- Authorization code
- Resource owner password credentials
- Client credentials
More information about the grant types can found from here.
Client Credentials is the grant type which goes closely with 2-legged OAuth. With the Client Credentials grant type, the client can request an access token using only its client credentials (or other supported means of authentication) when the client is requesting access to the protected resources under its control. Once the client makes this request to the authorization server - it will return back an access token to access the protected resource. The access token returned back to the client could be either of type bearer of MAC. The "mac" token type defined in ietf-oauth-v2-http-mac is utilized by issuing a MAC key together with the access token which is used to sign certain components of the HTTP requests by the client when accessing the protected resource. The MAC scheme requires the establishment of a shared symmetric key between the client and the server. This is often accomplished through a manual process such as client registration.