OAuth is an open standard for authorization, used by many enterprises and service providers to protect their resources. OAuth solves a different security problem from what HTTP basic authentication has been used for. The OAuth protocol allows client applications to access protected resources on behalf of the resource owner (typically, the application user).

If we look at the history of this protocol, OAuth version 1.0 was published as RFC 5849 in 2010. Later, the next evolution of OAuth, version 2.0, was published as RFC 6749 in 2012. Note that these two versions are different in their implementations and do not have many things in common. In this section, we will explore what the OAuth protocol and its details are. We will also discuss accessing OAuth-protected RESTful web APIs from a RESTful web service client.