Monkey patch asciidoctor gem to fix attributes

Locked attributes should not be able to be
overridden using a {counter}.
Added specs for updated kroki gem as well.

changelogs/unreleased/security-kroki-arbitraryfile-read-write.yml

+---
+title: Fix arbitrary read/write in AsciiDoctor and Kroki gems
+merge_request:
+author:
+type: security

config/initializers/asciidoctor_patch.rb

+# frozen_string_literal: true
+
+# Ensure that locked attributes can not be changed using a counter.
+# TODO: this can be removed once `asciidoctor` gem is > 2.0.12
+#       and https://github.com/asciidoctor/asciidoctor/issues/3939 is merged
+module Asciidoctor
+  module DocumentPatch
+    def counter(name, seed = nil)
+      return @parent_document.counter(name, seed) if @parent_document   # rubocop: disable Gitlab/ModuleWithInstanceVariables
+
+      unless attribute_locked? name
+        super
+      end
+    end
+  end
+end
+
+class Asciidoctor::Document
+  prepend Asciidoctor::DocumentPatch
+end

spec/lib/gitlab/asciidoc_spec.rb

@@ -92,6 +92,15 @@ module Gitlab
             expect(render(data[:input], context)).to include(data[:output])
           end
         end
+
+        it 'does not allow locked attributes to be overridden' do
+          input = <<~ADOC
+            {counter:max-include-depth:1234}
+            <|-- {max-include-depth}
+          ADOC
+
+          expect(render(input, {})).not_to include('1234')
+        end
       end
 
       context "images" do
@@ -543,6 +552,40 @@ module Gitlab
 
           expect(render(input, context)).to include(output.strip)
         end
+
+        it 'does not allow kroki-plantuml-include to be overridden' do
+          input = <<~ADOC
+            [plantuml, test="{counter:kroki-plantuml-include:/etc/passwd}", format="png"]
+            ....
+            class BlockProcessor
+
+            BlockProcessor <|-- {counter:kroki-plantuml-include}
+            ....
+          ADOC
+
+          output = <<~HTML
+            <div>
+            <div>
+            <a class=\"no-attachment-icon\" href=\"https://kroki.io/plantuml/png/eNpLzkksLlZwyslPzg4oyk9OLS7OL-LiQuUr2NTo6ipUJ-eX5pWkFlllF-VnZ-oW5CTmlZTm5uhm5iXnlKak1gIABQEb8A==\" target=\"_blank\" rel=\"noopener noreferrer\"><img src=\"data:image/gif;base64,R0lGODlhAQABAAAAACH5BAEKAAEALAAAAAABAAEAAAICTAEAOw==\" alt=\"Diagram\" class=\"lazy\" data-src=\"https://kroki.io/plantuml/png/eNpLzkksLlZwyslPzg4oyk9OLS7OL-LiQuUr2NTo6ipUJ-eX5pWkFlllF-VnZ-oW5CTmlZTm5uhm5iXnlKak1gIABQEb8A==\"></a>
+            </div>
+            </div>
+          HTML
+
+          expect(render(input, {})).to include(output.strip)
+        end
+
+        it 'does not allow kroki-server-url to be overridden' do
+          input = <<~ADOC
+            [plantuml, test="{counter:kroki-server-url:evilsite}", format="png"]
+            ....
+            class BlockProcessor
+
+            BlockProcessor
+            ....
+          ADOC
+
+          expect(render(input, {})).not_to include('evilsite')
+        end
       end
 
       context 'with Kroki and BlockDiag (additional format) enabled' do