register.cgi in Ikonboard 2.1.7b and earlier allows remote attackers to execute arbitrary commands via the SEND_MAIL parameter, which overwrites an internal program variable that references a program to be executed.
https://exchange.xforce.ibmcloud.com/vulnerabilities/5819
http://www.securityfocus.com/bid/2157
http://archives.neohapsis.com/archives/bugtraq/2000-12/0483.html