# HTTP Connection Request Smuggling

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## Connection state attacks <a href="#state" id="state"></a>

### **First-request validation**

Reverse proxies often use the **Host header** to identify **which back-end server to route** each request to, and have a whitelist of hosts that people are **allowed to access.**

However, some proxies only apply this **whitelist to the first request sent** over a given connection. This means attackers can gain **access to internal** websites by issuing a **request to an allowed** destination, **followed** by one for the **internal site** down the same connection:

```
GET / HTTP/1.1
Host: redacted

GET / HTTP/1.1
Host: intranet.redacted
```

Mercifully, this **mistake is quite rare**.

### **First-request routing**

First-request routing occurs when the front-end uses the **first request's Host header to decide** which back-end to route the request to, and then **routes all subsequent requests** from the same client connection down the **same back-end connection**.

```
GET / HTTP/1.1
Host: example.com

POST /pwreset HTTP/1.1
Host: psres.net
```

This could be chained with [**Host header attacks**](https://portswigger.net/web-security/host-header) like password reset poisoning, [**web cache poisoning**](https://portswigger.net/web-security/web-cache-poisoning), and gaining access to other virtual hosts.

{% hint style="info" %}
You can scan for these two flaws using the 'connection-state probe' option in HTTP Request Smuggler.
{% endhint %}

## References

* [https://portswigger.net/research/browser-powered-desync-attacks](https://portswigger.net/research/browser-powered-desync-attacks)

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