These articles discuss methods for securing the REXEC server for authorized users and preventing access to the REXEC server.
The Remote EXECution server (REXEC) receives and runs commands from an REXEC client. A REXEC client is typically a PC or UNIX® application that supports sending REXEC commands. The support that this server provides is similar to the capability that is available when you use the RCMD (Remote Command) sub-command for the FTP server.