Netbus 1.5 through 1.7 allows more than one client to be connected at the same time, but only prompts the first connection for authentication, which allows remote attackers to gain access.
https://exchange.xforce.ibmcloud.com/vulnerabilities/11982
https://euvd.enisa.europa.eu/vulnerability/EUVD-2003-1465
http://www.securityfocus.com/bid/7538