man HTML::Defang (3): Cleans HTML as well as CSS of scripting and other executable contents, and neutralises XSS attacks.

SYNOPSIS

my $InputHtml = "<html><body></body></html>";
my $Defang = HTML::Defang->new(
context => $Self,
fix_mismatched_tags => 1,
tags_to_callback => [ br embed img ],
tags_callback => \&DefangTagsCallback,
url_callback => \&DefangUrlCallback,
css_callback => \&DefangCssCallback,
attribs_to_callback => [ qw(border src) ],
attribs_callback => \&DefangAttribsCallback
);
my $SanitizedHtml = $Defang->defang($InputHtml);
# Callback for custom handling specific HTML tags
sub DefangTagsCallback {
my ($Self, $Defang, $OpenAngle, $lcTag, $IsEndTag, $AttributeHash, $CloseAngle, $HtmlR, $OutR) = @_;
# Explicitly defang this tag, eventhough safe
return DEFANG_ALWAYS if $lcTag eq 'br';
# Explicitly whitelist this tag, eventhough unsafe
return DEFANG_NONE if $lcTag eq 'embed';
# I am not sure what to do with this tag, so process as HTML::Defang normally would
return DEFANG_DEFAULT if $lcTag eq 'img';
}
# Callback for custom handling URLs in HTML attributes as well as style tag/attribute declarations
sub DefangUrlCallback {
my ($Self, $Defang, $lcTag, $lcAttrKey, $AttrValR, $AttributeHash, $HtmlR) = @_;
# Explicitly allow this URL in tag attributes or stylesheets
return DEFANG_NONE if $$AttrValR =~ /safesite.com/i;
# Explicitly defang this URL in tag attributes or stylesheets
return DEFANG_ALWAYS if $$AttrValR =~ /evilsite.com/i;
}
# Callback for custom handling style tags/attributes
sub DefangCssCallback {
my ($Self, $Defang, $Selectors, $SelectorRules, $Tag, $IsAttr) = @_;
my $i = 0;
foreach (@$Selectors) {
my $SelectorRule = $$SelectorRules[$i];
foreach my $KeyValueRules (@$SelectorRule) {
foreach my $KeyValueRule (@$KeyValueRules) {
my ($Key, $Value) = @$KeyValueRule;
# Comment out any '!important' directive
$$KeyValueRule[2] = DEFANG_ALWAYS if $Value =~ '!important';
# Comment out any 'position=fixed;' declaration
$$KeyValueRule[2] = DEFANG_ALWAYS if $Key =~ 'position' && $Value =~ 'fixed';
}
}
$i++;
}
}
# Callback for custom handling HTML tag attributes
sub DefangAttribsCallback {
my ($Self, $Defang, $lcTag, $lcAttrKey, $AttrValR, $HtmlR) = @_;
# Change all 'border' attribute values to zero.
$$AttrValR = '0' if $lcAttrKey eq 'border';
# Defang all 'src' attributes
return DEFANG_ALWAYS if $lcAttrKey eq 'src';
return DEFANG_NONE;
}

DESCRIPTION

This module accepts an input HTML and/or CSS string and removes any executable code including scripting, embedded objects, applets, etc., and neutralises any XSS attacks. A whitelist based approach is used which means only HTML known to be safe is allowed through.

HTML::Defang uses a custom html tag parser. The parser has been designed and tested to work with nasty real world html and to try and emulate as close as possible what browsers actually do with strange looking constructs. The test suite has been built based on examples from a range of sources such as http://ha.ckers.org/xss.html and http://imfo.ru/csstest/css_hacks/import.php to ensure that as many as possible XSS attack scenarios have been dealt with.

HTML::Defang can make callbacks to client code when it encounters the following:

When a specified tag is parsed
When a specified attribute is parsed
When a URL is parsed as part of an HTML attribute, or CSS property value.
When style data is parsed, as part of an HTML style attribute, or as part of an HTML <style> tag.

The callbacks include details about the current tag/attribute that is being parsed, and also gives a scalar reference to the input HTML. Querying pos() on the input HTML should indicate where the module is with parsing. This gives the client code flexibility in working with HTML::Defang.

HTML::Defang can defang whole tags, any attribute in a tag, any URL that appear as an attribute or style property, or any CSS declaration in a declaration block in a style rule. This helps to precisely block the most specific unwanted elements in the contents(for example, block just an offending attribute instead of the whole tag), while retaining any safe HTML/CSS.

CONSTRUCTOR

HTML::Defang->new(%Options)

Constructs a new HTML::Defang object. The following options are supported:

Options

tags_to_callback: Array reference of tags for which a call back should be made. If a tag in this array is parsed, the subroutine tags_callback() is invoked.
attribs_to_callback: Array reference of tag attributes for which a call back should be made. If an attribute in this array is parsed, the subroutine attribs_callback() is invoked.
tags_callback: Subroutine reference to be invoked when a tag listed in @$tags_to_callback is parsed.
attribs_callback: Subroutine reference to be invoked when an attribute listed in @$attribs_to_callback is parsed.
url_callback: Subroutine reference to be invoked when a URL is detected in an HTML tag attribute or a CSS property.
css_callback: Subroutine reference to be invoked when CSS data is found either as the contents of a 'style' attribute in an HTML tag, or as the contents of a <style> HTML tag.
fix_mismatched_tags: This property, if set, fixes mismatched tags in the HTML input. By default, tags present in the default %mismatched_tags_to_fix hash are fixed. This set of tags can be overridden by passing in an array reference $mismatched_tags_to_fix to the constructor. Any opened tags in the set are automatically closed if no corresponding closing tag is found. If an unbalanced closing tag is found, that is commented out.
mismatched_tags_to_fix: Array reference of tags for which the code would check for matching opening and closing tags. See the property $fix_mismatched_tags.
context: You can pass an arbitrary scalar as a 'context' value that's then passed as the first parameter to all callback functions. Most commonly this is something like '$Self'
allow_double_defang: If this is true, then tag names and attribute names which already begin with the defang string (``defang_'' by default) will have an additional copy of the defang string prepended if they are flagged to be defanged by the return value of a callback, or if the tag or attribute name is unknown.
The default is to assume that tag names and attribute names beginning with the defang string are already made safe, and need no further modification, even if they are flagged to be defanged by the return value of a callback. Any tag or attribute modifications made directly by a callback are still performed.
Debug: If set, prints debugging output.

CALLBACK METHODS

COMMON PARAMETERS

A number of the callbacks share the same parameters. These common parameters are documented here. Certain variables may have specific meanings in certain callbacks, so be sure to check the documentation for that method first before referring this section.

$context

You can pass an arbitrary scalar as a 'context' value that's then passed as the first parameter to all callback functions. Most commonly this is something like '$Self'

$Defang

Current HTML::Defang instance

$OpenAngle

Opening angle(<) sign of the current tag.

$lcTag

Lower case version of the HTML tag that is currently being parsed.

$IsEndTag

Has the value '/' if the current tag is a closing tag.

$AttributeHash

A reference to a hash containing the attributes of the current tag and their values. Each value is a scalar reference to the value, rather than just a scalar value. You can add attributes (remember to make it a scalar ref, eg $AttributeHash{``newattr''} = \``newval''), delete attributes, or modify attribute values in this hash, and any changes you make will be incorporated into the output HTML stream.

The attribute values will have any entity references decoded before being passed to you, and any unsafe values we be re-encoded back into the HTML stream.

So for instance, the tag:

  <div title="&lt;&quot;Hi there &#x003C;">

Will have the attribute hash:

  { title => \q[<"Hi there <] }

And will be turned back into the HTML on output:

  <div title="&lt;&quot;Hi there &lt;">

$CloseAngle

Anything after the end of last attribute including the closing HTML angle(>)

$HtmlR

A scalar reference to the input HTML. The input HTML is parsed using m/\G$SomeRegex/c constructs, so to continue from where HTML:Defang left, clients can use m/\G$SomeRegex/c for further processing on the input. This will resume parsing from where HTML::Defang left. One can also use the pos() function to determine where HTML::Defang left off. This combined with the add_to_output() method should give reasonable flexibility for the client to process the input.

$OutR

A scalar reference to the processed output HTML so far.

tags_callback($context, $Defang, $OpenAngle, $lcTag, $IsEndTag, $AttributeHash, $CloseAngle, $HtmlR, $OutR)

If $Defang->{tags_callback} exists, and HTML::Defang has parsed a tag preset in $Defang->{tags_to_callback}, the above callback is made to the client code. The return value of this method determines whether the tag is defanged or not. More details below.

Return values

DEFANG_NONE: The current tag will not be defanged.
DEFANG_ALWAYS: The current tag will be defanged.
DEFANG_DEFAULT: The current tag will be processed normally by HTML:Defang as if there was no callback method specified.

attribs_callback($context, $Defang, $lcTag, $lcAttrKey, $AttrVal, $HtmlR, $OutR)

If $Defang->{attribs_callback} exists, and HTML::Defang has parsed an attribute present in $Defang->{attribs_to_callback}, the above callback is made to the client code. The return value of this method determines whether the attribute is defanged or not. More details below.

Method parameters

$lcAttrKey: Lower case version of the HTML attribute that is currently being parsed.
$AttrVal: Reference to the HTML attribute value that is currently being parsed.
See $AttributeHash for details of decoding.

Return values

DEFANG_NONE: The current attribute will not be defanged.
DEFANG_ALWAYS: The current attribute will be defanged.
DEFANG_DEFAULT: The current attribute will be processed normally by HTML:Defang as if there was no callback method specified.

url_callback($context, $Defang, $lcTag, $lcAttrKey, $AttrVal, $AttributeHash, $HtmlR, $OutR)

If $Defang->{url_callback} exists, and HTML::Defang has parsed a URL, the above callback is made to the client code. The return value of this method determines whether the attribute containing the URL is defanged or not. URL callbacks can be made from <style> tags as well style attributes, in which case the particular style declaration will be commented out. More details below.

Method parameters

$lcAttrKey: Lower case version of the HTML attribute that is currently being parsed. However if this callback is made as a result of parsing a URL in a style attribute, $lcAttrKey will be set to the string style, or will be set to undef if this callback is made as a result of parsing a URL inside a style tag.
$AttrVal: Reference to the URL value that is currently being parsed.
$AttributeHash: A reference to a hash containing the attributes of the current tag and their values. Each value is a scalar reference to the value, rather than just a scalar value. You can add attributes (remember to make it a scalar ref, eg $AttributeHash{``newattr''} = \``newval''), delete attributes, or modify attribute values in this hash, and any changes you make will be incorporated into the output HTML stream. Will be set to undef if the callback is made due to URL in a <style> tag or attribute.

Return values

DEFANG_NONE: The current URL will not be defanged.
DEFANG_ALWAYS: The current URL will be defanged.
DEFANG_DEFAULT: The current URL will be processed normally by HTML:Defang as if there was no callback method specified.

css_callback($context, $Defang, $Selectors, $SelectorRules, $lcTag, $IsAttr, $OutR)

If $Defang->{css_callback} exists, and HTML::Defang has parsed a <style> tag or style attribtue, the above callback is made to the client code. The return value of this method determines whether a particular declaration in the style rules is defanged or not. More details below.

Method parameters

$Selectors

Reference to an array containing the selectors in a style tag or attribute.

$SelectorRules

Reference to an array containing the style declaration blocks of all selectors in a style tag or attribute. Consider the below CSS:

  a { b:c; d:e}
  j { k:l; m:n}

The declaration blocks will get parsed into the following data structure:

  [
    [
      [ "b", "c", DEFANG_DEFAULT ],
      [ "d", "e", DEFANG_DEFAULT ]
    ],
    [
      [ "k", "l", DEFANG_DEFAULT ],
      [ "m", "n", DEFANG_DEFAULT ]
    ]
  ]

So, generally each property:value pair in a declaration is parsed into an array of the form

  ["property", "value", X]

where X can be DEFANG_NONE, DEFANG_ALWAYS or DEFANG_DEFAULT, and DEFANG_DEFAULT the default value. A client can manipulate this value to instruct HTML::Defang to defang this property:value pair.

DEFANG_NONE - Do not defang

DEFANG_ALWAYS - Defang the style:property value

DEFANG_DEFAULT - Process this as if there is no callback specified

$IsAttr

True if the currently processed item is a style attribute. False if the currently processed item is a style tag.

METHODS

PUBLIC METHODS

defang($InputHtml)

Cleans up $InputHtml of any executable code including scripting, embedded objects, applets, etc., and defang any XSS attacks.

Method parameters

$InputHtml: The input HTML string that needs to be sanitized.

: Returns the cleaned HTML. If fix_mismatched_tags is set, any tags that appear in @$mismatched_tags_to_fix that are unbalanced are automatically commented or closed.

add_to_output($String)

Appends $String to the output after the current parsed tag ends. Can be used by client code in callback methods to add HTML text to the processed output. If the HTML text needs to be defanged, client code can safely call HTML::Defang->defang() recursively from within the callback.

Method parameters

$String: The string that is added after the current parsed tag ends.

INTERNAL METHODS

Generally these methods never need to be called by users of the class, because they'll be called internally as the appropriate tags are encountered, but they may be useful for some users in some cases.

defang_script($OutR, $HtmlR, $TagOps, $OpenAngle, $IsEndTag, $Tag, $TagTrail, $Attributes, $CloseAngle)

This method is invoked when a <script> tag is parsed. Defangs the <script> opening tag, and any closing tag. Any scripting content is also commented out, so browsers don't display them.

Returns 1 to indicate that the <script> tag must be defanged.

Method parameters

$OutR

A reference to the processed output HTML before the tag that is currently being parsed.

$HtmlR

A scalar reference to the input HTML.

$TagOps

Indicates what operation should be done on a tag. Can be undefined, integer or code reference. Undefined indicates an unknown tag to HTML::Defang, 1 indicates a known safe tag, 0 indicates a known unsafe tag, and a code reference indicates a subroutine that should be called to parse the current tag. For example, <style> and <script> tags are parsed by dedicated subroutines.

$OpenAngle

Opening angle(<) sign of the current tag.

$IsEndTag

Has the value '/' if the current tag is a closing tag.

$Tag

The HTML tag that is currently being parsed.

$TagTrail

Any space after the tag, but before attributes.

$Attributes

A reference to an array of the attributes and their values, including any surrouding spaces. Each element of the array is added by 'push' calls like below.

  push @$Attributes, [ $AttributeName, $SpaceBeforeEquals, $EqualsAndSubsequentSpace, $QuoteChar, $AttributeValue, $QuoteChar, $SpaceAfterAtributeValue ];

$CloseAngle

Anything after the end of last attribute including the closing HTML angle(>)

defang_style($OutR, $HtmlR, $TagOps, $OpenAngle, $IsEndTag, $Tag, $TagTrail, $Attributes, $CloseAngle, $IsAttr)

Builds a list of selectors and declarations from HTML style tags as well as style attributes in HTML tags and calls defang_stylerule() to do the actual defanging.

Returns 0 to indicate that style tags must not be defanged.

Method parameters

$IsAttr: Whether we are currently parsing a style attribute or style tag. $IsAttr will be true if we are currently parsing a style attribute.

: For a description of other parameters, see documentation of defang_script() method

cleanup_style($StyleString)

Helper function to clean up CSS data. This function directly operates on the input string without taking a copy.

Method parameters

$StyleString: The input style string that is cleaned.

defang_stylerule($SelectorsIn, $StyleRules, $lcTag, $IsAttr, $HtmlR, $OutR)

Defangs style data.

Method parameters

$SelectorsIn: An array reference to the selectors in the style tag/attribute contents.
$StyleRules: An array reference to the declaration blocks in the style tag/attribute contents.
$lcTag: Lower case version of the HTML tag that is currently being parsed.
$IsAttr: Whether we are currently parsing a style attribute or style tag. $IsAttr will be true if we are currently parsing a style attribute.
$HtmlR: A scalar reference to the input HTML.
$OutR: A scalar reference to the processed output so far.

defang_attributes($OutR, $HtmlR, $TagOps, $OpenAngle, $IsEndTag, $Tag, $TagTrail, $Attributes, $CloseAngle)

Defangs attributes, defangs tags, does tag, attrib, css and url callbacks.

Method parameters: For a description of the method parameters, see documentation of defang_script() method

cleanup_attribute($AttributeString)

Helper function to cleanup attributes

Method parameters

$AttributeString: The value of the attribute.

AUTHOR

Kurian Jose Aerthail <[email protected]>. Thanks to Rob Mueller <[email protected]> for initial code, guidance and support and bug fixes.

COPYRIGHT AND LICENSE

This library is free software; you can redistribute it and/or modify it under the same terms as Perl itself.

SYNOPSIS

DESCRIPTION

CONSTRUCTOR

CALLBACK METHODS

METHODS

AUTHOR

COPYRIGHT AND LICENSE

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