editproducts.cgi in Bugzilla 2.16.3 and earlier, when usebuggroups is enabled, does not properly remove group add privileges from a group that is being deleted, which allows users with those privileges to perform unauthorized additions to the next group that is assigned with the original group ID.
https://exchange.xforce.ibmcloud.com/vulnerabilities/13597
http://www.securityfocus.com/bid/8953
http://www.securityfocus.com/archive/1/343185
http://distro.conectiva.com.br/atualizacoes/?id=a&anuncio=000774