The comment_form_add_preview function in comment.module in Drupal before 4.7.6, and 5.x before 5.1, and vbDrupal, allows remote attackers with "post comments" privileges and access to multiple input filters to execute arbitrary code by previewing comments, which are not processed by "normal form validation routines."
https://exchange.xforce.ibmcloud.com/vulnerabilities/31940
http://www.vupen.com/english/advisories/2007/0415
http://www.vupen.com/english/advisories/2007/0406
http://www.securityfocus.com/bid/22306
http://secunia.com/advisories/23990