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13 <meta name="keywords" content="HTMLPurifier, HTML Purifier, HTML, filter, filtering, HTML_Safe, PEAR, comparison, kses, striptags, SafeHTMLChecker" />
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24 <p>
26 the end user has gone from passive consumer to active producer of content
30 </p>
32 <p>
33 Give the user too much control, however, and you set yourself up for <a
36 proven to be both a blessing and a curse, and the software that
37 processes it must strike a fine balance between security and usability.
38 How do we prevent users from injecting JavaScript or inserting malformed
43 come across this problem before, and many have tried (albeit
44 unsuccessfully) to solve this problem. We will analyze existing
45 libraries to demonstrate how they are ineffective and, of course, how
47 standards-compliance.
48 </p>
50 <p>
51 I will take no quarter and pull no punches: as of the time of writing,
53 for richly formatted documents. But, nonetheless, there is a necessary
54 disclaimer:
55 </p>
58 <p>
59 This comparison document was written by the author of HTML Purifier,
61 mean that it is biased: I have made every attempt to be <strong>factual and
62 fair</strong>, and I hope that you will agree, by the time you finish reading
64 filter out there today.
65 </p>
66 </div>
77 <thead>
78 <tr>
90 </tr>
91 </thead>
93 <tbody>
95 <tr>
107 </tr>
109 <tr>
121 </tr>
123 <tr>
135 </tr>
137 <tr>
149 </tr>
151 <tr>
163 </tr>
165 <tr>
177 </tr>
179 <tr>
191 </tr>
193 </tbody>
195 </table>
196 </div>
198 <p>
201 </p>
207 A clever person solves a problem.
208 A wise person avoids it.
209 </div>
211 </blockquote>
213 <p>
214 Before we jump into the weird and not-so-wonderful world of
218 number one method of taking user input and processing it into something
220 define their own markup syntax. <a
225 such markup languages (although it should be noted that Wikitext and
227 those who use it, anyway) are clear: simplicity and security.
228 </p>
231 <thead>
232 <tr>
235 </tr>
236 </thead>
237 <tbody>
238 <tr>
241 </tr>
242 <tr>
245 </tr>
246 <tr>
249 </tr>
250 <tr>
253 </tr>
254 <tr>
256 <td><tt><b>B</b> <i>i</i> <a href="http://www.example.com/">link</a></tt></td>
257 </tr>
258 <tr>
261 </tr>
262 </tbody>
263 </table>
266 <li>
267 Wikitext shown is modeled after <a
269 There are many variants of Wikitext currently extant.
270 </li>
271 <li>
272 Strictly speaking, the Markdown syntax is not equivalent: bold text
275 however, map those two semantic tags to the associated styling, so
276 many users assume that it really is italics (and use it improperly for,
277 say, book titles.)
278 </li>
279 </ol>
283 <p>
285 read. For example, compare:
286 </p>
288 <pre>
289 * Item 1
290 * Item 2
291 </pre>
295 <pre>
300 </pre>
302 <p>
303 Which would you prefer to edit? The answer seems obvious, but be careful
304 not to fall into the fallacy of <a
307 text) editor, which blows earlier choices out of the water in terms of
308 usability.
309 </p>
311 <p>
312 Note that rich text editors and alternate markup syntaxes are not
313 mutually exclusive, but, when push comes to shove, it's easier
315 markup language. And in the cases when it is done, you usually end up with
316 a live preview, not a true rich text editor.
317 </p>
320 <p>
322 editors aren't all that great.</q> There are many good arguments against
323 these editors, and <a
325 people have written essays</a> devoted to criticizing
327 said editors, the web poses another limitation: no JavaScript means no
328 editor, and no editor means... (gasp) manually typing in code.
329 </p>
330 <p>
331 Even the most dogmatic purist, however, should recognize that for all
333 There are steps you can take to mitigate the associated drawbacks of
334 these editors.
335 </p>
336 <p>
339 this is the case with any markup language that allows the smallest
342 eliminate the dialogue boxes that allow users to change colors or fonts
344 scheme, allowing users to select contextually correct formatting styles
345 for segments of text.
346 </p>
347 </blockquote>
349 <p>
350 Simplicity is also a double-edged sword. The moment any remotely
351 complex markup is needed, these lightweight markup languages fail to
352 produce. Sure you can make '''this text bold''' with Wikitext, but that
356 restrictive (besides the fact that their table markup is extraordinarily
357 complex).
358 </p>
362 <p>
365 the angled brackets with square brackets and omitting the occasional parameter
366 name? How much more un-original can you get? Somehow, I don't think BBCode
367 was meant to readable. <a
369 </p>
371 <blockquote>
372 BBCode was devised and put to use in order to provide a safer, easier
373 and more limited way of allowing users to format their messages.
375 which could be used to break/imitate parts of the layout, or run
376 JavaScript. Some implementations of BBCode have suffered problems related
378 security that was intended to be given by BBCode.
379 </blockquote>
383 <blockquote>
384 BBCode came to life when developers where too
386 and decided to invent their own markup language. As with all products of
387 laziness, the result is completely inconsistent, unstandardized, and
388 widely adopted.
389 </blockquote>
391 <p>
392 Well, developers, the whole point of HTML Purifier is that I do the
393 work so you can just execute the ridiculously simple
396 </p>
400 <p>
401 These alternative markup languages have their shiny points, and HTML
402 Purifier is not meant to replace them. However, a major reason for
404 using these languages?
405 </p>
409 <p>
410 Dave Raggett's
414 The premise is simple, the execution effective. Tidy is, in short, a great
416 </p>
418 <p>
419 It is not, however, a filter. I am often surprised when people ask
422 </p>
431 </blockquote>
433 <p>
440 </p>
442 <p>
443 This is not to say that I haven't seen Tidy be used in this sort of
445 output before shuttling it off to the browser. The developers, nevertheless,
446 agree that this is only a band-aid solution, and that the real way
447 to fix it is to fix the parser. Tidy's great, but in terms of security,
448 it's not suitable for untrusted sources.
449 </p>
453 <p>
454 Although <a href="http://www.owasp.org/index.php/Category:OWASP_AntiSamy_Project">OWASP AntiSamy</a> is implemented in Java and .NET, it is
455 worth a quick mention here because it purports to do the same thing
456 as HTML Purifier. The bottom line? It gets pretty close, but
457 it just doesn't have the same depth as HTML Purifier.
458 </p>
460 <p>
461 Architecturally speaking, OWASP AntiSamy is highly dependent on
464 the actual code for filtering is relatively light-weight. AntiSamy
467 </p>
469 <p>
471 control on the validation, the regular expression heavy approach
472 begins showing signs of stress when data-types are complex (e.g.
473 <abbr>URI</abbr>s), and <abbr>XML</abbr> Schema is ill-suited for large-scale <acronym>DOM</acronym> manipulation,
476 abilities, so long as it removes things it doesn't recognize by default
477 (something I find slightly perplexing in its policy files, since some
478 rules indicate things to be removed.)
479 </p>
483 <p>
484 I've ordered my analyses according to how bad a library is. The worst
485 is first, and then we move up the spectrum. I will point out the most
486 flagrant problems with the libraries, but note that I will omit more
489 points through. The ideal solution, however, must do all these things.
490 </p>
492 <p>
493 Note that besides striptags,
495 attacks. None of them (save Safe HTML Checker) fare very well
496 in the standards-compliance department though.
497 </p>
507 </table>
509 <p>
512 the classic solution for attempting to clean up
515 The fact that it doesn't validate attributes at all means that anyone can
517 </p>
519 <p>
520 While this can be bandaided with a series of regular expressions that strip out
522 quirky browser behavior), striptags() is fundamentally flawed and should not be
523 used.
524 </p>
528 <p>
529 Though its title may not imply it,
531 is a souped up version of striptags() with the ability to inspect
532 attributes. (Don't mind the hastily tacked on query escaping function).
533 </p>
546 </table>
548 <p>
549 PHP Input Filter implements an
551 performs very basic checks on whether or not tags and attributes have
552 been defined in the whitelist as well as some
554 the user to define what they'll permit.
555 </p>
557 <p>
558 With absolutely no checking of well-formedness, it is trivially easy
559 to trick the filter into leaving unclosed tags lying around. While to some
561 basic sanity checks like this must be implemented, otherwise a user
562 can mangle a website's layout.
563 </p>
565 <p>
566 More troubles: Woe to
569 layout not to be badly mutilated. To top things off,
570 the filter doesn't even preserve data properly: attributes have all
571 spaces stripped out of them. Stay away, stay away!
572 </p>
576 <p>
579 It should be noted that this is the same library as
581 branding (and a different version number).
582 </p>
595 </table>
597 <p>
598 HTML_Safe's mechanism of action involves parsing
601 validation and filtering as the handlers are called. HTML_Safe does a lot
604 is fundamentally flawed: blacklists.
605 </p>
607 <p>
608 This library maintains arrays of dangerous tags, attributes and
611 intelligently disabled by default in favor of a protocol whitelist.)
612 What this means is that HTML_Safe has no qualms of accepting input
614 the tags in those arrays. Scratch standards-compliance (and that was
615 without even considering proper nesting).
616 </p>
618 <p>
620 In the future, however, one of the infinitely many tags that HTML_Safe lets
621 through might just possibly be given special functionality by browser vendors.
623 solution puts you at a perpetual arms race against crackers who are constantly
624 discovering new and inventive ways to abuse tags and attributes that you
625 didn't blacklist.
626 </p>
630 <p>
635 </p>
648 </table>
650 <p>
651 To be truthful, I didn't do as comprehensive a code survey for kses
652 as I did for some of the other libraries. Out of
653 all the classes I've reviewed so far, kses was definitely the hardest to
654 understand.
655 </p>
657 <p>
658 kses's modus operandi is splitting up html with a monster regexp
660 suffers from the same problems as Input Filter: no well-formedness
661 checks leading to rampant runaway tags (and no standards-compliance).
662 WordPress, the primary user of kses today, had to implement their
663 own custom tag-balancing code to fix this problem: don't use this
664 library without some equivalent!
665 </p>
667 <p>
668 Its whitelist syntax, however, is the most complex of all these libraries,
669 so I'm going to take some time to argue why this particular implementation
670 is bad. The author of this library was thoughtful enough to provide some
671 basic constraint checks on attributes like maxlen and maxval. Now, barring
672 the fact that there simply aren't enough checks, and the fact that they are
673 all lumped together in one function, we now must wonder whether or not
674 the user will go through the trouble of specifying the maximum length
675 of a title attribute.
676 </p>
678 <p>
679 I have my opinions about inherent human laziness, but perhaps WordPress's
680 default filterset is the most telling example:
681 </p>
683 <pre>
684 $allowedposttags = array (
685 /* formatted and trimmed */
686 'hr' => array (
687 'align' => array (),
688 'noshade' => array (),
689 'size' => array (),
690 'width' => array ()
691 )
692 );
693 </pre>
695 <p>
696 Hmm... do I see a blatant lack of attribute constraints? Conclusion:
697 if the user can get away with not doing work, they will! The biggest
699 the whitelist. The whitelist is just as important as the code that uses
701 </p>
705 <p>
706 <a href="http://www.bioinformatics.org/phplabware/internal_utilities/htmLawed/index.php">htmLawed</a>
707 is kses on steroids. After looking at HTML Purifier and deciding that it was
708 too slow for him, Santosh Patnaik went ahead and rewrote the kses engine
709 with more features.
710 </p>
723 </table>
725 <p>
726 htmLawed improves standards-compliance, but it is not fully
727 standards-compliant; there are a number of cases which the author has
728 explicitly stated he will not fix. There are issues with content
731 </p>
733 <p>
737 to turn on htmLawed's security features! This is
739 design</a>. Sane defaults are important, because for every person who
740 does read the documentation, there is
742 one who doesn't (and is mislead by claims that <q>htmLawed is a single-file PHP
743 software that makes input text secure</q>), and is
744 surprised at some behavior.
746 any security restrictions.
747 </p>
749 <p>
750 I also disagree with some of the choices with regards to what elements are
753 but it is certainly not phishing safe. Forms can be
756 </p>
760 <p>
762 HTML Checker</a> is (to my knowledge) the first attempt to make a filter
765 search result must have done something right.
766 </p>
779 </table>
781 <p>
782 Indeed, it is quite a well-written piece of code. It demonstrates
783 knowledge of inline versus block elements, thus almost nearly getting
784 nesting correct (the only exception is an unimplemented omitted SGML
786 </p>
788 <p>
789 Unfortunately, part of the reason why it works so well is that it's
790 extremely restrictive. No styling, no tables, very few attributes.
791 Perfectly appropriate for blog comments, but then again, there's always
792 BBCode. This probably means that Safe HTML Checker has a different
793 goal than HTML Purifier.
794 </p>
796 <p>
798 < sign? The parser will complain with the cryptic message:
800 simple as just switching to a more permissive parser: Safe HTML Checker
801 relies on the fact that the parser will have matched up the tags for
802 them.
803 </p>
818 </table>
820 <p>
821 That table should say it all, but I'll add a few more features:
822 </p>
834 </table>
836 <p>
837 This is not to say that HTML Purifier doesn't have problems of its own.
838 It's big (while the others usually fit in one file, this one requires a huge
840 features.</a> But even with these deficiencies,
841 HTML Purifier is far better than the other libraries.
842 </p>
844 <p>
846 </p>
848 </div>
849 </div>
850 </body>
851 </html>