Class: Sanitize::Transformers::CleanElement

Inherits:
Object
  • Object
show all
Defined in:
lib/sanitize/transformers/clean_element.rb

Constant Summary collapse

REGEX_DATA_ATTR =

Matches a valid HTML5 data attribute name. The unicode ranges included here are a conservative subset of the full range of characters that are technically allowed, with the intent of matching the most common characters used in data attribute names while excluding uncommon or potentially misleading characters, or characters with the potential to be normalized into unsafe or confusing forms.

If you need data attr names with characters that aren't included here (such as combining marks, full-width characters, or CJK), please consider creating a custom transformer to validate attributes according to your needs.

http://www.whatwg.org/specs/web-apps/current-work/multipage/elements.html#embedding-custom-non-visible-data-with-the-data-*-attributes

/\Adata-(?!xml)[a-z_][\w.\u00E0-\u00F6\u00F8-\u017F\u01DD-\u02AF-]*\z/u
UNESCAPED_TEXT_ELEMENTS =

Elements whose content is treated as unescaped text by HTML parsers.

Set.new(%w[
  iframe
  noembed
  noframes
  noscript
  plaintext
  script
  style
  xmp
])
UNSAFE_LIBXML_ATTRS_A =

Attributes that need additional escaping on <a> elements due to unsafe libxml2 behavior.

Set.new(%w[
  name
])
UNSAFE_LIBXML_ATTRS_GLOBAL =

Attributes that need additional escaping on all elements due to unsafe libxml2 behavior.

Set.new(%w[
  action
  href
  src
])
UNSAFE_LIBXML_ESCAPE_CHARS =

Mapping of original characters to escape sequences for characters that should be escaped in attributes affected by unsafe libxml2 behavior.

{
  ' ' => '%20',
  '"' => '%22'
}
UNSAFE_LIBXML_ESCAPE_REGEX =

Regex that matches any single character that needs to be escaped in attributes affected by unsafe libxml2 behavior.

/[ "]/

Instance Method Summary collapse

Constructor Details

#initialize(config) ⇒ CleanElement

Returns a new instance of CleanElement.



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# File 'lib/sanitize/transformers/clean_element.rb', line 59

def initialize(config)
  @add_attributes          = config[:add_attributes]
  @attributes              = config[:attributes].dup
  @elements                = config[:elements]
  @protocols               = config[:protocols]
  @remove_all_contents     = false
  @remove_element_contents = Set.new
  @whitespace_elements     = {}

  @attributes.each do |element_name, attrs|
    unless element_name == :all
      @attributes[element_name] = Set.new(attrs).merge(@attributes[:all] || [])
    end
  end

  # Backcompat: if :whitespace_elements is a Set, convert it to a hash.
  if config[:whitespace_elements].is_a?(Set)
    config[:whitespace_elements].each do |element|
      @whitespace_elements[element] = {:before => ' ', :after => ' '}
    end
  else
    @whitespace_elements = config[:whitespace_elements]
  end

  if config[:remove_contents].is_a?(Enumerable)
    @remove_element_contents.merge(config[:remove_contents].map(&:to_s))
  else
    @remove_all_contents = !!config[:remove_contents]
  end
end

Instance Method Details

#call(env) ⇒ Object 



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# File 'lib/sanitize/transformers/clean_element.rb', line 90

def call(env)
  node = env[:node]
  return if node.type != Nokogiri::XML::Node::ELEMENT_NODE || env[:is_allowlisted]

  name = env[:node_name]

  # Delete any element that isn't in the config allowlist, unless the node has
  # already been deleted from the document.
  #
  # It's important that we not try to reparent the children of a node that has
  # already been deleted, since that seems to trigger a memory leak in
  # Nokogiri.
  unless @elements.include?(name) || node.parent.nil?
    # Elements like br, div, p, etc. need to be replaced with whitespace in
    # order to preserve readability.
    if @whitespace_elements.include?(name)
      node.add_previous_sibling(Nokogiri::XML::Text.new(@whitespace_elements[name][:before].to_s, node.document))

      unless node.children.empty?
        node.add_next_sibling(Nokogiri::XML::Text.new(@whitespace_elements[name][:after].to_s, node.document))
      end
    end

    unless node.children.empty?
      unless @remove_all_contents || @remove_element_contents.include?(name)
        node.add_previous_sibling(node.children)
      end
    end

    node.unlink
    return
  end

  attr_allowlist = @attributes[name] || @attributes[:all]

  if attr_allowlist.nil?
    # Delete all attributes from elements with no allowlisted attributes.
    node.attribute_nodes.each {|attr| attr.unlink }
  else
    allow_data_attributes = attr_allowlist.include?(:data)

    # Delete any attribute that isn't allowed on this element.
    node.attribute_nodes.each do |attr|
      attr_name = attr.name.downcase

      unless attr_allowlist.include?(attr_name)
        # The attribute isn't in the allowlist, but may still be allowed if
        # it's a data attribute.

        unless allow_data_attributes && attr_name.start_with?('data-') && attr_name =~ REGEX_DATA_ATTR
          # Either the attribute isn't a data attribute or arbitrary data
          # attributes aren't allowed. Remove the attribute.
          attr.unlink
          next
        end
      end

      # The attribute is allowed.

      # Remove any attributes that use unacceptable protocols.
      if @protocols.include?(name) && @protocols[name].include?(attr_name)
        attr_protocols = @protocols[name][attr_name]

        if attr.value =~ REGEX_PROTOCOL
          unless attr_protocols.include?($1.downcase)
            attr.unlink
            next
          end

        else
          unless attr_protocols.include?(:relative)
            attr.unlink
            next
          end
        end

        # Leading and trailing whitespace around URLs is ignored at parse
        # time. Stripping it here prevents it from being escaped by the
        # libxml2 workaround below.
        attr.value = attr.value.strip
      end

      # libxml2 >= 2.9.2 doesn't escape comments within some attributes, in an
      # attempt to preserve server-side includes. This can result in XSS since
      # an unescaped double quote can allow an attacker to inject a
      # non-allowlisted attribute.
      #
      # Sanitize works around this by implementing its own escaping for
      # affected attributes, some of which can exist on any element and some
      # of which can only exist on `<a>` elements.
      #
      # This fix is technically no longer necessary with Nokogumbo >= 2.0
      # since it no longer uses libxml2's serializer, but it's retained to
      # avoid breaking use cases where people might be sanitizing individual
      # Nokogiri nodes and then serializing them manually without Nokogumbo.
      #
      # The relevant libxml2 code is here:
      # <https://github.com/GNOME/libxml2/commit/960f0e275616cadc29671a218d7fb9b69eb35588>
      if UNSAFE_LIBXML_ATTRS_GLOBAL.include?(attr_name) ||
          (name == 'a' && UNSAFE_LIBXML_ATTRS_A.include?(attr_name))

        attr.value = attr.value.gsub(UNSAFE_LIBXML_ESCAPE_REGEX, UNSAFE_LIBXML_ESCAPE_CHARS)
      end
    end
  end

  # Add required attributes.
  if @add_attributes.include?(name)
    @add_attributes[name].each {|key, val| node[key] = val }
  end

  # Make a best effort to ensure that text nodes in invalid "unescaped text"
  # elements that are inside a math or svg namespace are properly escaped so
  # that they don't get parsed as HTML.
  #
  # Sanitize is explicitly documented as not supporting MathML or SVG, but
  # people sometimes allow `<math>` and `<svg>` elements in their custom
  # configs without realizing that it's not safe. This workaround makes it
  # slightly less unsafe, but you still shouldn't allow `<math>` or `<svg>`
  # because Nokogiri doesn't parse them the same way browsers do and Sanitize
  # can't guarantee that their contents are safe.
  unless node.namespace.nil?
    prefix = node.namespace.prefix

    if (prefix == 'math' || prefix == 'svg') && UNESCAPED_TEXT_ELEMENTS.include?(name)
      node.children.each do |child|
        if child.type == Nokogiri::XML::Node::TEXT_NODE
          child.content = CGI.escapeHTML(child.content)
        end
      end
    end
  end

  # Element-specific special cases.
  case name

  # If this is an allowlisted iframe that has children, remove all its
  # children. The HTML standard says iframes shouldn't have content, but when
  # they do, this content is parsed as text and is serialized verbatim without
  # being escaped, which is unsafe because legacy browsers may still render it
  # and execute `<script>` content. So the safe and correct thing to do is to
  # always remove iframe content.
  when 'iframe'
    if !node.children.empty?
      node.children.each do |child|
        child.unlink
      end
    end

  # Prevent the use of `<meta>` elements that set a charset other than UTF-8,
  # since Sanitize's output is always UTF-8.
  when 'meta'
    if node.has_attribute?('charset') &&
        node['charset'].downcase != 'utf-8'

      node['charset'] = 'utf-8'
    end

    if node.has_attribute?('http-equiv') &&
        node.has_attribute?('content') &&
        node['http-equiv'].downcase == 'content-type' &&
        node['content'].downcase =~ /;\s*charset\s*=\s*(?!utf-8)/

      node['content'] = node['content'].gsub(/;\s*charset\s*=.+\z/, ';charset=utf-8')
    end

  # A `<noscript>` element's content is parsed differently in browsers
  # depending on whether or not scripting is enabled. Since Nokogiri doesn't
  # support scripting, it always parses `<noscript>` elements as if scripting
  # is disabled. This results in edge cases where it's not possible to
  # reliably sanitize the contents of a `<noscript>` element because Nokogiri
  # can't fully replicate the parsing behavior of a scripting-enabled browser.
  # The safest thing to do is to simply remove all `<noscript>` elements.
  when 'noscript'
    node.unlink
  end
end