OAuth2 is one of the most popular specifications for API authentication today, though wrapping your head around it can be a challenge.

When you are using OAuth, you outsource user authentication and authorization to a central identity provider (IdP). Users sign in to the IdP and are granted time-bound permissions in the form of an access token. This token is presented to other applications, APIs, and services.

Auth0 SDKs make it really easy to add SSO to any app, on any platform. But sometimes, apps cannot be modified. What to do then? A very simple solution is to