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

Spring Security supports protecting endpoints 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 provides details on 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].
====

Let's take a look at how Bearer Token Authentication works within Spring Security.
First, we see that, like 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 resource `/private` for which it is not authorized.

image:{icondir}/number_2.png[] Spring Security's xref:servlet/authorization/authorize-requests.adoc#servlet-authorization-filtersecurityinterceptor[`FilterSecurityInterceptor`] 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/web/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.
Below is 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.
