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This vulnerability occurs when a **desyncronization** between **front-end proxies** and the **back-end** server allows an **attacker** to **send** an HTTP **request** that will be **interpreted** as a **single request** by the **front-end** proxies (load balance/reverse-proxy) and **as 2 request** by the **back-end** server.\
The **Front-End** (a load-balance / Reverse Proxy) **process** the _**content-length**_ or the _**transfer-encoding**_ header and the **Back-end** server **process the other** one provoking a **desyncronization** between the 2 systems.\
This could be very critical as **an attacker will be able to send one request** to the reverse proxy that will be **interpreted** by the **back-end** server **as 2 different requests**. The **danger** of this technique resides in the fact the **back-end** server **will interpret** the **2nd request injected** as if it **came from the next client** and the **real request** of that client will be **part** of the **injected request**.
* **Content-Length**: This header uses a **decimal number** to indicate the **number** of **bytes** of the **body** of the request. The body is expected to end in the last character, **a new line is not needed in the end of the request**.
* **Transfer-Encoding:** This header uses in the **body** an **hexadecimal number** to indicate the **number** of **bytes** of the **next chunk**. The **chunk** must **end** with a **new line** but this new line **isn't counted** by the length indicator. This transfer method must end with a **chunk of size 0 followed by 2 new lines**: `0`
So, request smuggling attacks involve placing both the `Content-Length` header and the `Transfer-Encoding` header into a single HTTP request and manipulating these so that the front-end and back-end servers process the request differently. The exact way in which this is done depends on the behaviour of the two servers:
* **CL.TE**: the front-end server uses the `Content-Length` header and the back-end server uses the `Transfer-Encoding` header.
* **TE.CL**: the front-end server uses the `Transfer-Encoding` header and the back-end server uses the `Content-Length` header.
* **TE.TE**: the front-end and back-end servers both support the `Transfer-Encoding` header, but one of the servers can be induced not to process it by obfuscating the header in some way.
Here, the **front-end** server uses the **`Content-Length`** header and the **back-end** server uses the **`Transfer-Encoding`** header. We can perform a simple HTTP request smuggling attack as follows:
Note how `Content-Length` indicate the **bodies request length is 30 bytes long** (_remember that HTTP uses as new line, so 2bytes each new line_), so the reverse proxy **will send the complete request** to the back-end, and the back-end will process the `Transfer-Encoding` header leaving the `GET /404 HTTP/1.1` as the **begging of the next request** (BTW, the next request will be appended to `Foo:x<Next request starts here>`).
Here, the front-end server uses the `Transfer-Encoding` header and the back-end server uses the `Content-Length` header. We can perform a simple HTTP request smuggling attack as follows:
In this case the **reverse-proxy** will **send the hole request** to the **back-end** as the **`Transfer-encoding`** indicates so. But, the **back-end** is going to **process** only the **`7b`** (4bytes) as indicated in the `Content-Lenght` .Therefore, the next request will be the one starting by `GET /404 HTTP/1.1`
_Also note that the Content-Length of the embedded request will indicate the length of the next request that is going to b appended to the **x** parameter. If it's too small, only a few bytes will be appended, and if to large (bigger that the length of the next request) and error will be thrown for the next request._
Here, the front-end and back-end servers both support the `Transfer-Encoding` header, but one of the servers can be induced not to process it by obfuscating the header in some way.\
Depending on the server (reverse-proxy or backing) that **stops processing** the **TE** header, you will find a **CL.TE vulnerability** or a **TE.CL vulnerability**.
Since the front-end server uses the `Content-Length` header, it will forward only part of this request, omitting the `0`. The back-end server uses the `Transfer-Encoding` header, processes the first chunk, and then waits for the next chunk to arrive. This will cause an observable time delay.
Sometimes, instead of getting a timeout you receive a 400 bad request from the final host like in the following scenario, where a CL.TE payload is sent:
Since the front-end server uses the `Transfer-Encoding` header, it will forward only part of this request, omitting the `X`. The back-end server uses the `Content-Length` header, expects more content in the message body, and waits for the remaining content to arrive. This will cause an observable time delay.
In the [Basic Examples](./#basic-examples) we already saw `CL.TE` and `TE.CL` examples of how to poison a clients request to ask for `/404` provoking a 404 response when the client was asking for any other resource.
Some important considerations should be kept in mind when attempting to confirm request smuggling vulnerabilities via interference with other requests:
* The "attack" request and the "normal" request should be sent to the server using different network connections. Sending both requests through the same connection won't prove that the vulnerability exists.
* The "attack" request and the "normal" request should use the same URL and parameter names, as far as possible. This is because many modern applications route front-end requests to different back-end servers based on the URL and parameters. Using the same URL and parameters increases the chance that the requests will be processed by the same back-end server, which is essential for the attack to work.
* When testing the "normal" request to detect any interference from the "attack" request, you are in a race with any other requests that the application is receiving at the same time, including those from other users. You should send the "normal" request immediately after the "attack" request. If the application is busy, you might need to perform multiple attempts to confirm the vulnerability.
* In some applications, the front-end server functions as a load balancer, and forwards requests to different back-end systems according to some load balancing algorithm. If your "attack" and "normal" requests are forwarded to different back-end systems, then the attack will fail. This is an additional reason why you might need to try several times before a vulnerability can be confirmed.
* If your attack succeeds in interfering with a subsequent request, but this wasn't the "normal" request that you sent to detect the interference, then this means that another application user was affected by your attack. If you continue performing the test, this could have a disruptive effect on other users, and you should exercise caution.
Abusing hop-by-hop headers you could indicate the proxy to **delete the header Content-Length or Transfer-Encoding so a HTTP request smuggling is possible to abuse**.
```
Connection: Content-Lentgh
```
For **more information about hop-by-hop headers** visit:
Some times the **front-end proxies will perform some security checks**. You can avoid them by abusing HTTP Request Smuggling as you will be able to **bypass the protections**. For example, in this example you **cannot access `/admin` from the outside** and the front-end proxy is checking that, but this **proxy isn't checking the embedded request**:
In many applications, the **front-end server performs some rewriting of requests** before they are forwarded to the back-end server, typically by adding some additional request headers.\
One common thing to do is to **add to the request the header**`X-Forwarded-For: <IP of the client>` or some similar header so the back-end knows the IP of the client.\
Sometimes, if you can **find which new values are appended** to the request you could be able to **bypass protections** and **access hidden information**/**endpoints**.
For discovering how is the proxy rewriting the request you need to **find a POST parameter that the back-end will reflect it's value** on the response. Then, use this parameter the last one and use an exploit like this one:
In this case the next request will be appended after `search=` which is also **the parameter whose value is going to be reflected** on the response, therefore it's going to **reflect the headers of the next request**.
Note that **only the length indicated in the `Content-Length` header of the embedded request is going to be reflected**. If you use a low number, only a few bytes will be reflected, if you use a bigger number than the length of all the headers, then the embedded request will throw and error. Then, you should **start** with a **small number** and **increase** it until you see all you wanted to see.\
Note also that this **technique is also exploitable with a TE.CL** vulnerability but the request must end with `search=\r\n0`. However, independently of the new line characters the values are going to be appended to the search parameter.
If you can find a POST request which is going to save the contents of one of the parameters you can append the following request as the value of that parameter in order to store the quest of the next client:
In this case, the value of the **parameter comment** is going to be **saved inside a comment** of a post in the page that is **publicly available**, so a **comment will appear with the content of the next request**.
_One limitation with this technique is that it will generally only capture data up until the parameter delimiter that is applicable for the smuggled request. For URL-encoded form submissions, this will be the `&` character, meaning that the content that is stored from the victim user's request will end at the first `&`, which might even appear in the query string._
Note also that this **technique is also exploitable with a TE.CL** vulnerability but the request must end with `search=\r\n0`. However, independently of the new line characters the values are going to be appended to the search parameter.
If the web page is also **vulnerable to Reflected XSS**, you can abuse HTTP Request Smuggling to attack clients of the web. The exploitation of Reflected XSS from HTTP Request Smuggling have some advantages:
* It can be used to **exploit** XSS behavior in parts of the request that **cannot be trivially controlled in a normal reflected XSS attack**, such as HTTP request headers.
### Using HTTP request smuggling to turn an on-site redirect into an open redirect <a href="#using-http-request-smuggling-to-turn-an-on-site-redirect-into-an-open-redirect" id="using-http-request-smuggling-to-turn-an-on-site-redirect-into-an-open-redirect"></a>
Many applications perform on-site redirects from one URL to another and place the hostname from the request's `Host` header into the redirect URL. An example of this is the default behavior of Apache and IIS web servers, where a request for a folder without a trailing slash receives a redirect to the same folder including the trailing slash:
This behavior is normally considered harmless, but it can be exploited in a request smuggling attack to redirect other users to an external domain. For example:
The smuggled request will trigger a redirect to the attacker's website, which will affect the next user's request that is processed by the back-end server. For example:
Here, the user's request was for a JavaScript file that was imported by a page on the web site. The attacker can fully compromise the victim user by returning their own JavaScript in the response.
### Using HTTP request smuggling to perform web cache poisoning <a href="#using-http-request-smuggling-to-perform-web-cache-poisoning" id="using-http-request-smuggling-to-perform-web-cache-poisoning"></a>
If any part of the **front-end infrastructure performs caching of content** (generally for performance reasons) the it **might be possible to poison that cache modifying the response of the server**.
We have already see how to modify the expected returned value from the server to a 404 (in the [Basic Examples](./#basic-examples)), in a similar way you could make the server return the content of /index.html when the poisoned request is asking for `/static/include.js` . This way, the content of the `/static/include.js` will be cached with the content of `/index.html` making `/static/include.js` inaccessible to the clients (DoS?).
Notice this is even more interesting if you find some **Open Redirect** or some **on-site redirect to open redirect** (last section). Because, you could be able to **change the cache values** of `/static/include.js` with the **ones of a script controlled by you** (making a **general XSS to all the clients** that try to download the new version of`/static/include.js`).
In this example it's going to be shown how you can exploit a **cache poisoning + on-site redirect to open redirect** to modify the contents of the cache of `/static/include.js` to **serve JS code controlled** by the attacker:
Note how the embedded request is asking for `/post/next?postId=3` This request is going to be redirected to `/post?postId=4` and **will use the value of the Host header** to indicate the domain. Therefore, you can **modify the Host header** to point the attackers server and the redirect will use that domain (**on-site redirect to open redirect**).
Then, **after poisoning the socket**, you need to send a **GET request** to \*\*`/static/include.js`\*\*this request will be **poisoned** by the **on-site redirect to open redirect** request and will **grab the contents of the attacker controlled script**.
### Using HTTP request smuggling to perform web cache deception <a href="#using-http-request-smuggling-to-perform-web-cache-deception" id="using-http-request-smuggling-to-perform-web-cache-deception"></a>
> **What is the difference between web cache poisoning and web cache deception?**
>
> * In **web cache poisoning**, the attacker causes the application to store some malicious content in the cache, and this content is served from the cache to other application users.
> * In **web cache deception**, the attacker causes the application to store some sensitive content belonging to another user in the cache, and the attacker then retrieves this content from the cache.
In this variant, the attacker smuggles a request that returns some sensitive user-specific content. For example:
If the **poison reaches a client that was accessing some static content** like `/someimage.png` that was going to be **cached**. The contents of `/private/messages` of the victim will be cached in `/someimage.png` and the attacker will be able to steal them.\
Note that the **attacker doesn't know which static content the victim was trying to access** so probably the best way to test this is to perform the attack, wait a few seconds and **load all** the static contents and **search for the private data**.
[Image from here.](https://twitter.com/SpiderSec/status/1200413390339887104?ref\_src=twsrc%5Etfw%7Ctwcamp%5Etweetembed%7Ctwterm%5E1200413390339887104\&ref\_url=https%3A%2F%2Ftwitter.com%2FSpiderSec%2Fstatus%2F1200413390339887104)
* [https://github.com/bahruzjabiyev/t-reqs-http-fuzzer](https://github.com/bahruzjabiyev/t-reqs-http-fuzzer): This tool is a grammar-based HTTP Fuzzer useful to find weird request smuggling discrepancies.
* 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)
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