> For the complete documentation index, see [llms.txt](https://angelica.gitbook.io/hacktricks/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://angelica.gitbook.io/hacktricks/pentesting-web/http-connection-request-smuggling.md).
# HTTP Connection Request Smuggling
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**This is a summary of the post** [**https://portswigger.net/research/browser-powered-desync-attacks**](https://portswigger.net/research/browser-powered-desync-attacks)
## Connection State Attacks
### First-request Validation
When routing requests, reverse proxies might depend on the **Host header** to determine the destination back-end server, often relying on a whitelist of hosts that are permitted access. However, a vulnerability exists in some proxies where the whitelist is only enforced on the initial request in a connection. Consequently, attackers could exploit this by first making a request to an allowed host and then requesting an internal site through the same connection:
```
GET / HTTP/1.1
Host: [allowed-external-host]
GET / HTTP/1.1
Host: [internal-host]
```
### First-request Routing
In some configurations, a front-end server may use the **Host header of the first request** to determine the back-end routing for that request, and then persistently route all subsequent requests from the same client connection to the same back-end connection. This can be demonstrated as:
```
GET / HTTP/1.1
Host: example.com
POST /pwreset HTTP/1.1
Host: psres.net
```
This issue can potentially be combined with [Host header attacks](https://portswigger.net/web-security/host-header), such as password reset poisoning or [web cache poisoning](https://portswigger.net/web-security/web-cache-poisoning), to exploit other vulnerabilities or gain unauthorized access to additional virtual hosts.
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To identify these vulnerabilities, the 'connection-state probe' feature in HTTP Request Smuggler can be utilized.
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