Host validation bypass via connection state attack | Dec 28, 2022

Introduction

Welcome to my another writeup! In this Portswigger Labs lab, you'll learn: Host validation bypass via connection state attack! Without further ado, let's dive in.

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

Background

This lab is vulnerable to routing-based SSRF via the Host header. Although the front-end server may initially appear to perform robust validation of the Host header, it makes assumptions about all requests on a connection based on the first request it receives.

To solve the lab, exploit this behavior to access an internal admin panel located at 192.168.0.1/admin, then delete the user carlos.

Exploitation

Home page:

Now, we can try to modify the Host HTTP header in Burp Repeater:

However, it redirects me to the lab domain.

What if I supply multiple Host header?

Duplicate header names are not allowed.

How about indenting HTTP headers with a space character?

Hmm… Still the same.

In the lab's background, it said:

This lab is vulnerable to routing-based SSRF via the Host header. Although the front-end server may initially appear to perform robust validation of the Host header, it makes assumptions about all requests on a connection based on the first request it receives.

Hmm… What if I send a normal Host header on the first request, then in the second request I send a malicious Host header, which points to 192.168.0.1?

To do so, I'll use 2 Burp Repeater tabs:

• Tab 1: GET /, normal Host header:

• Tab 2: GET /admin, Host header change to 192.168.0.1:

• Add both tabs to a new group:

• Change the send mode to Send group in sequence (single connection):

• Change the Connection header to keep-alive:

• Click Send group (single connection):

As you can see, the second request has successfully accessed the admin panel!

Now, in order to delete user carlos, we need to send a POST request to /admin/delete, with parameter csrf, and username.

Let's modify the second tab:

• Change the location to /admin/delete:

• Change the request method:

• Add parameter csrf, username with correct value:

• Send the request:

We did it!

What we've learned:

1. Host validation bypass via connection state attack