Admin Control Panel v2.0

✎ Editing: enduser-youtube.html ← Back

<?xml version="1.0" encoding="UTF-8"?>

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"

"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">

<title>Embedding YouTube Videos - HTML Purifier</title>

</head><body>

<h1 class="subtitled">Embedding YouTube Videos</h1>

<div class="subtitle">...as well as other dangerous active content</div>

<div id="filing">Filed under End-User</div>

<div id="index">Return to the <a href="index.html">index</a>.</div>

<div id="home"><a href="http://htmlpurifier.org/">HTML Purifier</a> End-User Documentation</div>

<p>Clients like their YouTube videos. It gives them a warm fuzzy feeling when

they see a neat little embedded video player on their websites that can play

the latest clips from their documentary &quot;Fido and the Bones of Spring&quot;.

All joking aside, the ability to embed YouTube videos or other active

content in their pages is something that a lot of people like.</p>

<p>This is a <em>bad</em> idea. The moment you embed anything untrusted,

you will definitely be slammed by a manner of nasties that can be

embedded in things from your run of the mill Flash movie to

<a href="http://blog.spywareguide.com/2006/12/myspace_phish_attack_leads_use.html">Quicktime movies</a>.

Even <code>img</code> tags, which HTML Purifier allows by default, can be

dangerous. Be distrustful of anything that tells a browser to load content

from another website automatically.</p>

<p>Luckily for us, however, whitelisting saves the day. Sure, letting users

include any old random flash file could be dangerous, but if it's

from a specific website, it probably is okay. If no amount of pleading will

convince the people upstairs that they should just settle with just linking

to their movies, you may find this technique very useful.</p>

<h2>Looking in</h2>

<p>Below is custom code that allows users to embed

YouTube videos. This is not favoritism: this trick can easily be adapted for

other forms of embeddable content.</p>

<p>Usually, websites like YouTube give us boilerplate code that you can insert

into your documents. YouTube's code goes like this:</p>

<pre>

&lt;object width=&quot;425&quot; height=&quot;350&quot;&gt;

&lt;param name=&quot;movie&quot; value=&quot;http://www.youtube.com/v/AyPzM5WK8ys&quot; /&gt;

&lt;param name=&quot;wmode&quot; value=&quot;transparent&quot; /&gt;

&lt;embed src=&quot;http://www.youtube.com/v/AyPzM5WK8ys&quot;

type=&quot;application/x-shockwave-flash&quot;

wmode=&quot;transparent&quot; width=&quot;425&quot; height=&quot;350&quot; /&gt;

&lt;/object&gt;

</pre>

<p>There are two things to note about this code:</p>

<ol>

<li><code>&lt;embed&gt;</code> is not recognized by W3C, so if you want

standards-compliant code, you'll have to get rid of it.</li>

<li>The code is exactly the same for all instances, except for the

identifier <tt>AyPzM5WK8ys</tt> which tells us which movie file

to retrieve.</li>

</ol>

<p>What point 2 means is that if we have code like <code>&lt;span

class=&quot;youtube-embed&quot;&gt;AyPzM5WK8ys&lt;/span&gt;</code> your

application can reconstruct the full object from this small snippet that

passes through HTML Purifier <em>unharmed</em>.

<p>And the corresponding usage:</p>

<pre>&lt;?php

$config-&gt;set('Filter.YouTube', true);

?&gt;</pre>

<p>There is a bit going in the two code snippets, so let's explain.</p>

<ol>

<li>This is a Filter object, which intercepts the HTML that is

coming into and out of the purifier. You can add as many

filter objects as you like. <code>preFilter()</code>

processes the code before it gets purified, and <code>postFilter()</code>

processes the code afterwards. So, we'll use <code>preFilter()</code> to

replace the object tag with a <code>span</code>, and <code>postFilter()</code>

to restore it.</li>

<li>The first preg_replace call replaces any YouTube code users may have

embedded into the benign span tag. Span is used because it is inline,

and objects are inline too. We are very careful to be extremely

restrictive on what goes inside the span tag, as if an errant code

gets in there it could get messy.</li>

<li>The HTML is then purified as usual.</li>

<li>Then, another preg_replace replaces the span tag with a fully fledged

object. Note that the embed is removed, and, in its place, a data

attribute was added to the object. This makes the tag standards

compliant! It also breaks Internet Explorer, so we add in a bit of

conditional comments with the old embed code to make it work again.

It's all quite convoluted but works.</li>

</ol>

<h2>Warning</h2>

<p>There are a number of possible problems with the code above, depending

on how you look at it.</p>

<h3>Cannot change width and height</h3>

<p>The width and height of the final YouTube movie cannot be adjusted. This

is because I am lazy. If you really insist on letting users change the size

of the movie, what you need to do is package up the attributes inside the

span tag (along with the movie ID). It gets complicated though: a malicious

user can specify an outrageously large height and width and attempt to crash

the user's operating system/browser. You need to either cap it by limiting

the amount of digits allowed in the regex or using a callback to check the

number.</p>

<h3>Trusts media's host's security</h3>

<p>By allowing this code onto our website, we are trusting that YouTube has

tech-savvy enough people not to allow their users to inject malicious

code into the Flash files.  An exploit on YouTube means an exploit on your

site.  Even though YouTube is run by the reputable Google, it

<a href="http://ha.ckers.org/blog/20061213/google-xss-vuln/">doesn't</a>

mean they are

<a href="http://ha.ckers.org/blog/20061208/xss-in-googles-orkut/">invulnerable.</a>

You're putting a certain measure of the job on an external provider (just as

you have by entrusting your user input to HTML Purifier), and

it is important that you are cognizant of the risk.</p>

<h3>Poorly written adaptations compromise security</h3>

<p>This should go without saying, but if you're going to adapt this code

for Google Video or the like, make sure you do it <em>right</em>. It's

extremely easy to allow a character too many in <code>postFilter()</code> and

suddenly you're introducing XSS into HTML Purifier's XSS free output. HTML

Purifier may be well written, but it cannot guard against vulnerabilities

introduced after it has finished.</p>

<p>If you write a filter for your favorite video destination (or anything

like that, for that matter), send it over and it might get included

with the core!</p>

</body>

</html>

<!-- vim: et sw=4 sts=4

-->

Cancel