Security is the most important aspect of an operating system. Due to its unique authentication module, Linux is considered as more secured than other operating systems. Linux consists of PAM. PAM is Pluggable Authentication Modules. It provides a layer between applications and actual authentication mechanism. It is a library of loadable modules which are called by the application for authentication. It also allows the administrator to control when a user can log in. All PAM applications are configured in the directory â€ś/etc/pam.dâ€ť or in a file â€ś/etc/pam.confâ€ť. PAM is controlled using the configuration file or the configuration directory.