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Loofah

Status

System Status
Concourse Concourse CI
Code Climate Code Climate
Version Eye Version Eye

Description

Loofah is a general library for manipulating and transforming HTML/XML documents and fragments. It's built on top of Nokogiri and libxml2, so it's fast and has a nice API.

Loofah excels at HTML sanitization (XSS prevention). It includes some nice HTML sanitizers, which are based on HTML5lib's whitelist, so it most likely won't make your codes less secure. (These statements have not been evaluated by Netexperts.)

ActiveRecord extensions for sanitization are available in the loofah-activerecord gem.

Features

  • Easily write custom scrubbers for HTML/XML leveraging the sweetness of Nokogiri (and HTML5lib's whitelists).
  • Common HTML sanitizing tasks are built-in:
    • Strip unsafe tags, leaving behind only the inner text.
    • Prune unsafe tags and their subtrees, removing all traces that they ever existed.
    • Escape unsafe tags and their subtrees, leaving behind lots of < and > entities.
    • Whitewash the markup, removing all attributes and namespaced nodes.
  • Common HTML transformation tasks are built-in:
    • Add the nofollow attribute to all hyperlinks.
  • Format markup as plain text, with or without sensible whitespace handling around block elements.
  • Replace Rails's strip_tags and sanitize view helper methods.

Compare and Contrast

Loofah is one of two known Ruby XSS/sanitization solutions that guarantees well-formed and valid markup (the other is Sanitize, which also uses Nokogiri).

Loofah works on XML, XHTML and HTML documents.

Also, it's pretty fast. Here is a benchmark comparing Loofah to other commonly-used libraries (ActionView, Sanitize, HTML5lib and HTMLfilter):

Lastly, Loofah is extensible. It's super-easy to write your own custom scrubbers for whatever document manipulation you need. You don't like the built-in scrubbers? Build your own, like a boss.

The Basics

Loofah wraps Nokogiri in a loving embrace. Nokogiri is an excellent HTML/XML parser. If you don't know how Nokogiri works, you might want to pause for a moment and go check it out. I'll wait.

Loofah presents the following classes:

  • Loofah::HTML::Document and Loofah::HTML::DocumentFragment
  • Loofah::XML::Document and Loofah::XML::DocumentFragment
  • Loofah::Scrubber

The documents and fragments are subclasses of the similar Nokogiri classes.

The Scrubber represents the document manipulation, either by wrapping a block,

span2div = Loofah::Scrubber.new do |node|
  node.name = "div" if node.name == "span"
end

or by implementing a method.

Side Note: Fragments vs Documents

Generally speaking, unless you expect to have a DOCTYPE and a single root node, you don't have a document, you have a fragment. For HTML, another rule of thumb is that documents have html and body tags, and fragments usually do not.

HTML fragments should be parsed with Loofah.fragment. The result won't be wrapped in html or body tags, won't have a DOCTYPE declaration, head elements will be silently ignored, and multiple root nodes are allowed.

XML fragments should be parsed with Loofah.xml_fragment. The result won't have a DOCTYPE declaration, and multiple root nodes are allowed.

HTML documents should be parsed with Loofah.document. The result will have a DOCTYPE declaration, along with html, head and body tags.

XML documents should be parsed with Loofah.xml_document. The result will have a DOCTYPE declaration and a single root node.

Loofah::HTML::Document and Loofah::HTML::DocumentFragment

These classes are subclasses of Nokogiri::HTML::Document and Nokogiri::HTML::DocumentFragment, so you get all the markup fixer-uppery and API goodness of Nokogiri.

The module methods Loofah.document and Loofah.fragment will parse an HTML document and an HTML fragment, respectively.

Loofah.document(unsafe_html).is_a?(Nokogiri::HTML::Document)         # => true
Loofah.fragment(unsafe_html).is_a?(Nokogiri::HTML::DocumentFragment) # => true

Loofah injects a scrub! method, which takes either a symbol (for built-in scrubbers) or a Loofah::Scrubber object (for custom scrubbers), and modifies the document in-place.

Loofah overrides to_s to return HTML:

unsafe_html = "ohai! <div>div is safe</div> <script>but script is not</script>"

doc = Loofah.fragment(unsafe_html).scrub!(:prune)
doc.to_s    # => "ohai! <div>div is safe</div> "

and text to return plain text:

doc.text    # => "ohai! div is safe "

Also, to_text is available, which does the right thing with whitespace around block-level elements.

doc = Loofah.fragment("<h1>Title</h1><div>Content</div>")
doc.text    # => "TitleContent"           # probably not what you want
doc.to_text # => "\nTitle\n\nContent\n"   # better

Loofah::XML::Document and Loofah::XML::DocumentFragment

These classes are subclasses of Nokogiri::XML::Document and Nokogiri::XML::DocumentFragment, so you get all the markup fixer-uppery and API goodness of Nokogiri.

The module methods Loofah.xml_document and Loofah.xml_fragment will parse an XML document and an XML fragment, respectively.

Loofah.xml_document(bad_xml).is_a?(Nokogiri::XML::Document)         # => true
Loofah.xml_fragment(bad_xml).is_a?(Nokogiri::XML::DocumentFragment) # => true

Nodes and NodeSets

Nokogiri::XML::Node and Nokogiri::XML::NodeSet also get a scrub! method, which makes it easy to scrub subtrees.

The following code will apply the employee_scrubber only to the employee nodes (and their subtrees) in the document:

Loofah.xml_document(bad_xml).xpath("//employee").scrub!(employee_scrubber)

And this code will only scrub the first employee node and its subtree:

Loofah.xml_document(bad_xml).at_xpath("//employee").scrub!(employee_scrubber)

Loofah::Scrubber

A Scrubber wraps up a block (or method) that is run on a document node:

# change all <span> tags to <div> tags
span2div = Loofah::Scrubber.new do |node|
  node.name = "div" if node.name == "span"
end

This can then be run on a document:

Loofah.fragment("<span>foo</span><p>bar</p>").scrub!(span2div).to_s
# => "<div>foo</div><p>bar</p>"

Scrubbers can be run on a document in either a top-down traversal (the default) or bottom-up. Top-down scrubbers can optionally return Scrubber::STOP to terminate the traversal of a subtree. Read below and in the Loofah::Scrubber class for more detailed usage.

Here's an XML example:

# remove all <employee> tags that have a "deceased" attribute set to true
bring_out_your_dead = Loofah::Scrubber.new do |node|
  if node.name == "employee" and node["deceased"] == "true"
    node.remove
    Loofah::Scrubber::STOP # don't bother with the rest of the subtree
  end
end
Loofah.xml_document(File.read('plague.xml')).scrub!(bring_out_your_dead)

=== Built-In HTML Scrubbers

Loofah comes with a set of sanitizing scrubbers that use HTML5lib's whitelist algorithm:

doc.scrub!(:strip)       # replaces unknown/unsafe tags with their inner text
doc.scrub!(:prune)       #  removes unknown/unsafe tags and their children
doc.scrub!(:escape)      #  escapes unknown/unsafe tags, like this: &lt;script&gt;
doc.scrub!(:whitewash)   #  removes unknown/unsafe/namespaced tags and their children,
                         #          and strips all node attributes

Loofah also comes with some common transformation tasks:

doc.scrub!(:nofollow)    #     adds rel="nofollow" attribute to links
doc.scrub!(:unprintable) #  removes unprintable characters from text nodes

See Loofah::Scrubbers for more details and example usage.

Chaining Scrubbers

You can chain scrubbers:

Loofah.fragment("<span>hello</span> <script>alert('OHAI')</script>") \
      .scrub!(:prune) \
      .scrub!(span2div).to_s
# => "<div>hello</div> "

Shorthand

The class methods Loofah.scrub_fragment and Loofah.scrub_document are shorthand.

Loofah.scrub_fragment(unsafe_html, :prune)
Loofah.scrub_document(unsafe_html, :prune)
Loofah.scrub_xml_fragment(bad_xml, custom_scrubber)
Loofah.scrub_xml_document(bad_xml, custom_scrubber)

are the same thing as (and arguably semantically clearer than):

Loofah.fragment(unsafe_html).scrub!(:prune)
Loofah.document(unsafe_html).scrub!(:prune)
Loofah.xml_fragment(bad_xml).scrub!(custom_scrubber)
Loofah.xml_document(bad_xml).scrub!(custom_scrubber)

View Helpers

Loofah has two "view helpers": Loofah::Helpers.sanitize and Loofah::Helpers.strip_tags, both of which are drop-in replacements for the Rails ActionView helpers of the same name. These are no longer required automatically. You must require loofah/helpers.

Requirements

  • Nokogiri >= 1.5.9

Installation

Unsurprisingly:

  • gem install loofah

Support

The bug tracker is available here:

And the mailing list is on librelist:

And the IRC channel is #loofah on freenode.

Security

See SECURITY.md for vulnerability reporting details.

"Secure by Default"

Some tools may incorrectly report Loofah as a potential security vulnerability.

Loofah depends on Nokogiri, and it's possible to use Nokogiri in a dangerous way (by enabling its DTDLOAD option and disabling its NONET option). This specifically allows the opportunity for an XML External Entity (XXE) vulnerability if the XML data is untrusted.

However, Loofah never enables this Nokogiri configuration; Loofah never enables DTDLOAD, and it never disables NONET, thereby protecting you by default from this XXE vulnerability.

Related Links

Authors

Featuring code contributed by:

  • Aaron Patterson
  • John Barnette
  • Josh Owens
  • Paul Dix
  • Luke Melia

And a big shout-out to Corey Innis for the name, and feedback on the API.

Thank You

The following people have generously donated via the Pledgie badge on the Loofah github page:

  • Bill Harding

Historical Note

This library was formerly known as Dryopteris, which was a very bad name that nobody could spell properly.

License

Distributed under the MIT License. See MIT-LICENSE.txt for details.