The history (revision control) function in TWiki 02-Sep-2004 and earlier allows remote attackers to execute arbitrary code via shell metacharacters, as demonstrated via the rev parameter to TWikiUsers.
https://euvd.enisa.europa.eu/vulnerability/EUVD-2005-2877
http://www.securityfocus.com/bid/14834
http://www.kb.cert.org/vuls/id/757181
http://twiki.org/cgi-bin/view/Codev/SecurityAlertExecuteCommandsWithRev