Secure ASP.NET Web API with Windows Azure AD and Microsoft OWIN Components

As the role of the Web API becomes more prominent, so does the need to ensure you can use it confidently in high-value scenarios, where sensitive data and operations might be exposed. The industry is clearly converging on a solution for securing REST APIs that relies on the OAuth 2.0 standard.

http://msdn.microsoft.com/en-us/magazine/dn463788.aspx