2015-02-21 08:16:38 +01:00
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HTML Purifier
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by Edward Z. Yang
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There are a number of ad hoc HTML filtering solutions out there on the web
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(some examples including HTML_Safe, kses and SafeHtmlChecker.class.php) that
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claim to filter HTML properly, preventing malicious JavaScript and layout
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breaking HTML from getting through the parser. None of them, however,
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demonstrates a thorough knowledge of neither the DTD that defines the HTML
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nor the caveats of HTML that cannot be expressed by a DTD. Configurable
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filters (such as kses or PHP's built-in striptags() function) have trouble
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validating the contents of attributes and can be subject to security attacks
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due to poor configuration. Other filters take the naive approach of
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blacklisting known threats and tags, failing to account for the introduction
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of new technologies, new tags, new attributes or quirky browser behavior.
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However, HTML Purifier takes a different approach, one that doesn't use
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specification-ignorant regexes or narrow blacklists. HTML Purifier will
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decompose the whole document into tokens, and rigorously process the tokens by:
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removing non-whitelisted elements, transforming bad practice tags like <font>
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into <span>, properly checking the nesting of tags and their children and
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validating all attributes according to their RFCs.
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To my knowledge, there is nothing like this on the web yet. Not even MediaWiki,
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which allows an amazingly diverse mix of HTML and wikitext in its documents,
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gets all the nesting quirks right. Existing solutions hope that no JavaScript
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will slip through, but either do not attempt to ensure that the resulting
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output is valid XHTML or send the HTML through a draconic XML parser (and yet
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still get the nesting wrong: SafeHtmlChecker.class.php does not prevent <a>
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tags from being nested within each other).
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This document no longer is a detailed description of how HTMLPurifier works,
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as those descriptions have been moved to the appropriate code. The first
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draft was drawn up after two rough code sketches and the implementation of a
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forgiving lexer. You may also be interested in the unit tests located in the
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tests/ folder, which provide a living document on how exactly the filter deals
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with malformed input.
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In summary (see corresponding classes for more details):
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1. Parse document into an array of tag and text tokens (Lexer)
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2. Remove all elements not on whitelist and transform certain other elements
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into acceptable forms (i.e. <font>)
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3. Make document well formed while helpfully taking into account certain quirks,
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such as the fact that <p> tags traditionally are closed by other block-level
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elements.
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4. Run through all nodes and check children for proper order (especially
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important for tables).
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5. Validate attributes according to more restrictive definitions based on the
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RFCs.
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6. Translate back into a string. (Generator)
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HTML Purifier is best suited for documents that require a rich array of
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HTML tags. Things like blog comments are, in all likelihood, most appropriately
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written in an extremely restrictive set of markup that doesn't require
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all this functionality (or not written in HTML at all), although this may
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be changing in the future with the addition of levels of filtering.
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vim: et sw=4 sts=4
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