"... The Webmaster must ensure that all CGI scripts placed on any Web server have been through a process to find and fix security holes. ..."
A CGI script can, intentionally or otherwise, do anything that the user it runs as can do. Typically, CGI scripts run as the same user as the Web server. On most UNIX systems, the Apache Web server is used and by default, Apache runs as user nobody. By convention, nobody is a user for unprivileged operations. Some may think that something running as nobody could not do much to compromise a Web server, but there are many ways security can be compromised.