Merge branch 'bvl-validate-urls-in-markdown-using-uri' into 'security'

Add correct `rel` attributes to external links when rendering markdown

See merge request !2086

changelogs/unreleased/bvl-validate-urls-in-markdown-using-uri.yml

+---
+title: Validate URLs in markdown using URI to detect the host correctly
+merge_request:
+author:

lib/banzai/filter/external_link_filter.rb

@@ -2,16 +2,17 @@ module Banzai
   module Filter
     # HTML Filter to modify the attributes of external links
     class ExternalLinkFilter < HTML::Pipeline::Filter
+      SCHEMES = ['http', 'https', nil].freeze
+
       def call
         links.each do |node|
-          href = href_to_lowercase_scheme(node["href"].to_s)
+          uri = uri(node['href'].to_s)
+          next unless uri
 
-          unless node["href"].to_s == href
-            node.set_attribute('href', href)
-          end
+          node.set_attribute('href', uri.to_s)
 
-          if href =~ %r{\A(https?:)?//[^/]} && external_url?(href)
-            node.set_attribute('rel', 'nofollow noreferrer')
+          if SCHEMES.include?(uri.scheme) && external_url?(uri)
+            node.set_attribute('rel', 'nofollow noreferrer noopener')
             node.set_attribute('target', '_blank')
           end
         end
@@ -21,27 +22,26 @@ module Banzai
 
       private
 
+      def uri(href)
+        URI.parse(href)
+      rescue URI::InvalidURIError
+        nil
+      end
+
       def links
         query = 'descendant-or-self::a[@href and not(@href = "")]'
         doc.xpath(query)
       end
 
-      def href_to_lowercase_scheme(href)
-        scheme_match = href.match(/\A(\w+):\/\//)
-
-        if scheme_match
-          scheme_match.to_s.downcase + scheme_match.post_match
-        else
-          href
-        end
-      end
+      def external_url?(uri)
+        # Relative URLs miss a hostname
+        return false unless uri.hostname
 
-      def external_url?(url)
-        !url.start_with?(internal_url)
+        uri.hostname != internal_url.hostname
       end
 
       def internal_url
-        @internal_url ||= Gitlab.config.gitlab.url
+        @internal_url ||= URI.parse(Gitlab.config.gitlab.url)
       end
     end
   end

spec/lib/banzai/filter/external_link_filter_spec.rb

 require 'spec_helper'
 
+shared_examples 'an external link with rel attribute' do
+  it 'adds rel="nofollow" to external links' do
+    expect(doc.at_css('a')).to have_attribute('rel')
+    expect(doc.at_css('a')['rel']).to include 'nofollow'
+  end
+
+  it 'adds rel="noreferrer" to external links' do
+    expect(doc.at_css('a')).to have_attribute('rel')
+    expect(doc.at_css('a')['rel']).to include 'noreferrer'
+  end
+
+  it 'adds rel="noopener" to external links' do
+    expect(doc.at_css('a')).to have_attribute('rel')
+    expect(doc.at_css('a')['rel']).to include 'noopener'
+  end
+end
+
 describe Banzai::Filter::ExternalLinkFilter, lib: true do
   include FilterSpecHelper
 
@@ -22,49 +39,51 @@ describe Banzai::Filter::ExternalLinkFilter, lib: true do
   context 'for root links on document' do
     let(:doc) { filter %q(<a href="https://google.com/">Google</a>) }
 
-    it 'adds rel="nofollow" to external links' do
-      expect(doc.at_css('a')).to have_attribute('rel')
-      expect(doc.at_css('a')['rel']).to include 'nofollow'
+    it_behaves_like 'an external link with rel attribute'
   end
 
-    it 'adds rel="noreferrer" to external links' do
-      expect(doc.at_css('a')).to have_attribute('rel')
-      expect(doc.at_css('a')['rel']).to include 'noreferrer'
+  context 'for nested links on document' do
+    let(:doc) { filter %q(<p><a href="https://google.com/">Google</a></p>) }
+
+    it_behaves_like 'an external link with rel attribute'
+  end
+
+  context 'for invalid urls' do
+    it 'skips broken hrefs' do
+      doc = filter %q(<p><a href="don't crash on broken urls">Google</a></p>)
+      expected = %q(<p><a href="don't%20crash%20on%20broken%20urls">Google</a></p>)
+
+      expect(doc.to_html).to eq(expected)
     end
   end
 
-  context 'for nested links on document' do
-    let(:doc) { filter %q(<p><a href="https://google.com/">Google</a></p>) }
+  context 'for links with a username' do
+    context 'with a valid username' do
+      let(:doc) { filter %q(<a href="https://user@google.com/">Google</a>) }
 
-    it 'adds rel="nofollow" to external links' do
-      expect(doc.at_css('a')).to have_attribute('rel')
-      expect(doc.at_css('a')['rel']).to include 'nofollow'
+      it_behaves_like 'an external link with rel attribute'
     end
 
-    it 'adds rel="noreferrer" to external links' do
-      expect(doc.at_css('a')).to have_attribute('rel')
-      expect(doc.at_css('a')['rel']).to include 'noreferrer'
+    context 'with an impersonated username' do
+      let(:internal) { Gitlab.config.gitlab.url }
+
+      let(:doc) { filter %Q(<a href="https://#{internal}@example.com" target="_blank">Reverse Tabnabbing</a>) }
+
+      it_behaves_like 'an external link with rel attribute'
     end
   end
 
   context 'for non-lowercase scheme links' do
-    let(:doc_with_http) { filter %q(<p><a href="httP://google.com/">Google</a></p>) }
-    let(:doc_with_https) { filter %q(<p><a href="hTTpS://google.com/">Google</a></p>) }
+    context 'with http' do
+      let(:doc) { filter %q(<p><a href="httP://google.com/">Google</a></p>) }
 
-    it 'adds rel="nofollow" to external links' do
-      expect(doc_with_http.at_css('a')).to have_attribute('rel')
-      expect(doc_with_https.at_css('a')).to have_attribute('rel')
-
-      expect(doc_with_http.at_css('a')['rel']).to include 'nofollow'
-      expect(doc_with_https.at_css('a')['rel']).to include 'nofollow'
+      it_behaves_like 'an external link with rel attribute'
     end
 
-    it 'adds rel="noreferrer" to external links' do
-      expect(doc_with_http.at_css('a')).to have_attribute('rel')
-      expect(doc_with_https.at_css('a')).to have_attribute('rel')
+    context 'with https' do
+      let(:doc) { filter %q(<p><a href="hTTpS://google.com/">Google</a></p>) }
 
-      expect(doc_with_http.at_css('a')['rel']).to include 'noreferrer'
-      expect(doc_with_https.at_css('a')['rel']).to include 'noreferrer'
+      it_behaves_like 'an external link with rel attribute'
     end
 
     it 'skips internal links' do
@@ -84,14 +103,6 @@ describe Banzai::Filter::ExternalLinkFilter, lib: true do
   context 'for protocol-relative links' do
     let(:doc) { filter %q(<p><a href="//google.com/">Google</a></p>) }
 
-    it 'adds rel="nofollow" to external links' do
-      expect(doc.at_css('a')).to have_attribute('rel')
-      expect(doc.at_css('a')['rel']).to include 'nofollow'
-    end
-
-    it 'adds rel="noreferrer" to external links' do
-      expect(doc.at_css('a')).to have_attribute('rel')
-      expect(doc.at_css('a')['rel']).to include 'noreferrer'
-    end
+    it_behaves_like 'an external link with rel attribute'
   end
 end