env x=`(){ :;};echo exploit` bash -c 'cat /etc/passwd'What is the Shellshock bash vulnerability attempting to...

To extract private information, attackers are using a couple of techniques. The simplest extraction attacks are in the form:

() {:;}; /bin/cat /etc/passwd

That reads the password file /etc/passwd, and adds it to the response from the web server. So an attacker injecting this code through the Shellshock vulnerability would see the password file dumped out onto their screen as part of the web page returned.

References: https://blog.cloudflare.com/inside-shellshock/