The world-wide standard for securing TCP connections began as the Secure Sockets Layer, or SSL. SSL 3.0 was succeeded by Transport Layer Security (TLS) in 1999, and together they are often known as SSL/TLS. SSL 3.0 is now obsolete and considered a security risk. It is disabled by default in CompleteFTP.
Recently (March 2021), the Internet Engineering Task Force (IETF) announced that TLS 1.0 and 1.1 are formally deprecated. They should no longer be enabled because of their lack of support for recommended cryptographic algorithms. CompleteFTP server administrators should disable TLS 1.0 and TLS 1.1. This can be done via the CompleteFTP manager application. In the FTP/FTPS Settings, navigate to Advanced FTP/FTPS Settings, and then Security Settings. Set the Minimum SSL Version to TLS 1.2, and click on Apply Changes.