Neuter YAML to help mitigate CVE-2013-0156-style attacks.

make_yaml_safe.rb

# The fact that YAML.load will instantiate arbitrary ruby objects
# means that calling `YAML.load` on untrusted data is virtually always
# equivalent to executing arbitrary code in a complex app.

# This code fragment globally neuters YAML to disable this behavior,
# which should (hopefully) cut off all such attacks from the start.

# I don't promise this closes all possible attacks, but this closes
# off the trivial case.  You should audit and upgrade all your
# dependencies, as well.

# Now with Psych/Ruby 1.9 support by @ebroder

def self.make_yaml_safe!
  # Make syck-based YAML safe
  require 'yaml'
  require 'syck'
  case RUBY_VERSION
  when /^1\.8/
    syckmod = YAML
  else
    syckmod = Syck
  end
 # I've left in Symbol because it's common to rely on loading YAML config files
  # with symbol keys. If you don't need them, remove it from this list --
  # loading attacker-provided symbols is an easy memory leak, and can potentially
  # be used to tickle CVE-2012-5664-style bugs
  whitelist_classes = [String, Hash, Symbol, Float, Array,
                       TrueClass, FalseClass, Integer,
                       Time, Date, NilClass]
  syckmod.tagged_classes.delete_if { |k,v| !whitelist_classes.include?(v) }
  syckmod.tagged_classes.freeze
 
  # Make psych-based (1.9) YAML safe
  if defined?(Psych)
    Psych.const_set("UnsafeYAML", Class.new(StandardError))
    Psych.module_eval do
      def self.load(yaml, *args)
        result = parse(yaml, *args)
        check_safety(result)
        result ? result.to_ruby : result
      end
 
      private
      def self.check_safety(o)
        check_node(o)
        case o
        when Psych::Nodes::Scalar
        when Psych::Nodes::Sequence
          o.children.each {|child| check_safety(child)}
        when Psych::Nodes::Mapping
          o.children.each {|child| check_safety(child)}
        when Psych::Nodes::Document
          check_safety(o.root)
        when Psych::Nodes::Stream
          o.children.each {|child| check_safety(child)}
        when Psych::Nodes::Alias
        else
          raise Psych::UnsafeYAML.new("Found unknown node type: #{o.class}")
        end
      end

      def self.check_node(n)
        unless n.tag.nil? || ['!ruby/sym', '!ruby/symbol'].include?(n.tag)
          raise Psych::UnsafeYAML.new("Found node with tag: #{n.tag}")
        end
      end
    end

    # Force the default engine back to Psych, since we required Syck.
    YAML::ENGINE.yamler = 'psych'
  end
end


make_yaml_safe!

FWIW, I removed Syck references for Ruby 2.0 here:

https://gist.github.com/thbar/9770758