diff --git a/lib/reynard.rb b/lib/reynard.rb
index c56695f..186ec6a 100644
--- a/lib/reynard.rb
+++ b/lib/reynard.rb
@@ -60,6 +60,21 @@ def self.http
     end
   end
 
+  def self.permitted_classes
+    @permitted_classes = parse_permitted_classes(ENV['REYNARD_PERMITTED_CLASSES']) || []
+  end
+
+  def self.parse_permitted_classes(env_value)
+    return nil unless env_value
+
+    env_value.split(',').map(&:strip).map do |class_name|
+      Object.const_get(class_name)
+    rescue NameError
+      warn "Warning: Could not find class #{class_name}"
+      nil
+    end.compact
+  end
+
   private
 
   def build_context
diff --git a/lib/reynard/specification.rb b/lib/reynard/specification.rb
index 04b6481..c561ec9 100644
--- a/lib/reynard/specification.rb
+++ b/lib/reynard/specification.rb
@@ -105,7 +105,7 @@ def self.media_type_matches?(media_type, expression)
 
     def read
       File.open(@filename, encoding: 'UTF-8') do |file|
-        YAML.safe_load(file, aliases: true)
+        YAML.safe_load(file, aliases: true, permitted_classes: Reynard.permitted_classes)
       end
     end