          alert('Please include significantly more detail about exactly what problem you are trying to solve.');
          return false;
        }
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    </script><p>
     <input onclick="return checkFeedbackForm(form)" type="submit" value="Submit feedback"><small>(Note: Your IP address and user agent will be publicly recorded for spam prevention purposes.)</small>
    </p>
   </fieldset></form><p>You can also e-mail feedback to <a href="mailto:public-webapps@w3.org">public-webapps@w3.org</a> (<a href="mailto:public-webapps-request@w3.org?subject=subscribe">subscribe</a>,
  <a href="http://lists.w3.org/Archives/Public/public-webapps/">archives</a>),
  or <a href="mailto:whatwg@whatwg.org">whatwg@whatwg.org</a> (<a href="http://lists.whatwg.org/listinfo.cgi/whatwg-whatwg.org">subscribe</a>,
  <a href="http://lists.whatwg.org/pipermail/whatwg-whatwg.org/">archives</a>).
  All feedback is welcome.</p><p>
  <strong>Implementors who are not taking part in the
  discussions are likely to find the specification changing out from
  under them in incompatible ways.</strong> Vendors interested in
  implementing this specification before it eventually reaches the
  Candidate Recommendation stage should join the aforementioned
  mailing lists and take part in the discussions.<div id="multipage-common">
  </div><p>The latest
  stable version of the editor's draft of this specification is always
  available on <a href="http://dev.w3.org/html5/webstorage/">the W3C CVS server</a>
  and in the <a href="http://svn.whatwg.org/webapps/">WHATWG
  Subversion repository</a>. The <a href="http://www.whatwg.org/specs/web-apps/current-work/complete.html">latest
  editor's working copy</a> (which may contain unfinished text in the
  process of being prepared) contains the latest draft text of this
  specification (amongst others). For more details, please see the <a href="http://wiki.whatwg.org/wiki/FAQ#What_are_the_various_versions_of_the_spec.3F">WHATWG
  FAQ</a>.<p>Notifications of changes to this specification are sent along
  with notifications of changes to related specifications using the
  following mechanisms:<dl><dt>E-mail notifications of changes</dt>
   <dd>Commit-Watchers mailing list (complete source diffs): <a href="http://lists.whatwg.org/listinfo.cgi/commit-watchers-whatwg.org">http://lists.whatwg.org/listinfo.cgi/commit-watchers-whatwg.org</a></dd>
   <dt>Browsable version-control record of all changes:</dt>
   <dd>CVSWeb interface with side-by-side diffs: <a href="http://dev.w3.org/cvsweb/html5/">http://dev.w3.org/cvsweb/html5/</a></dd>
   <dd>Annotated summary with unified diffs: <a href="http://html5.org/tools/web-apps-tracker">http://html5.org/tools/web-apps-tracker</a></dd>
   <dd>Raw Subversion interface: <code>svn checkout http://svn.whatwg.org/webapps/</code></dd>
  </dl><p>The W3C <a href="http://www.w3.org/2008/webapps/">Web Applications
  Working Group</a> is the W3C working group responsible for this
  specification's progress along the W3C Recommendation track.
  This specification is the 25 October 2011 Last Call Working Draft.
  The Last Call review period will end on 15 November 2011.
  </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="http://www.w3.org/2004/01/pp-impl/42538/status" rel="disclosure">public list of
  any patent disclosures</a> made in connection with the deliverables
  of the group; that page also includes instructions for disclosing a
  patent. An individual who has actual knowledge of a patent which the
  individual believes contains <a href="http://www.w3.org/Consortium/Patent-Policy-20040205/#def-essential">Essential
  Claim(s)</a> must disclose the information in accordance with <a href="http://www.w3.org/Consortium/Patent-Policy-20040205/#sec-Disclosure">section
  6 of the W3C Patent Policy</a>.<h2 class="no-num no-toc" id="issues">Issues</h2><p>The use of the storage mutex to avoid race conditions is
  currently considered by certain implementors to be too high a
  performance burden, to the point where allowing data corruption is
  considered preferable. Alternatives that do not require a
  user-agent-wide per-origin script lock are eagerly sought after. If
  reviewers have any suggestions, they are urged to send them to the
  addresses given in the previous section.<p>More details regarding this issue are available in these
  e-mails (as well as <a href="http://lists.whatwg.org/mmsearch.cgi/whatwg-whatwg.org?config=whatwg-whatwg.org&amp;restrict=&amp;exclude=&amp;method=and&amp;format=short&amp;sort=revtime&amp;words=storage+mutex">numerous others</a>):<ul><li><a href="http://lists.whatwg.org/htdig.cgi/whatwg-whatwg.org/2009-September/023059.html">http://lists.whatwg.org/htdig.cgi/whatwg-whatwg.org/2009-September/023059.html</a></li>
   <li><a href="http://lists.whatwg.org/htdig.cgi/whatwg-whatwg.org/2009-December/024277.html">http://lists.whatwg.org/htdig.cgi/whatwg-whatwg.org/2009-December/024277.html</a></li>
  </ul><h2 class="no-num no-toc" id="contents">Table of Contents</h2>
<ol class="toc">
 <li><a href="#introduction"><span class="secno">1 </span>Introduction</a></li>
 <li><a href="#conformance-requirements"><span class="secno">2 </span>Conformance requirements</a>
  <ol>
   <li><a href="#dependencies"><span class="secno">2.1 </span>Dependencies</a></ol></li>
 <li><a href="#terminology"><span class="secno">3 </span>Terminology</a></li>
 <li><a href="#storage"><span class="secno">4 </span>The API</a>
  <ol>
   <li><a href="#the-storage-interface"><span class="secno">4.1 </span>The <code>Storage</code> interface</a></li>
   <li><a href="#the-sessionstorage-attribute"><span class="secno">4.2 </span>The <code title="dom-sessionStorage">sessionStorage</code> attribute</a></li>
   <li><a href="#the-localstorage-attribute"><span class="secno">4.3 </span>The <code title="dom-localStorage">localStorage</code> attribute</a>
    <ol>
     <li><a href="#security-localStorage"><span class="secno">4.3.1 </span>Security</a></ol></li>
   <li><a href="#the-storage-event"><span class="secno">4.4 </span>The <code title="event-storage">storage</code> event</a>
    <ol>
     <li><a href="#event-definition"><span class="secno">4.4.1 </span>Event definition</a></ol></li>
   <li><a href="#threads"><span class="secno">4.5 </span>Threads</a></ol></li>
 <li><a href="#disk-space"><span class="secno">5 </span>Disk space</a></li>
 <li><a href="#privacy"><span class="secno">6 </span>Privacy</a>
  <ol>
   <li><a href="#user-tracking"><span class="secno">6.1 </span>User tracking</a></li>
   <li><a href="#sensitivity-of-data"><span class="secno">6.2 </span>Sensitivity of data</a></ol></li>
 <li><a href="#security-storage"><span class="secno">7 </span>Security</a>
  <ol>
   <li><a href="#dns-spoofing-attacks"><span class="secno">7.1 </span>DNS spoofing attacks</a></li>
   <li><a href="#cross-directory-attacks"><span class="secno">7.2 </span>Cross-directory attacks</a></li>
   <li><a href="#implementation-risks"><span class="secno">7.3 </span>Implementation risks</a></ol></li>
 <li><a class="no-num" href="#references">References</a></li>
 <li><a class="no-num" href="#acknowledgements">Acknowledgements</a></ol>
<hr><h2 id="introduction"><span class="secno">1 </span>Introduction</h2><p><i>This section is non-normative.</i><p>This specification introduces two related mechanisms, similar to
  HTTP session cookies, for storing structured data on the client
  side. <a href="#refsCOOKIES">[COOKIES]</a><p>The first is designed for scenarios where the user is carrying
  out a single transaction, but could be carrying out multiple
  transactions in different windows at the same time.<p>Cookies don't really handle this case well. For example, a user
  could be buying plane tickets in two different windows, using the
  same site. If the site used cookies to keep track of which ticket
  the user was buying, then as the user clicked from page to page in
  both windows, the ticket currently being purchased would "leak" from
  one window to the other, potentially causing the user to buy two
  tickets for the same flight without really noticing.<p>To address this, this specification introduces the <code title="dom-sessionStorage"><a href="#dom-sessionstorage">sessionStorage</a></code> IDL attribute.
  Sites can add data to the session storage, and it will be accessible
  to any page from the same site opened in that window.</p><div class="example">

   <p>For example, a page could have a checkbox that the user ticks to
   indicate that he wants insurance:</p>

   <pre>&lt;label&gt;
 &lt;input type="checkbox" onchange="sessionStorage.insurance = checked ? 'true' : ''"&gt;
 I want insurance on this trip.
&lt;/label&gt;</pre>

   <p>A later page could then check, from script, whether the user had
   checked the checkbox or not:</p>

   <pre>if (sessionStorage.insurance) { ... }</pre>

   <p>If the user had multiple windows opened on the site, each one
   would have its own individual copy of the session storage object.</p>

  </div><p>The second storage mechanism is designed for storage that spans
  multiple windows, and lasts beyond the current session. In
  particular, Web applications may wish to store megabytes of user
  data, such as entire user-authored documents or a user's mailbox, on
  the client side for performance reasons.<p>Again, cookies do not handle this case well, because they are
  transmitted with every request.<p>The <code title="dom-localStorage"><a href="#dom-localstorage">localStorage</a></code> IDL
  attribute is used to access a page's local storage area.<div class="example">

   <p>The site at example.com can display a count of how many times
   the user has loaded its page by putting the following at the bottom
   of its page:</p>

   <pre>&lt;p&gt;
  You have viewed this page
  &lt;span id="count"&gt;an untold number of&lt;/span&gt;
  time(s).
&lt;/p&gt;
&lt;script&gt;
  if (!localStorage.pageLoadCount)
    localStorage.pageLoadCount = 0;
  localStorage.pageLoadCount = parseInt(localStorage.pageLoadCount) + 1;
  document.getElementById('count').textContent = localStorage.pageLoadCount;
&lt;/script&gt;</pre>

  </div><p>Each site has its own separate storage area.<h2 id="conformance-requirements"><span class="secno">2 </span>Conformance requirements</h2><p>All diagrams, examples, and notes in this specification are
  non-normative, as are all sections explicitly marked non-normative.
  Everything else in this specification is normative.<p>The key words "MUST", "MUST NOT", "REQUIRED",  "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and
  "OPTIONAL" in the normative parts of this document are to be
  interpreted as described in RFC2119. For readability, these words do
  not appear in all uppercase letters in this specification. <a href="#refsRFC2119">[RFC2119]</a><p>Requirements phrased in the imperative as part of algorithms
  (such as "strip any leading space characters" or "return false and
  abort these steps") are to be interpreted with the meaning of the
  key word ("must", "should", "may", etc) used in introducing the
  algorithm.<p>Some conformance requirements are phrased as requirements on
  attributes, methods or objects. Such requirements are to be
  interpreted as requirements on user agents.<p>Conformance requirements phrased as algorithms or specific steps
  may be implemented in any manner, so long as the end result is
  equivalent. (In particular, the algorithms defined in this
  specification are intended to be easy to follow, and not intended to
  be performant.)<p>The only conformance class defined by this specification is user
  agents.<p>User agents may impose implementation-specific limits on
  otherwise unconstrained inputs, e.g. to prevent denial of service
  attacks, to guard against running out of memory, or to work around
  platform-specific limitations.<p>When support for a feature is disabled (e.g. as an emergency
  measure to mitigate a security problem, or to aid in development, or
  for performance reasons), user agents must act as if they had no
  support for the feature whatsoever, and as if the feature was not
  mentioned in this specification. For example, if a particular
  feature is accessed via an attribute in a Web IDL interface, the
  attribute itself would be omitted from the objects that implement
  that interface &mdash; leaving the attribute on the object but
  making it return null or throw an exception is insufficient.<h3 id="dependencies"><span class="secno">2.1 </span>Dependencies</h3><p>This specification relies on several other underlying
  specifications.<dl><dt>HTML</dt>

   <dd>

    <p>Many fundamental concepts from HTML are used by this
    specification. <a href="#refsHTML">[HTML]</a></p>

   </dd>

   <dt>WebIDL</dt>

   <dd>

    <p>The IDL blocks in this specification use the semantics of the
    WebIDL specification. <a href="#refsWEBIDL">[WEBIDL]</a></p>

   </dd>

  </dl><h2 id="terminology"><span class="secno">3 </span>Terminology</h2><p>The construction "a <code title="">Foo</code> object", where
  <code title="">Foo</code> is actually an interface, is sometimes
  used instead of the more accurate "an object implementing the
  interface <code title="">Foo</code>".<p>The term DOM is used to refer to the API set made available to
  scripts in Web applications, and does not necessarily imply the
  existence of an actual <code>Document</code> object or of any other
  <code>Node</code> objects as defined in the DOM Core
  specifications. <a href="#refsDOMCORE">[DOMCORE]</a><p>An IDL attribute is said to be <em>getting</em> when its value is
  being retrieved (e.g. by author script), and is said to be
  <em>setting</em> when a new value is assigned to it.<p>The term "JavaScript" is used to refer to ECMA262, rather than
  the official term ECMAScript, since the term JavaScript is more
  widely known. <a href="#refsECMA262">[ECMA262]</a><h2 id="storage"><span class="secno">4 </span>The API</h2><h3 id="the-storage-interface"><span class="secno">4.1 </span>The <code><a href="#storage-0">Storage</a></code> interface</h3><pre class="idl">interface <dfn id="storage-0">Storage</dfn> {
  readonly attribute unsigned long <a href="#dom-storage-length" title="dom-Storage-length">length</a>;
  DOMString? <a href="#dom-storage-key" title="dom-Storage-key">key</a>(unsigned long index);
  getter DOMString <a href="#dom-storage-getitem" title="dom-Storage-getItem">getItem</a>(DOMString key);
  setter creator void <a href="#dom-storage-setitem" title="dom-Storage-setItem">setItem</a>(DOMString key, DOMString value);
  deleter void <a href="#dom-storage-removeitem" title="dom-Storage-removeItem">removeItem</a>(DOMString key);
  void <a href="#dom-storage-clear" title="dom-Storage-clear">clear</a>();
};</pre><p>Each <code><a href="#storage-0">Storage</a></code> object provides access to a list of
  key/value pairs, which are sometimes called items. Keys are
  strings. Any string (including the empty string) is a valid
  key. Values are similarly strings.<p>Each <code><a href="#storage-0">Storage</a></code> object is associated with a list of
  key/value pairs when it is created, as defined in the sections on
  the <code title="dom-sessionStorage"><a href="#dom-sessionstorage">sessionStorage</a></code> and <code title="dom-localStorage"><a href="#dom-localstorage">localStorage</a></code> attributes. Multiple
  separate objects implementing the <code><a href="#storage-0">Storage</a></code> interface can
  all be associated with the same list of key/value pairs
  simultaneously.<p>The <dfn id="dom-storage-length" title="dom-Storage-length"><code>length</code></dfn>
  attribute must return the number of key/value pairs currently
  present in the list associated with the object.<p>The <dfn id="dom-storage-key" title="dom-Storage-key"><code>key(<var title="">n</var>)</code></dfn> method must return the name of the
  <var title="">n</var>th key in the list. The order of keys is
  user-agent defined, but must be consistent within an object so long
  as the number of keys doesn't change. (Thus, <a href="#dom-storage-setitem" title="dom-Storage-setItem">adding</a> or <a href="#dom-storage-removeitem" title="dom-Storage-removeItem">removing</a> a key may change the
  order of the keys, but merely changing the value of an existing key
  must not.)  If <var title="">n</var> is  greater than or equal to the number of key/value pairs
  in the object, then this method must return null.<p>The <span>supported property names</span> on a
  <code><a href="#storage-0">Storage</a></code> object are the keys of each key/value pair
  currently present in the list associated with the object.<p>The <dfn id="dom-storage-getitem" title="dom-Storage-getItem"><code>getItem(<var title="">key</var>)</code></dfn> method must return
  the current value associated with
  the given <var title="">key</var>. If the given <var title="">key</var> does not exist in the list associated with the
  object then this method must return null.
  <p>The <dfn id="dom-storage-setitem" title="dom-Storage-setItem"><code>setItem(<var title="">key</var>, <var title="">value</var>)</code></dfn> method
  must first check if a key/value pair
  with the given <var title="">key</var> already exists in the list
  associated with the object.<p>If it does not, then a new key/value pair must be added to the
  list, with the given <var title="">key</var> and with its value set
  to <var title="">value</var>.<p>If the given <var title="">key</var> <em>does</em> exist in the
  list, then it must have its value updated to <var title="">value</var>.<p>If it couldn't set the new value, the method must throw an
  <code>QuotaExceededError</code> exception. (Setting could fail if,
  e.g., the user has disabled storage for the site, or if the quota
  has been exceeded.)<p>The <dfn id="dom-storage-removeitem" title="dom-Storage-removeItem"><code>removeItem(<var title="">key</var>)</code></dfn> method must cause the key/value
  pair with the given <var title="">key</var> to be removed from the
  list associated with the object, if it exists. If no item with that
  key exists, the method must do nothing.<p>The <code title="dom-Storage-setItem"><a href="#dom-storage-setitem">setItem()</a></code> and <code title="dom-Storage-removeItem"><a href="#dom-storage-removeitem">removeItem()</a></code> methods must be
  atomic with respect to failure. In the case of failure, the method
  does nothing. That is, changes to the data storage area must either
  be successful, or the data storage area must not be changed at
  all.<p>The <dfn id="dom-storage-clear" title="dom-Storage-clear"><code>clear()</code></dfn>
  method must atomically cause the list associated with the object to
  be emptied of all key/value pairs, if there are any. If there are
  none, then the method must do nothing.<p class="note">When the <code title="dom-Storage-setItem"><a href="#dom-storage-setitem">setItem()</a></code>, <code title="dom-Storage-removeItem"><a href="#dom-storage-removeitem">removeItem()</a></code>, and <code title="dom-Storage-clear"><a href="#dom-storage-clear">clear()</a></code> methods are invoked, events
  are fired on other <code>Document</code> objects that can access the
  newly stored or removed data, as defined in the sections on the
  <code title="dom-sessionStorage"><a href="#dom-sessionstorage">sessionStorage</a></code> and <code title="dom-localStorage"><a href="#dom-localstorage">localStorage</a></code> attributes.</p><p class="note">This specification does not require that the above
  methods wait until the data has been physically written to
  disk. Only consistency in what different scripts accessing the same
  underlying list of key/value pairs see is required.<h3 id="the-sessionstorage-attribute"><span class="secno">4.2 </span>The <code title="dom-sessionStorage"><a href="#dom-sessionstorage">sessionStorage</a></code> attribute</h3><pre class="idl">[NoInterfaceObject]
interface <dfn id="windowsessionstorage">WindowSessionStorage</dfn> {
  readonly attribute <a href="#storage-0">Storage</a> <a href="#dom-sessionstorage" title="dom-sessionStorage">sessionStorage</a>;
};
<span>Window</span> implements <a href="#windowsessionstorage">WindowSessionStorage</a>;</pre><p>The <dfn id="dom-sessionstorage" title="dom-sessionStorage"><code>sessionStorage</code></dfn>
  attribute represents the set of storage areas specific to the
  current <span>top-level browsing context</span>.<p>Each <span>top-level browsing context</span> has a unique set of
  session storage areas, one for each <span>origin</span>.<p>User agents should not expire data from a browsing context's
  session storage areas, but may do so when the user requests that
  such data be deleted, or when the UA detects that it has limited
  storage space, or for security reasons. User agents should always
  avoid deleting data while a script that could access that data is
  running. When a top-level browsing context is destroyed (and
  therefore permanently inaccessible to the user) the data stored in
  its session storage areas can be discarded with it, as the API
  described in this specification provides no way for that data to
  ever be subsequently retrieved.<p class="note">The lifetime of a browsing context can be unrelated
  to the lifetime of the actual user agent process itself, as the user
  agent may support resuming sessions after a restart.<p>When a new <code>Document</code> is created in a <span>browsing
  context</span> which has a <span>top-level browsing context</span>,
  the user agent must check to see if that <span>top-level browsing
  context</span> has a session storage area for that document's
  <span>origin</span>. If it does, then that is the
  <code>Document</code>'s assigned session storage area. If it does
  not, a new storage area for that document's <span>origin</span> must
  be created, and then <em>that</em> is the <code>Document</code>'s
  assigned session storage area. A <code>Document</code>'s assigned
  storage area does not change during the lifetime of a
  <code>Document</code>, even in the case of a <span>nested browsing
  context</span> (e.g. in an <code>iframe</code>) being moved to
  another <span>parent browsing context</span>.<p>The <code title="dom-sessionStorage"><a href="#dom-sessionstorage">sessionStorage</a></code>
  attribute must return a <code><a href="#storage-0">Storage</a></code> object associated with
  the <code>Document</code>'s assigned session storage area, if any,
  or null if there isn't one. Each <code>Document</code> object must
  have a separate object for its <code>Window</code>'s <code title="dom-sessionStorage"><a href="#dom-sessionstorage">sessionStorage</a></code> attribute.<p>When a new <span>top-level browsing context</span> is created by
  cloning an existing <span>browsing context</span>, the new browsing
  context must start with the same session storage areas as the
  original, but the two sets must from that point on be considered
  separate, not affecting each other in any way.<p>When a new <span>top-level browsing context</span> is created by
  a <span title="concept-script">script</span> in an existing
  <span>browsing context</span>, or by the user following a link in an
  existing browsing context, or in some other way related to a
  specific <code>Document</code>, then the session storage area of the
  <span>origin</span> of that <code>Document</code> must be copied
  into the new browsing context when it is created. From that point
  on, however, the two session storage areas must be considered
  separate, not affecting each other in any way.<p id="sessionStorageEvent">When the <code title="dom-Storage-setItem"><a href="#dom-storage-setitem">setItem()</a></code>, <code title="dom-Storage-removeItem"><a href="#dom-storage-removeitem">removeItem()</a></code>, and <code title="dom-Storage-clear"><a href="#dom-storage-clear">clear()</a></code> methods are called on a
  <code><a href="#storage-0">Storage</a></code> object <var title="">x</var> that is associated
  with a session storage area, if the methods did something, then in
  every <code>Document</code> object whose <code>Window</code>
  object's <code title="dom-sessionStorage"><a href="#dom-sessionstorage">sessionStorage</a></code>
  attribute's <code><a href="#storage-0">Storage</a></code> object is associated with the same
  storage area, other than <var title="">x</var>, a <code title="event-storage"><a href="#event-storage">storage</a></code> event must be fired, as <a href="#event-storage" title="event-storage">described below</a>.<h3 id="the-localstorage-attribute"><span class="secno">4.3 </span>The <code title="dom-localStorage"><a href="#dom-localstorage">localStorage</a></code> attribute</h3><pre class="idl">[NoInterfaceObject]
interface <dfn id="windowlocalstorage">WindowLocalStorage</dfn> {
  readonly attribute <a href="#storage-0">Storage</a> <a href="#dom-localstorage" title="dom-localStorage">localStorage</a>;
};
<span>Window</span> implements <a href="#windowlocalstorage">WindowLocalStorage</a>;</pre><p>The <dfn id="dom-localstorage" title="dom-localStorage"><code>localStorage</code></dfn>
  object provides a <code><a href="#storage-0">Storage</a></code> object for an
  <span>origin</span>.<p>User agents must have a set of local storage areas, one for each
  <span>origin</span>.<p>User agents should expire data from the local storage areas only
  for security reasons or when requested to do so by the user. User
  agents should always avoid deleting data while a script that could
  access that data is running.<p>When the <code title="dom-localStorage"><a href="#dom-localstorage">localStorage</a></code>
  attribute is accessed, the user agent must run the following steps,
  which are known as the <dfn id="storage-object-initialization-steps"><code>Storage</code> object
  initialization steps</dfn>:</p><ol><li><p>The user agent may throw a <code>SecurityError</code>
   exception instead of returning a <code><a href="#storage-0">Storage</a></code> object if the
   request violates a policy decision (e.g. if the user agent is
   configured to not allow the page to persist data).</li>

   <li><p>If the <code>Document</code>'s <span>origin</span> is not a
   scheme/host/port tuple, then throw a <code>SecurityError</code>
   exception and abort these steps.</li>

   <li><p>Check to see if the user agent has allocated a local storage
   area for the <span>origin</span> of the <code>Document</code> of
   the <code>Window</code> object on which the attribute was accessed.
   If it has not, create a new storage area for that
   <span>origin</span>.</li>

   <li><p>Return the <code><a href="#storage-0">Storage</a></code> object associated with that
   origin's local storage area. Each <code>Document</code> object must
   have a separate object for its <code>Window</code>'s <code title="dom-localStorage"><a href="#dom-localstorage">localStorage</a></code> attribute.</p>

  </ol><p id="localStorageEvent">When the <code title="dom-Storage-setItem"><a href="#dom-storage-setitem">setItem()</a></code>, <code title="dom-Storage-removeItem"><a href="#dom-storage-removeitem">removeItem()</a></code>, and <code title="dom-Storage-clear"><a href="#dom-storage-clear">clear()</a></code> methods are called on a
  <code><a href="#storage-0">Storage</a></code> object <var title="">x</var> that is associated
  with a local storage area, if the methods did something, then in
  every <code>Document</code> object whose <code>Window</code>
  object's <code title="dom-localStorage"><a href="#dom-localstorage">localStorage</a></code>
  attribute's <code><a href="#storage-0">Storage</a></code> object is associated with the same
  storage area, other than <var title="">x</var>, a <code title="event-storage"><a href="#event-storage">storage</a></code> event must be fired, as <a href="#event-storage" title="event-storage">described below</a>.<p id="localStorageMutex">Whenever the properties of a <code title="dom-localStorage"><a href="#dom-localstorage">localStorage</a></code> attribute's
  <code><a href="#storage-0">Storage</a></code> object are to be examined, returned, set, or
  deleted, whether as part of a direct property access, when checking
  for the presence of a property, during property enumeration, when
  determining the number of properties present, or as part of the
  execution of any of the methods or attributes defined on the
  <code><a href="#storage-0">Storage</a></code> interface, the user agent must first
  <span>obtain the storage mutex</span>.<h4 id="security-localStorage"><span class="secno">4.3.1 </span>Security</h4><p>User agents must throw a <code>SecurityError</code> exception
  whenever any of the members of a <code><a href="#storage-0">Storage</a></code> object
  originally returned by the <code title="dom-localStorage"><a href="#dom-localstorage">localStorage</a></code> attribute are accessed
  by scripts whose <span>effective script origin</span> is not the
  <span title="same origin">same</span> as the <span>origin</span> of
  the <code>Document</code> of the <code>Window</code> object on which
  the <code title="dom-localStorage"><a href="#dom-localstorage">localStorage</a></code> attribute was
  accessed.<p class="note">This means <code><a href="#storage-0">Storage</a></code> objects are neutered
  when the <code title="dom-document-domain">document.domain</code>
  attribute is used.<h3 id="the-storage-event"><span class="secno">4.4 </span>The <code title="event-storage"><a href="#event-storage">storage</a></code> event</h3><p>The <dfn id="event-storage" title="event-storage"><code>storage</code></dfn> event
  is fired when a storage area changes, as described in the previous
  two sections (<a href="#sessionStorageEvent">for session
  storage</a>, <a href="#localStorageEvent">for local
  storage</a>).<p>When this happens, the user agent must <span>queue a task</span>
  to fire an event with the name <code><a href="#storage-0">storage</a></code>, which does not
  bubble and is not cancelable, and which uses the
  <code><a href="#storageevent">StorageEvent</a></code> interface, at each <code>Window</code>
  object whose <code>Document</code> object has a <code><a href="#storage-0">Storage</a></code>
  object that is affected.<p class="note">This includes <code>Document</code> objects that are
  not <span>fully active</span>, but events fired on those are ignored
  by the <span>event loop</span> until the <code>Document</code>
  becomes <span>fully active</span> again.<p>The <span>task source</span> for this task is the <span>DOM
  manipulation task source</span>.<p>If the event is being fired due to an invocation of the <code title="dom-Storage-setItem"><a href="#dom-storage-setitem">setItem()</a></code> or <code title="dom-Storage-removeItem"><a href="#dom-storage-removeitem">removeItem()</a></code> methods, the
  event must have its <code title="dom-StorageEvent-key"><a href="#dom-storageevent-key">key</a></code>
  attribute initialized to the name of the key in question, its <code title="dom-StorageEvent-oldValue"><a href="#dom-storageevent-oldvalue">oldValue</a></code> attribute initialized to
  the old value of the key in question, or null if the key is newly
  added, and its <code title="dom-StorageEvent-newValue"><a href="#dom-storageevent-newvalue">newValue</a></code> attribute initialized to
  the new value of the key in question, or null if the key was
  removed.<p>Otherwise, if the event is being fired due to an invocation of
  the <code title="dom-Storage-clear"><a href="#dom-storage-clear">clear()</a></code> method, the event
  must have its <code title="dom-StorageEvent-key"><a href="#dom-storageevent-key">key</a></code>, <code title="dom-StorageEvent-oldValue"><a href="#dom-storageevent-oldvalue">oldValue</a></code>, and <code title="dom-StorageEvent-newValue"><a href="#dom-storageevent-newvalue">newValue</a></code> attributes
  initialized to null.<p>In addition, the event must have its <code title="dom-StorageEvent-url"><a href="#dom-storageevent-url">url</a></code> attribute initialized to
  <span title="the document's address">the address of the
  document</span> whose <code><a href="#storage-0">Storage</a></code> object was affected; and
  its <code title="dom-StorageEvent-storageArea"><a href="#dom-storageevent-storagearea">storageArea</a></code>
  attribute initialized to the <code><a href="#storage-0">Storage</a></code> object from the
  <code>Window</code> object of the target <code>Document</code> that
  represents the same kind of <code><a href="#storage-0">Storage</a></code> area as was
  affected (i.e. session or local).<h4 id="event-definition"><span class="secno">4.4.1 </span>Event definition</h4><pre class="idl">[Constructor(DOMString type, optional <a href="#storageeventinit">StorageEventInit</a> eventInitDict)]
interface <dfn id="storageevent">StorageEvent</dfn> : <span>Event</span> {
  readonly attribute DOMString <a href="#dom-storageevent-key" title="dom-StorageEvent-key">key</a>;
  readonly attribute DOMString? <a href="#dom-storageevent-oldvalue" title="dom-StorageEvent-oldValue">oldValue</a>;
  readonly attribute DOMString? <a href="#dom-storageevent-newvalue" title="dom-StorageEvent-newValue">newValue</a>;
  readonly attribute DOMString <a href="#dom-storageevent-url" title="dom-StorageEvent-url">url</a>;
  readonly attribute <a href="#storage-0">Storage</a>? <a href="#dom-storageevent-storagearea" title="dom-StorageEvent-storageArea">storageArea</a>;
};

dictionary <dfn id="storageeventinit">StorageEventInit</dfn> : <span>EventInit</span> {
  DOMString key;
  DOMString? oldValue;
  DOMString? newValue;
  DOMString url;
  <a href="#storage-0">Storage</a>? storageArea;
};</pre><p>The <dfn id="dom-storageevent-key" title="dom-StorageEvent-key"><code>key</code></dfn>
  attribute must return the value it was initialized to. When the
  object is created, this attribute must be initialized to the empty
  string. It represents the key being changed.<p>The <dfn id="dom-storageevent-oldvalue" title="dom-StorageEvent-oldValue"><code>oldValue</code></dfn>
  attribute must return the value it was initialized to. When the
  object is created, this attribute must be initialized to null. It
  represents the old value of the key being changed.<p>The <dfn id="dom-storageevent-newvalue" title="dom-StorageEvent-newValue"><code>newValue</code></dfn>
  attribute must return the value it was initialized to. When the
  object is created, this attribute must be initialized to null. It
  represents the new value of the key being changed.<p>The <dfn id="dom-storageevent-url" title="dom-StorageEvent-url"><code>url</code></dfn>
  attribute must return the value it was initialized to. When the
  object is created, this attribute must be initialized to the empty
  string. It represents the address of the document whose key
  changed.<p>The <dfn id="dom-storageevent-storagearea" title="dom-StorageEvent-storageArea"><code>storageArea</code></dfn>
  attribute must return the value it was initialized to. When the
  object is created, this attribute must be initialized to null. It
  represents the <code><a href="#storage-0">Storage</a></code> object that was affected.<h3 id="threads"><span class="secno">4.5 </span>Threads</h3><p>Because of <a href="#localStorageMutex">the use</a> of the
  <span>storage mutex</span>, multiple browsing contexts will be able
  to access the local storage areas simultaneously in such a manner
  that scripts cannot detect any concurrent script execution.<p>Thus, the <code title="dom-Storage-length"><a href="#dom-storage-length">length</a></code>
  attribute of a <code><a href="#storage-0">Storage</a></code> object, and the value of the
  various properties of that object, cannot change while a script is
  executing, other than in a way that is predictable by the script
  itself.<h2 id="disk-space"><span class="secno">5 </span>Disk space</h2><p>User agents should limit the total amount of space allowed for
  storage areas.<p>User agents should guard against sites storing data under the
  origins other affiliated sites, e.g. storing up to the limit in
  a1.example.com, a2.example.com, a3.example.com, etc, circumventing
  the main example.com storage limit.<p>User agents may prompt the user when quotas are reached, allowing
  the user to grant a site more space. This enables sites to store
  many user-created documents on the user's computer, for
  instance.<p>User agents should allow users to see how much space each domain
  is using.</p><p>A mostly arbitrary limit of five megabytes per
  <span>origin</span> is recommended. Implementation feedback is
  welcome and will be used to update this suggestion in the
  future.<h2 id="privacy"><span class="secno">6 </span>Privacy</h2><h3 id="user-tracking"><span class="secno">6.1 </span>User tracking</h3><p>A third-party advertiser (or any entity capable of getting
  content distributed to multiple sites) could use a unique identifier
  stored in its local storage area to track a user across multiple
  sessions, building a profile of the user's interests to allow for
  highly targeted advertising. In conjunction with a site that is
  aware of the user's real identity (for example an e-commerce site
  that requires authenticated credentials), this could allow
  oppressive groups to target individuals with greater accuracy than
  in a world with purely anonymous Web usage.<p>There are a number of techniques that can be used to mitigate the
  risk of user tracking:<dl><dt>Blocking third-party storage</dt>
   <dd>

    <p>User agents may restrict access to the <code title="dom-localStorage"><a href="#dom-localstorage">localStorage</a></code> objects to scripts
    originating at the domain of the top-level document of the
    <span>browsing context</span>, for instance denying access to the
    API for pages from other domains running in
    <code>iframe</code>s.</p>

   </dd>

   <dt>Expiring stored data</dt>
   <dd>

    <p>User agents may, if so configured by the user, automatically
    delete stored data after a period of time.</p>

    <p>For example, a user agent could be configured to treat
    third-party local storage areas as session-only storage, deleting
    the data once the user had closed all the <span title="browsing
    context">browsing contexts</span> that could access it.</p>

    <p>This can restrict the ability of a site to track a user, as the
    site would then only be able to track the user across multiple
    sessions when he authenticates with the site itself (e.g. by
    making a purchase or logging in to a service).</p>

    <p>However, this also reduces the usefulness of the API as a
    long-term storage mechanism. It can also put the user's data at
    risk, if the user does not fully understand the implications of
    data expiration.</p>

    
   </dd>

   <dt>Treating persistent storage as cookies</dt>
   <dd>

    <p>If users attempt to protect their privacy by clearing cookies
    without also clearing data stored in the local storage area, sites
    can defeat those attempts by using the two features as redundant
    backup for each other. User agents should present the interfaces
    for clearing these in a way that helps users to understand this
    possibility and enables them to delete data in all persistent
    storage features simultaneously. <a href="#refsCOOKIES">[COOKIES]</a></p>

   </dd>

   <dt>Site-specific white-listing of access to local storage
   areas</dt>
   <dd>

    <p>User agents may allow sites to access session storage areas in
    an unrestricted manner, but require the user to authorize access
    to local storage areas.</p>

   </dd>

   <dt>Origin-tracking of stored data</dt>
   <dd>

    <p>User agents may record the <span title="origin">origins</span>
    of sites that contained content from third-party origins that
    caused data to be stored.</p>

    <p>If this information is then used to present the view of data
    currently in persistent storage, it would allow the user to make
    informed decisions about which parts of the persistent storage to
    prune. Combined with a blacklist ("delete this data and prevent
    this domain from ever storing data again"), the user can restrict
    the use of persistent storage to sites that he trusts.</p>

   </dd>

   <dt>Shared blacklists</dt>
   <dd>

    <p>User agents may allow users to share their persistent storage
    domain blacklists.</p>

    <p>This would allow communities to act together to protect their
    privacy.</p>

   </dd>

  </dl><p>While these suggestions prevent trivial use of this API for user
  tracking, they do not block it altogether. Within a single domain, a
  site can continue to track the user during a session, and can then
  pass all this information to the third party along with any
  identifying information (names, credit card numbers, addresses)
  obtained by the site. If a third party cooperates with multiple
  sites to obtain such information, a profile can still be
  created.<p>However, user tracking is to some extent possible even with no
  cooperation from the user agent whatsoever, for instance by using
  session identifiers in URLs, a technique already commonly used for
  innocuous purposes but easily repurposed for user tracking (even
  retroactively). This information can then be shared with other
  sites, using using visitors' IP addresses and other user-specific
  data (e.g. user-agent headers and configuration settings) to combine
  separate sessions into coherent user profiles.<h3 id="sensitivity-of-data"><span class="secno">6.2 </span>Sensitivity of data</h3><p>User agents should treat persistently stored data as potentially
  sensitive; it's quite possible for e-mails, calendar appointments,
  health records, or other confidential documents to be stored in this
  mechanism.<p>To this end, user agents should ensure that when deleting data,
  it is promptly deleted from the underlying storage.<h2 id="security-storage"><span class="secno">7 </span>Security</h2><h3 id="dns-spoofing-attacks"><span class="secno">7.1 </span>DNS spoofing attacks</h3><p>Because of the potential for DNS spoofing attacks, one cannot
  guarantee that a host claiming to be in a certain domain really is
  from that domain. To mitigate this, pages can use TLS. Pages using
  TLS can be sure that only the user, software working on behalf of
  the user, and other pages using TLS that have certificates
  identifying them as being from the same domain, can access their
  storage areas.<h3 id="cross-directory-attacks"><span class="secno">7.2 </span>Cross-directory attacks</h3><p>Different authors sharing one host name, for example users
  hosting content on <code>geocities.com</code>, all share one local
  storage object. There is no feature to restrict the access by
  pathname. Authors on shared hosts are therefore recommended to avoid
  using these features, as it would be trivial for other authors to
  read the data and overwrite it.<p class="note">Even if a path-restriction feature was made
  available, the usual DOM scripting security model would make it
  trivial to bypass this protection and access the data from any
  path.<h3 id="implementation-risks"><span class="secno">7.3 </span>Implementation risks</h3><p>The two primary risks when implementing these persistent storage
  features are letting hostile sites read information from other
  domains, and letting hostile sites write information that is then
  read from other domains.<p>Letting third-party sites read data that is not supposed to be
  read from their domain causes <em>information leakage</em>, For
  example, a user's shopping wishlist on one domain could be used by
  another domain for targeted advertising; or a user's
  work-in-progress confidential documents stored by a word-processing
  site could be examined by the site of a competing company.<p>Letting third-party sites write data to the persistent storage of
  other domains can result in <em>information spoofing</em>, which is
  equally dangerous. For example, a hostile site could add items to a
  user's wishlist; or a hostile site could set a user's session
  identifier to a known ID that the hostile site can then use to track
  the user's actions on the victim site.<p>Thus, strictly following the <span>origin</span> model described
  in this specification is important for user security.<h2 class="no-num" id="references">References</h2><p>All references are normative unless marked "Non-normative".</p><dl><dt id="refsCOOKIES">[COOKIES]</dt>
      <dd><cite><a href="http://tools.ietf.org/html/rfc6265">HTTP State Management Mechanism</a></cite>, A. Barth. IETF.</dd>

   <dt id="refsDOMCORE">[DOMCORE]</dt>
   <dd><cite><a href="http://dvcs.w3.org/hg/domcore/raw-file/tip/Overview.html">Web DOM Core</a></cite>, A. van Kesteren. W3C.</dd>

   <dt id="refsECMA262">[ECMA262]</dt>
   <dd><cite><a href="http://www.ecma-international.org/publications/standards/Ecma-262.htm">ECMAScript
   Language Specification</a></cite>. ECMA.</dd>

   <dt id="refsHTML">[HTML]</dt>
   <dd><cite><a href="http://www.w3.org/TR/html5/">HTML5</a></cite>,
   I. Hickson. W3C.</dd>

   <dt id="refsRFC2119">[RFC2119]</dt>
   <dd><cite><a href="http://tools.ietf.org/html/rfc2119">Key words for use in
   RFCs to Indicate Requirement Levels</a></cite>, S. Bradner. IETF.</dd>

   <dt id="refsWEBIDL">[WEBIDL]</dt>
      <dd><cite><a href="http://dev.w3.org/2006/webapi/WebIDL/">Web
   IDL</a></cite>, C. McCormack. W3C.</dd>

  </dl><h2 class="no-num" id="acknowledgements">Acknowledgements</h2><p>For a full list of acknowledgements, please see the HTML
  specification. <a href="#refsHTML">[HTML]</a>
Received on Friday, 21 October 2011 12:15:00 GMT

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