An issue was discovered in Webmin <=1.920. The parameter old in password_change.cgi contains a command injection vulnerability.
https://www.exploit-db.com/exploits/47230
https://attackerkb.com/topics/hxx3zmiCkR/webmin-password-change-cgi-command-injection
http://www.webmin.com/security.html
http://www.pentest.com.tr/exploits/DEFCON-Webmin-1920-Unauthenticated-Remote-Command-Execution.html
http://packetstormsecurity.com/files/154485/Webmin-1.920-Remote-Code-Execution.html
http://packetstormsecurity.com/files/154197/Webmin-1.920-password_change.cgi-Backdoor.html
http://packetstormsecurity.com/files/154141/Webmin-Remote-Comman-Execution.html
http://packetstormsecurity.com/files/154141/Webmin-1.920-Remote-Command-Execution.html