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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`
HTTP request smuggling attacks are crafted by sending ambiguous requests that exploit discrepancies in how front-end and back-end servers interpret the `Content-Length` (CL) and `Transfer-Encoding` (TE) headers. These attacks can manifest in different forms, primarily as **CL.TE**, **TE.CL**, and **TE.TE**. Each type represents a unique combination of how the front-end and back-end servers prioritize these headers. The vulnerabilities arise from the servers processing the same request in different ways, leading to unexpected and potentially malicious outcomes.
#### CL.TE Vulnerability (Content-Length used by Front-End, Transfer-Encoding used by Back-End)
- **Front-End (CL):** Processes the request based on the `Content-Length` header.
- **Back-End (TE):** Processes the request based on the `Transfer-Encoding` header.
- **Attack Scenario:**
- The attacker sends a request where the `Content-Length` header's value does not match the actual content length.
- The front-end server forwards the entire request to the back-end, based on the `Content-Length` value.
- The back-end server processes the request as chunked due to the `Transfer-Encoding: chunked` header, interpreting the remaining data as a separate, subsequent request.
- **Example:**
```
POST / HTTP/1.1
Host: vulnerable-website.com
Content-Length: 30
Connection: keep-alive
Transfer-Encoding: chunked
0
GET /404 HTTP/1.1
Foo: x
```
#### TE.CL Vulnerability (Transfer-Encoding used by Front-End, Content-Length used by Back-End)
- **Front-End (TE):** Processes the request based on the `Transfer-Encoding` header.
- **Back-End (CL):** Processes the request based on the `Content-Length` header.
- **Attack Scenario:**
- The attacker sends a chunked request where the chunk size (`7b`) and actual content length (`Content-Length: 4`) do not align.
- The front-end server, honoring `Transfer-Encoding`, forwards the entire request to the back-end.
- The back-end server, respecting `Content-Length`, processes only the initial part of the request (`7b` bytes), leaving the rest as part of an unintended subsequent request.
- **Example:**
```
POST / HTTP/1.1
Host: vulnerable-website.com
Content-Length: 4
Connection: keep-alive
Transfer-Encoding: chunked
7b
GET /404 HTTP/1.1
Host: vulnerable-website.com
Content-Type: application/x-www-form-urlencoded
Content-Length: 30
x=
0
```
#### TE.TE Vulnerability (Transfer-Encoding used by both, with obfuscation)
- **Servers:** Both support `Transfer-Encoding`, but one can be tricked into ignoring it via obfuscation.
- **Attack Scenario:**
- The attacker sends a request with obfuscated `Transfer-Encoding` headers.
- Depending on which server (front-end or back-end) fails to recognize the obfuscation, a CL.TE or TE.CL vulnerability may be exploited.
- The unprocessed part of the request, as seen by one of the servers, becomes part of a subsequent request, leading to smuggling.
- **Example:**
```
POST / HTTP/1.1
Host: vulnerable-website.com
Transfer-Encoding: xchunked
Transfer-Encoding : chunked
Transfer-Encoding: chunked
Transfer-Encoding: x
Transfer-Encoding: chunked
Transfer-Encoding: x
Transfer-Encoding:[tab]chunked
[space]Transfer-Encoding: chunked
X: X[\n]Transfer-Encoding: chunked
Transfer-Encoding
: chunked
```
#### **CL.CL Scenario (Content-Length used by both Front-End and Back-End):**
- Both servers process the request based solely on the `Content-Length` header.
- This scenario typically does not lead to smuggling, as there's alignment in how both servers interpret the request length.
- **Example:**
```
POST / HTTP/1.1
Host: vulnerable-website.com
Content-Length: 16
Connection: keep-alive
Normal Request
```
#### **CL != 0 Scenario:**
- Refers to scenarios where the `Content-Length` header is present and has a value other than zero, indicating that the request body has content.
- It's crucial in understanding and crafting smuggling attacks, as it influences how servers determine the end of a request.
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**.
Identifying HTTP request smuggling vulnerabilities can often be achieved using timing techniques, which rely on observing how long it takes for the server to respond to manipulated requests. These techniques are particularly useful for detecting CL.TE and TE.CL vulnerabilities. Besides these methods, there are other strategies and tools that can be used to find such vulnerabilities:
### Finding CL.TE Vulnerabilities Using Timing Techniques
- **Method:**
- Send a request that, if the application is vulnerable, will cause the back-end server to wait for additional data.
- **Example:**
```
POST / HTTP/1.1
Host: vulnerable-website.com
Transfer-Encoding: chunked
Connection: keep-alive
Content-Length: 4
1
A
0
```
- **Observation:**
- The front-end server processes the request based on `Content-Length` and cuts off the message prematurely.
- The back-end server, expecting a chunked message, waits for the next chunk that never arrives, causing a delay.
- **Indicators:**
- Timeouts or long delays in response.
- Receiving a 400 Bad Request error from the back-end server, sometimes with detailed server information.
### Finding TE.CL Vulnerabilities Using Timing Techniques
- **Method:**
- Send a request that, if the application is vulnerable, will cause the back-end server to wait for additional data.
- **Example:**
```
POST / HTTP/1.1
Host: vulnerable-website.com
Transfer-Encoding: chunked
Connection: keep-alive
Content-Length: 6
0
X
```
- **Observation:**
- The front-end server processes the request based on `Transfer-Encoding` and forwards the entire message.
- The back-end server, expecting a message based on `Content-Length`, waits for additional data that never arrives, causing a delay.
### Other Methods to Find Vulnerabilities
- **Differential Response Analysis:**
- Send slightly varied versions of a request and observe if the server responses differ in an unexpected way, indicating a parsing discrepancy.
- **Using Automated Tools:**
- Tools like Burp Suite's 'HTTP Request Smuggler' extension can automatically test for these vulnerabilities by sending various forms of ambiguous requests and analyzing the responses.
- **Content-Length Variance Tests:**
- Send requests with varying `Content-Length` values that are not aligned with the actual content length and observe how the server handles such mismatches.
- **Transfer-Encoding Variance Tests:**
- Send requests with obfuscated or malformed `Transfer-Encoding` headers and monitor how differently the front-end and back-end servers respond to such manipulations.
### HTTP Request Smuggling Vulnerability Testing
After confirming the effectiveness of timing techniques, it's crucial to verify if client requests can be manipulated. A straightforward method is to attempt poisoning your requests, for instance, making a request to `/` yield a 404 response. The `CL.TE` and `TE.CL` examples previously discussed in [Basic Examples](./#basic-examples) demonstrate how to poison a client's request to elicit a 404 response, despite the client aiming to access a different resource.
**Key Considerations**
When testing for request smuggling vulnerabilities by interfering with other requests, bear in mind:
* **Distinct Network Connections:** The "attack" and "normal" requests should be dispatched over separate network connections. Utilizing the same connection for both doesn't validate the vulnerability's presence.
* **Consistent URL and Parameters:** Aim to use identical URLs and parameter names for both requests. Modern applications often route requests to specific back-end servers based on URL and parameters. Matching these increases the likelihood that both requests are processed by the same server, a prerequisite for a successful attack.
* **Timing and Racing Conditions:** The "normal" request, meant to detect interference from the "attack" request, competes against other concurrent application requests. Therefore, send the "normal" request immediately following the "attack" request. Busy applications may necessitate multiple trials for conclusive vulnerability confirmation.
* **Load Balancing Challenges:** Front-end servers acting as load balancers may distribute requests across various back-end systems. If the "attack" and "normal" requests end up on different systems, the attack won't succeed. This load balancing aspect may require several attempts to confirm a vulnerability.
* **Unintended User Impact:** If your attack inadvertently impacts another user's request (not the "normal" request you sent for detection), this indicates your attack influenced another application user. Continuous testing could disrupt other users, mandating a cautious approach.
Sometimes, front-end proxies enforce security measures, scrutinizing incoming requests. However, these measures can be circumvented by exploiting HTTP Request Smuggling, allowing unauthorized access to restricted endpoints. For instance, accessing `/admin` might be prohibited externally, with the front-end proxy actively blocking such attempts. Nonetheless, this proxy may neglect to inspect embedded requests within a smuggled HTTP request, leaving a loophole for bypassing these restrictions.
Consider the following examples illustrating how HTTP Request Smuggling can be used to bypass front-end security controls, specifically targeting the `/admin` path which is typically guarded by the front-end proxy:
In the CL.TE attack, the `Content-Length` header is leveraged for the initial request, while the subsequent embedded request utilizes the `Transfer-Encoding: chunked` header. The front-end proxy processes the initial `POST` request but fails to inspect the embedded `GET /admin` request, allowing unauthorized access to the `/admin` path.
Conversely, in the TE.CL attack, the initial `POST` request uses `Transfer-Encoding: chunked`, and the subsequent embedded request is processed based on the `Content-Length` header. Similar to the CL.TE attack, the front-end proxy overlooks the smuggled `GET /admin` request, inadvertently granting access to the restricted `/admin` path.
Applications often employ a **front-end server** to modify incoming requests before passing them to the back-end server. A typical modification involves adding headers, such as `X-Forwarded-For: <IP of the client>`, to relay the client's IP to the back-end. Understanding these modifications can be crucial, as it might reveal ways to **bypass protections** or **uncover concealed information or endpoints**.
To investigate how a proxy alters a request, locate a POST parameter that the back-end echoes in the response. Then, craft a request, using this parameter last, similar to the following:
In this structure, subsequent request components are appended after `search=`, which is the parameter reflected in the response. This reflection will expose the headers of the subsequent request.
It's important to align the `Content-Length` header of the nested request with the actual content length. Starting with a small value and incrementing gradually is advisable, as too low a value will truncate the reflected data, while too high a value can cause the request to error out.
This technique is also applicable in the context of a TE.CL vulnerability, but the request should terminate with `search=\r\n0`. Regardless of the newline characters, the values will append to the search parameter.
This method primarily serves to understand the request modifications made by the front-end proxy, essentially performing a self-directed investigation.
It's feasible to capture the requests of the next user by appending a specific request as the value of a parameter during a POST operation. Here's how this can be accomplished:
By appending the following request as the value of a parameter, you can store the subsequent client's request:
In this scenario, the **comment parameter** is intended to store the contents within a post's comment section on a publicly accessible page. Consequently, the subsequent request's contents will appear as a comment.
However, this technique has limitations. Generally, it captures data only up to the parameter delimiter used in the smuggled request. For URL-encoded form submissions, this delimiter is the `&` character. This means the captured content from the victim user's request will stop at the first `&`, which may even be part of the query string.
Additionally, it's worth noting that this approach is also viable with a TE.CL vulnerability. In such cases, the request should conclude with `search=\r\n0`. Regardless of newline characters, the values will be appended to the search parameter.
In scenarios where a website is susceptible to Reflected XSS through the User-Agent header, the following payload demonstrates how to exploit this vulnerability:
1. Initiating a `POST` request, seemingly typical, with a `Transfer-Encoding: chunked` header to indicate the start of smuggling.
2. Following with a `0`, marking the end of the chunked message body.
3. Then, a smuggled `GET` request is introduced, where the `User-Agent` header is injected with a script, `<script>alert(1)</script>`, triggering the XSS when the server processes this subsequent request.
By manipulating the `User-Agent` through smuggling, the payload bypasses normal request constraints, thus exploiting the Reflected XSS vulnerability in a non-standard but effective manner.
### 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>
Applications often redirect from one URL to another by using the hostname from the `Host` header in the redirect URL. This is common with web servers like Apache and IIS. For instance, requesting a folder without a trailing slash results in a redirect to include the slash:
In this scenario, a user's request for a JavaScript file is hijacked. The attacker can potentially compromise the user by serving malicious JavaScript in 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>
Web cache poisoning can be executed if any component of the **front-end infrastructure caches content**, typically to enhance performance. By manipulating the server's response, it's possible to **poison the cache**.
Previously, we observed how server responses could be altered to return a 404 error (refer to [Basic Examples](./#basic-examples)). Similarly, it’s feasible to trick the server into delivering `/index.html` content in response to a request for `/static/include.js`. Consequently, the `/static/include.js` content gets replaced in the cache with that of `/index.html`, rendering `/static/include.js` inaccessible to users, potentially leading to a Denial of Service (DoS).
This technique becomes particularly potent if an **Open Redirect vulnerability** is discovered or if there's an **on-site redirect to an open redirect**. Such vulnerabilities can be exploited to replace the cached content of `/static/include.js` with a script under the attacker's control, essentially enabling a widespread Cross-Site Scripting (XSS) attack against all clients requesting the updated `/static/include.js`.
Below is an illustration of exploiting **cache poisoning combined with an on-site redirect to open redirect**. The objective is to alter the cache content of `/static/include.js` to serve JavaScript code controlled by the attacker:
Note the embedded request targeting `/post/next?postId=3`. This request will be redirected to `/post?postId=4`, utilizing the **Host header value** to determine the domain. By altering the **Host header**, the attacker can redirect the request to their domain (**on-site redirect to open redirect**).
After successful **socket poisoning**, a **GET request** for `/static/include.js` should be initiated. This request will be contaminated by the prior **on-site redirect to open redirect** request and fetch the content of the script controlled by the attacker.
### 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.
If this smuggled request poisons a cache entry intended for static content (e.g., `/someimage.png`), the victim's sensitive data from `/private/messages` might be cached under the static content's cache entry. Consequently, the attacker could potentially retrieve these cached sensitive data.
* [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.
<summary><strong>Learn AWS hacking from zero to hero with</strong><ahref="https://training.hacktricks.xyz/courses/arte"><strong>htARTE (HackTricks AWS Red Team Expert)</strong></a><strong>!</strong></summary>
* If you want to see your **company advertised in HackTricks** or **download HackTricks in PDF** Check the [**SUBSCRIPTION PLANS**](https://github.com/sponsors/carlospolop)!
* **Join the** 💬 [**Discord group**](https://discord.gg/hRep4RUj7f) or the [**telegram group**](https://t.me/peass) or **follow** us on **Twitter** 🐦 [**@carlospolopm**](https://twitter.com/hacktricks_live)**.**
* **Share your hacking tricks by submitting PRs to the** [**HackTricks**](https://github.com/carlospolop/hacktricks) and [**HackTricks Cloud**](https://github.com/carlospolop/hacktricks-cloud) github repos.
