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First of all, this technique **abuses a HTTP Request Smuggling vulnerability**, so you need to know what that is:
The **main****difference** between this technique and a common HTTP Request smuggling is that **instead** of **attacking** the **request** of the **victim****by adding a prefix to it**, we are going to **leak or modify the response the victim receives**. This is done by, instead of sending 1 request and a half to abuse the HTTP Request smuggling, **send 2 complete requests to desynchronise the proxies responses queue**.
This is because we are going to be able to **desynchronise the response queue** so the **response** from the **legit****request** of the **victim is sent to the attacker**, or by **injecting attackers controlled content in the response to the victim**.
HTTP/1.1 allows to ask for **different resources without needing to wait for previous ones**. Therefore, if there is a **proxy** in the **middle**, it's the proxies task to **maintain a synchronised match of requests sent to the backend and responses coming from it**.
However, there is a problem desynchronising the responses queue. If an attacker send a HTTP Response smuggling attack and the responses to the **initial request and the smuggled one are responded immediately**, the smuggled response won't be inserted inside the queue of the victim response but will **just be discarded as an error**.
Therefore, it's needed that the **smuggled****request****takes more time to be processed** inside the back-end server. Therefore, by the time the smuggled request is processed, the communication with the attacker will be over.
If in this specific situation a **victim has sent a request** and the **smuggled request is responded before** the legitimate request, the **smuggled response will be sent to the victim**. Therefore, the attacker will be **controlling the request "performed" by the victim**.
Moreover, is the **attacker then perform a request** and the **legitimate response** to the **victim** request is **answered****before** the attackers request. The **response to the victim is going to be sent to the attacker**, **stealing** the response to the victim (which can contains for example the header **Set-Cookie**).
Another **interesting difference** with common **HTTP Request Smuggling** is that, in a common smuggling attack, the **goal** is to **modify the beginning of the victims request** so it perform an unexpected action. In a **HTTP Response smuggling attack**, as you are **sending full requests**, you can **inject in one payload tens of responses** that will be **desynchronising tens of users** that will be **receiving** the **injected****responses**.
Apart from being able to **distribute more easily tens of exploits** across legitimate users, this could also be used to cause a **DoS** in the server.
As explained previously, in order to abuse this technique, it's needed that the **first smuggled message** into the server **requires a lot of time to be processed**.
This **time consuming request is enough** if we just want to **try to steal the victims response.** But if you want to perform a more complex exploit this will be a common structure for the exploit.
First of all the **initial** request abusing **HTTP****Request****smuggling**, then the **time consuming request** and then **1 or more payload requests** that whose responses will be sent to the victims.
As with HTTP Request Smuggling known payloads, you can **steal the victims request** with one important difference: In this case you just need the **send content to be reflected in the response**, **no persistent storage** is needed.
Then, once the **initial request** (blue) was **processed** and **while** the **sleepy** one is being processed (yellow) the **next request that arrives from a victim** is going to be **appended in the queue just after the reflected parameter**:
Then, the **victim** will **receive** the **response to the sleepy** request and if in the meantime the **attacker****sent****another****request**, the **response from the reflected content request will be sent to him**.
Up to this point, we have learned how to abuse HTTP Request Smuggling attacks to **control** the **request****whose****response** a **client** is going to **receive** and how you can then **steal the response that was meant for the victim**.
But it's still possible to **desynchronise even** more the responses.
There are interesting requests like **HEAD** request that are specified to not have **any content inside the responses body** and that should (must) **contain the Content-Length** of the request like **if it was a GET request**.
Therefore, if an attacker **injects** a **HEAD** request, like in this images:
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Then, **once the blue one is responded to the attacker**, the next victims request is going to be introduced in the queue:
Then, the **victim** will **receive** the **response** from the **HEAD** request, which is **going to contain a Content-Length but no content at all**. Therefore, the proxy **won't send this response** to the victim, but will **wait** for some **content**, which actually is going to be **response to the yellow request** (also injected by the attacker):
Following the previous example, knowing that you can **control the body** of the request whose response is going to receive the victim and that a **HEAD****response** usually contains in its headers the **Content-Type and the Content-Length**, you can **send a request like the following** one to **cause XSS** in the victim without the page being vulnerable to XSS:
Abusing the previously commented response desynchronisation Content Confusion attack, i**f the cache stores the response to the request performed by the victim and this response is an injected one causing a XSS, then the cache is poisoned**.
Note that in this case if the **"victim" is the attacker** he can now perform **cache poisoning in arbitrary URLs** as he can **control the URL that is going to be cached** with the malicious response.
This attack is similar to the previous one, but **instead of injecting a payload inside the cache, the attacker will be caching victim information inside of the cache:**
The **goal** of this attack is to abuse again the **response****desynchronisation** in order to **make the proxy send a 100% attacker generated response**.
In order to achieve this, the attacker needs to find an endpoint of the web application that is **reflecting some values inside the response** and **know the content length of the HEAD response**.
However, note how the **reflected data had a size according to the Content-Length** of the **HEAD** response that **generated a valid HTTP response in the response queue**.
Therefore, the **next request of the second victim** will be **receiving** as **response something completely crafted by the attacker**. As the response is completely crafted by the attacker he can also **make the proxy cache the response**.
* Don't forget to check this video explaining all these techniques really good: [https://www.youtube.com/watch?v=suxDcYViwao\&t=1343s](https://www.youtube.com/watch?v=suxDcYViwao\&t=1343s)
* Do you work in a **cybersecurity company**? Do you want to see your **company advertised in HackTricks**? or do you want to have access to the **latest version of the PEASS or download HackTricks in PDF**? Check the [**SUBSCRIPTION PLANS**](https://github.com/sponsors/carlospolop)!
* Discover [**The PEASS Family**](https://opensea.io/collection/the-peass-family), our collection of exclusive [**NFTs**](https://opensea.io/collection/the-peass-family)
* Get the [**official PEASS & HackTricks swag**](https://peass.creator-spring.com)
* **Join the** [**💬**](https://emojipedia.org/speech-balloon/) [**Discord group**](https://discord.gg/hRep4RUj7f) or the [**telegram group**](https://t.me/peass) or **follow** me on **Twitter** [**🐦**](https://github.com/carlospolop/hacktricks/tree/7af18b62b3bdc423e11444677a6a73d4043511e9/\[https:/emojipedia.org/bird/README.md)[**@carlospolopm**](https://twitter.com/carlospolopm)**.**
* **Share your hacking tricks by submitting PRs to the [hacktricks repo](https://github.com/carlospolop/hacktricks) and [hacktricks-cloud repo](https://github.com/carlospolop/hacktricks-cloud)**.
