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

CRLFuzz is a powerful Linux tool written in Go that scans for CRLF (Carriage Return Line Feed) vulnerabilities in web applications. CRLF vulnerabilities occur when applications mishandle line termination characters, potentially leading to HTTP response splitting, cross-site scripting (XSS), and session hijacking attacks.

This tutorial covers the installation process, demonstrates CRLFuzz usage with various command-line options, and explains how to effectively identify CRLF vulnerabilities to strengthen web application security.

What are CRLF Vulnerabilities

CRLF vulnerabilities arise when user input containing carriage return (\r) and line feed (
) characters is not properly sanitized. Attackers can exploit this to inject malicious headers into HTTP responses, leading to:

• HTTP Response Splitting Injecting additional HTTP responses

• Header Injection Adding unauthorized headers to responses

• Cross-site Scripting Executing malicious scripts in browsers

• Session Hijacking Compromising user sessions

Installation and Setup

Follow these steps to install CRLFuzz on your Linux system:

1. Verify Go Installation

go version

If Go is not installed, download it from the official Go website (https://golang.org/dl/).

2. Clone the Repository

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

3. Build the Binary

cd crlfuzz
go build
chmod +x crlfuzz

Basic Usage

Simple Vulnerability Scan

To perform a basic CRLF vulnerability scan, provide the target URL:

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

CRLFuzz automatically injects various CRLF payloads to test for vulnerabilities in the target application.

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

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

Using Custom Payloads

Create a custom wordlist file for targeted testing:

echo -e "X-Custom-Header: CRLF\r\nInjected: true" > wordlist.txt
echo -e "Location: http://evil.com\r<br>\r<br><script>alert('XSS')</script>" >> wordlist.txt

Execute the scan using the custom wordlist:

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

[+] URL: https://example.com/
[+] Payloads: 2
[+] Wordlist: wordlist.txt
[+] Concurrency: 100

200 - OK | Length: 125
    - X-Custom-Header: CRLF
    - Location: http://evil.com

Advanced Configuration Options

Concurrency Control

Adjust the concurrency level to balance scan speed and server load:

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

Request Delay

Add delays between requests to avoid overwhelming the target server:

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

Proxy Configuration

Route requests through a proxy for analysis or anonymity:

./crlfuzz fuzz --url https://example.com/ --proxy http://127.0.0.1:8080

Command-Line Options Summary

Interpreting Results

CRLFuzz output provides valuable information about potential vulnerabilities:

• 200 OK Normal response, payload may not have triggered vulnerability

• 302 Found Potential redirection attack if location header is controlled

• 400 Bad Request Server rejected malformed request (good security)

• 500 Internal Server Error Possible server-side processing issue

Look for responses where injected headers appear in the output, indicating successful CRLF injection.

Conclusion

CRLFuzz is an effective tool for identifying CRLF vulnerabilities in web applications. By systematically testing various payload combinations, it helps security professionals discover header injection flaws that could lead to serious attacks. Regular scanning with CRLFuzz can significantly improve web application security posture.