Pure-FTPd External Authentication Bash Environment Variable Code Injection (Shellshock)

This module exploits the Shellshock vulnerability, a flaw in how the Bash shell
handles external environment variables. This module targets the Pure-FTPd FTP
server when it has been compiled with the --with-extauth flag and an external
Bash script is used for authentication. If the server is not set up this way,
the exploit will fail, even if the version of Bash in use is vulnerable.