Sanitize `vbscript:` links

Closes https://dev.gitlab.org/gitlab/gitlabhq/issues/2660

lib/banzai/filter/sanitization_filter.rb

@@ -7,6 +7,8 @@ module Banzai
     #
     # Extends HTML::Pipeline::SanitizationFilter with a custom whitelist.
     class SanitizationFilter < HTML::Pipeline::SanitizationFilter
+      UNSAFE_PROTOCOLS = %w(javascript :javascript data vbscript).freeze
+
       def whitelist
         whitelist = super
 
@@ -62,7 +64,7 @@ module Banzai
           return unless node.name == 'a'
           return unless node.has_attribute?('href')
 
-          if node['href'].start_with?('javascript', ':javascript', 'data')
+          if node['href'].start_with?(*UNSAFE_PROTOCOLS)
             node.remove_attribute('href')
           end
         end

spec/lib/banzai/filter/sanitization_filter_spec.rb

@@ -170,6 +170,13 @@ describe Banzai::Filter::SanitizationFilter, lib: true do
       expect(output.to_html).to eq '<a>XSS</a>'
     end
 
+    it 'disallows vbscript links' do
+      input = '<a href="vbscript:alert(document.domain)">XSS</a>'
+      output = filter(input)
+
+      expect(output.to_html).to eq '<a>XSS</a>'
+    end
+
     it 'allows non-standard anchor schemes' do
       exp = %q{<a href="irc://irc.freenode.net/git">IRC</a>}
       act = filter(exp)