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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:
# 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 =[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 =[tr td th].freeze)
TABLE = 'table'.freeze
TABLE_SECTIONS =[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.
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
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
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
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?( }
# 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| == TABLE }
# A more limited sanitization whitelist. This includes all attributes,
# protocols, and transformers from WHITELIST but with a more locked down
# set of allowed elements.
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)
# 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)