Fix xss for Markdown elements where [[_TOC_]] is enabled

changelogs/unreleased/security-2682-fix-xss-for-markdown-toc.yml

+---
+title: Fix XSS vulnerability for table of content generation
+merge_request:
+author:
+type: security

lib/banzai/filter/table_of_contents_filter.rb

@@ -92,7 +92,7 @@ module Banzai
         def text
           return '' unless node
 
-          @text ||= node.text
+          @text ||= EscapeUtils.escape_html(node.text)
         end
 
         private

spec/lib/banzai/filter/table_of_contents_filter_spec.rb

@@ -139,5 +139,14 @@ describe Banzai::Filter::TableOfContentsFilter do
         expect(items[5].ancestors).to include(items[4])
       end
     end
+
+    context 'header text contains escaped content' do
+      let(:content) { '<img src="x" onerror="alert(42)">' }
+      let(:results) { result(header(1, content)) }
+
+      it 'outputs escaped content' do
+        expect(doc.inner_html).to include(content)
+      end
+    end
   end
 end