Jan 26, 2023 by Thibault Debatty | 498 views
In previous blog posts, we have already illustrated two web application vulnerabilities: brute force login cracking and SQL injection. In this post we illustrate a 3rd vulnerability, unrestricted file upload, and show how it can be exploited using a web shell.
Unrestricted file upload vulnerability happens when :
This means that the attacker is able to upload an executable script, that will be executed by the server. One category of such web scripts is called web shell. This creates a shell interface that the attacker can access using his browser . A web shell can be used to:
To illustrate this danger, we will use the Upload web application from Cylab Play, our collection of vulnerable apps. And we will deploy the app using Play with Docker:
This app is a simple photo gallery, where users can upload pictures. The vulnerability arises from the fact that the app is not validating or filtering the type of uploaded files...
To run the attack, instead of uploading an image, we will upload a web shell. The app is written in PHP, so we need a PHP web shell. These are pretty easy to find. This GitHub repo lists some of them:
For this demo, I will use c99, an old but general-purpose web shell:
And I will upload the c99 shell on the gallery:
When c99 is uploaded, it will not be displayed on screen (as it is not an image), but we can check the source of the page to find where the file is stored on the server:
Now if we manually enter this address in the URL bar of the browser, the server will execute the file, because it is a PHP script! This gives us this (nice) interface:
C99 is a general purpose web shell, that allows to browse, download and modify files on the server. For this demo I have simply modified the web page, but this could be used to steal data, run a crypto miner, or start a larger attack...