Fixing Ffuf Host Header Problems In HTTP/2 Fuzzing

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Fixing ffuf Host Header Problems in HTTP/2 Fuzzing

Hey folks, if you're like me, you're probably diving deep into web application security, specifically fuzzing with tools like ffuf. One common hurdle when testing HTTP/2 applications is properly setting the Host header, especially when dealing with virtual hosts (vhosts). I recently ran into some headaches with this, and I figured I'd share how to get things working smoothly. Let's break down the issue and how to resolve it.

The Problem: ffuf and HTTP/2 Host Header Limitations

So, the deal is this: you're trying to fuzz an application, and you need to specify a different Host header than the one derived from the URL. This is super important when you're targeting vhosts or applications that rely on the Host header for routing. You're probably thinking, "No problem, I'll just use -H 'Host: y_domain' or include it in my req.raw file." And for HTTP/1.1, that works like a charm. But when you switch to HTTP/2 with the -http2 flag, you might hit a snag.

Specifically, ffuf can sometimes throw an error like "invalid http2 host header" if the Host header value is not formatted correctly. This is because HTTP/2 has stricter requirements for the :authority pseudo-header, which essentially replaces the Host header. The :authority header in HTTP/2 must be a valid origin, which means it should typically be just the domain name (e.g., example.com), and not include a path (e.g., example.com/hello). This difference in how HTTP/2 handles host information compared to HTTP/1.1 is the root of the problem.

Let's get into some specific examples to clarify what's going on and how you can get around it, keeping in mind that the primary goal is often to fuzz different virtual hosts or endpoints.

If you try a command like this:

ffuf -u <url_x_domain> -request req.raw -http2 -w wordlist:FUZZ

and req.raw contains Host: y_domain, you might encounter the issue. Also:

ffuf -u <url_x_domain> -X POST -d "p=FUZZ" -http2 -w wordlist:FUZZ -H "Host: y_domain"

If the y_domain is something like example.com/hello, you're likely to see that "invalid http2 host header" error, which is frustrating because the tool is preventing you from testing your attack surface.

So, what's the fix?

Understanding HTTP/2 and the :authority Header

Before we jump into solutions, it's worth taking a quick peek under the hood at HTTP/2. In HTTP/1.1, the Host header tells the server which website you're trying to reach on the same IP address. But in HTTP/2, this information is conveyed through the :authority pseudo-header. Pseudo-headers start with a colon (:) and are part of the HTTP/2 framing layer. The :authority header combines the host and port, which must be a valid origin (e.g., example.com:443).

The crucial difference here is that the :authority header (and by extension, the host) in HTTP/2 cannot include a path. This is a departure from HTTP/1.1 where the Host header could, in some cases, be combined with the request path to construct the full URL. If you try to include a path in the :authority header, you're going to run into problems, hence the "invalid http2 host header" error.

Basically, the webserver relies on the host header to understand which site or application is being requested on the same IP. HTTP/2 and :authority just make it a bit more strict. Understanding this fundamental difference is crucial for troubleshooting.

Workarounds and Solutions for ffuf

Alright, let's get down to the nitty-gritty and explore some solutions to get ffuf working with your intended Host header and vhost fuzzing.

Solution 1: Manipulating the URL

One approach is to manipulate the URL itself to reflect the target domain. This is useful when the path isn't critical for the initial request.

Here's how you might adapt your command:

  1. Modify the Target URL: Instead of the original URL, use a URL that only contains the base domain.
  2. Use -H for the Host: Use the -H flag to set the Host header to the full domain and path you need.
ffuf -u https://example.com -H "Host: example.com/hello" -http2 -w wordlist:FUZZ

In this example, the base URL is https://example.com, and the Host header is set to example.com/hello. ffuf will then send requests with the specified Host header. This will instruct the server to route the request to the virtual host or application hosted at example.com/hello.

This approach works well if the initial part of the request (the part before the path) is crucial for the routing and the rest of the path can be set within your fuzzing payloads. However, if the path itself is critical in the initial part of your request, you may have to go to other options.

Solution 2: Crafting Custom Requests (req.raw file)

Another approach involves crafting your requests more manually using a req.raw file. This allows you more control over the HTTP/2 framing and the :authority header. This approach can be more flexible, but also more complex.

Here's how to do it:

  1. Create a req.raw file: This file will contain the raw HTTP request, including the headers and any data.
  2. Set the :authority header: In your req.raw file, ensure the :authority header is set correctly to the domain and port.
  3. Include the path in the request: You can define the entire path (including the domain) within the request path. Although this may not work, depending on your goal.

Here's an example req.raw file:

POST /hello HTTP/2
Host: example.com
User-Agent: Mozilla/5.0
Content-Type: application/x-www-form-urlencoded

p=FUZZ

And the ffuf command:

ffuf -u https://example.com -request req.raw -http2 -w wordlist:FUZZ

This method allows you to define your request with greater precision. Just remember that with HTTP/2, you are setting Host instead of the :authority.

Solution 3: Combining with Proxy Tools

If you're still having trouble, or if you need to examine the traffic more closely, consider using a proxy tool like Burp Suite or OWASP ZAP in conjunction with ffuf. While Burp Suite might not have the same performance as ffuf, you can use it to craft the requests and then use ffuf to send those crafted requests with the desired headers.

Here's the idea:

  1. Configure Burp Suite (or ZAP): Set up the proxy to intercept and modify requests.
  2. Craft the Request: Manually create the request in Burp Suite, including the correct Host header and any other necessary headers.
  3. Send to ffuf: Save the modified request, then use ffuf to send the request via the proxy.
ffuf -u http://127.0.0.1:8080 -request burp_request.txt -http2 -w wordlist:FUZZ

In this case, burp_request.txt would contain your crafted HTTP request (saved from Burp Suite), and ffuf would send that request through the proxy at http://127.0.0.1:8080. This approach is helpful for complex scenarios where you need more control over request construction and can also help you inspect the traffic to ensure everything is being sent correctly.

Troubleshooting Tips

  • Verify the Host header: Double-check that the Host header is formatted correctly and that it matches the expected domain. An easy typo can cause all kinds of headaches.
  • Inspect the traffic: Use a tool like Wireshark or the browser's developer tools to examine the actual HTTP/2 traffic being sent. This can help you identify any problems with the headers or the request structure.
  • Update ffuf: Make sure you have the latest version of ffuf. Older versions might have bugs or limitations that have been addressed in newer releases.
  • Test with other tools: Compare your results with other tools like curl or h2load to see if they behave differently. This can help you isolate the problem.

Conclusion: Fuzzing HTTP/2 with ffuf

So there you have it, folks! Fuzzing HTTP/2 applications with ffuf can be a bit tricky because of the HTTP/2 standards and the specifics of how ffuf implements HTTP/2. However, by understanding the :authority header, using these workarounds (manipulating the URL, crafting custom requests, and combining with proxy tools), and taking these troubleshooting steps, you can successfully fuzz and test those HTTP/2 applications. The key is understanding how HTTP/2 handles host information, adapting your commands accordingly, and always verifying that your requests are being sent as expected. Happy fuzzing!