Re-escape whole HTML content instead of only match

When we un-escape HTML text to find references in it, we should then
re-escape the whole text again, not only found matches.

Because we replace matches with milestone/label links (which contain
HTML tags we don't want to escape again), we re-escape HTML text
with placeholders instead of these links and then replace placeholders
in the escaped text.

changelogs/unreleased/security-fix-markdown-xss.yml

+---
+title: Make sure HTML text is always escaped when replacing label/milestone references.
+merge_request:
+author:
+type: security

lib/banzai/filter/abstract_reference_filter.rb

@@ -7,6 +7,14 @@ module Banzai
     class AbstractReferenceFilter < ReferenceFilter
       include CrossProjectReference
 
+      # REFERENCE_PLACEHOLDER is used for re-escaping HTML text except found
+      # reference (which we replace with placeholder during re-scaping).  The
+      # random number helps ensure it's pretty close to unique. Since it's a
+      # transitory value (it never gets saved) we can initialize once, and it
+      # doesn't matter if it changes on a restart.
+      REFERENCE_PLACEHOLDER = "_reference_#{SecureRandom.hex(16)}_"
+      REFERENCE_PLACEHOLDER_PATTERN = %r{#{REFERENCE_PLACEHOLDER}(\d+)}.freeze
+
       def self.object_class
         # Implement in child class
         # Example: MergeRequest
@@ -389,6 +397,14 @@ module Banzai
       def escape_html_entities(text)
         CGI.escapeHTML(text.to_s)
       end
+
+      def escape_with_placeholders(text, placeholder_data)
+        escaped = escape_html_entities(text)
+
+        escaped.gsub(REFERENCE_PLACEHOLDER_PATTERN) do |match|
+          placeholder_data[$1.to_i]
+        end
+      end
     end
   end
 end

lib/banzai/filter/label_reference_filter.rb

@@ -14,24 +14,24 @@ module Banzai
         find_labels(parent_object).find(id)
       end
 
-      def self.references_in(text, pattern = Label.reference_pattern)
-        unescape_html_entities(text).gsub(pattern) do |match|
-          yield match, $~[:label_id].to_i, $~[:label_name], $~[:project], $~[:namespace], $~
-        end
-      end
-
       def references_in(text, pattern = Label.reference_pattern)
-        unescape_html_entities(text).gsub(pattern) do |match|
+        labels = {}
+        unescaped_html = unescape_html_entities(text).gsub(pattern) do |match|
           namespace, project = $~[:namespace], $~[:project]
           project_path = full_project_path(namespace, project)
           label = find_label(project_path, $~[:label_id], $~[:label_name])
 
           if label
-            yield match, label.id, project, namespace, $~
+            labels[label.id] = yield match, label.id, project, namespace, $~
+            "#{REFERENCE_PLACEHOLDER}#{label.id}"
           else
-            escape_html_entities(match)
+            match
           end
         end
+
+        return text if labels.empty?
+
+        escape_with_placeholders(unescaped_html, labels)
       end
 
       def find_label(parent_ref, label_id, label_name)

lib/banzai/filter/milestone_reference_filter.rb

@@ -51,15 +51,21 @@ module Banzai
         # default implementation.
         return super(text, pattern) if pattern != Milestone.reference_pattern
 
-        unescape_html_entities(text).gsub(pattern) do |match|
+        milestones = {}
+        unescaped_html = unescape_html_entities(text).gsub(pattern) do |match|
           milestone = find_milestone($~[:project], $~[:namespace], $~[:milestone_iid], $~[:milestone_name])
 
           if milestone
-            yield match, milestone.id, $~[:project], $~[:namespace], $~
+            milestones[milestone.id] = yield match, milestone.id, $~[:project], $~[:namespace], $~
+            "#{REFERENCE_PLACEHOLDER}#{milestone.id}"
           else
-            escape_html_entities(match)
+            match
           end
         end
+
+        return text if milestones.empty?
+
+        escape_with_placeholders(unescaped_html, milestones)
       end
 
       def find_milestone(project_ref, namespace_ref, milestone_id, milestone_name)

lib/gitlab/markdown_cache.rb

@@ -3,8 +3,8 @@
 module Gitlab
   module MarkdownCache
     # Increment this number every time the renderer changes its output
+    CACHE_COMMONMARK_VERSION        = 17
     CACHE_COMMONMARK_VERSION_START  = 10
-    CACHE_COMMONMARK_VERSION        = 16
 
     BaseError = Class.new(StandardError)
     UnsupportedClassError = Class.new(BaseError)

spec/lib/banzai/filter/label_reference_filter_spec.rb

@@ -10,6 +10,11 @@ describe Banzai::Filter::LabelReferenceFilter do
   let(:label)     { create(:label, project: project) }
   let(:reference) { label.to_reference }
 
+  it_behaves_like 'HTML text with references' do
+    let(:resource) { label }
+    let(:resource_text) { resource.title }
+  end
+
   it 'requires project context' do
     expect { described_class.call('') }.to raise_error(ArgumentError, /:project/)
   end

spec/lib/banzai/filter/milestone_reference_filter_spec.rb

@@ -329,6 +329,10 @@ describe Banzai::Filter::MilestoneReferenceFilter do
     it_behaves_like 'cross-project / same-namespace complete reference'
     it_behaves_like 'cross project shorthand reference'
     it_behaves_like 'references with HTML entities'
+    it_behaves_like 'HTML text with references' do
+      let(:resource) { milestone }
+      let(:resource_text) { "#{resource.class.reference_prefix}#{resource.title}" }
+    end
   end
 
   shared_context 'group milestones' do
@@ -340,6 +344,10 @@ describe Banzai::Filter::MilestoneReferenceFilter do
     it_behaves_like 'String-based multi-word references in quotes'
     it_behaves_like 'referencing a milestone in a link href'
     it_behaves_like 'references with HTML entities'
+    it_behaves_like 'HTML text with references' do
+      let(:resource) { milestone }
+      let(:resource_text) { "#{resource.class.reference_prefix}#{resource.title}" }
+    end
 
     it 'does not support references by IID' do
       doc = reference_filter("See #{Milestone.reference_prefix}#{milestone.iid}")

spec/support/shared_examples/lib/banzai/filters/reference_filter_shared_examples.rb

+# frozen_string_literal: true
+
+RSpec.shared_examples 'HTML text with references' do
+  let(:markdown_prepend) { "<img src=\"\" onerror=alert(`bug`)>" }
+
+  it 'preserves escaped HTML text and adds valid references' do
+    reference = resource.to_reference(format: :name)
+
+    doc = reference_filter("#{markdown_prepend}#{reference}")
+
+    expect(doc.to_html).to start_with(markdown_prepend)
+    expect(doc.text).to eq %(<img src="" onerror=alert(`bug`)>#{resource_text})
+  end
+
+  it 'preserves escaped HTML text if there are no valid references' do
+    reference = "#{resource.class.reference_prefix}invalid"
+    text = "#{markdown_prepend}#{reference}"
+
+    doc = reference_filter(text)
+
+    expect(doc.to_html).to eq text
+  end
+end