SquirrelMail 1.2.5 and earlier allows authenticated SquirrelMail users to execute arbitrary commands by modifying the THEME variable in a cookie.
http://www.iss.net/security_center/static/8671.php
http://archives.neohapsis.com/archives/bugtraq/2002-03/0386.html
http://archives.neohapsis.com/archives/bugtraq/2002-03/0350.html