Apache Software Foundation (ASF) Struts before 1.2.9 allows remote attackers to bypass validation via a request with a 'org.apache.struts.taglib.html.Constants.CANCEL' parameter, which causes the action to be canceled but would not be detected from applications that do not use the isCancelled check.
https://exchange.xforce.ibmcloud.com/vulnerabilities/25612
http://www.vupen.com/english/advisories/2006/1205
http://www.securityfocus.com/bid/17342
http://struts.apache.org/struts-doc-1.2.9/userGuide/release-notes.html
http://securitytracker.com/id?1015856
http://secunia.com/advisories/20117
http://secunia.com/advisories/19493
http://lists.suse.com/archive/suse-security-announce/2006-May/0004.html