hacktricks/pentesting-web/http-request-smuggling/browser-http-request-smuggling.md

# Browser HTTP Request Smuggling

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## CL.0/H2.0 browser-compatible desync

This vulnerability occurs when the **Content Length** (CL) header is being completely **ignored** by the **backend server**. Then, the back-end treats the **body** as the **start of the second request's method**. Ignoring the CL is equivalent to treating it as having a value of 0, so this is a CL.0 desync - a [known](https://i.blackhat.com/USA-20/Wednesday/us-20-Klein-HTTP-Request-Smuggling-In-2020-New-Variants-New-Defenses-And-New-Challenges.pdf) but lesser-explored attack class.

![](<../../.gitbook/assets/image (3).png>)

The attack was possible because the back-end server simply **wasn't expecting a POST request**.

{% hint style="warning" %}
Note that this vulnerability is being **triggered** by a completely **valid**, specification-compliant **HTTP request**. This meant the **front-end has zero chance of protecting** against it, and it could even be triggered by a browser.
{% endhint %}

The only **difference** between **CL.0** and **H2.0** is that the second one is using **HTTP2** (which has an implicit content-length header) but the **backend isn't using that either**.

## Client-Side Desync

Traditional desync attacks **poison** the **connection** between a **front-end and back-end** server, and are therefore impossible on websites that don't use a front-end/back-end architecture. These are **server-side desync** from now on. Most **server-side desyncs** can only be triggered by a **custom HTTP client issuing a malformed request.**

The ability for a **browser to cause a desync** enables a whole new class of threat called **client-side desync** (CSD).\
A CSD attack starts with the **victim visiting the attacker's website**, which then makes their browser send **two cross-domain requests to the vulnerable website**. The **first** request is crafted to **desync the browser's connection** and make the **second request trigger** a harmful response, typically giving the attacker control of the victim's account.

### Detect

A CSD vector is a HTTP request with **two** **key** properties.

First, the **server must ignore the request's Content-Length (CL)**. This typically happens because the request either **triggered a server error**, or the server simply **wasn't expecting a POST request** to the chosen endpoint. Try targeting **static files** and **server-level redirects**, and triggering errors via **overlong-URLs**, and **semi-malformed** ones like /%2e%2e.

Secondly, the request must be **triggerable in a web-browser cross-domain**. Browsers severely restrict control over cross-domain requests, so you have limited control over headers, and if your request has a body you'll need to use the HTTP POST method. Ultimately you only **control** the **URL**, plus a few odds and ends like the **Referer header**, the **body**, and **latter part of the Content-Type.**

#### CL ignore testing

The way to test this missconfig is to **send 2 requests and smuggle one** in the **middle**. If the **smuggled** connection **affected** the response of the **second** **request**, it means that it's **vulnerable**:

![](<../../.gitbook/assets/image (1).png>)

{% hint style="warning" %}
Note that you **cannot** test this vuln by just sending a **Content-Length bigger** than the one sent and **looking for a timeout** because some servers **respond** even if they **didn't receive the whole body**.
{% endhint %}

It's important to note whether the **target website supports HTTP**/2. CSD attacks typically exploit HTTP/1.1 connection reuse and web **browsers prefer to use HTTP/2** whenever possible, so if the target **website supports HTTP/2 your attacks are unlikely to work**. There's one **exception**; some **forward proxies don't support HTTP/2** so you can exploit anyone using them. This includes corporate proxies, certain intrusive VPNs and even some security tools.

### Confirm

First, select a site to launch the attack from. This site must be **accessed over HTTPS** and located on a **different domain than the target**.

Next, ensure that you **don't have a proxy configured**, then browse to your attack site. Open the **developer tools** and switch to the **Network tab**. To help with debugging potential issues later, I recommend making the following adjustments:

* Select the **"Preserve log"** checkbox.
* Right-click on the column headers and **enable the "Connection ID" column**.

Switch to the developer console and execute JavaScript to replicate your attack sequence using fetch(). This may look something like:

```javascript
fetch('https://example.com/', {
  method: 'POST',
     body: "GET /hopefully404 HTTP/1.1\r\nX: Y", // malicious prefix
     mode: 'no-cors', // ensure connection ID is visible
     credentials: 'include' // poison 'with-cookies' pool
}).then(() => {
     location = 'https://example.com/' // use the poisoned connection
})
```

I've set the fetch mode **'no-cors'** to ensure Chrome **displays the connection ID** in the Network tab. I've also set **credentials: 'include'** as Chrome has [**two separate connection pools**](https://www.chromium.org/developers/design-documents/network-stack/preconnect) - one for requests with cookies and one for requests without. You'll usually want to exploit **navigations**, and those **use the 'with-cookies' pool**, so it's worth getting into the habit of always poisoning that pool.

When you execute this, you should see **two requests** in the Network tab with the **same connection ID**, and the **second** one should trigger a **404**:

![](../../.gitbook/assets/image.png)

If this works as expected, congratulations - you've found yourself a client-side desync!

### Exploitation - Store

One option is to identify functionality on the target site that lets you **store text data**, and craft the prefix so that your victim's cookies, authentication headers, or password end up being **stored somewhere you can retrieve them**. This attack flow works [almost identically to server-side request smuggling](https://portswigger.net/web-security/request-smuggling/exploiting#capturing-other-users-requests), so I won't dwell on it.

### Exploitation - **Chain\&pivot**

Under normal circumstances, many classes of **server-side attack** can only be launched by an attacker with direct access to the target website as they **rely on HTTP requests that browsers refuse to send**, like **tampering** with **HTTP headers** - web cache poisoning, most server-side request smuggling, host-header attacks, User-Agent based [SQLi](https://portswigger.net/web-security/sql-injection), CSRF JSON Content-type and numerous others.

The simplest path to a successful attack came from two key techniques usually used for server-side desync attacks: [**JavaScript resource poisoning via Host-header redirects**](https://portswigger.net/web-security/request-smuggling/exploiting#using-http-request-smuggling-to-turn-an-on-site-redirect-into-an-open-redirect), and using the [**HEAD method**](https://portswigger.net/web-security/request-smuggling/advanced/request-tunnelling#non-blind-request-tunnelling-using-head) to splice together a response with harmful HTML. Both techniques needed to be **adapted** to overcome some novel challenges associated with operating in the **victim's browser**.

## Exploit Examples

### Stacked HEAD example

* Coloured exploit

![](<../../.gitbook/assets/image (2).png>)

* **JS exploit**

```javascript
fetch('https://www.capitalone.ca/assets', {
    method: 'POST',
    
        // use a cache-buster to delay the response
        body: `HEAD /404/?cb=${Date.now()} HTTP/1.1\r\nHost: www.capitalone.ca\r\n\r\nGET /x?x=<script>alert(1)</script> HTTP/1.1\r\nX: Y`,
        credentials: 'include',
        mode: 'cors' // throw an error instead of following redirect
}).catch(() => {
        location = 'https://www.capitalone.ca/'
})va
```

Explanation:

* **Abuse of CL.0** in /assets (it redirects to /assets/ and doesn't check the CL)
* **Smuggle** a **HEAD** request (because HEAD responses still contains a content-length)
* **Smuggle** a **GET** request whose **content** is going be **reflected** in the response with the payload.
  * Because of the **content-length of the HEAD** req, the **response** of this request will be the **body of the HEAD req**
* Set **cors mode**. Normally this isn't done, but in this case the **response** of the server to de **initial** **POST** is a **redirect** that if **followed** the **exploit won't work**. Therefore, **cors mode** is used to **trigger** an **error** and **redirect** the victim with the **`catch`**.

### **Host header redirect + client-side cache poisoning**

## **References**

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