Add noreferrer scrubber

README.md

@@ -230,7 +230,9 @@ doc.scrub!(:whitewash)   #  removes unknown/unsafe/namespaced tags and their chi
 Loofah also comes with some common transformation tasks:
 
 ``` ruby
-doc.scrub!(:nofollow)    #     adds rel="nofollow" attribute to links
+doc.scrub!(:nofollow)    #  adds rel="nofollow" attribute to links
+doc.scrub!(:noopener)    #  adds rel="noopener" attribute to links
+doc.scrub!(:noreferrer)  #  adds rel="noreferrer" attribute to links
 doc.scrub!(:unprintable) #  removes unprintable characters from text nodes
 doc.scrub!(:targetblank) #     adds target="_blank" attribute to links
 ```

lib/loofah/scrubbers.rb

@@ -78,6 +78,14 @@ module Loofah
   #     Loofah.html5_fragment(link_farmers_markup).scrub!(:noopener)
   #     => "ohai! <a href='http://www.myswarmysite.com/' rel="noopener">I like your blog post</a>"
   #
+  #  === Loofah::Scrubbers::NoReferrer / scrub!(:noreferrer)
+  #
+  #  +:noreferrer+ adds a rel="noreferrer" attribute to all links
+  #
+  #     link_farmers_markup = "ohai! <a href='http://www.myswarmysite.com/'>I like your blog post</a>"
+  #     Loofah.html5_fragment(link_farmers_markup).scrub!(:noreferrer)
+  #     => "ohai! <a href='http://www.myswarmysite.com/' rel="noreferrer">I like your blog post</a>"
+  #
   #
   #  === Loofah::Scrubbers::Unprintable / scrub!(:unprintable)
   #
@@ -271,6 +279,28 @@ def scrub(node)
       end
     end
 
+    #
+    #  === scrub!(:noreferrer)
+    #
+    #  +:noreferrer+ adds a rel="noreferrer" attribute to all links
+    #
+    #     link_farmers_markup = "ohai! <a href='http://www.myswarmysite.com/'>I like your blog post</a>"
+    #     Loofah.html5_fragment(link_farmers_markup).scrub!(:noreferrer)
+    #     => "ohai! <a href='http://www.myswarmysite.com/' rel="noreferrer">I like your blog post</a>"
+    #
+    class NoReferrer < Scrubber
+      def initialize # rubocop:disable Lint/MissingSuper
+        @direction = :top_down
+      end
+
+      def scrub(node)
+        return CONTINUE unless (node.type == Nokogiri::XML::Node::ELEMENT_NODE) && (node.name == "a")
+
+        append_attribute(node, "rel", "noreferrer")
+        STOP
+      end
+    end
+
     # This class probably isn't useful publicly, but is used for #to_text's current implemention
     class NewlineBlockElements < Scrubber # :nodoc:
       def initialize # rubocop:disable Lint/MissingSuper
@@ -328,6 +358,7 @@ def scrub(node)
       strip: Strip,
       nofollow: NoFollow,
       noopener: NoOpener,
+      noreferrer: NoReferrer,
       targetblank: TargetBlank,
       newline_block_elements: NewlineBlockElements,
       unprintable: Unprintable,

test/integration/test_scrubbers.rb

@@ -31,6 +31,12 @@ class IntegrationTestScrubbers < Loofah::TestCase
   NOOPENER_WITH_REL_FRAGMENT = '<a href="http://www.example.com/" rel="nofollow">Click here</a>'
   NOOPENER_WITH_REL_RESULT = '<a href="http://www.example.com/" rel="nofollow noopener">Click here</a>'
 
+  NOREFERRER_FRAGMENT = '<a href="http://www.example.com/">Click here</a>'
+  NOREFERRER_RESULT = '<a href="http://www.example.com/" rel="noreferrer">Click here</a>'
+
+  NOREFERRER_WITH_REL_FRAGMENT = '<a href="http://www.example.com/" rel="noopener">Click here</a>'
+  NOREFERRER_WITH_REL_RESULT = '<a href="http://www.example.com/" rel="noopener noreferrer">Click here</a>'
+
   UNPRINTABLE_FRAGMENT = "<b>Lo\u2029ofah ro\u2028cks!</b><script>x\u2028y</script>"
   UNPRINTABLE_RESULT = "<b>Loofah rocks!</b><script>xy</script>"
 
@@ -443,6 +449,28 @@ def html5?
           end
         end
 
+        context ":noreferrer" do
+          context "for a hyperlink without a 'rel' attribute" do
+            it "add a 'noreferrer' attribute to hyperlinks" do
+              doc = klass.parse("<div>#{NOREFERRER_FRAGMENT}</div>")
+              result = doc.scrub!(:noreferrer)
+
+              assert_equal NOREFERRER_RESULT, doc.xpath("./div").inner_html
+              assert_equal doc, result
+            end
+          end
+
+          context "for a hyperlink that does have a rel attribute" do
+            it "appends 'noreferrer' to 'rel' attribute" do
+              doc = klass.parse("<div>#{NOREFERRER_WITH_REL_FRAGMENT}</div>")
+              result = doc.scrub!(:noreferrer)
+
+              assert_equal NOREFERRER_WITH_REL_RESULT, doc.xpath("./div").inner_html
+              assert_equal doc, result
+            end
+          end
+        end
+
         context ":unprintable" do
           it "removes unprintable unicode characters" do
             doc = klass.parse("<div>#{UNPRINTABLE_FRAGMENT}</div>")