PlatinumFTP 1.0.18, and possibly earlier versions, allows remote attackers to cause a denial of service (server crash) via multiple connection attempts with a \ (backslash) in the username.
https://exchange.xforce.ibmcloud.com/vulnerabilities/19674
https://euvd.enisa.europa.eu/vulnerability/EUVD-2005-0780
http://www.securityfocus.com/bid/12790
http://www.securityfocus.com/archive/1/455609/100/0/threaded