Spring Security provides a number of approaches by which your application can authenticate. It also allows you to write a custom authentication mechanism if these provided default approaches don't fit your requirements. Because of this extensibility, you can even use the legacy application against which authentication can be done. The book has a dedicated chapters (Chapter 3, Authentication Using SAML, LDAP, and OAuth/OIDC and Chapter 4, Authentication Using CAS and JAAS) where we will cover various authentications mechanisms, such as OAuth, LDAP, and SAML, in more detail.