Authentication bypass via information disclosure | Dec 16, 2022

Introduction

Welcome to my another writeup! In this Portswigger Labs lab, you'll learn: Authentication bypass via information disclosure! Without further ado, let's dive in.

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

Background

This lab's administration interface has an authentication bypass vulnerability, but it is impractical to exploit without knowledge of a custom HTTP header used by the front-end.

To solve the lab, obtain the header name then use it to bypass the lab's authentication. Access the admin interface and delete Carlos's account.

You can log in to your own account using the following credentials: wiener:peter

Exploitation

Home page:

Login as user wiener:

After using gobuster, we can see the admin panel is in /admin:

┌──(root🌸siunam)-[~/ctf/Portswigger-Labs/Information-Disclosure/ID-4]
└─# gobuster dir -u https://0aaf009f04cd8875c0660ff5003900a4.web-security-academy.net/ -w /usr/share/wordlists/dirb/common.txt
[...]
/Admin                (Status: 401) [Size: 2348]
/ADMIN                (Status: 401) [Size: 2348]
/admin                (Status: 401) [Size: 2348]

However, when I reach there:

Hmm… It only allows local users, which is using the localhost(127.0.0.1) IP address.

But, when I change the method from GET to TRACE, something interesting happend:

It has a custom HTTP header called X-Custom-IP-Authorization, which contain my IP address!

Now, what if I also include that custom header in my request?

I'm authenticated!!

In Burp Suite, we can add that custom header to every request I send:

Go to "Proxy" -> "Options":

In "Match and Replace" session, click "Add", and leave the "Match" condition blank, but in the "Replace" field, enter X-Custom-IP-Authorization: 127.0.0.1

Now, let's go to /admin with the custom header!

Delete user carlos:

We did it!

What we've learned:

Authentication bypass via information disclosure