[[oauth2resourceserver]]
= OAuth 2.0 Resource Server
:figures: servlet/oauth2

Spring Security supports protecting endpoints by using two forms of OAuth 2.0 https://tools.ietf.org/html/rfc6750.html[Bearer Tokens]:

* https://tools.ietf.org/html/rfc7519[JWT]
* Opaque Tokens

This is handy in circumstances where an application has delegated its authority management to an https://tools.ietf.org/html/rfc6749[authorization server] (for example, Okta or Ping Identity).
This authorization server can be consulted by resource servers to authorize requests.

This section details how Spring Security provides support for OAuth 2.0 https://tools.ietf.org/html/rfc6750.html[Bearer Tokens].

[NOTE]
====
Working samples for both {gh-samples-url}/servlet/spring-boot/java/oauth2/resource-server/jwe[JWTs] and {gh-samples-url}/servlet/spring-boot/java/oauth2/resource-server/opaque[Opaque Tokens] are available in the {gh-samples-url}[Spring Security Samples repository].
====

Now we can consider how Bearer Token Authentication works within Spring Security.
First, we see that, as with xref:servlet/authentication/passwords/basic.adoc#servlet-authentication-basic[Basic Authentication], the https://tools.ietf.org/html/rfc7235#section-4.1[WWW-Authenticate] header is sent back to an unauthenticated client:

.Sending WWW-Authenticate Header
[.invert-dark]
image::{figures}/bearerauthenticationentrypoint.png[]

The figure above builds off our xref:servlet/architecture.adoc#servlet-securityfilterchain[`SecurityFilterChain`] diagram.

image:{icondir}/number_1.png[] First, a user makes an unauthenticated request to the `/private` resource for which the user is not authorized.

image:{icondir}/number_2.png[] Spring Security's xref:servlet/authorization/authorize-http-requests.adoc[`AuthorizationFilter`] indicates that the unauthenticated request is _Denied_ by throwing an `AccessDeniedException`.

image:{icondir}/number_3.png[] Since the user is not authenticated, xref:servlet/architecture.adoc#servlet-exceptiontranslationfilter[`ExceptionTranslationFilter`] initiates _Start Authentication_.
The configured xref:servlet/authentication/architecture.adoc#servlet-authentication-authenticationentrypoint[`AuthenticationEntryPoint`] is an instance of {security-api-url}org/springframework/security/oauth2/server/resource/authentication/BearerTokenAuthenticationEntryPoint.html[`BearerTokenAuthenticationEntryPoint`], which sends a `WWW-Authenticate` header.
The `RequestCache` is typically a `NullRequestCache` that does not save the request, since the client is capable of replaying the requests it originally requested.

When a client receives the `WWW-Authenticate: Bearer` header, it knows it should retry with a bearer token.
The following image shows the flow for the bearer token being processed:

[[oauth2resourceserver-authentication-bearertokenauthenticationfilter]]
.Authenticating Bearer Token
[.invert-dark]
image::{figures}/bearertokenauthenticationfilter.png[]

The figure builds off our xref:servlet/architecture.adoc#servlet-securityfilterchain[`SecurityFilterChain`] diagram.

image:{icondir}/number_1.png[] When the user submits their bearer token, the `BearerTokenAuthenticationFilter` creates a `BearerTokenAuthenticationToken` which is a type of xref:servlet/authentication/architecture.adoc#servlet-authentication-authentication[`Authentication`] by extracting the token from the `HttpServletRequest`.

image:{icondir}/number_2.png[] Next, the `HttpServletRequest` is passed to the `AuthenticationManagerResolver`, which selects the `AuthenticationManager`. The `BearerTokenAuthenticationToken` is passed into the `AuthenticationManager` to be authenticated.
The details of what `AuthenticationManager` looks like depends on whether you're configured for xref:servlet/oauth2/resource-server/jwt.adoc#oauth2resourceserver-jwt-minimalconfiguration[JWT] or xref:servlet/oauth2/resource-server/opaque-token.adoc#oauth2resourceserver-opaque-minimalconfiguration[opaque token].

image:{icondir}/number_3.png[] If authentication fails, then __Failure__

* The xref:servlet/authentication/architecture.adoc#servlet-authentication-securitycontextholder[SecurityContextHolder] is cleared out.
* The `AuthenticationEntryPoint` is invoked to trigger the WWW-Authenticate header to be sent again.

image:{icondir}/number_4.png[] If authentication is successful, then __Success__.

* The xref:servlet/authentication/architecture.adoc#servlet-authentication-authentication[Authentication] is set on the xref:servlet/authentication/architecture.adoc#servlet-authentication-securitycontextholder[SecurityContextHolder].
* The `BearerTokenAuthenticationFilter` invokes `FilterChain.doFilter(request,response)` to continue with the rest of the application logic.
