3 files changed, 50 insertions, 4 deletions
diff --git a/ext/psych/lib/psych/class_loader.rb b/ext/psych/lib/psych/class_loader.rb
index a5d1a7a4a9..088373cd66 100644
--- a/ext/psych/lib/psych/class_loader.rb
+++ b/ext/psych/lib/psych/class_loader.rb
@@ -86,7 +86,7 @@ module Psych
         if @symbols.include? sym
           super
         else
-          raise DisallowedClass, 'Symbol'
+          raise DisallowedClass.new('load', 'Symbol')
         end
       end
 
@@ -96,7 +96,7 @@ module Psych
         if @classes.include? klassname
           super
         else
-          raise DisallowedClass, klassname
+          raise DisallowedClass.new('load', klassname)
         end
       end
     end
diff --git a/ext/psych/lib/psych/exception.rb b/ext/psych/lib/psych/exception.rb
index fac0c42b9f..f473b95a3b 100644
--- a/ext/psych/lib/psych/exception.rb
+++ b/ext/psych/lib/psych/exception.rb
@@ -7,8 +7,8 @@ module Psych
   end
 
   class DisallowedClass < Exception
-    def initialize klass_name
-      super "Tried to load unspecified class: #{klass_name}"
+    def initialize action, klass_name
+      super "Tried to #{action} unspecified class: #{klass_name}"
     end
   end
 end
diff --git a/ext/psych/lib/psych/visitors/yaml_tree.rb b/ext/psych/lib/psych/visitors/yaml_tree.rb
index bf7c0bb8ca..05748dd81c 100644
--- a/ext/psych/lib/psych/visitors/yaml_tree.rb
+++ b/ext/psych/lib/psych/visitors/yaml_tree.rb
@@ -535,5 +535,51 @@ module Psych
         end
       end
     end
+
+    class RestrictedYAMLTree < YAMLTree
+      DEFAULT_PERMITTED_CLASSES = {
+        TrueClass => true,
+        FalseClass => true,
+        NilClass => true,
+        Integer => true,
+        Float => true,
+        String => true,
+        Array => true,
+        Hash => true,
+      }.compare_by_identity.freeze
+
+      def initialize emitter, ss, options
+        super
+        @permitted_classes = DEFAULT_PERMITTED_CLASSES.dup
+        Array(options[:permitted_classes]).each do |klass|
+          @permitted_classes[klass] = true
+        end
+        @permitted_symbols = {}.compare_by_identity
+        Array(options[:permitted_symbols]).each do |symbol|
+          @permitted_symbols[symbol] = true
+        end
+        @aliases = options.fetch(:aliases, false)
+      end
+
+      def accept target
+        if !@aliases && @st.key?(target)
+          raise BadAlias, "Tried to dump an aliased object"
+        end
+
+        unless @permitted_classes[target.class]
+          raise DisallowedClass.new('dump', target.class.name || target.class.inspect)
+        end
+
+        super
+      end
+
+      def visit_Symbol sym
+        unless @permitted_symbols[sym]
+          raise DisallowedClass.new('dump', "Symbol(#{sym.inspect})")
+        end
+
+        super
+      end
+    end
   end
 end