Sanitize output by dependency linkers

When there are URLs defined in some dependency file (e.g. Gemfile,
gemspec, etc), they get converted to links. We are not sanitizing
it so if some `javascript:` code is added as a URL, it can cause
XSS vulnerability.

changelogs/unreleased/security-pb-fix-xss-dependency-link.yml

+---
+title: Sanitize output by dependency linkers
+merge_request:
+author:
+type: security

lib/gitlab/dependency_linker/base_linker.rb

@@ -7,6 +7,8 @@ module Gitlab
       GIT_INVALID_URL_REGEX = /^git\+#{URL_REGEX}/.freeze
       REPO_REGEX = %r{[^/'" ]+/[^/'" ]+}.freeze
 
+      include ActionView::Helpers::SanitizeHelper
+
       class_attribute :file_type
 
       def self.support?(blob_name)
@@ -62,7 +64,10 @@ module Gitlab
       end
 
       def link_tag(name, url)
-        %{<a href="#{ERB::Util.html_escape_once(url)}" rel="nofollow noreferrer noopener" target="_blank">#{ERB::Util.html_escape_once(name)}</a>}.html_safe
+        sanitize(
+          %{<a href="#{ERB::Util.html_escape_once(url)}" rel="nofollow noreferrer noopener" target="_blank">#{ERB::Util.html_escape_once(name)}</a>},
+          attributes: %w[href rel target]
+        )
       end
 
       # Links package names based on regex.

spec/lib/gitlab/dependency_linker/base_linker_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe Gitlab::DependencyLinker::BaseLinker do
+  let(:linker_class) do
+    Class.new(described_class) do
+      def link_dependencies
+        link_regex(%r{^(?<name>https?://[^ ]+)}, &:itself)
+      end
+    end
+  end
+
+  let(:plain_content) do
+    <<~CONTENT
+      http://\\njavascript:alert(1)
+      https://gitlab.com/gitlab-org/gitlab
+    CONTENT
+  end
+
+  let(:highlighted_content) do
+    <<~CONTENT
+      <span><span>http://</span><span>\\n</span><span>javascript:alert(1)</span></span>
+      <span><span>https://gitlab.com/gitlab-org/gitlab</span></span>
+    CONTENT
+  end
+
+  let(:linker) { linker_class.new(plain_content, highlighted_content) }
+
+  describe '#link' do
+    subject { linker.link }
+
+    it 'only converts valid links' do
+      expect(subject).to eq(
+        <<~CONTENT
+          <span><span>#{link('http://')}</span><span>#{link('\n', url: '%5Cn')}</span><span>#{link('javascript:alert(1)', url: nil)}</span></span>
+          <span><span>#{link('https://gitlab.com/gitlab-org/gitlab')}</span></span>
+        CONTENT
+      )
+    end
+  end
+
+  def link(text, url: text)
+    attrs = [
+      'rel="nofollow noreferrer noopener"',
+      'target="_blank"'
+    ]
+
+    attrs.unshift(%{href="#{url}"}) if url
+
+    %{<a #{attrs.join(' ')}>#{text}</a>}
+  end
+end