Merge branch 'dev_issue_15331' into 'master'

Fixes window.opener bug

Adds `noreferrer` value to rel attribute for external links

REF: https://gitlab.com/gitlab-org/gitlab-ce/issues/15331

See merge request !1953

lib/banzai/filter/external_link_filter.rb

 module Banzai
   module Filter
-    # HTML Filter to add a `rel="nofollow"` attribute to external links
-    #
+    # HTML Filter to modify the attributes of external links
     class ExternalLinkFilter < HTML::Pipeline::Filter
       def call
         doc.search('a').each do |node|
@@ -15,7 +14,7 @@ module Banzai
           # Skip internal links
           next if link.start_with?(internal_url)
 
-          node.set_attribute('rel', 'nofollow')
+          node.set_attribute('rel', 'nofollow noreferrer')
         end
 
         doc

spec/features/markdown_spec.rb

@@ -165,7 +165,12 @@ describe 'GitLab Markdown', feature: true do
     describe 'ExternalLinkFilter' do
       it 'adds nofollow to external link' do
         link = doc.at_css('a:contains("Google")')
-        expect(link.attr('rel')).to match 'nofollow'
+        expect(link.attr('rel')).to include('nofollow')
+      end
+
+      it 'adds noreferrer to external link' do
+        link = doc.at_css('a:contains("Google")')
+        expect(link.attr('rel')).to include('noreferrer')
       end
 
       it 'ignores internal link' do

spec/lib/banzai/filter/external_link_filter_spec.rb

@@ -24,6 +24,14 @@ describe Banzai::Filter::ExternalLinkFilter, lib: true do
     doc = filter(act)
 
     expect(doc.at_css('a')).to have_attribute('rel')
-    expect(doc.at_css('a')['rel']).to eq 'nofollow'
+    expect(doc.at_css('a')['rel']).to include 'nofollow'
+  end
+
+  it 'adds rel="noreferrer" to external links' do
+    act = %q(<a href="https://google.com/">Google</a>)
+    doc = filter(act)
+
+    expect(doc.at_css('a')).to have_attribute('rel')
+    expect(doc.at_css('a')['rel']).to include 'noreferrer'
   end
 end