Michael Salzer Guestbox 0.6, and other versions before 0.8, allows remote attackers to post an admin comment to a guestbook entry via a certain modified form, possibly related to the nummer parameter.
https://exchange.xforce.ibmcloud.com/vulnerabilities/24797
http://www.vupen.com/english/advisories/2006/0675
http://www.securityfocus.com/archive/1/426663/100/0/threaded
http://www.securityfocus.com/archive/1/425495/100/0/threaded