full-disclosure - Internet Explorer 6 Meta Refresh Parsing Weakness

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Date: Thu Aug 18 01:47:15 2005
From: info at moritz-naumann.com (Moritz Naumann)
Subject: Internet Explorer 6 Meta Refresh Parsing Weakness

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SA0001

+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
+++++ Internet Explorer 6 Meta Refresh Parsing Weakness +++++
+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++


PUBLISHED ON
  Aug 17, 2005


PUBLISHED AT
  http://moritz-naumann.com/adv/0001/ie6meta/0001.txt


PUBLISHED BY
  Moritz Naumann IT Consulting & Services
  Hamburg/Germany
  http://moritz-naumann.com/

  info AT moritz HYPHON naumann D0T com
  GPG key: http://moritz-naumann.com/keys/0x277F060C.asc


AFFECTED PRODUCT OR SERVICE
  Microsoft Internet Explorer
  http://www.microsoft.com/windows/ie/


AFFECTED VERSION
  Version 6 up to release 6.0.2900.2180 (SP2 + all patches)
  Possibly versions < 6.0 (untested)


BACKGROUND
  While the format of
    META http-equiv="refresh"
  and
    META name="refresh"
  type HTML headers was never exactly defined by they W3C, web
  browsers have been interpreting this instruction since early
  releases. Web application developers got used to the clients'
  behaviour and using this tag to initiate URL redirections
  became common.

  As most web browsers, Internet Explorer 6 interprets this tag,
  too. However, in contrary to other web browsers, IE6's HTML
  parser uses a pretty loose rule set which facilitates
  injection of malicious code into it when browsing web
  applications which insufficiently sanitize user supplied
  input.

  For example, a web application may use the following PHP code
  (redirect.php) to redirect a web browser to a different URL:

    <!-- Common HTML 4.01 header except "</head>" goes here -->
    <?php
    $goto  = $_GET["goto"]; // Input sanitization omitted
    $meta1 = '<META http-equiv="refresh" content="0; URL=http://';
    $meta2 = '">';
    echo $meta.$goto;
    ?>
    </head>
    <!-- Common HTML 4.01 <body> goes here -->

  Assuming this script is hosted in the web root on example.org,
  the following HTML code would be returned on a request to
  http://example.org/redirect.php?goto=localhost :

    <!-- Common HTML 4.01 header except "</head>" goes here -->
    <META http-equiv="refresh" content="0; URL=http://localhost">
    </head>
    <!-- Common HTML 4.01 <body> goes here -->

  Obviously, a web application developer must make sure that no
  malicious code can be injected along the 'goto' parameter passed
  via the HTTP GET method. A common method to sanitize user input
  would be to hardcode the protocol part of the URL ('http://')
  contained in 'goto', and to URL-encode any double quotes. This
  would assumely make it difficult to inject any malicious client
  side code.


ISSUE
  Unlickily, and in contrary to other web browsers, Internet
  Explorer 6 allows multiple 'URL=' parts in the 'content'
  attribute and will only interpret the last value given.
  Resulting from this, it is still possible to inject code into a
  web application using the input sanitization described above
  which will be executed when using Internet Explorer 6.

  For example, Internet Explorer 6 will interpret the following
  statement:

  URL parameter:
    goto=;URL=javascript:alert('XSS');
  Resulting META tag:
    content="0; URL=http://;URL=javascript:alert('XSS');">
  Resulting behaviour:
    Displays Javascript alert with text 'XSS'

  Making use of Internet Explorers loose parsing, a code such as
  this value of the 'goto' URL parameter will work, too:

    %20%20%20%20%20;UrL=jaVAscRIpt:alert('XSS');

  will work, too. As any of ';', 'UrL', '=', 'jaVAscRIpt' and ':'
  may be legal content passed to the traget web site (think of a
  search term passed to a search engine), sanitizing this is not
  too easy.

  As the expected behaviour would be that a web browser would
  either return an error message for incorrect syntax or would
  attempt to interpret anything after the first 'URL=' part as the
  target URL, Internet Explorer behaves in a pretty uncommon way. A
  fix on the user agent side would be the best solution for this
  issue.


WORKAROUND
  Client: Disable META REFRESH in Security Settings for the Internet
  Zone.
  Server: Perform thorough sanitization on your web applications.


SOLUTIONS
  Microsoft will not provide a patch.


TIMELINE
  Aug 04, 2005: Vendor informed
  Aug 04, 2005: First vendor reply
  Aug 17, 2005: Vendor finishes investigation, declares itself
  unaccountabile


CREDIT
  N/A


LICENSE
  Creative Commons Attribution-ShareAlike License Germany
  http://creativecommons.org/licenses/by-sa/2.0/de/


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