The original version of the protocol (SSH-1, dated 1995) has been revised in 1996, and SSH-2 was created offering improved security and new features. The two versions of the protocol are incompatible with each other. Since SSH-1 has inherent design flaws and is vulnerable to attacks, it is now generally considered obsolete and its use should not be permitted. Practically all server and client applications today support SSH-2, however, software not supporting SSH-2 may still be in use by some organizations, posing a considerable security vulnerability to them.

The SSH proxy supports only the SSHv2 protocol (SECSH).