ProFTPD 1.2.2rc2, and possibly other versions, does not properly verify reverse-resolved hostnames by performing forward resolution, which allows remote attackers to bypass ACLs or cause an incorrect client hostname to be logged.
https://exchange.xforce.ibmcloud.com/vulnerabilities/7126
http://www.securityfocus.com/bid/3310
http://www.securityfocus.com/archive/1/212805
http://www.mandriva.com/security/advisories?name=MDKSA-2002:005
http://distro.conectiva.com.br/atualizacoes/?id=a&anuncio=000450