Insecure Default Variable Initialization

Demonstrations

The following examples help to illustrate the nature of this weakness and describe methods or techniques which can be used to mitigate the risk.

Note that the examples here are by no means exhaustive and any given weakness may have many subtle varieties, each of which may require different detection methods or runtime controls.

Example One

This code attempts to login a user using credentials from a POST request:

// $user and $pass automatically set from POST request
if (login_user($user,$pass)) {
  $authorized = true;
}
...

if ($authorized) {
  generatePage();
}

Because the $authorized variable is never initialized, PHP will automatically set $authorized to any value included in the POST request if register_globals is enabled. An attacker can send a POST request with an unexpected third value 'authorized' set to 'true' and gain authorized status without supplying valid credentials.

Here is a fixed version:

$user = $_POST['user'];
$pass = $_POST['pass'];
$authorized = false;
if (login_user($user,$pass)) {
  $authorized = true;
}
...

This code avoids the issue by initializing the $authorized variable to false and explicitly retrieving the login credentials from the $_POST variable. Regardless, register_globals should never be enabled and is disabled by default in current versions of PHP.

See Also