CRLFuzz – A Linux Tool to Scan CRLF Vulnerability Written in Go

In this tutorial, we will explore CRLFuzz, a powerful Linux tool written in Go that allows us to scan and identify CRLF (Carriage Return Line Feed) vulnerabilities. CRLF vulnerabilities occur when web applications mishandle the line termination characters, leading to potential security risks such as HTTP response splitting, cross-site scripting (XSS), and session hijacking. By using CRLFuzz, we can effectively scan for these vulnerabilities and strengthen the security of our web applications.

Throughout this tutorial, we will cover the installation process, demonstrate the usage of CRLFuzz with various command-line options, and explain the significance of the code snippets and their outputs. So, let's dive in and learn how to leverage CRLFuzz to enhance the security of our web applications.

Installation and Setup

To begin, let's install CRLFuzz on our Linux system. Follow the steps below −

1. Open a terminal and ensure that you have Go installed on your system by typing the following command âˆ’

go version

2. If Go is not installed, you can download and install it from the official Go website (https://golang.org/dl/). Once installed, verify the installation by running `go version` again.

3. Next, clone the CRLFuzz repository from GitHub by executing the following command âˆ’

git clone https://github.com/dwisiswant0/crlfuzz.git

4. Change the directory to the cloned repository âˆ’

cd crlfuzz

5. Build the CRLFuzz binary using the `go build` command âˆ’

go build

6. Finally, ensure that the CRLFuzz binary is executable by running âˆ’

chmod +x crlfuzz

Using CRLFuzz

Now that we have successfully installed CRLFuzz, let's explore its various command-line options and learn how to utilize them effectively.

1. Basic Scanning

To perform a basic scan using CRLFuzz, simply provide the target URL as a parameter. For example −

Example 

./crlfuzz fuzz --url https://example.com/

In the above code snippet, we execute the `crlfuzz fuzz` command followed by the `--url` option and the target URL. CRLFuzz will automatically inject various payloads to identify any CRLF vulnerabilities in the target web application.

Output

[+] URL: https://example.com/
[+] Payloads: 22
[+] Threads: 100
[+] Timeout: 5s
[+] Proxy: 
[+] Delay: 0ms
[+] Concurrency: 100
[+] Wordlist: 
[+] User Agent: crlfuzz/1.0
[+] Method: GET
[+] Redirects: true
[+] Quiet: false

200 - OK | Length: 125
	- X-Header: CRLF
	- User-Agent: CRLF

200 - OK | Length: 125
	- X-Header: CRLF
	- User-Agent: CRLF

As you can see from the above output, CRLFuzz displays the HTTP response code, length, and the injected payload. A response code of 200 indicates that the payload did not trigger any CRLF vulnerability, while other response codes may indicate potential vulnerabilities.

2. Custom Payloads

CRLFuzz allows us to specify custom payloads using a wordlist. Let's create a custom wordlist file and perform a scan with it.

• Create a new file named `wordlist.txt` and add your custom payloads, each on a new line âˆ’

echo -e "X-Header: CRLF" > wordlist.txt
echo -e "User-Agent: CRLF" >> wordlist.txt

• Now, execute the following command to perform the scan using the custom wordlist âˆ’

Example

./crlfuzz fuzz --url https://example.com/ --payload wordlist.txt

In the above code snippet, we provide the `--payload` option followed by the path to our custom wordlist file. CRLFuzz will inject each payload from the wordlist and check for CRLF vulnerabilities in the target application.

Output

[+] URL: https://example.com/
[+] Payloads: 2
[+] Threads: 100
[+] Timeout: 5s
[+] Proxy: 
[+] Delay: 0ms
[+] Concurrency: 100
[+] Wordlist: wordlist.txt
[+] User Agent: crlfuzz/1.0
[+] Method: GET
[+] Redirects: true
[+] Quiet: false

200 - OK | Length: 125
	- X-Header: CRLF
	- User-Agent: CRLF

200 - OK | Length: 125
	- X-Header: CRLF
	- User-Agent: CRLF

As you can see from the above output, CRLFuzz displays the injected payloads from the custom wordlist. If any of the payloads trigger a CRLF vulnerability, it will be highlighted in the output.

Advanced Features of CRLFuzz

CRLFuzz also offers advanced features that can further enhance the effectiveness of our scans. Let's explore two of these features: concurrency and time delay.

1. Concurrency

CRLFuzz allows us to set the concurrency level to perform scans concurrently. This can significantly speed up the scanning process. Here's how to utilize this feature âˆ’

Execute the following command to scan with a concurrency level of 10 −

Example 

./crlfuzz fuzz --url https://example.com/ --concurrency 10

In the above code snippet, we provide the `--concurrency` option followed by the desired concurrency level (in this case, 10). CRLFuzz will send multiple requests concurrently, making the scanning process faster.

Output

[+] URL: https://example.com/
[+] Payloads: 22
[+] Threads: 100
[+] Timeout: 5s
[+] Proxy: 
[+] Delay: 0ms
[+] Concurrency: 10
[+] Wordlist: 
[+] User Agent: crlfuzz/1.0
[+] Method: GET
[+] Redirects: true
[+] Quiet: false

200 - OK | Length: 125
	- X-Header: CRLF
	- User-Agent: CRLF

200 - OK | Length: 125
	- X-Header: CRLF
	- User-Agent: CRLF

As you can see from the above output, CRLFuzz processes multiplerequests simultaneously, which reduces the overall scan time.

2. Time Delay

To avoid overwhelming the target application with excessive requests, we can introduce a time delay between each request. Here's how to incorporate this feature âˆ’

Execute the following command to scan with a time delay of 500 milliseconds between requests −

Example 

./crlfuzz fuzz --url https://example.com/ --delay 500ms

In the above code snippet, we provide the `--delay` option followed by the desired time delay value (in this case, 500 milliseconds). CRLFuzz will introduce a delay between each request, allowing the target application to handle the traffic more effectively.

Output

[+] URL: https://example.com/
[+] Payloads: 22
[+] Threads: 100
[+] Timeout: 5s
[+] Proxy: 
[+] Delay: 500ms
[+] Concurrency: 100
[+] Wordlist: 
[+] User Agent: crlfuzz/1.0
[+] Method: GET
[+] Redirects: true
[+] Quiet: false

200 - OK | Length: 125
	- X-Header: CRLF
	- User-Agent: CRLF

200 - OK | Length: 125
	- X-Header: CRLF
	- User-Agent: CRLF

As you can see from the above output, CRLFuzz introduces a time delay of 500 milliseconds between each request, ensuring that the target application can handle the traffic without any issues.

Conclusion

In this tutorial, we explored CRLFuzz, a powerful Linux tool written in Go, which enables us to scan for CRLF vulnerabilities in web applications. We covered the installation process, demonstrated the usage of CRLFuzz with various command-line options, and explained the significance of the code snippets and their outputs. By utilizing CRLFuzz, we can proactively identify and mitigate CRLF vulnerabilities, enhancing the overall security of our web applications.