If an operating system is not secure then it is not successful. In comparison to other operating systems, Linux is the most secure operating system as it consists of Pluggable Authentication Modules. A secure layer is created between the authentication process and applications. It is because of PAM only by which an admin can give access to other users to log into the system. You can find the configuration of PAM applications in the “/etc/pam.d” or “/etc/pam.conf” directory.