2006-11-20 02:47:00 +00:00
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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 various methods for allowing IDs in documents safely in HTML Purifier." />
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<link rel="stylesheet" type="text/css" href="./style.css" />
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<title>IDs - HTML Purifier</title>
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</head><body>
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<h1 class="subtitled">IDs</h1>
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<div class="subtitle">What they are, why you should(n't) wear them, and how to deal with it</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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2007-01-18 22:38:40 +00:00
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<div id="home"><a href="http://hp.jpsband.org/">HTML Purifier</a> End-User Documentation</div>
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2006-11-20 02:47:00 +00:00
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<p>Prior to HTML Purifier 1.2.0, this library blithely accepted user input that
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looked like this:</p>
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<pre><a id="fragment">Anchor</a></pre>
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<p>...presenting an attractive vector for those that would destroy standards
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compliance: simply set the ID to one that is already used elsewhere in the
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document and voila: validation breaks. There was a half-hearted attempt to
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prevent this by allowing users to blacklist IDs, but I suspect that no one
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really bothered, and thus, with the release of 1.2.0, IDs are now <em>removed</em>
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by default.</p>
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<p>IDs, however, are quite useful functionality to have, so if users start
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complaining about broken anchors you'll probably want to turn them back on
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with %HTML.EnableAttrID. But before you go mucking around with the config
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object, it's probably worth to take some precautions to keep your page
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validating. Why?</p>
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<ol>
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<li>Standards-compliant pages are good</li>
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<li>Duplicated IDs interfere with anchors. If there are two id="foobar"s in a
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document, which spot does a browser presented with the fragment #foobar go
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to? Most browsers opt for the first appearing ID, making it impossible
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to references the second section. Similarly, duplicated IDs can hijack
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client-side scripting that relies on the IDs of elements.</li>
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</ol>
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<p>You have (currently) four ways of dealing with the problem.</p>
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<h2 class="subtitled">Blacklisting IDs</h2>
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<div class="subsubtitle">Good for pages with single content source and stable templates</div>
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<p>Keeping in terms with the
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<acronym title="Keep It Simple, Stupid">KISS</acronym> principle, let us
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deal with the most obvious solution: preventing users from using any IDs that
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appear elsewhere on the document. The method is simple:</p>
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<pre>$config->set('HTML', 'EnableAttrID', true);
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$config->set('Attr', 'IDBlacklist' array(
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'list', 'of', 'attributes', 'that', 'are', 'forbidden'
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));</pre>
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<p>That being said, there are some notable drawbacks. First of all, you have to
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know precisely which IDs are being used by the HTML surrounding the user code.
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This is easier said than done: quite often the page designer and the system
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coder work separately, so the designer has to constantly be talking with the
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coder whenever he decides to add a new anchor. Miss one and you open yourself
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to possible standards-compliance issues.</p>
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<p>Furthermore, this position becomes untenable when a single web page must hold
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multiple portions of user-submitted content. Since there's obviously no way
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to find out before-hand what IDs users will use, the blacklist is helpless.
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And even since HTML Purifier validates each segment seperately, perhaps doing
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so at different times, it would be extremely difficult to dynamically update
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the blacklist inbetween runs.</p>
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<p>Finally, simply destroying the ID is extremely un-userfriendly behavior: after
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all, they might have simply specified a duplicate ID by accident.</p>
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<p>Thus, we get to our second method.</p>
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<h2 class="subtitled">Namespacing IDs</h2>
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<div class="subsubtitle">Lazy developer's way, but needs user education</div>
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<p>This method, too, is quite simple: add a prefix to all user IDs. With this
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code:</p>
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<pre>$config->set('HTML', 'EnableAttrID', true);
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$config->set('Attr', 'IDPrefix', 'user_');</pre>
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<p>...this:</p>
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<pre><a id="foobar">Anchor!</a></pre>
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<p>...turns into:</p>
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<pre><a id="user_foobar">Anchor!</a></pre>
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<p>As long as you don't have any IDs that start with user_, collisions are
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guaranteed not to happen. The drawback is obvious: if a user submits
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id="foobar", they probably expect to be able to reference their page with
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#foobar. You'll have to tell them, "No, that doesn't work, you have to add
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user_ to the beginning."</p>
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<p>And yes, things get hairier. Even with a nice prefix, we still have done
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nothing about multiple HTML Purifier outputs on one page. Thus, we have
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a second configuration value to piggy-back off of: %Attr.IDPrefixLocal:</p>
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<pre>$config->set('Attr', 'IDPrefixLocal', 'comment' . $id . '_');</pre>
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<p>This new attributes does nothing but append on to regular IDPrefix, but is
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special in that it is volatile: it's value is determined at run-time and
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cannot possibly be cordoned into, say, a .ini config file. As for what to
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put into the directive, is up to you, but I would recommend the ID number
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the text has been assigned in the database. Whatever you pick, however, it
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has to be unique and stable for the text you are validating. Note, however,
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that we require that %Attr.IDPrefix be set before you use this directive.</p>
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<p>And also remember: the user has to know what this prefix is too!</p>
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<h2>Abstinence</h2>
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<p>You may not want to bother. That's okay too, just don't enable IDs.</p>
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<p>Personally, I would take this road whenever user-submitted content would be
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possibly be shown together on one page. Why a blog comment would need to use
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anchors is beyond me.</p>
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<h2>Denial</h2>
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<p>To revert back to pre-1.2.0 behavior, simply:</p>
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<pre>$config->set('HTML', 'EnableAttrID', true);</pre>
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<p>Don't come crying to me when your page mysteriously stops validating, though.</p>
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<div id="version">$Id$</div>
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</body>
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</html>
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