DVWA - File inclusion

Starting the challenge

Refer to the post start DVWA with Docker to learn how to start DVWA.

I will mostly use Burp Suite to solve the challenges. To configure Burp suite refer to the post configure burp suite for DVWA.

Click on the File inclusion button on the left menu to access the challenge.

Low Level

Understanding the application

We reach a page allowing us to choose between three files.

If we select the first file we get our IP address.

The third file gives us even more information :

File 3
Welcome back admin
Your IP address is: 172.17.0.1
Your user-agent address is: Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:67.0) Gecko/20100101 Firefox/67.0
You came from: http://localhost/vulnerabilities/fi/?page=include.php
I'm hosted at: localhost

What is interesting is the way the page seem to be loaded : with php includes. If we look at the URLs they are constructed like this host?page=<filename> :

• http://localhost/vulnerabilities/fi/?page=file1.php
• http://localhost/vulnerabilities/fi/?page=file2.php
• http://localhost/vulnerabilities/fi/?page=file3.php

Inclusion vulnerability

If the code uses PHP includes we might be able to trick the server into serving other files located on a server we control.

For instance if I set up a file index.php with the following code on my server

<?php phpinfo() ?>

then the vulnerable site will load my index.php file and execute my code which will display php information.

To do so I replace file1.php by <my_url> in the URL.

We can go further and upload a script that will let me run commands on the vulnerable server.

Vulnerable code

The code is very light :

<?php

// The page we wish to display
$file = $_GET[ 'page' ];

?>

It justs retrieves the content of the parameter page and loads it as a filepath. The problem here is that the user input is trusted not to go outside of the perimeter thought for the application.

If only files file1.php, file2.php and file3.php should be readable by the user, the code should implement a whitelist to only allow those files.

Moreover, the other files on the server can have specific permission sets disallowing the user www-data from reading them (or listing the directories).

Medium level

In this level the previous attack does not work. It seems that the developer has implemented a system of regular expressions to validate the input passed in the page parameter.

But it is difficult to think of everything. If we configure a burp intruder with the list of local file inclusions we can see that /etc/passwd returns some information.

And there we managed to retrieve the list of users. If we can upload files we could upload our own php script and get a shell on the server.

Vulnerable code

The code strips http: and relative paths ../ from the user’s input but fails to take into account absolute paths.

<?php

// The page we wish to display
$file = $_GET[ 'page' ];

// Input validation
$file = str_replace( array( "http://", "https://" ), "", $file );
$file = str_replace( array( "../", "..\"" ), "", $file );

?>

In addition to that, the server is misconfigured and allows any user to access sensitive files.

Remediation

The server should only serve files related to the website. The developper should implement a whitelist of files to display.

High level

In this level the direct file inclusion /etc/passwd does not seem to work. It might be that the developer implemented a control to ensure that the parameter starts with file.

However there are techniques to bypass these kind of filters. For instance we can try %00 or %0A.

If we try the following URL we will dump the passwd file :

http://localhost/vulnerabilities/fi/?page=file1.php%0A/../../../../../../etc/passwd

You can learn more about null-byte injection here.

Vulnerable code

<?php

// The page we wish to display
$file = $_GET[ 'page' ];

// Input validation
if( !fnmatch( "file*", $file ) && $file != "include.php" ) {
    // This isn't the page we want!
    echo "ERROR: File not found!";
    exit;
}

?>

Impossible level

The impossible level implements a whitelist :

<?php

// The page we wish to display
$file = $_GET[ 'page' ];

// Only allow include.php or file{1..3}.php
if( $file != "include.php" && $file != "file1.php" && $file != "file2.php" && $file != "file3.php" ) {
    // This isn't the page we want!
    echo "ERROR: File not found!";
    exit;
}

?>