guestbook.pl cleanses user-inserted SSI commands by removing text between "<!--" and "-->" separators, which allows remote attackers to execute arbitrary commands when guestbook.pl is run on Apache 1.3.9 and possibly other versions, since Apache allows other closing sequences besides "-->".
Matt Wright FormMail 1.9 and earlier allows remote attackers to bypass the HTTP_REFERER check and conduct unauthorized activities via (1) a blank referer, (2) a spoofed referer with a trusted domain/URL after the beginning of the referer, or (3) a spoofed referer with a trusted domain/URL in the beginning (hostname) portion of the referer.
FormMail.pl in FormMail 1.6 and earlier allows a remote attacker to send anonymous email (spam) by modifying the recipient and message parameters.
WWWBoard has a default username and default password.
FormMail CGI program allows remote execution of commands.