install/index.php in WebCalendar before 1.2.5 allows remote attackers to execute arbitrary code via the form_single_user_login parameter.
https://www.exploit-db.com/exploits/18775
https://packetstormsecurity.com/files/112332/WebCalendar-1.2.4-Remote-Code-Execution.html
https://packetstormsecurity.com/files/112323/WebCalendar-1.2.4-Pre-Auth-Remote-Code-Injection.html
https://euvd.enisa.europa.eu/vulnerability/EUVD-2012-1513
http://sourceforge.net/projects/webcalendar/files/webcalendar%201.2/1.2.5/