htmLawed documentation

1.1  Example uses

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*  Filtering of text submitted as comments on blogs to allow only certain HTML elements

*  Making RSS/Atom newsfeed item content standard-compliant: often one uses an excerpt from an HTML document for the content, and with unbalanced tags, non-numerical entities, etc., such excerpts may not be XML-compliant

*  Text processing for stricter XML standard-compliance: e.g., to have lowercased x in hexadecimal numeric entities becomes necessary if an XHTML document with MathML content needs to be served as application/xml

1.2  Features

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Key: * security feature, ^ standard compliance, ~ requires setting right options, ` different from Kses

*  HTML in input may be highly ill-written; htmLawed will make it more secure and standard-compliant
*  output can be used in HTML 4, XHTML 1.0, XHTML 1.1, or even generic XML documents  ^~`

*  options to restrict elements  ^~`
*  proper closure of empty elements like img  ^`
*  deprecated elements like u can be transformed  ^~`
*  HTML comments and CDATA sections can be permitted  ^~`
*  script elements can be permitted  ~

*  options to restrict attributes, including element-specifically  ^~`
*  removal of invalid attributes  ^`
*  element and attribute names are lower-cased  ^
*  provides required attributes, like action for form, when missing  ^`
*  deprecated attributes can be transformed  ^~`
*  attributes declared only once  ^`

*  options to restrict attribute values, including element-specifically  ^~`
*  a value is declared for empty (minimized) attributes like checked  ^
*  attributes with potentially dangerous values (that can cause buffer overflows and denial of service attacks) can be removed after checking their lengths or values  *~
*  unique id attribute values can be ensured  ^~`
*  attribute values are enclosed in double-quotes  ^
*  standard attribute values are lower-cased (like type="password")  ^`

*  attribute-specific URL protocol/scheme restriction  *~`
*  dynamic expressions in style values can be disabled  *~`

*  non-numeric, named character entities not in the HTML standard are neutralized  ^`
*  hexadecimal numeric entities may be made decimal ones, or vice versa  ^~`
*  HTML-specific named character entities can be converted to numeric ones for generic XML use  ^~`

*  removes null characters from input  *
*  neutralizes potentially dangerous proprietary Netscape Javascript entities  *
*  replaces soft-hyphen character (code-point, decimally, 173; a vulnerability in some older versions of the Opera and Mozilla [Firefox] browsers) in attribute values with spaces  *

*  common invalid characters not allowed in HTML or XML are removed  ^`
*  characters from Microsoft applications like Word that are discouraged in HTML or XML can be replaced with good ones  ^~`
*  entities for characters not allowed in and for most characters discouraged in HTML or XML are neutralized  ^`
*  appropriately neutralizes <, &, ", and > characters  ^*`

*  understands improperly spaced tag content (like, spread over more than a line) and properly spaces them  `
*  attempts to balance tags for well-formedness  ^~`
*  attempts to permit only validly nested tags  ^~`
*  option to remove or neutralize bad content ^~`
*  attempts to rectify common errors of plain-text misplacement (e.g., directly inside blockquote) ^~`

*  fast, non-OOP code of ~45 kb incurring peak basal memory usage of ~0.5 MB
*  compatible with pre-exisiting code using Kses (the filter used by WordPress)

*  optional anti-spam measures such as addition of rel="nofollow" and link-disabling  ~`
*  optionally makes relative URLs absolute, and vice versa  ~`

*  optionally mark & to identify the entities for &, < and > introduced by htmLawed  ~`

*  independent of character encoding of input and does not affect it
*  won't change formatting of element content by affecting line-breaks, spaces or tabs outside tags but normalizes white spaces in tag content
*  tolerance for ill-written HTML to a certain degree

1.3  History

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htmLawed was developed for use with LabWiki, a wiki software developed at PHP Labware, as a suitable software could not be found. Existing PHP software like Kses and HTMLPurifier were deemed inadequate, slow or resource-intensive, or dependent on external applications like HTML Tidy.

htmLawed started as a modification of Ulf Harnhammar's Kses (version 0.2.2) sofware, and is compatible with code that uses Kses; see section 2.6.

1.4  License & copyright

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htmLawed is free and open-source software licensed under GPL license version 3, and copyrighted by Santosh Patnaik, MD, PhD.

1.5  Terms used here

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*  administrator - or admin; person setting up the code to pass input through htmLawed; also, user
*  attributes - name-value pairs like href="http://x.com" in opening tags
*  author - writer
*  entity - markup like &gt; and &#160; used to refer to a character
*  element - HTML element like a and img
*  element content -  content between the opening and closing tags of an element, like click of <a href="x">click</a>
*  HTML - implies XHTML unless specified otherwise
*  input - text string given to htmLawed to process
*  processing - involves filtering, correction, etc., of input
*  safe - absence or reduction of certain characters and HTML elements and attributes in the input that can otherwise potentially and circumstantially expose web-site users to security vulnerabilities like cross-site scripting attacks (XSS)
*  scheme - URL protocol like http and ftp
*  specs - standard specifications
*  style property - terms like border and height for which declarations are made in values for the style attribute of elements
*  tag - markers like <a href="x"> and </a> delineating element content; the opening tag can contain attributes
*  tag content - consists of tag markers < and >, element names like div, and possibly attributes
*  user - administrator
*  writer - end-user like a blog commenter providing the input that is to be processed; also, author

2.1  Simple

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The input text to be processed, $text, is passed as an argument of type string; htmLawed() returns the processed string:

    $processed = htmLawed($text);

Note: If input is from a $_GET or $_POST value, and magic quotes are enabled on the PHP setup, run stripslashes() on yhe input before passing to htmLawed.

By default, htmLawed will process the text allowing all valid HTML elements/tags, secure URL scheme/CSS style properties, etc. It will allow CDATA sections and HTML comments, balance tags, and ensure proper nesting of elements. Such actions can be configured using two other optional arguments -- $config and $spec:

    $processed = htmLawed($text, $config, $spec);

These extra parameters are detailed below. Some examples are shown in section 2.9.

Note: For maximum protection against XSS and other scripting attacks (e.g., by disallowing Javascript code), consider using the safe parameter; see section 3.6.

2.2  Configuring htmLawed using the $config parameter

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$config instructs htmLawed on how to tackle certain tasks. When $config is not specified, or not set as an array (e.g., $config = 1), htmLawed will take default actions. One or many of the task-action/value-specification pairs can be specified in $config as array key-value pairs (when a pair is not specified, htmLawed will take the default action for that task):

    $config = array('comment'=>0, 'cdata'=>1);
    $processed = htmLawed($text, $config);

Or,

    $processed = htmLawed($text, array('comment'=>0, 'cdata'=>1));

Below are the possible task-action / value-specification combinations. In PHP code, values that are integers should not be quoted and should be used as numeric types (unless meant as string/text).

Key: * default, ^ different default when htmLawed is used in the Kses-compatible mode (see section 2.6), ~ different default when valid_xhtml is set to 1 (see section 3.5), " different default when safe is set to 1 (see section 3.6)

abs_url
Make URLs absolute or relative; $config["base_url"] needs to be set; see section 3.4.4

-1 - make relative
0 - no action  *
1 - make absolute

and_mark
Mark & characters in the original input; see section 3.2

anti_link_spam
Anti-spam; see section 3.4.7

0 - no measure taken  *
array("regex1", "regex2") - will ensure a rel attribute with nofollow in its value in case the href attribute value matches the regular expression pattern regex1, and/or will remove href if its value matches the regular expression pattern regex2. E.g., array("/./", "/://\W*(?!(abc\.com|xyz\.org))/"). This is a parameter for advanced usage. See section 3.4.7 for more.

anti_mail_spam
Anti-spam; see section 3.4.7

0 - no measure taken  *
word - @ in mail address in href attribute value is replaced with word -- a word of admin's choice, like NOSPAM@ and AT.

balance
Balance tags for well-formedness and proper nesting; see section 3.3.3

0 - no
1 - yes  *

base_url
Base URL value that needs to be set if $config["abs_url"] is not 0; see section 3.4.4

cdata
Handling of CDATA sections; see section 3.3.1

0 - don't consider CDATA sections as markup and proceed as if plain text  ^"
1 - remove
2 - allow, but neutralize any <, >, and & inside by converting them to named entities
3 - allow  *

clean_ms_char
Replace discouraged characters introduced by Microsoft Word, etc.; see section 3.1

0 - no  *
1 - yes
2 - yes, but replace special single & double quotes with ordinary ones

comment
Handling of HTML comments; see section 3.3.1

0 - don't consider comments as markup and proceed as if plain text  ^"
1 - remove
2 - allow, but neutralize any <, >, and & inside by converting to named entities
3 - allow  *

css_expression
Allow dynamic CSS expression by not removing the expression from CSS property values in style attributes; see section 3.4.7

0 - remove  *
1 - allow  ^

deny_attribute
Denied HTML attributes; see section 3.4

0 - none  *
string - dictated by values in string
on* (like onfocus) attributes not allowed - "

elements
Allowed HTML elements; see section 3.3

* -center -dir -font -isindex -menu -s -strike -u -  ~
applet, embed, iframe, object, script not allowed - "

hexdec_entity
Allow hexadecimal numeric entities and do not convert to the more widely accepted decimal ones, or convert decimal to hexadecimal ones; see section 3.2

0 - no
1 - yes  *
2 - convert decimal to hexadecimal ones

hook
Name of an optional hook function to pre-process input string, and optionally $config or $htm, before htmLawed starts its main work; see section 3.7

0 - no hook function  *
name - name is name of the hook function (kses_hook  ^)

keep_bad
Neutralize bad tags by converting < and > to entities, or remove them; see section 3.3.3

0 - remove  ^
1 - neutralize both tags and element content
2 - remove tags but neutralize element content
3 and 4 - like 1 and 2 but remove if text (pcdata) is invalid in parent element
5 and 6 * -  like 3 and 4 but line-breaks, tabs and spaces are left

lc_std_val
For XHTML compliance, predefined, standard attribute values, like get for the method attribute of form, must be lowercased; see section 3.4.5

0 - no  ^
1 - yes  *

make_tag_strict
Transform/remove these non-strict XHTML elements, even if they are allowed by the admin: applet center dir embed font isindex menu s strike u; see section 3.3.2

0 - no  ^
1 - yes, but leave applet, embed and isindex elements that currently can't be transformed  *
2 - yes, removing applet, embed and isindex elements and their contents (nested elements remain)  ~

named_entity
Allow non-universal named HTML entities, or convert to numeric ones; see section 3.2

0 - convert
1 - allow  *

no_deprecated_attr
Allow deprecated attributes or transform them; see section 3.4.6

0 - allow  ^
1 - transform, but name attributes for a and map are retained  *
2 - transform

parent
Name of the parent element, possibly imagined, that will hold the input; see section 3.3

safe
Magic parameter to make input the most secure against XSS without needing to specify other relevant $config parameters; see section 3.6

0 - no  *
1 - will auto-adjust other relevant $config parameters (indicated by " in this list)

schemes
Array of attribute-specific, comma-separated, lower-cased list of schemes (protocols) allowed in attributes accepting URLs; * covers all unspecified attributes; see section 3.4.3

href: aim, feed, file, ftp, gopher, http, https, irc, mailto, news, nntp, sftp, ssh, telnet; *:file, http, https  *
*: ftp, gopher, http, https, mailto, news, nntp, telnet  ^
href: aim, feed, file, ftp, gopher, http, https, irc, mailto, news, nntp, sftp, ssh, telnet; style: nil; *:file, http, https  "

show_setting
Name of a variable to assign the finalized $config and $spec values; see section 3.8

unique_ids
ID attribute value checks; see section 3.4.2

0 - no  ^
1 - remove duplicate and/or invalid ones  *
word - remove invalid ones and replace duplicate ones with new and unique ones based on the word; the admin-specified word, like my_, should begin with a letter (a-z) and can contain letters, digits, ., _, -, and :.

valid_xhtml
Magic parameter to make input the most valid XHTML without needing to specify other relevant $config parameters; see section 3.5

0 - no  *
1 - will auto-adjust other relevant $config parameters (indicated by ~ in this list)

xml:lang
Auto-adding xml:lang attribute; see section 3.4.1

0 - no  *
1 - add if lang attribute is present
2 - add if lang attribute is present, and remove lang  ~

2.3  Extra HTML specifications using the $spec parameter

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The $spec argument should be used to not allow an otherwise legal attribute for an element, or to restrict the attribute's values. $spec is specified as a string of text containing one or more rules, with multiple rules separated from each other by a semi-colon (;). E.g.,

    $spec = 'i=-*; td, tr=style, id, -*; a=id(match="/[a-z][a-z\d.:\-`"]*/i"/minval=2), href(maxlen=100/minlen=34); img=-width,-alt';
    $processed = htmLawed($text, $config, $spec);

Or,

    $processed = htmLawed($text, $config, 'i=-*; td, tr=style, id, -*; a=id(match="/[a-z][a-z\d.:\-`"]*/i"/minval=2), href(maxlen=100/minlen=34); img=-width,-alt');

A rule begins with an HTML element name(s) (rule-element), for which the rule applies, followed by an equal (=) sign. A rule-element may represent multiple elements if comma (,)-separated element names are used. E.g., th,td,tr=.

Rest of the rule consists of comma-separated HTML attribute names. A minus (-) character before an attribute means that the attribute is not permitted inside the rule-element. E.g., -width. To deny all atributes, -* can be used.

   Following shows examples of rule excerpts with rule-element a and the attributes that are being permitted:

   *  a= - all
   *  a=id - all
   *  a=href, title, -id, -onclick - all except id and onclick
   *  a=*, id, -id - all except id
   *  a=-* - none
   *  a=-*, href, title - none except href and title
   *  a=-*, -id, href, title - none except href and title

Rules regarding attribute values are optionally specified inside round brackets after attribute names in slash ('/')-separated parameter = value pairs. E.g., title(maxlen=30/minlen=5). None, or one or more of the following parameters may be specified:

*  oneof - one or more choices separated by | that the value should match; if only one choice is provided, then the value must match that choice

*  noneof - one or more choices separated by | that the value should not match

*  maxlen and minlen - upper and lower limits for the number of characters in the attribute value; specified in numbers

*  maxval and minval - upper and lower limits for the numerical value specified in the attribute value; specified in numbers

*  match and nomatch - pattern that the attribute value should or should not match; specified as PHP/PCRE-compatible regular expressions with delimiters and possibly modifiers

*  default - a value to force on the attribute if the value provided by the writer does not fit any of the specified parameters

If default is not set and the attribute value does not satisfy any of the specified parameters, then the attribute is removed. The default value can also be used to force all attribute declarations to take the same value (by getting the values declared illegal by setting, e.g., maxlen to -1).

Examples with input <input title="WIDTH" value="10em" /><input title="length" value="5" />:

   Rule: input=title(maxlen=60/minlen=6), value
   Output: <input value="10em" /><input title="length" value="5" />

   Rule: input=title(), value(maxval=8/default=6)
   Output: <input title="WIDTH" value="6" /><input title="length" value="5" />

   Rule: input=title(nomatch=$w.d$i), value(match=$em$/default=6em)
   Output: <input value="10em" /><input title="length" value="6em" />

   Rule: input=title(oneof=height|depth/default=depth), value(noneof=5|6)
   Output: <input title="depth" value="10em" /><input title="depth" />

Special characters: The characters ;, ,, /, (, ), |, ~ and space have special meanings in the rules. Words in the rules that use such characters, or the characters themselves, should be flanked by double-quotes ("). A back-tick (`) can be used to escape a literal " inside such words. An example rule illustrating this is input=value(maxlen=30/match="/^\w/i"/default="your `"ID`"").

Note: To deny an attribute for all elements for which it is legal, $config["deny_attribute"] can be used instead of $spec.

2.4  Performance time and memory usage

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As expected, the time and memory used by htmLawed depends on the size of the input and on the number of elements in it. These are also increased by certain $config values. In particular, balancing ($config["balance"] = 1) can increase the processing time by a third or so.

One can use the page for testing to evaluate performance and the effects of different types of input and $config.

2.5  Some security risks to keep in mind

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When setting the parameters/arguments (like those to allow certain HTML elements) for use with htmLawed, potentially dangerous code may get through. This may not be a problem if the authors are trusted.

For example, following increase security risks:

*  Allowing script, applet, embed, iframe or object elements, or certain of their attributes like allowscriptaccess

*  Allowing HTML comments (some Internet Explorer versions are vulnerable with, e.g., <!--[if gte IE 4]><script>alert("xss");</script><![endif]-->

Unsafe HTML can be removed by setting $config appropriately. E.g., $config["elements"] = "* -script" (section 3.3), $config["safe"] = 1 (section 3.6), etc.

2.6  Using without modifying old kses() code

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The Kses PHP script is used by many applications (like WordPress). It is possible to have such applications use htmLawed instead since it is compatible with code that calls the kses() function declared in the Kses file (usually named kses.php). E.g., application code like this will continue to work after replacing Kses with htmLawed:

    $comment_filtered = kses($comment_input, array('a'=>array(), 'b'=>array(), 'i'=>array()));

For some of the $config parameters, htmLawed will use values other than the default ones. These are indicated by ^ in section 2.2. To force htmLawed to use other values, function kses() in the htmLawed code should be edited -- a few configurable parameters/variables need to be changed.

If the application uses a Kses file that has the kses() function declared, then, to have the application use htmLawed instead of Kses, simply rename htmLawed.php (to kses.php, e.g.) and replace the Kses file (or just replace the code in the Kses file with the htmLawed code). If the kses() function in the Kses file had been renamed by the application developer (e.g., in WordPress, it is named wp_kses()), then appropriately rename the kses() function in the htmLawed code.

If the Kses file used by the application has been highly altered by the application developers, then one may need a different approach. E.g., with WordPress, it is best to copy the htmLawed code to wp_includes/kses.php, rename the newly added function kses() to wp_kses(), and delete the code for the original wp_kses() function.

If the Kses code has a non-empty hook function (e.g., wp_kses_hook() in case of WordPress), then the code for htmLawed's kses_hook() function should be appropriately edited. However, the requirement of the hook function should be re-evaluated considering that htmLawed has extra capabilities. With WordPress, the hook function is an essential one. The following code is suggested for the htmLawed kses_hook() in case of WordPress:

    function kses_hook($string, &$cf, &$spec){
    // kses compatibility
    $allowed_html = $spec;
    $allowed_protocols = array();
    foreach($cf['schemes'] as $v){
     foreach($v as $k2=>$v2){
      if(!in_array($k2, $allowed_protocols)){
       $allowed_protocols[] = $k2;
      }
     }
    }
    return wp_kses_hook($string, $allowed_html, $allowed_protocols);
    // eof
    }

2.7  Tolerance for ill-written HTML

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htmLawed can work with ill-written HTML code in the input. However, ill-written HTML may not be read as HTML and be considered mere plain text by htmLawed. Following indicate the degree of looseness that htmLawed can work with, and can be provided in instructions to writers:

*  Tags must be flanked by < and > with no > inside -- any needed > should be put in as &gt; instead. It is possible for tag content (element name and attributes) to be spread over many lines instead of being on one. A space is possible between the tag content and >, like <div > and <img / >, but not after the <.

*  Element and attribute names may not be in lower-case.

*  Attribute string of elements may be liberally spaced with tabs, line-breaks, etc.

*  Attribute values may be un-quoted or single-quoted.

*  Entities must end with ;. Left-padding of numeric entities (like, &#0160;, &x07ff;) with 0 is okay as long as the number of characters between between the & and the ; does not exceed 8.

*  Named character entities must be properly cased. E.g., &Lt; or &TILDE; will not be let through without modification.

*  HTML comments should not be inside element tags (okay between tags), and should begin with <!-- and end with -->. Characters like <, >, and & may be allowed inside depending on $config, but any --> inside should be put in as --&gt;. Any -- inside will be automatically converted to -, and a space will be added before the comment delimiter -->.

*  CDATA sections should not be inside element tags, and can be in element content only if plain text is allowed for that element. They should begin with <[CDATA[ and end with ]]>. Characters like <, >, and & may be allowed inside depending on $config, but any ]]> inside should be put in as ]]&gt;.

*  For attribute values, character entities &lt;, &gt; and &amp; should be used instead of characters < and >, and & (when & is not part of a character entity). This applies even for Javascript code in values of attributes like onclick.

*  Characters <, >, & and " that are part of actual Javascript, etc., code in script elements should be used as such and not be put in as entities like &gt;. Otherwise, though the HTML will be valid, the code may fail to work. Further, if such characters have to be used, then they should be put inside CDATA sections.

*  Simple instructions like "an opening tag cannot be present between two closing tags" and "nested elements should be closed in the reverse order of how they were opened" can help authors write balanced HTML. If tags are imbalanced, htmLawed will try to balance them, but in the process, depending on $config["keep_bad"], some code may be lost.

*  Input authors should be notified of admin-specified allowed elements, attributes, configuration values (like conversion of named entities to numeric ones), etc.

*  With $config["unique_ids"] not 0 and the id attribute being permitted, writers should carefully avoid using duplicate or invalid id values as even though htmLawed will correct/remove the values, the final output may not be the one desired. E.g., when <a id="home"></a><input id="home" /><label for="home"></label> is processed into
<a id="home"></a><input id="prefix_home" /><label for="home"></label>.

*  Note that even if intended HTML is lost in a highly ill-written input, the processed output will be more secure and standard-compliant.

*  For URLs, unless $config["scheme"] is appropriately set, writers should avoid using escape characters or entities in schemes. E.g., htt&#112; (which many browsers will read as the harmless http) may be considered bad by htmLawed.

* htmLawed will attempt to put plain text present directly inside blockquote, form, map and noscript elements (illegal as per the specs) inside auto-generated div elements.

2.8  Limitations & work-arounds

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htmLawed's main objective is to make the input text more HTML-standard compliant and secure for web visitors, and free of HTML elements and attributes considered undesirable by the administrator. Some of its limitations, regardless of this objective, are noted below. Future versions might address some of them.

*  htmLawed is meant for input that goes into the body of HTML documents. HTML's head-level elements are not supported, nor are the frameset elements frameset, frame and noframes.

*  htmLawed doesn't beautify HTML code text by formatting it with indentations, etc. It does, however, properly white-space tag content (remving line-breaks, tabs, etc.).

*  It cannot transform the non-standard embed elements to the standard-compliant object elements. Yet, it can allow embed elements if permitted (embed is widely used and supported).

*  The only non-standard element that may be permitted is embed; others like noembed and nobr cannot be permitted without modifying the htmLawed code.

*  It cannot handle input that has non-HTML code like SVG and MathML. One way around is to break the input into pieces and passing only those without non-HTML code to htmLawed. Another is described in section 3.9. A third way may be to some how take advantage of the $config["and_mark"] parameter (see section 3.2).

*  By default, htmLawed won't check many attribute values for standard compliance. E.g., width="20m" with the dimension in non-standard m is let through. Implementing universal and strict attribute value checks can make htmLawed slow and resource-intensive. Admins can partially implement such features using $spec.

*  The attributes, deprecated (which can be transformed too) or not, that it supports are largely those that are in the specs. Only very few of the proprietary attributes are supported.

*  Except for contained URLs and dynamic expressions (also optional), htmLawed does not check CSS style property values. Again, this keeps htmLawed fast. Admins can partially implement this feature using $spec. Perhaps the best option is to disallow style but allow class attributes with the right oneof or match values for class, and have the various class style properties in .css CSS stylesheet files.

*  htmLawed does not parse emoticons, decode BBcode, or wikify, auto-converting text to proper HTML. Similarly, it won't convert line-breaks to br elements. Such functions are beyond its purview. Admins should use other code to pre- or post-process the input for such purposes.

*  htmLawed cannot be used to have links force-opened in new windows (by auto-adding appropriate target and onclick attributes to a). Admins should look at Javascript-based DOM-modifying solutions for this.

*  Nesting-based checks are not possible. E.g., one cannot disallow p elements specifically inside td while permitting it elsewhere.

*  Except for optionally converting absolute or relative URLs to the other type, htmLawed will not alter URLs (e.g., to change the value of query strings or to convert http to https. Having absolute URLs may be a standard-requirement, e.g., when HTML is embedded in email messages, whereas altering URLs for other purposes is beyond htmLawed's goals.

*  Pairs of opening and closing tags that do not enclose any content (like <em></em>) are not removed. This may be against the standard specs for certain elements, e.g., in the case of <table></table>.

*  htmLawed does not check for certain element orderings described in the standard specs (e.g., in a table, tbody is allowed before tfoot).

*  htmLawed does not check the number of nested elements. E.g., it will allow two caption in table, illegal as per the specs.

*  htmLawed does not filter the \r\n character sequence (or its various encodings) from attribute values to prevent against HTTP response splitting (HRS) attacks (this vulnerability can arise when input data is used to generate HTTP headers). If input data is being used for generating HTTP headers, then, post-htmLawed processing, admins should look for and neutralize such sequences before the headers are created.

*  htmLawed does not correct certain possible attribute-based security vulnerabilities (e.g., <a href="http://x%22+style=%22background-image:xss">x</a>). Theses arise when browsers mis-identify markup in escaped text, defeating the very purpose of escaping text (a bad browser will read the given example as <a href="http://x" style="background-image:xss">x</a>).

*  htmLawed does not remove all invalid characters. Those that are not removed are extremely unlikely to be in the input. Simple preg_replace code can be used to remove them if you want to (see section 3.1).

2.9  Examples

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A blog administrator wants to allow only a, em, strike, strong and u in comments, but needs strike and u transformed to span for better XHTML 1-strict compliance. Further, he wants the a links to be to http or https resources.

    $processed = htmLawed($in, array('elements'=>'a, em, strike, strong, u', 'make_tag_strict'=>1, 'safe'=>1, 'schemes'=>'*:http, https'), 'a=href');

An author uses a custom-made web application to load content on his web-site. He is the only one using that application and the content he generates has all types of HTML, including scripts. The web application uses htmLawed primarily as a tool to correct errors that creep in while writing HTML and to take care of the occasional bad characters in copy-paste text introduced by Microsoft Office. The web application provides a preview before submitted input is added to the content. For the previewing process, htmLawed is set up as follows:

    $processed = htmLawed($in, array('css_expression'=>1, 'keep_bad'=>1, 'make_tag_strict'=>1, 'schemes'=>'*:*', 'valid_xhtml'=>1));

For the final submission process, keep_bad is set to 6. A value of 1 for the preview process allows the author to note and correct any HTML mistake without losing any of the typed text.

3.1  Invalid/dangerous characters

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Valid characters (more correctly, their code-points) in HTML or XML are, hexadecimally, 9, a, d, 20 to d7ff, and e000 to 10ffff (except fffe and ffff), or decimally, 9, 10, 13, 32 to 55295, and 57344 to 1114111 (except 65534 and 65535). htmLawed removes the invalid characters 0 to 8, b, c, and e to 1f. It does not check for the remaining invalid characters; for various reasons, it is extremely unlikely for any of those characters to be in the input. Simple code like the following can be run on the htmLawed output should you want to get rid of any of those remaining characters:

    $out = preg_replace('`[\xd800-\xdfff\xfffe-\xffff\x110000-]`', '', $out);

Characters that are discouraged (see section 5.1) but not invalid are not removed by htmLawed.

It (function hl_tag()) also replaces the potentially dangerous (in some Mozilla [Firefox] and Opera browsers) soft-hyphen character (code-point, hexadecimally, ad, or decimally, 173) in attribute values with spaces. Where required, the characters <, >, &, and " are converted to entities.

With $config["clean_ms_char"] set as 1 or 2, many of the discouraged characters (code-points, decimally, 127 to 159 except 133) that many Microsoft applications incorrectly use (often as as per the Windows 1252 or Cp-1252 encoding system), and the character for code-point, decimally, 133, are converted to appropriate decimal numerical entities (or removed for a few cases)-- see appendix in section 5.4. This can help avoid some display issues arising from copying-pasting of content.

With $config["clean_ms_char"] set as 2, characters for code-points, hexadecimally, 82, 91, and 92 (for special single-quotes), and 84, 93, and 94 (for special double-quotes) are converted to ordinary single and double quotes respectively and not to entities.

The character values are replaced with entities/characters and not character values referred to by the entities/characters to keep this task independent of the character-encoding of input text.

The $config["clean_ms_char"] parameter need not be used if authors do not copy-paste Microsoft-created text or if the input text is not believed to use the Windows 1252 or Cp-1252 encoding. Further, the input form and the web-pages displaying it or its content should have the character encoding appropriately marked-up.

3.2  Character references/entities

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Valid character entities take the form &*; where * is #x followed by a hexadecimal number (hexadecimal numeric entity; like &#xA0; for non-breaking space), or alphanumeric like gt (external or named entity; like &nbsp; for non-breaking space), or # followed by a number (decimal numeric entity; like &#160; for non-breaking space). Character entities referring to the soft-hyphen character (code-point, hexadecimally, ad) in attribute values are always replaced with spaces.

htmLawed (function hl_ent()):

*  Neutralizes entities with multiple leading zeroes or missing semi-colons (potentially dangerous)

*  Lowercases the X (XML compliance) and A-F of hexadecimal numeric entities

*  Neutralizes all entities referring to characters that are invalid (see section 3.1)

*  Neutralizes entities referring to characters that are discouraged (code-points, hexadecimally, 7f to 84, 86 to 9f, and fdd0 to fddf, or decimally, 127 to 132, 134 to 159, and 64991 to 64976). Entities referring to the remaining discouraged characters (see section 5.1 for a full list) are let through.

*  Neutralizes named entities that are not in HTML specs.

*  Optionally converts valid HTML-specific named entities except &gt;, &lt;, &quot;, and &amp; to decimal numeric ones (but hexadecimal if $config["hexdec_entity"] is 2) for generic XML compliance. For this, $config["named_entity"] should be 1.

*  Optionally converts hexadecimal numeric entities to the more widely supported decimal ones. For this, $config["hexdec_entity"] should be 0.

*  Optionally converts decimal numeric entities to the hexadecimal ones. For this, $config["hexdec_entity"] should be 2.

Neutralization involves entitification & with &amp;.

Note: Although htmLawed does not convert entities to the actual characters represented by them, you can easily do so by running the htmLawed output through PHP's html_entity_decode function.

Note: If $config["and_mark"] is set, and set to a value other than 0, then the & characters in the original input are replaced with the control character for the code-point, hexadecimally, 6 (\x06; & characters introduced by htmLawed, e.g., after converting < to &lt;, are not affected). This allows one to distinguish, say, an &gt; introduced by htmLawed and an &gt; put in by the input writer, and can be helpful in further processing of the htmLawed-processed text (e.g., to identify the character sequence o(><)o to generate an emoticon image). When this feature is active, admins should ensure that the htmLawed output is not directly used in web pages or XML documents as the presence of the \x06 can break documents. Before use in such documents, and preferably before any storage, any remaining \x06 should be changed back to &, e.g., with:

    $final = str_replace("\x06", '&', $prelim);

Also, see section 3.9.

3.3  HTML elements

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htmLawed can be configured to allow only certain HTML elements (tags) in the input. Un-permitted elements (just tag-content, and not element-content), based on $config["keep_bad"], are either neutralized (converted to plain text by entitification of < and >) or removed.

E.g., with only em permitted:

  Input:

      <em>My</em> website is <a href="http://a.com>a.com</a>.

  Output, with $config["keep_bad"] = 0:

      <em>My</em> website is a.com.

  Example output, with $config["keep_bad"] not 0:

      <em>My</em> website is &lt;a href=""&gt;a.com&lt;/a&gt;.

See section 3.3.3 for differences between the various non-zero $config["keep_bad"] values.

htmLawed by default permits these 86 elements:

    a, abbr, acronym, address, applet, area, b, bdo, big, blockquote, br, button, caption, center, cite, code, col, colgroup, dd, del, dfn, dir, div, dl, dt, em, embed, fieldset, font, form, h1, h2, h3, h4, h5, h6, hr, i, iframe, img, input, ins, isindex, kbd, label, legend, li, map, menu, noscript, object, ol, optgroup, option, p, param, pre, q, rb, rbc, rp, rt, rtc, ruby, s, samp, script, select, small, span, strike, strong, sub, sup, table, tbody, td, textarea, tfoot, th, thead, tr, tt, u, ul, var

Except for embed (included because of its wide-spread use) and the Ruby elements (rb, rbc, rp, rt, rtc, ruby; part of XHTML 1.1), these are all the elements in the HTML 4/XHTML 1 specs. Strict-specific specs. exclude center, dir, font, isindex, menu, s, strike, and u.

With $config["safe"] = 1, the default set will exclude applet, embed, iframe, object and script; see section 3.6.

When $config["elements"], which specifies allowed elements, is properly defined, and neither empty nor set to 0 or *, the default set is not used. To have elements added to or removed from the default set, a +/- notation is used. E.g., *-script-object implies that only script and object are disallowed, whereas *+embed means that noembed is also allowed. Elements can also be specified as comma separated names. E.g., a, b, i means only a, b and i are permitted. In this notation, *, + and - have no significance and can actually cause a mis-reading.

Some more xample $config["elements"] values indicating permitted elements (note that empty spaces are liberally allowed for clarity):

*  a, blockquote, code, em, strong -- only a, blockquote, code, em, and strong
*  *-script -- all excluding script
*  * -center -dir -font -isindex -menu -s -strike -u -- only XHTML-Strict elements
*  *+noembed-script -- all including noembed excluding script

Some mis-usages (and the resulting permitted elements) that can be avoided:

*  -* -- none; one should rather use, e.g., the htmlspecialchars() PHP function on the input than htmLawed
*  *, -script -- all except script; admin probably meant *-script
*  -*, a, em, strong -- all; admin probably meant a, em, strong
*  * -- all; admin need not have set elements
*  *-form+form -- all; a + will always over-ride any -
*  *, noembed -- only noembed; admin probably meant *+noembed
*  a, +b, i -- only a and i; admin probably meant a, b, i

Basically, when using the +/- notation, do not use commas (,), and vice versa, and use * with the former but not with the latter.

Note: Even if an element that is not in the default set is allowed through $config["elements"], like noembed in the last example, it will eventually be removed during tag balancing unless such balancing is turned off ($config["balance"] set to 0). Currently, the only way around this, which actually is simple, is to edit the various arrays in the function hl_bal() to accommodate the element and its nesting properties.

A possibly second way to specify allowed elements is to set $config["parent"] to an element name that supposedly will hold the input, and to set $config["balance"] to 1. During tag balancing (see section 3.3.3), all elements that cannot legally nest inside the parent element will be removed. The parent element is auto-reset to div if $config["parent"] is empty, body, or an element not in htmLawed's default set of 86 elements.

Tag transformation is possible for improving XHTML-Strict compliance -- most of the deprecated elements are removed or converted to valid XHTML-Strict ones; see section 3.3.2.

3.4  Attributes

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htmLawed will only permit attributes in the HTML specs (including deprecated ones). It also permits some attributes for use with the embed element (the non-standard embed element is supported in htmLawed because of its widespread use), and the the xml:space attribute (valid only in XHTML 1.1). List of these 111 attributes and elements they are allowed in is in section 5.2.

When $config["deny_attribute"] is not set, or set to 0, or empty (""), all the 111 attributes are permitted. Otherwise, $config["deny_attribute"] can be set as a list of comma-separated names of the denied attributes. on* can be used to refer to the group of potentially dangerous, script-accepting attributes: onblur, onchange, onclick, ondblclick, onfocus, onkeydown, onkeypress, onkeyup, onmousedown, onmousemove, onmouseout, onmouseover, onmouseup, onreset, onselect and onsubmit.

With $config["safe"] = 1, the on* attributes are disallowed.

htmLawed (function hl_tag()) also:

*  Lower-cases attribute names
*  Removes duplicate attributes (last one stays)
*  Gives attributes the form name="value" and single-spaces them, removing unnecessary white space characters
*  Provides required attributes (see section 3.4.1)
*  Double-quotes values and escapes any " inside
*  Removes unnecessary white-spaces
*  Replaces possibly dangerous soft-hyphens (#xad) in the values with spaces

3.5  Simple configuration directive for most valid XHTML

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If $config["valid_xhtml"] is set to 1, some relevant $config parameters (indicated by ~ in section 2.2) are auto-adjusted. This allows one to pass the $config argument with a simpler value. If a value for a parameter auto-set through valid_xhtml is still manually provided, then that value will over-ride the auto-set value. E.g., for the unique_ids parameter.

3.6  Simple configuration directive for most safe HTML

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Safe HTML refers to HTML that is restricted to reduce the vulnerability for scripting attacks (such as XSS) based on HTML code which otherwise may be legal and compliant with the HTML standard specs. When elements such as script and object, and attributes such as onmouseover and style are allowed in the input text, an input writer can introduce HTML code for such attacks. Note that what is considered safe depends on the nature of the web application and the trust-level accorded to its users.

htmLawed allows an admin to use $config["safe"] to auto-adjust multiple $config parameters (such as elements which declares the allowed element-set), which otherwise would have to be manually set. The relevant parameters are indicated by " in section 2.2). Thus, one can pass the $config argument with a simpler value.

With the value of 1, htmLawed considers CDATA sections and comments as plain text, and disallows the applet, embed, iframe, object and script elements, and the on* attributes like onclick. Further URLs with schemes (see section 3.4.3) are neutralized so that, e.g., style="moz-binding:url(http://danger)" becomes style="moz-binding:url(denied:http://danger)" while style="moz-binding:url(ok)" remains intact.

Admins, however, may still want to completely deny the style attribute, e.g., with code like

    $processed = htmLawed($text, array('safe'=>1, 'deny_attribute'=>'on*, style'));

There are $config parameters like css_expression that are not affected by the value set for safe but whose default values still contribute towards a more safe output.

If a value for a parameter auto-set through safe is still manually provided, then that value can over-ride the auto-set value. E.g., with $config["safe"] = 1 and $config["elements"] = "*+script", script, but not applet, is allowed.

A page illustrating the efficacy of htmLawed's anti-XSS abilities with safe set to 1 against XSS vectors listed by RSnake may be available here.

3.7  Using a hook function

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If $config["hook"] is not set to 0, then htmLawed will allow preliminarily processed input to be altered by a hook function (name set in $config["hook"]) before starting the main work (but after handling of characters, entities, HTML comments and CDATA sections -- see code for function htmLawed()).

The hook function also allows one to alter the finalized values of $config and $spec.

3.8  Obtaining finalized parameter values

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htmLawed can be made to assign the finalized $config and $spec values to a variable named in $config["show_setting"]. The variable, made global by htmLawed, is set as an array with three keys: config, with the $config value, spec, with the $spec value, and time, with a value that is the Unix time (the output of PHP's microtime() function) at which the value was assigned. Admins should use a PHP-compliant variable name (e.g., one that does not begin with a numerical digit) that does not conflict with variable names in their non-htmLawed code.

The values, which are also post-hook function (if any), can be used to auto-generate information (on, e.g., the elements that are permitted) for input writers.

3.9  Retaining non-HTML tags in input with mixed markup

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htmLawed does not remove certain characters that though invalid are nevertheless discouraged in HTML documents as per the specs (see section 5.1). This can be utilized to deal with input that contains mixed markup. Input that may have HTML markup as well as some other markup that is based on the <, > and & characters is considered to have mixed markup. The non-HTML markup can be rather proprietary (like markup for emoticons/smileys), or standard (like MathML or SVG). Or it can be programming code meant for execution/evaluation (such as embedded PHP code).

To deal with such mixed markup, the input text can be pre-processed to hide the non-HTML markup by specifically replacing the <, > and & characters with some of the discouraged characters. Post-htmLawed processing, the replacements are reverted.

An example (HTML + PHP):

    $text = preg_replace('`<\?php(.+?)\?>`sm', "\x83?php\\1?\x84", $text);
    $processed = htmLawed($text);
    $processed = preg_replace('`\x83\?php(.+?)\?\x84`sm', '<?php$1?>', $processed);

Admins may also be able to use $config["and_mark"] to deal with mixed markup; see section 3.2.

4.1  Support

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A careful reading of this documentation may answer many questions.

Software updates and forum-based community-support may be found at http://www.bioinformatics.org/phplabware/internal_utilities/htmLawed. For general PHP issues (not htmLawed-specific), support may be found through internet searches and at http://php.net.

4.2  Known issues

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See section 2.8.

Readers are advised to cross-check information given in this document.

4.3  Change-log

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v1.0.9 - 11 June 2008. Fixed bug in invalid HTML code-point entity check

v1.0.8 - 15 May 2008. bordercolor attribute for table, td and tr

v1.0.7 - 1 May 2008. wmode attribute for embed; $config["show_setting"] introduced; improved elements evaluation

v1.0.6 - 20 April 2008. $config["and_mark"] introduced

v1.0.5 - 12 March 2008. style URL schemes essentially disallowed when $config safe is on; improved regex for CSS expression search

v1.0.4 - 10 March 2008. Improved corrections for blockquote, form, map and noscript

v1.0.3 - 3 March 2008. Improved documentation; character entities for soft-hyphens are now replaced with spaces (instead of being removed); a bug allowing td directly inside table fixed; safe $config parameter added

v1.0.2 - 13 February 2008. Improved implementation for $config["keep_bad"]

v1.0.1 - 7 November 2007. Improved regex for identifying URLs, protocols and dynamic expressions (hl_tag() and hl_prot()); no error display with hl_regex()

v1.0 - 2 November 2007. First release

4.4  Testing

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To test htmLawed using a form-based interface, a demo web-page is provided (htmLawed.php and htmLawedTest.php should be in the same directory on the web-server). Input can be typed in or copy-pasted. A file with test-cases is provided with the htmLawed distribution.

4.5  Upgrade, & old versions

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Upgrading is as simple as replacing the previous version of htmLawed.php (assuming it was not modified for customized features). As htmLawed output is almost always used in static documents, upgrading should not affect old, finalized content.

Old versions of htmLawed may be available online. E.g., for version 1.0, check http://www.bioinformatics.org/phplabware/downloads/htmLawed1.zip, for 1.1.1, ...111.zip, and for 1.1.10, ...1110.zip.

4.6  Comparison with HTMLPurifier

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The HTMLPurifier PHP library by Edward Yang (http://htmlpurifier.org) is an excellent, OOP (object-oriented programming)-based, filtering script. Compared to htmLawed, it:

*  does not support PHP versions older than 5.0 (HTMLPurifier dropped PHP 4 support after releasing version 2.1; currently it is at version 3.0)

*  is atleast 15 times bigger (scores of files totalling more than 750 kb)

*  consumes 10-15 times more RAM memory

*  is expectedly many times slower

*  does not allow admins to fully allow all valid HTML (e.g., form or script elements are always considered illegal)

*  lacks some of the extra features of htmLawed (like entity conversions).

However, HTMLPurifier has finer checks for character encodings and attribute values, can log warnings and errors, and does not use the less commercialization-friendly GPL license.

4.7  Using through application plug-ins/modules

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Plug-ins/modules to implement htmLawed filtering in applications such as Drupal and DokuWiki may have been developed. Please check the application websites and the forum on the htmLawed site.

4.8  Using in non-PHP applications

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Non-PHP applications written in Python, Ruby, etc., may be able to use htmLawed through system calls to the PHP engine. Such code may have been documented on the internet. Also check the forum on the htmLawed site.

4.9  Donate

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A donation in any currency and amount to appreciate or support this software can be sent by PayPal to this email address: drpatnaik at yahoo dot com.

4.10  Acknowledgements

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Ulf Harnhammer, Lukasz Pilorz, Edward Yang, and many anonymous users.

Thank you!

5.1  Characters discouraged in XHTML

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Characters represented by these hexadecimal code-points are not invalid, even though some validators may issue messages stating otherwise.

7f to 84, 86 to 9f, fdd0 to fddf, 1fffe, 1ffff, 2fffe, 2ffff, 3fffe, 3ffff, 4fffe, 4ffff, 5fffe, 5ffff, 6fffe, 6ffff, 7fffe, 7ffff, 8fffe, 8ffff, 9fffe, 9ffff, afffe, affff, bfffe, bffff, cfffe, cffff, dfffe, dffff, efffe, effff, ffffe, fffff, 10fffe and 10ffff

5.2  Valid attribute-element combinations

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Valid attribute-element combinations as per W3C specs:

*  includes deprecated attributes (marked ^), attributes for the non-standard embed element (marked *), and the proprietary bordercolor (marked ~)
*  only non-frameset, HTML body elements
*  name for a and map, and lang are invalid in XHTML 1.1
*  target is valid for a in XHTML 1.1 and higher
*  xml:space is only for XHTML 1.1

abbr - td, th
accept - form, input
accept-charset - form
accesskey - a, area, button, input, label, legend, textarea
action - form
align - caption^, embed, applet, iframe, img^, input^, object^, legend^, table^, hr^, div^, h1^, h2^, h3^, h4^, h5^, h6^, p^, col, colgroup, tbody, td, tfoot, th, thead, tr
alt - applet, area, img, input
archive - applet, object
axis - td, th
bgcolor - embed, table^, tr^, td^, th^
border - table, img^, object^
bordercolor~ - table, td, tr
cellpadding - table
cellspacing - table
char - col, colgroup, tbody, td, tfoot, th, thead, tr
charoff - col, colgroup, tbody, td, tfoot, th, thead, tr
charset - a, script
checked - input
cite - blockquote, q, del, ins
classid - object
clear - br^
code - applet
codebase - object, applet
codetype - object
color - font
cols - textarea
colspan - td, th
compact - dir, dl^, menu, ol^, ul^
coords - area, a
data - object
datetime - del, ins
declare - object
defer - script
dir - bdo
disabled - button, input, optgroup, option, select, textarea
enctype - form
face - font
for - label
frame - table
frameborder - iframe
headers - td, th
height - embed, iframe, td^, th^, img, object, applet
href - a, area
hreflang - a
hspace - applet, img^, object^
ismap - img, input
label - option, optgroup
language - script^
longdesc - img, iframe
marginheight - iframe
marginwidth - iframe
maxlength - input
method - form
model* - embed
multiple - select
name - button, embed, textarea, applet^, select, form^, iframe^, img^, a^, input, object, map^, param
nohref - area
noshade - hr^
nowrap - td^, th^
object - applet
onblur - a, area, button, input, label, select, textarea
onchange - input, select, textarea
onfocus - a, area, button, input, label, select, textarea
onreset - form
onselect - input, textarea
onsubmit - form
pluginspage* - embed
pluginurl* - embed
prompt - isindex
readonly - textarea, input
rel - a
rev - a
rows - textarea
rowspan - td, th
rules - table
scope - td, th
scrolling - iframe
selected - option
shape - area, a
size - hr^, font, input, select
span - col, colgroup
src - embed, script, input, iframe, img
standby - object
start - ol^
summary - table
tabindex - a, area, button, input, object, select, textarea
target - a^, area, form
type - a, embed, object, param, script, input, li^, ol^, ul^, button
usemap - img, input, object
valign - col, colgroup, tbody, td, tfoot, th, thead, tr
value - input, option, param, button, li^
valuetype - param
vspace - applet, img^, object^
width - embed, hr^, iframe, img, object, table, td^, th^, applet, col, colgroup, pre^
wmode - embed
xml:space - pre, script, style

These are allowed in all but the shown elements:

class - param, script
dir - applet, bdo, br, iframe, param, script
id - script
lang - applet, br, iframe, param, script
onclick - applet, bdo, br, font, iframe, isindex, param, script
ondblclick - applet, bdo, br, font, iframe, isindex, param, script
onkeydown - applet, bdo, br, font, iframe, isindex, param, script
onkeypress - applet, bdo, br, font, iframe, isindex, param, script
onkeyup - applet, bdo, br, font, iframe, isindex, param, script
onmousedown - applet, bdo, br, font, iframe, isindex, param, script
onmousemove - applet, bdo, br, font, iframe, isindex, param, script
onmouseout - applet, bdo, br, font, iframe, isindex, param, script
onmouseover - applet, bdo, br, font, iframe, isindex, param, script
onmouseup - applet, bdo, br, font, iframe, isindex, param, script
style - param, script
title - param, script
xml:lang - applet, br, iframe, param, script

5.3  CSS 2.1 properties accepting URLs

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background
background-image
content
cue-after
cue-before
cursor
list-style
list-style-image
play-during

5.4  Microsoft Windows 1252 character replacements

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Key: d double, l left, q quote, r right, s. single

Code-point (decimal) - hexadecimal value - replacement entity - represented character

127 - 7f - (removed)
128 - 80 - &#8364; - euro
129 - 81 - (removed)
130 - 82 - &#8218; - baseline s. q
131 - 83 - &#402; - florin
132 - 84 - &#8222; - baseline d q
133 - 85 - &#8230; - ellipsis
134 - 86 - &#8224; - dagger
135 - 87 - &#8225; - d dagger
136 - 88 - &#710; - circumflex accent
137 - 89 - &#8240; - permile
138 - 8a - &#352; - S Hacek
139 - 8b - &#8249; - l s. guillemet
140 - 8c - &#338; - OE ligature
141 - 8d - (removed)
142 - 8e - &#381; - Z dieresis
143 - 8f - (removed)
144 - 90 - (removed)
145 - 91 - &#8216; - l s. q
146 - 92 - &#8217; - r s. q
147 - 93 - &#8220; - l d q
148 - 94 - &#8221; - r d q
149 - 95 - &#8226; - bullet
150 - 96 - &#8211; - en dash
151 - 97 - &#8212; - em dash
152 - 98 - &#732; - tilde accent
153 - 99 - &#8482; - trademark
154 - 9a - &#353; - s Hacek
155 - 9b - &#8250; - r s. guillemet
156 - 9c - &#339; - oe ligature
157 - 9d - (removed)
158 - 9e - &#382; - z dieresis
159 - 9f - &#376; - Y dieresis

5.5  URL format

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An absolute URL has a protocol or scheme, a network location or hostname, and, optional path, parameters, query and fragment segments. Thus, an absolute URL has this generic structure:

    (scheme) : (//network location) /(path) ;(parameters) ?(?query) #(fragment)

The schemes can only contain letters, digits, +, . and -. Hostname is the portion after the // and up to the first / (if any; else, up to the end) when : is followed by a // (e.g., abc.com in ftp://abc.com/def); otherwise, it consists of everything after the : (e.g., def@abc.com in mailto:def@abc.com').

Relative URLs do not have explicit schemes and network locations; such values are inherited from a base URL.

5.6  Brief on htmLawed code

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Much of the code's logic and reasoning can be understood from the documentation above.

The output of htmLawed is a text string containing the processed input. There is no custom error tracking, etc.

Function arguments for htmLawed are:

*  $in - 1st argument; a text string; the input text to be processed. Any extraneous slashes added by PHP when magic quotes are enabled should be removed beforehand using PHP's stripslashes function.

*  $cf - 2nd argument; an associative array; optional. The array has keys with names like balance and keep_bad, and the values, which can be boolean, string, or array, depending on the key, are read to accordingly set the configurable parameters (indicated by the keys). All configurable parameters receive some default value if the value to be used is not specified by the user through $cf. Finalized $cf is thus a filtered and possibly larger array.

*  $spec - 3rd argument; a text string; optional. The string has rules, writted in an htmLawed-designated format, specifying element-specific attribute and attribute value restrictions. Function hl_spec is used to convert the string to an associative-array for internal use. Finalized $spec is thus an array.

Finalized $cf and $spec are made global variables while htmLawed is at work. Values of any pre-existing global variables with same names are noted, and their valueds are restored after htmLawed finishes processing the input. Depending on $cf, another global variable hl_Ids, to track id attribute values for uniqueness, may be set. Unlike the other two variables, this one is not reset (or unset) post-processing.

Except for the main function htmLawed and the functions kses and kses_hook, htmLawed function names are name-spaced using the hl_ prefix. The functions and their roles are:

*  hl_attrval - checking attribute values against $spec
*  hl_bal - tag balancing
*  hl_cmtcd - handling CDATA sections and HTML comments
*  hl_ent - entity handling
*  hl_prot - checking a URL scheme/protocol
*  hl_regex - checking syntax of a regular expression
*  hl_spec - converting user-supplied $spec value to one used by htmLawed internally
*  hl_tag - handling tags
*  hl_tag2 - transforming tags
*  hl_version - reporting htmLawed version
*  htmLawed - main function
*  kses - main function of kses
*  kses_hook - hook function of kses

The last two are for compatibility with pre-existing code using the kses script. htmLawed's kses() basically passes on the filtering task to htmLawed() function after deciphering $cf and $spec from the argument values supplied to it. kses_hook() is "blank" and is meant for being filled with custom code if the  kses script users were using one.

htmLawed() finalizes $spec (with the help of hl_spec()) and $cf, and globalizes them. Finalization of $cf involves setting default values if an inappropriate or invalid one is supplied. This includes calling hl_regex() to check well-formedness of regular expression patterns if such expressions are user-supplied through $cf. htmLawed() then removes invalid characters like nulls and x01 and appropriately handles entities using hl_ent(). HTML comments and CDATA sections are identified and treated as per $cf values with the help of hl_cmtcd(). When retained, &lt; and &gt; of their markups are replaced with control characters until the end to avoid their being mis-read as tag markup. htmLawed() identifies tags using regex and processes them with the help of hl_tag() --  a large function that analyzes tag content, filtering it as per HTML standards, $config and $spec. Among other things, hl_tag() transforms deprecated elements using hl_tag2(), removes attributes from closing tags, checks attribute values as per $spec rules using hl_attrval(), and checks URL protocols using hl_prot. htmLawed() performs tag balancing and nesting checks at the end with a call to hl_bal().