Mastering HTTP Request Smuggling: Bypassing Firewalls and Filters

HTTP Request Smuggling (HRS) is a powerful exploitation technique that allows attackers to interfere with the communication between web servers and intermediaries, such as proxies or firewalls. For ethical hackers and bug bounty hunters, mastering this attack can open the door to bypassing security filters, performing unauthorized actions, and potentially compromising entire applications.

In this post, we’ll dive deep into the mechanics of HTTP Request Smuggling, provide a practical guide to performing this attack, and offer insights into how to identify vulnerable applications.

HTTP Request Smuggling exploits discrepancies between how different web servers or proxies interpret HTTP requests. The attack works by crafting a request that is interpreted differently by each layer of the application infrastructure (e.g., front-end server, proxy, back-end server). This confusion can allow attackers to:

Bypass security filters: Firewalls and proxies may fail to properly validate or sanitize requests.Execute unauthorized actions: Attackers can trigger actions on a vulnerable back-end server that are not visible to the front-end server.Access internal resources: By smuggling requests past the perimeter security, attackers may gain access to otherwise restricted internal resources.

Before diving into practical steps, let’s clarify some key terms related to HTTP Request Smuggling:

Chunked Encoding: A transfer encoding where data is sent in chunks, and each chunk is prefixed with its size.Content-Length Header: The header that indicates the size of the body of the request.HTTP Pipelining: A technique where multiple HTTP requests are sent in a single connection without waiting for the responses.

At its core, HTTP Request Smuggling involves crafting an ambiguous request that is interpreted differently by two systems in the application stack. For example, the front-end server might interpret the request in one way, while the back-end server interprets it differently, causing one of the servers to miss part of the request.

Crafting the Malicious Request: An attacker might create a request where one system (e.g., the proxy server) interprets the headers differently from the other system (e.g., the back-end server).Request Flow: The front-end system processes part of the request and forwards the rest to the back-end system, which then processes it as a new request.Exploiting the Discrepancy: Since the back-end system processes the rest of the request, it might trigger unintended actions, such as an internal request that is not validated by the front-end.

To successfully carry out an HTTP Request Smuggling attack, the attacker needs to carefully craft requests that exploit vulnerabilities in the request processing mechanism. Here’s how you can approach it:

Overview: This technique exploits the inconsistency between how different HTTP parsers handle chunked transfer encoding. While some servers might allow chunked transfer encoding to be interpreted as a separate request, others might see it as part of the same request.

How to Perform the Attack:

Send a request with a valid Content-Length header and a chunked body.Manipulate the chunk sizes to smuggle a second request.If the front-end server forwards the request to the back-end server, the back-end might process the second request without being validated by the front-end.

Example:

POST / HTTP/1.1
Host: target.com
Content-Length: 13
Transfer-Encoding: chunked

POST /attack HTTP/1.1
Host: target.com
Content-Length: 0

Overview: This attack takes advantage of discrepancies between the Content-Length and the Transfer-Encoding headers. Some servers process Content-Length first, while others prioritize Transfer-Encoding.

How to Perform the Attack:

Craft a request with both Content-Length and Transfer-Encoding headers, specifying different body lengths in each.This causes the server to treat the body differently and may allow the attacker to smuggle an additional request.

Example:

POST / HTTP/1.1
Host: target.com
Content-Length: 15
Transfer-Encoding: chunked

POST /exploit HTTP/1.1
Host: target.com
Content-Length: 0

Overview: In this attack, multiple requests are sent in a single HTTP request. Some proxies or firewalls process the first request, while others may forward subsequent requests in a different order.

How to Perform the Attack:

Send multiple HTTP requests in one go, using HTTP pipelining to confuse the server into processing them differently.

Not all web servers and proxies are vulnerable to HTTP Request Smuggling. Identifying vulnerable targets is crucial for maximizing the impact of this attack.

Look for Misconfigured Proxies

Proxies that don’t properly validate requests or that forward multiple requests in one connection are prime targets for HTTP Request Smuggling.

Test for HTTP Protocol Differences

Test if the server and proxy handle the Content-Length and Transfer-Encoding headers differently. A vulnerable configuration will typically be inconsistent in how it processes these headers.

Examine the HTTP Stack

Servers that use older or poorly configured HTTP stacks are more likely to have vulnerabilities that can be exploited via HTTP Request Smuggling.

Mastering HTTP Request Smuggling is a valuable skill for bug bounty hunters and penetration testers. This technique allows attackers to bypass firewalls, proxies, and other security filters, potentially gaining unauthorized access to internal systems. By understanding the core principles, learning the different techniques for smuggling requests, and using the right tools, you can effectively discover and exploit these vulnerabilities in web applications.