Understanding Arbitrary File Upload Vulnerablilities

As the name suggests Arbitrary File Upload Vulnerabilities is a type of vulnerability which occurs in web applications if the file type uploaded is not checked, filtered or sanitized.

The main danger of these kind of vulnerabilities is that the attacker can upload a malicious PHP , ASP etc. script and execute it. The main idea is to get the access to the server and execute desired code. for example an Attacker who have gained access to such kind of vulnerability can upload a malicious shell script and further can control the machine to execute desired commands, which would lead to a full compromise of the server and the victim’s server gets owned.

In this tutorial we’ll be looking at a a basic example of a Vulnerable Script and How to exploit it. So let’s get started.

Proof of Concept

For the demonstration of a realistic scenario, I have created a basic vulnerable PHP script.

Upload.php

Code:

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Arbitary file upload ( POC )


   
  Hey all this is a sample php script to upload image files , This script doesn't contains file type checking code which makes it prone to Arbitary file upload vulnerbility. 


   
  


  

Upload


   
  

  




      
          
Upload File 
          

          

      
      


   
  
   
  

In the above script we simply ask the user to input the file to be uploaded and without even checking what the file-type is or its extension we upload it.

This is a basic example of how these bugs occur.

How to exploit it

Now to exploit this common bug is yet simpler, the hacker can simply download any Web Shell-Scripts , Written in PHP , ASP etc.

Some PHP Shells :-

Ani-Shell
[ R57 Shell
C99 Shell

Note: These shells are not intended to be used as this way, author is not responsible for the way in which the user uses it.

Now to exploit this vulnerability the hacker have to carry out some steps :-

Upload the Shell



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