Shellshock is a security bug found in Bash, a command shell program commonly used on Linux and UNIX systems. The bug is whats known as a Remote Code Execution vulnerability, or RCE. This means that someone who isnt already logged on to your computer might be able to trick Bash into running a program that it wasnt supposed to do. The problem is that, unlike the semi-colon example above, no-one knew what to watch out for until this bug was found.”]
Source: https://nakedsecurity.sophos.com/2014/09/25/bash-shellshock-vulnerability-what-you-need-to-know/