sanitization_filter.rb

# frozen_string_literal: true

HTML::Pipeline.require_dependency('sanitize', 'SanitizationFilter')

module HTML
  class Pipeline
    # HTML filter with sanization routines and whitelists. This module defines
    # what HTML is allowed in user provided content and fixes up issues with
    # unbalanced tags and whatnot.
    #
    # See the Sanitize docs for more information on the underlying library:
    #
    # https://github.com/rgrove/sanitize/#readme
    #
    # Context options:
    #   :whitelist      - The sanitizer whitelist configuration to use. This
    #                     can be one of the options constants defined in this
    #                     class or a custom sanitize options hash.
    #   :anchor_schemes - The URL schemes to allow in <a href> attributes. The
    #                     default set is provided in the ANCHOR_SCHEMES
    #                     constant in this class. If passed, this overrides any
    #                     schemes specified in the whitelist configuration.
    #
    # This filter does not write additional information to the context.
    class SanitizationFilter < Filter
      LISTS     = Set.new(%w[ul ol].freeze)
      LIST_ITEM = 'li'.freeze

      # List of table child elements. These must be contained by a <table> element
      # or they are not allowed through. Otherwise they can be used to break out
      # of places we're using tables to contain formatted user content (like pull
      # request review comments).
      TABLE_ITEMS = Set.new(%w[tr td th].freeze)
      TABLE = 'table'.freeze
      TABLE_SECTIONS = Set.new(%w[thead tbody tfoot].freeze)

      # These schemes are the only ones allowed in <a href> attributes by default.
      ANCHOR_SCHEMES = ['http', 'https', 'mailto', 'xmpp', :relative, 'github-windows', 'github-mac', 'irc', 'ircs'].freeze

      # The main sanitization whitelist. Only these elements and attributes are
      # allowed through by default.
      WHITELIST = {
        elements: %w[
          h1 h2 h3 h4 h5 h6 h7 h8 br b i strong em a pre code img tt
          div ins del sup sub p ol ul table thead tbody tfoot blockquote
          dl dt dd kbd q samp var hr ruby rt rp li tr td th s strike summary
          details caption figure figcaption
          abbr bdo cite dfn mark small span time wbr
        ].freeze,
        remove_contents: ['script'].freeze,
        attributes: {
          'a'          => ['href'].freeze,
          'img'        => %w[src longdesc].freeze,
          'div'        => %w[itemscope itemtype].freeze,
          'blockquote' => ['cite'].freeze,
          'del'        => ['cite'].freeze,
          'ins'        => ['cite'].freeze,
          'q'          => ['cite'].freeze,
          all: %w[abbr accept accept-charset
                  accesskey action align alt
                  aria-describedby aria-hidden aria-label aria-labelledby
                  axis border cellpadding cellspacing char
                  charoff charset checked
                  clear cols colspan color
                  compact coords datetime dir
                  disabled enctype for frame
                  headers height hreflang
                  hspace ismap label lang
                  maxlength media method
                  multiple name nohref noshade
                  nowrap open prompt readonly rel rev
                  rows rowspan rules scope
                  selected shape size span
                  start summary tabindex target
                  title type usemap valign value
                  vspace width itemprop].freeze
        }.freeze,
        protocols: {
          'a'          => { 'href' => ANCHOR_SCHEMES }.freeze,
          'blockquote' => { 'cite' => ['http', 'https', :relative].freeze },
          'del'        => { 'cite' => ['http', 'https', :relative].freeze },
          'ins'        => { 'cite' => ['http', 'https', :relative].freeze },
          'q'          => { 'cite' => ['http', 'https', :relative].freeze },
          'img'        => {
            'src'      => ['http', 'https', :relative].freeze,
            'longdesc' => ['http', 'https', :relative].freeze
          }.freeze
        },
        transformers: [
          # Top-level <li> elements are removed because they can break out of
          # containing markup.
          lambda { |env|
            name = env[:node_name]
            node = env[:node]
            if name == LIST_ITEM && node.ancestors.none? { |n| LISTS.include?(n.name) }
              node.replace(node.children)
            end
          },

          # Table child elements that are not contained by a <table> are removed.
          lambda { |env|
            name = env[:node_name]
            node = env[:node]
            if (TABLE_SECTIONS.include?(name) || TABLE_ITEMS.include?(name)) && node.ancestors.none? { |n| n.name == TABLE }
              node.replace(node.children)
            end
          }
        ].freeze
      }.freeze

      # A more limited sanitization whitelist. This includes all attributes,
      # protocols, and transformers from WHITELIST but with a more locked down
      # set of allowed elements.
      LIMITED = WHITELIST.merge(
        elements: %w[b i strong em a pre code img ins del sup sub mark abbr p ol ul li]
      )

      # Strip all HTML tags from the document.
      FULL = { elements: [] }.freeze

      # Sanitize markup using the Sanitize library.
      def call
        Sanitize.clean_node!(doc, whitelist)
      end

      # The whitelist to use when sanitizing. This can be passed in the context
      # hash to the filter but defaults to WHITELIST constant value above.
      def whitelist
        whitelist = context[:whitelist] || WHITELIST
        anchor_schemes = context[:anchor_schemes]
        return whitelist unless anchor_schemes
        whitelist = whitelist.dup
        whitelist[:protocols] = (whitelist[:protocols] || {}).dup
        whitelist[:protocols]['a'] = (whitelist[:protocols]['a'] || {}).merge('href' => anchor_schemes)
        whitelist
      end
    end
  end
end