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git-svn-id: http://htmlpurifier.org/svnroot/htmlpurifier/trunk@619 48356398-32a2-884e-a903-53898d9a118a
179 lines
8.4 KiB
HTML
179 lines
8.4 KiB
HTML
<?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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<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>Sample</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="embed-youtube">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>.</p>
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<pre>
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<?php
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class HTMLPurifierX_PreserveYouTube extends HTMLPurifier
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{
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function purify($html, $config = null) {
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$pre_regex = '#<object[^>]+>.+?'.
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'http://www.youtube.com/v/([A-Za-z0-9]+).+?</object>#';
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$pre_replace = '<span class="youtube-embed">\1</span>';
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$html = preg_replace($pre_regex, $pre_replace, $html);
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$html = parent::purify($html, $config);
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$post_regex = '#<span class="youtube-embed">([A-Za-z0-9]+)</span>#';
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$post_replace = '<object width="425" height="350" '.
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'data="http://www.youtube.com/v/\1">'.
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'<param name="movie" value="http://www.youtube.com/v/\1"></param>'.
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'<param name="wmode" value="transparent"></param>'.
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'<!--[if IE]>'.
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'<embed src="http://www.youtube.com/v/\1"'.
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'type="application/x-shockwave-flash"'.
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'wmode="transparent" width="425" height="350" />'.
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'<![endif]-->'.
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'</object>';
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$html = preg_replace($post_regex, $post_replace, $html);
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return $html;
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}
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}
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$purifier = new HTMLPurifierX_PreserveYouTube();
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$html_still_with_youtube = $purifier->purify($html_with_youtube);
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?>
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</pre>
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<p>There is a bit going on here, so let's explain.</p>
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<ol>
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<li>The class uses the prefix <code>HTMLPurifierX</code> because it's
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userspace code. Don't use <code>HTMLPurifier</code> in front of your
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class, since it might clobber another class in the library.</li>
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<li>In order to keep the interface compatible, we've extended HTMLPurifier
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into a new class that preserves the YouTube videos. This means that
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all you have to do is replace all instances of
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<code>new HTMLPurifier</code> to <code>new
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HTMLPurifierX_PreserveYouTube</code>. There's other ways to go about
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doing this: if you were calling a function that wrapped HTML Purifier,
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you could paste the PHP right there. If you wanted to be really
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fancy, you could make a decorator for HTMLPurifier.</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 the final section 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>Future plans</h2>
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<p>It would probably be a good idea if this code was added to the core
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library. Look out for the inclusion of this into the core as a decorator
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or the like.</p>
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</body>
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</html> |