Directory traversal attack
A directory traversal (or path traversal) consists in exploiting insufficient security validation / sanitization of user-supplied input file names, so that characters representing "traverse to parent directory" are passed through to the file APIs.
It's sample code.
<?php $template = 'red.php'; if (isset($_COOKIE['TEMPLATE'])) $template = $_COOKIE['TEMPLATE']; include ("/home/users/phpguru/templates/" . $template); ?>
The repeated ../ characters after /home/users/phpguru/templates/ has caused include() to traverse to the root directory, and then include the Unix password file /etc/passwd.