User authentication service for desktop applications?

There are paid services like Stormpath ( link ) or you can use oauth2 to do the so-called "social login" where users can use the accounts of their social networks like Facebook, twitter besides github, Google.

Take a careful look at this need before implementing it as it may be more laborious than creating your own. If it's just for user authentication then oauth2 might be interesting, but if you add it you need to control access to the system modules, permissions and etc. it will not help you.