index.php in ActiveCampaign KnowledgeBuilder 2.4 and earlier allows remote attackers to cause a denial of service (CPU consumption) via an invalid category parameter, which causes a large number of SQL queries to be processed.
http://www.vupen.com/english/advisories/2005/2587
http://secunia.com/advisories/17732
http://pridels0.blogspot.com/2005/11/activecampaign-knowledgebuilder-vuln.html