File path traversal, traversal sequences stripped with superfluous URL-decode | Dec 12, 2022

Introduction

Welcome to my another writeup! In this Portswigger Labs lab, you’ll learn: File path traversal, traversal sequences stripped with superfluous URL-decode! Without further ado, let’s dive in.

Overall difficulty for me (From 1-10 stars): ★☆☆☆☆☆☆☆☆☆

Background

This lab contains a file path traversal vulnerability in the display of product images.

The application blocks input containing path traversal sequences. It then performs a URL-decode of the input before using it.

To solve the lab, retrieve the contents of the /etc/passwd file.

Exploitation

Home page:

In the previous labs, we found that there is a file path traversal vulnerability in the display of product images:

Also, in the lab background, it said:

The application blocks input containing path traversal sequences. It then performs a URL-decode of the input before using it.

To bypass that, we can use double URL encoding:

To do so, I’ll use CyberChef to do URL encoding:

Now, we can use %252E%252E%252F as ../:

# Before URL encoded
/image?filename=../../../etc/passwd

# After double URL encoded
/image?filename=%252E%252E%252F%252E%252E%252F%252E%252E%252F/etc/passwd

What we’ve learned:

File path traversal, traversal sequences stripped with superfluous URL-decode