2015-02-21 08:16:38 +01:00
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<?xml version="1.0" encoding="UTF-8"?>
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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
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"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
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<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en"><head>
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<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
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<meta name="description" content="Explains how to safely allow the embedding of flash from trusted sites in HTML Purifier." />
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<link rel="stylesheet" type="text/css" href="./style.css" />
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<title>Embedding YouTube Videos - HTML Purifier</title>
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</head><body>
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<h1 class="subtitled">Embedding YouTube Videos</h1>
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<div class="subtitle">...as well as other dangerous active content</div>
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<div id="filing">Filed under End-User</div>
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<div id="index">Return to the <a href="index.html">index</a>.</div>
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<div id="home"><a href="http://htmlpurifier.org/">HTML Purifier</a> End-User Documentation</div>
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<p>Clients like their YouTube videos. It gives them a warm fuzzy feeling when
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they see a neat little embedded video player on their websites that can play
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the latest clips from their documentary "Fido and the Bones of Spring".
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All joking aside, the ability to embed YouTube videos or other active
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content in their pages is something that a lot of people like.</p>
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<p>This is a <em>bad</em> idea. The moment you embed anything untrusted,
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you will definitely be slammed by a manner of nasties that can be
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embedded in things from your run of the mill Flash movie to
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<a href="http://blog.spywareguide.com/2006/12/myspace_phish_attack_leads_use.html">Quicktime movies</a>.
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Even <code>img</code> tags, which HTML Purifier allows by default, can be
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dangerous. Be distrustful of anything that tells a browser to load content
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from another website automatically.</p>
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<p>Luckily for us, however, whitelisting saves the day. Sure, letting users
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include any old random flash file could be dangerous, but if it's
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from a specific website, it probably is okay. If no amount of pleading will
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convince the people upstairs that they should just settle with just linking
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to their movies, you may find this technique very useful.</p>
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<h2>Looking in</h2>
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<p>Below is custom code that allows users to embed
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YouTube videos. This is not favoritism: this trick can easily be adapted for
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other forms of embeddable content.</p>
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<p>Usually, websites like YouTube give us boilerplate code that you can insert
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into your documents. YouTube's code goes like this:</p>
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<pre>
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<object width="425" height="350">
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<param name="movie" value="http://www.youtube.com/v/AyPzM5WK8ys" />
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<param name="wmode" value="transparent" />
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<embed src="http://www.youtube.com/v/AyPzM5WK8ys"
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type="application/x-shockwave-flash"
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wmode="transparent" width="425" height="350" />
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</object>
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</pre>
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<p>There are two things to note about this code:</p>
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<ol>
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<li><code><embed></code> is not recognized by W3C, so if you want
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standards-compliant code, you'll have to get rid of it.</li>
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<li>The code is exactly the same for all instances, except for the
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identifier <tt>AyPzM5WK8ys</tt> which tells us which movie file
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to retrieve.</li>
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</ol>
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<p>What point 2 means is that if we have code like <code><span
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class="youtube-embed">AyPzM5WK8ys</span></code> your
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application can reconstruct the full object from this small snippet that
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passes through HTML Purifier <em>unharmed</em>.
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<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>
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<p>And the corresponding usage:</p>
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<pre><?php
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$config->set('Filter.YouTube', true);
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?></pre>
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<p>There is a bit going in the two code snippets, so let's explain.</p>
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<ol>
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<li>This is a Filter object, which intercepts the HTML that is
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coming into and out of the purifier. You can add as many
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filter objects as you like. <code>preFilter()</code>
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processes the code before it gets purified, and <code>postFilter()</code>
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processes the code afterwards. So, we'll use <code>preFilter()</code> to
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replace the object tag with a <code>span</code>, and <code>postFilter()</code>
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to restore it.</li>
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<li>The first preg_replace call replaces any YouTube code users may have
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embedded into the benign span tag. Span is used because it is inline,
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and objects are inline too. We are very careful to be extremely
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restrictive on what goes inside the span tag, as if an errant code
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gets in there it could get messy.</li>
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<li>The HTML is then purified as usual.</li>
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<li>Then, another preg_replace replaces the span tag with a fully fledged
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object. Note that the embed is removed, and, in its place, a data
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attribute was added to the object. This makes the tag standards
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compliant! It also breaks Internet Explorer, so we add in a bit of
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conditional comments with the old embed code to make it work again.
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It's all quite convoluted but works.</li>
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</ol>
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<h2>Warning</h2>
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<p>There are a number of possible problems with the code above, depending
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on how you look at it.</p>
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<h3>Cannot change width and height</h3>
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<p>The width and height of the final YouTube movie cannot be adjusted. This
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is because I am lazy. If you really insist on letting users change the size
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of the movie, what you need to do is package up the attributes inside the
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span tag (along with the movie ID). It gets complicated though: a malicious
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user can specify an outrageously large height and width and attempt to crash
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the user's operating system/browser. You need to either cap it by limiting
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the amount of digits allowed in the regex or using a callback to check the
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number.</p>
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<h3>Trusts media's host's security</h3>
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<p>By allowing this code onto our website, we are trusting that YouTube has
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tech-savvy enough people not to allow their users to inject malicious
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code into the Flash files. An exploit on YouTube means an exploit on your
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site. Even though YouTube is run by the reputable Google, it
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<a href="http://ha.ckers.org/blog/20061213/google-xss-vuln/">doesn't</a>
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mean they are
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<a href="http://ha.ckers.org/blog/20061208/xss-in-googles-orkut/">invulnerable.</a>
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You're putting a certain measure of the job on an external provider (just as
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you have by entrusting your user input to HTML Purifier), and
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it is important that you are cognizant of the risk.</p>
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<h3>Poorly written adaptations compromise security</h3>
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<p>This should go without saying, but if you're going to adapt this code
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for Google Video or the like, make sure you do it <em>right</em>. It's
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extremely easy to allow a character too many in <code>postFilter()</code> and
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suddenly you're introducing XSS into HTML Purifier's XSS free output. HTML
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Purifier may be well written, but it cannot guard against vulnerabilities
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introduced after it has finished.</p>
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<h2>Help out!</h2>
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<p>If you write a filter for your favorite video destination (or anything
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like that, for that matter), send it over and it might get included
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with the core!</p>
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</body>
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</html>
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<!-- vim: et sw=4 sts=4
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-->
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