Rule:  --Sid:2211--Summary:This event is generated when an attempt is made to access guestserver.cgi on an internal web server. This may indicate an attempt to exploit a remote command execution vulnerability in Lars Ellingsen's Guestbook system.--Impact:Remote execution of arbitrary code.--Detailed Information:Lars Ellingsen's Guestbook system is a CGI application for web-based guestbooks. It contains a parsing vulnerability in guestserver.cgi where an attacker can place executable code within pipe characters (|) in front of an email address in the email value of a guestbook form. Because the pipe metacharacter is not properly filtered, code placed in the email value is executed with the security context of the web server.--Affected Systems:Any web server running Lars Ellingsen's Guestbook system.--Attack Scenarios:An attacker prepends shell commands between pipe characters to an email address in the email value of a guestbook entry. When the form data is submitted, the web server attempts to execute the commands. --Ease of Attack:Simple. Exploits exist.--False Positives:If a legitimate remote user accesses guestserver.cgi on an internal web server, this rule may generate an event.--False Negatives:None known.--Corrective Action:Because Lars Ellingsen's guestbook system does not appear to be currently maintained, you may want to use a different guestbook application. As a workaround, you can change the 1 that appears on the line beneath <-guestbook.mailto_guest-> to 0 in the guestbook.config file.--Contributors:Sourcefire Research TeamBrian Caswell <bmc@sourcefire.com>Nigel Houghton <nigel.houghton@sourcefire.com>Sourcefire Technical Publications TeamJennifer Harvey <jennifer.harvey@sourcefire.com>-- Additional References:--