1 files changed, 27 insertions, 0 deletions
diff --git a/lib/banzai/filter/pathological_markdown_filter.rb b/lib/banzai/filter/pathological_markdown_filter.rb
new file mode 100644
index 00000000000..0f94150c7a1
--- /dev/null
+++ b/lib/banzai/filter/pathological_markdown_filter.rb
@@ -0,0 +1,27 @@
+# frozen_string_literal: true
+
+module Banzai
+  module Filter
+    class PathologicalMarkdownFilter < HTML::Pipeline::TextFilter
+      # It's not necessary for this to be precise - we just need to detect
+      # when there are a non-trivial number of unclosed image links.
+      # So we don't really care about code blocks, etc.
+      # See https://gitlab.com/gitlab-org/gitlab/-/issues/370428
+      REGEX = /!\[(?:[^\]])+?!\[/.freeze
+      DETECTION_MAX = 10
+
+      def call
+        count = 0
+
+        @text.scan(REGEX) do |_match|
+          count += 1
+          break if count > DETECTION_MAX
+        end
+
+        return @text if count <= DETECTION_MAX
+
+        "_Unable to render markdown - too many unclosed markdown image links detected._"
+      end
+    end
+  end
+end