Example implementations relate to network authentication system extensions. For example, network authentication system extensions may be installed and executed using a system. The system may comprise a processing resource and a memory resource storing readable instructions. The instructions may cause the processing resource to initiate an extension system and initiate an application programming interface (API) to manage a plurality of installed extensions in the extension system, where the installed extensions operate on a network authentication system. The instructions may also cause the processing resource to define a particular instance of an installed extension among the plurality of installed extensions and execute installed extensions on the network authentication system.