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  1. <?xml version="1.0" encoding="UTF-8"?>
  2. <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
  3. "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
  4. <html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en"><head>
  5. <meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
  6. <meta name="description" content="Explains how to safely allow the embedding of flash from trusted sites in HTML Purifier." />
  7. <link rel="stylesheet" type="text/css" href="./style.css" />
  8. <title>Embedding YouTube Videos - HTML Purifier</title>
  9. </head><body>
  10. <h1 class="subtitled">Embedding YouTube Videos</h1>
  11. <div class="subtitle">...as well as other dangerous active content</div>
  12. <div id="filing">Filed under End-User</div>
  13. <div id="index">Return to the <a href="index.html">index</a>.</div>
  14. <div id="home"><a href="http://htmlpurifier.org/">HTML Purifier</a> End-User Documentation</div>
  15. <p>Clients like their YouTube videos. It gives them a warm fuzzy feeling when
  16. they see a neat little embedded video player on their websites that can play
  17. the latest clips from their documentary &quot;Fido and the Bones of Spring&quot;.
  18. All joking aside, the ability to embed YouTube videos or other active
  19. content in their pages is something that a lot of people like.</p>
  20. <p>This is a <em>bad</em> idea. The moment you embed anything untrusted,
  21. you will definitely be slammed by a manner of nasties that can be
  22. embedded in things from your run of the mill Flash movie to
  23. <a href="http://blog.spywareguide.com/2006/12/myspace_phish_attack_leads_use.html">Quicktime movies</a>.
  24. Even <code>img</code> tags, which HTML Purifier allows by default, can be
  25. dangerous. Be distrustful of anything that tells a browser to load content
  26. from another website automatically.</p>
  27. <p>Luckily for us, however, whitelisting saves the day. Sure, letting users
  28. include any old random flash file could be dangerous, but if it's
  29. from a specific website, it probably is okay. If no amount of pleading will
  30. convince the people upstairs that they should just settle with just linking
  31. to their movies, you may find this technique very useful.</p>
  32. <h2>Looking in</h2>
  33. <p>Below is custom code that allows users to embed
  34. YouTube videos. This is not favoritism: this trick can easily be adapted for
  35. other forms of embeddable content.</p>
  36. <p>Usually, websites like YouTube give us boilerplate code that you can insert
  37. into your documents. YouTube's code goes like this:</p>
  38. <pre>
  39. &lt;object width=&quot;425&quot; height=&quot;350&quot;&gt;
  40. &lt;param name=&quot;movie&quot; value=&quot;http://www.youtube.com/v/AyPzM5WK8ys&quot; /&gt;
  41. &lt;param name=&quot;wmode&quot; value=&quot;transparent&quot; /&gt;
  42. &lt;embed src=&quot;http://www.youtube.com/v/AyPzM5WK8ys&quot;
  43. type=&quot;application/x-shockwave-flash&quot;
  44. wmode=&quot;transparent&quot; width=&quot;425&quot; height=&quot;350&quot; /&gt;
  45. &lt;/object&gt;
  46. </pre>
  47. <p>There are two things to note about this code:</p>
  48. <ol>
  49. <li><code>&lt;embed&gt;</code> is not recognized by W3C, so if you want
  50. standards-compliant code, you'll have to get rid of it.</li>
  51. <li>The code is exactly the same for all instances, except for the
  52. identifier <tt>AyPzM5WK8ys</tt> which tells us which movie file
  53. to retrieve.</li>
  54. </ol>
  55. <p>What point 2 means is that if we have code like <code>&lt;span
  56. class=&quot;youtube-embed&quot;&gt;AyPzM5WK8ys&lt;/span&gt;</code> your
  57. application can reconstruct the full object from this small snippet that
  58. passes through HTML Purifier <em>unharmed</em>.
  59. <a href="http://repo.or.cz/w/htmlpurifier.git?a=blob;hb=HEAD;f=library/HTMLPurifier/Filter/YouTube.php">Show me the code!</a></p>
  60. <p>And the corresponding usage:</p>
  61. <pre>&lt;?php
  62. $config-&gt;set('Filter.YouTube', true);
  63. ?&gt;</pre>
  64. <p>There is a bit going in the two code snippets, so let's explain.</p>
  65. <ol>
  66. <li>This is a Filter object, which intercepts the HTML that is
  67. coming into and out of the purifier. You can add as many
  68. filter objects as you like. <code>preFilter()</code>
  69. processes the code before it gets purified, and <code>postFilter()</code>
  70. processes the code afterwards. So, we'll use <code>preFilter()</code> to
  71. replace the object tag with a <code>span</code>, and <code>postFilter()</code>
  72. to restore it.</li>
  73. <li>The first preg_replace call replaces any YouTube code users may have
  74. embedded into the benign span tag. Span is used because it is inline,
  75. and objects are inline too. We are very careful to be extremely
  76. restrictive on what goes inside the span tag, as if an errant code
  77. gets in there it could get messy.</li>
  78. <li>The HTML is then purified as usual.</li>
  79. <li>Then, another preg_replace replaces the span tag with a fully fledged
  80. object. Note that the embed is removed, and, in its place, a data
  81. attribute was added to the object. This makes the tag standards
  82. compliant! It also breaks Internet Explorer, so we add in a bit of
  83. conditional comments with the old embed code to make it work again.
  84. It's all quite convoluted but works.</li>
  85. </ol>
  86. <h2>Warning</h2>
  87. <p>There are a number of possible problems with the code above, depending
  88. on how you look at it.</p>
  89. <h3>Cannot change width and height</h3>
  90. <p>The width and height of the final YouTube movie cannot be adjusted. This
  91. is because I am lazy. If you really insist on letting users change the size
  92. of the movie, what you need to do is package up the attributes inside the
  93. span tag (along with the movie ID). It gets complicated though: a malicious
  94. user can specify an outrageously large height and width and attempt to crash
  95. the user's operating system/browser. You need to either cap it by limiting
  96. the amount of digits allowed in the regex or using a callback to check the
  97. number.</p>
  98. <h3>Trusts media's host's security</h3>
  99. <p>By allowing this code onto our website, we are trusting that YouTube has
  100. tech-savvy enough people not to allow their users to inject malicious
  101. code into the Flash files. An exploit on YouTube means an exploit on your
  102. site. Even though YouTube is run by the reputable Google, it
  103. <a href="http://ha.ckers.org/blog/20061213/google-xss-vuln/">doesn't</a>
  104. mean they are
  105. <a href="http://ha.ckers.org/blog/20061208/xss-in-googles-orkut/">invulnerable.</a>
  106. You're putting a certain measure of the job on an external provider (just as
  107. you have by entrusting your user input to HTML Purifier), and
  108. it is important that you are cognizant of the risk.</p>
  109. <h3>Poorly written adaptations compromise security</h3>
  110. <p>This should go without saying, but if you're going to adapt this code
  111. for Google Video or the like, make sure you do it <em>right</em>. It's
  112. extremely easy to allow a character too many in <code>postFilter()</code> and
  113. suddenly you're introducing XSS into HTML Purifier's XSS free output. HTML
  114. Purifier may be well written, but it cannot guard against vulnerabilities
  115. introduced after it has finished.</p>
  116. <h2>Help out!</h2>
  117. <p>If you write a filter for your favorite video destination (or anything
  118. like that, for that matter), send it over and it might get included
  119. with the core!</p>
  120. </body>
  121. </html>
  122. <!-- vim: et sw=4 sts=4
  123. -->