This indicates an attempt to execute potentially malicious commands via the simplestguest.cgi program.
The simplestguest.cgi CGI program by Leif Wright is designed to coordinate guestbook submissions from website visitors. Due to inadequate user input checking, a remote attacker can execute arbitrary commands on a target system via shell metacharacters in the guestbook parameter.
Any unprotected Leif M. Wright simplestguest.cgi 2.0 is vulnerable to the attack.
Attackers can execute arbitrary commands on the victim system.
Apply appropriate patches or Upgrade the system to the latest non-vulnerable version.