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159 lines
6.3 KiB
Plaintext
159 lines
6.3 KiB
Plaintext
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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 seeks to detail the inner workings of HTML Purifier. 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:
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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).
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The rest of this document is pending moving into their associated classes.
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== STAGE 4 - check attributes ==
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STATUS: F (currently implementing core/i18n)
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While we're doing all this nesting hocus-pocus, attributes are also being
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checked. The reason why we need this to be done with the nesting stuff
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is if a REQUIRED attribute is not there, we might need to kill the tag (or
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replace it with data). Fortunantely, this is rare enough that we only have
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to worry about it for certain things:
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* ! bdo - dir > replace with span, preserve attributes
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* ! img - src, alt > if only alt is missing, insert filename, else remove img
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* basefont - size
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* param - name
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* applet - width, height
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* map - id
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* area - alt
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* form - action
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* optgroup - label
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* textarea - rows, cols
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As you can see, only two of them we would remotely consider for our simplified
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tag set. But each has a different set of challenges. For the img tag, we'd
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have to be careful about deleting it. If we do hit a snag, we can supply
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a default "blank" image.
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So after that's all said and done, each of the different types of content
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inside the attributes needs to be handled differently.
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ContentType(s) [RFC2045]
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Charset(s) [RFC2045]
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LanguageCode [RFC3066] (NMTOKEN)
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Character [XML][2.2] (a single character)
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Number /^\d+$/
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LinkTypes [HTML][6.12] <space>
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MediaDesc [HTML][6.13] <comma>
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URI/UriList [RFC2396] <space>
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Datetime (ISO date format)
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Script ...
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StyleSheet [CSS] (complex)
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Text CDATA
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FrameTarget NMTOKEN
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Length (pixel, percentage) (?:px suffix allowed?)
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MultiLength (pixel, percentage, or relative)
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Pixels (integer)
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// map attributes omitted
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ImgAlign (top|middle|bottom|left|right)
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Color #NNNNNN, #NNN or color name (translate it
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Black = #000000 Green = #008000
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Silver = #C0C0C0 Lime = #00FF00
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Gray = #808080 Olive = #808000
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White = #FFFFFF Yellow = #FFFF00
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Maroon = #800000 Navy = #000080
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Red = #FF0000 Blue = #0000FF
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Purple = #800080 Teal = #008080
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Fuchsia= #FF00FF Aqua = #00FFFF
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// plus some directly in the spec
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Everything else is either ID, or defined as a certain set of values.
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Unless we use reflection (which then we have to make sure the attribute exists),
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we probably want to have a function like...
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validate($type, $value) where $type is like ContentType or Number
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and then pass it to a switch.
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The final problem is CSS. Get intimate with the syntax here:
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http://www.w3.org/TR/CSS21/syndata.html and also note the "bad" CSS elements
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that HTML_Safe defines to help determine a whitelist.
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----
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<!ENTITY % coreattrs
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"id ID #IMPLIED
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class CDATA #IMPLIED
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style %StyleSheet; #IMPLIED
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title %Text; #IMPLIED"
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>
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<!ENTITY % i18n
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"lang %LanguageCode; #IMPLIED
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xml:lang %LanguageCode; #IMPLIED
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dir (ltr|rtl) #IMPLIED"
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>
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<!ENTITY % attrs "%coreattrs; %i18n;">
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----
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These are the elements that only have %attrs:
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ul, dl, dt, dd, address, span, em, strong, dfn, code, samp, kbd, var,
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cite, abbr, acronym, sub, sup, tt, i, b, big, small, u, s, strike
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These are the elements that only have %attrs and need an alignment transform
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div, p, h1, h2, h3, h4, h5, h6
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----
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Prepend style transformations, as CSS takes precedence.
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