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.
TWiki allows arbitrary shell command execution via the Include function
The ImageGalleryPlugin (ImageGalleryPlugin.pm) in Twiki allows remote attackers to execute arbitrary commands via certain commands that generate thumbnails.
The (1) rdiff and (2) preview scripts in TWiki 4.0 and 4.0.1 ignore access control settings, which allows remote attackers to read restricted areas and access restricted content in TWiki topics.