The Linux security module developed by the U.S. National Security Agency is SELinux.
Security-Enhanced Linux provides mandatory access control, or MAC, for Linux systems. Instead of relying only on traditional discretionary permissions—where file owners and privileged users decide access—SELinux applies security policies that can confine processes, services, files, sockets, and other kernel-managed objects. A compromised service can therefore be restricted from accessing resources outside its approved security context.
The NSA released the first version of SELinux to the open-source community under the GNU General Public License on 22 December 2000. The project drew on earlier NSA security research, including the Flask architecture, and was later integrated into the mainline Linux kernel during the 2.6 development series.
AppArmor, Smack, and TOMOYO are also Linux security modules, but they are distinct projects with different origins and policy models. Another common misunderstanding is that SELinux replaces Unix permissions entirely: it supplements traditional discretionary access control with an additional, policy-driven enforcement layer.