[[configuration-model]]
= Configuration Model

[[default-configuration]]
== Default configuration

`OAuth2AuthorizationServerConfiguration` is a `@Configuration` that provides the minimal default configuration for an OAuth2 authorization server.

`OAuth2AuthorizationServerConfiguration` uses xref:configuration-model.adoc#customizing-the-configuration[`OAuth2AuthorizationServerConfigurer`] to apply the default configuration and registers a `SecurityFilterChain` `@Bean` composed of all the infrastructure components supporting an OAuth2 authorization server.

[TIP]
`OAuth2AuthorizationServerConfiguration.applyDefaultSecurity(HttpSecurity)` is a convenience (`static`) utility method that applies the default OAuth2 security configuration to `HttpSecurity`.

The OAuth2 authorization server `SecurityFilterChain` `@Bean` is configured with the following default protocol endpoints:

* xref:protocol-endpoints.adoc#oauth2-authorization-endpoint[OAuth2 Authorization endpoint]
* xref:protocol-endpoints.adoc#oauth2-device-authorization-endpoint[OAuth2 Device Authorization Endpoint]
* xref:protocol-endpoints.adoc#oauth2-device-verification-endpoint[OAuth2 Device Verification Endpoint]
* xref:protocol-endpoints.adoc#oauth2-token-endpoint[OAuth2 Token endpoint]
* xref:protocol-endpoints.adoc#oauth2-token-introspection-endpoint[OAuth2 Token Introspection endpoint]
* xref:protocol-endpoints.adoc#oauth2-token-revocation-endpoint[OAuth2 Token Revocation endpoint]
* xref:protocol-endpoints.adoc#oauth2-authorization-server-metadata-endpoint[OAuth2 Authorization Server Metadata endpoint]
* xref:protocol-endpoints.adoc#jwk-set-endpoint[JWK Set endpoint]

[NOTE]
The JWK Set endpoint is configured *only* if a `JWKSource<SecurityContext>` `@Bean` is registered.

The following example shows how to use `OAuth2AuthorizationServerConfiguration` to apply the minimal default configuration:

[source,java]
----
@Configuration
@Import(OAuth2AuthorizationServerConfiguration.class)
public class AuthorizationServerConfig {

	@Bean
	public RegisteredClientRepository registeredClientRepository() {
		List<RegisteredClient> registrations = ...
		return new InMemoryRegisteredClientRepository(registrations);
	}

	@Bean
	public JWKSource<SecurityContext> jwkSource() {
		RSAKey rsaKey = ...
		JWKSet jwkSet = new JWKSet(rsaKey);
		return (jwkSelector, securityContext) -> jwkSelector.select(jwkSet);
	}

}
----

[IMPORTANT]
The https://datatracker.ietf.org/doc/html/rfc6749#section-4.1[authorization_code grant] requires the resource owner to be authenticated. Therefore, a user authentication mechanism *must* be configured in addition to the default OAuth2 security configuration.

https://openid.net/specs/openid-connect-core-1_0.html[OpenID Connect 1.0] is disabled in the default configuration. The following example shows how to enable OpenID Connect 1.0 by initializing the `OidcConfigurer`:

[source,java]
----
@Bean
public SecurityFilterChain authorizationServerSecurityFilterChain(HttpSecurity http) throws Exception {
	OAuth2AuthorizationServerConfiguration.applyDefaultSecurity(http);

	http.getConfigurer(OAuth2AuthorizationServerConfigurer.class)
		.oidc(Customizer.withDefaults());	// Initialize `OidcConfigurer`

	return http.build();
}
----

In addition to the default protocol endpoints, the OAuth2 authorization server `SecurityFilterChain` `@Bean` is configured with the following OpenID Connect 1.0 protocol endpoints:

* xref:protocol-endpoints.adoc#oidc-provider-configuration-endpoint[OpenID Connect 1.0 Provider Configuration endpoint]
* xref:protocol-endpoints.adoc#oidc-logout-endpoint[OpenID Connect 1.0 Logout endpoint]
* xref:protocol-endpoints.adoc#oidc-user-info-endpoint[OpenID Connect 1.0 UserInfo endpoint]

[NOTE]
The xref:protocol-endpoints.adoc#oidc-client-registration-endpoint[OpenID Connect 1.0 Client Registration endpoint] is disabled by default because many deployments do not require dynamic client registration.

[TIP]
`OAuth2AuthorizationServerConfiguration.jwtDecoder(JWKSource<SecurityContext>)` is a convenience (`static`) utility method that can be used to register a `JwtDecoder` `@Bean`, which is *REQUIRED* for the xref:protocol-endpoints.adoc#oidc-user-info-endpoint[OpenID Connect 1.0 UserInfo endpoint] and the xref:protocol-endpoints.adoc#oidc-client-registration-endpoint[OpenID Connect 1.0 Client Registration endpoint].

The following example shows how to register a `JwtDecoder` `@Bean`:

[source,java]
----
@Bean
public JwtDecoder jwtDecoder(JWKSource<SecurityContext> jwkSource) {
	return OAuth2AuthorizationServerConfiguration.jwtDecoder(jwkSource);
}
----

The main intent of `OAuth2AuthorizationServerConfiguration` is to provide a convenient method to apply the minimal default configuration for an OAuth2 authorization server. However, in most cases, customizing the configuration will be required.

[[customizing-the-configuration]]
== Customizing the configuration

`OAuth2AuthorizationServerConfigurer` provides the ability to fully customize the security configuration for an OAuth2 authorization server.
It lets you specify the core components to use - for example, xref:core-model-components.adoc#registered-client-repository[`RegisteredClientRepository`],  xref:core-model-components.adoc#oauth2-authorization-service[`OAuth2AuthorizationService`], xref:core-model-components.adoc#oauth2-token-generator[`OAuth2TokenGenerator`], and others.
Furthermore, it lets you customize the request processing logic for the protocol endpoints – for example, xref:protocol-endpoints.adoc#oauth2-authorization-endpoint[authorization endpoint], xref:protocol-endpoints.adoc#oauth2-device-authorization-endpoint[device authorization endpoint], xref:protocol-endpoints.adoc#oauth2-device-verification-endpoint[device verification endpoint], xref:protocol-endpoints.adoc#oauth2-token-endpoint[token endpoint], xref:protocol-endpoints.adoc#oauth2-token-introspection-endpoint[token introspection endpoint], and others.

`OAuth2AuthorizationServerConfigurer` provides the following configuration options:

[source,java]
----
@Bean
public SecurityFilterChain authorizationServerSecurityFilterChain(HttpSecurity http) throws Exception {
	OAuth2AuthorizationServerConfigurer authorizationServerConfigurer =
		new OAuth2AuthorizationServerConfigurer();
	http.apply(authorizationServerConfigurer);

	authorizationServerConfigurer
		.registeredClientRepository(registeredClientRepository) <1>
		.authorizationService(authorizationService) <2>
		.authorizationConsentService(authorizationConsentService)   <3>
		.authorizationServerSettings(authorizationServerSettings) <4>
		.tokenGenerator(tokenGenerator) <5>
		.clientAuthentication(clientAuthentication -> { })  <6>
		.authorizationEndpoint(authorizationEndpoint -> { })    <7>
		.deviceAuthorizationEndpoint(deviceAuthorizationEndpoint -> { })    <8>
		.deviceVerificationEndpoint(deviceVerificationEndpoint -> { })  <9>
		.tokenEndpoint(tokenEndpoint -> { })    <10>
		.tokenIntrospectionEndpoint(tokenIntrospectionEndpoint -> { })  <11>
		.tokenRevocationEndpoint(tokenRevocationEndpoint -> { })    <12>
		.authorizationServerMetadataEndpoint(authorizationServerMetadataEndpoint -> { })    <13>
		.oidc(oidc -> oidc
			.providerConfigurationEndpoint(providerConfigurationEndpoint -> { })    <14>
			.logoutEndpoint(logoutEndpoint -> { })  <15>
			.userInfoEndpoint(userInfoEndpoint -> { })  <16>
			.clientRegistrationEndpoint(clientRegistrationEndpoint -> { })  <17>
		);

	return http.build();
}
----
<1> `registeredClientRepository()`: The xref:core-model-components.adoc#registered-client-repository[`RegisteredClientRepository`] (*REQUIRED*) for managing new and existing clients.
<2> `authorizationService()`: The xref:core-model-components.adoc#oauth2-authorization-service[`OAuth2AuthorizationService`] for managing new and existing authorizations.
<3> `authorizationConsentService()`: The xref:core-model-components.adoc#oauth2-authorization-consent-service[`OAuth2AuthorizationConsentService`] for managing new and existing authorization consents.
<4> `authorizationServerSettings()`: The xref:configuration-model.adoc#configuring-authorization-server-settings[`AuthorizationServerSettings`] (*REQUIRED*) for customizing configuration settings for the OAuth2 authorization server.
<5> `tokenGenerator()`: The xref:core-model-components.adoc#oauth2-token-generator[`OAuth2TokenGenerator`] for generating tokens supported by the OAuth2 authorization server.
<6> `clientAuthentication()`: The configurer for xref:configuration-model.adoc#configuring-client-authentication[OAuth2 Client Authentication].
<7> `authorizationEndpoint()`: The configurer for the xref:protocol-endpoints.adoc#oauth2-authorization-endpoint[OAuth2 Authorization endpoint].
<8> `deviceAuthorizationEndpoint()`: The configurer for the xref:protocol-endpoints.adoc#oauth2-device-authorization-endpoint[OAuth2 Device Authorization endpoint].
<9> `deviceVerificationEndpoint()`: The configurer for the xref:protocol-endpoints.adoc#oauth2-device-verification-endpoint[OAuth2 Device Verification endpoint].
<10> `tokenEndpoint()`: The configurer for the xref:protocol-endpoints.adoc#oauth2-token-endpoint[OAuth2 Token endpoint].
<11> `tokenIntrospectionEndpoint()`: The configurer for the xref:protocol-endpoints.adoc#oauth2-token-introspection-endpoint[OAuth2 Token Introspection endpoint].
<12> `tokenRevocationEndpoint()`: The configurer for the xref:protocol-endpoints.adoc#oauth2-token-revocation-endpoint[OAuth2 Token Revocation endpoint].
<13> `authorizationServerMetadataEndpoint()`: The configurer for the xref:protocol-endpoints.adoc#oauth2-authorization-server-metadata-endpoint[OAuth2 Authorization Server Metadata endpoint].
<14> `providerConfigurationEndpoint()`: The configurer for the xref:protocol-endpoints.adoc#oidc-provider-configuration-endpoint[OpenID Connect 1.0 Provider Configuration endpoint].
<15> `logoutEndpoint()`: The configurer for the xref:protocol-endpoints.adoc#oidc-logout-endpoint[OpenID Connect 1.0 Logout endpoint].
<16> `userInfoEndpoint()`: The configurer for the xref:protocol-endpoints.adoc#oidc-user-info-endpoint[OpenID Connect 1.0 UserInfo endpoint].
<17> `clientRegistrationEndpoint()`: The configurer for the xref:protocol-endpoints.adoc#oidc-client-registration-endpoint[OpenID Connect 1.0 Client Registration endpoint].

[[configuring-authorization-server-settings]]
== Configuring Authorization Server Settings

`AuthorizationServerSettings` contains the configuration settings for the OAuth2 authorization server.
It specifies the `URI` for the protocol endpoints as well as the https://datatracker.ietf.org/doc/html/rfc8414#section-2[issuer identifier].
The default `URI` for the protocol endpoints are as follows:

[source,java]
----
public final class AuthorizationServerSettings extends AbstractSettings {

	...

	public static Builder builder() {
		return new Builder()
			.authorizationEndpoint("/oauth2/authorize")
			.deviceAuthorizationEndpoint("/oauth2/device_authorization")
			.deviceVerificationEndpoint("/oauth2/device_verification")
			.tokenEndpoint("/oauth2/token")
			.tokenIntrospectionEndpoint("/oauth2/introspect")
			.tokenRevocationEndpoint("/oauth2/revoke")
			.jwkSetEndpoint("/oauth2/jwks")
			.oidcLogoutEndpoint("/connect/logout")
			.oidcUserInfoEndpoint("/userinfo")
			.oidcClientRegistrationEndpoint("/connect/register");
	}

	...

}
----

[NOTE]
`AuthorizationServerSettings` is a *REQUIRED* component.

[TIP]
xref:configuration-model.adoc#default-configuration[`@Import(OAuth2AuthorizationServerConfiguration.class)`] automatically registers an `AuthorizationServerSettings` `@Bean`, if not already provided.

The following example shows how to customize the configuration settings and register an `AuthorizationServerSettings` `@Bean`:

[source,java]
----
@Bean
public AuthorizationServerSettings authorizationServerSettings() {
	return AuthorizationServerSettings.builder()
		.issuer("https://example.com")
		.authorizationEndpoint("/oauth2/v1/authorize")
		.deviceAuthorizationEndpoint("/oauth2/v1/device_authorization")
		.deviceVerificationEndpoint("/oauth2/v1/device_verification")
		.tokenEndpoint("/oauth2/v1/token")
		.tokenIntrospectionEndpoint("/oauth2/v1/introspect")
		.tokenRevocationEndpoint("/oauth2/v1/revoke")
		.jwkSetEndpoint("/oauth2/v1/jwks")
		.oidcLogoutEndpoint("/connect/v1/logout")
		.oidcUserInfoEndpoint("/connect/v1/userinfo")
		.oidcClientRegistrationEndpoint("/connect/v1/register")
		.build();
}
----

The `AuthorizationServerContext` is a context object that holds information of the Authorization Server runtime environment.
It provides access to the `AuthorizationServerSettings` and the "`current`" issuer identifier.

[NOTE]
If the issuer identifier is not configured in `AuthorizationServerSettings.builder().issuer(String)`, it is resolved from the current request.

[NOTE]
The `AuthorizationServerContext` is accessible through the `AuthorizationServerContextHolder`, which associates it with the current request thread by using a `ThreadLocal`.

[[configuring-client-authentication]]
== Configuring Client Authentication

`OAuth2ClientAuthenticationConfigurer` provides the ability to customize https://datatracker.ietf.org/doc/html/rfc6749#section-2.3[OAuth2 client authentication].
It defines extension points that let you customize the pre-processing, main processing, and post-processing logic for client authentication requests.

`OAuth2ClientAuthenticationConfigurer` provides the following configuration options:

[source,java]
----
@Bean
public SecurityFilterChain authorizationServerSecurityFilterChain(HttpSecurity http) throws Exception {
	OAuth2AuthorizationServerConfigurer authorizationServerConfigurer =
		new OAuth2AuthorizationServerConfigurer();
	http.apply(authorizationServerConfigurer);

	authorizationServerConfigurer
		.clientAuthentication(clientAuthentication ->
			clientAuthentication
				.authenticationConverter(authenticationConverter)   <1>
				.authenticationConverters(authenticationConvertersConsumer) <2>
				.authenticationProvider(authenticationProvider) <3>
				.authenticationProviders(authenticationProvidersConsumer)   <4>
				.authenticationSuccessHandler(authenticationSuccessHandler) <5>
				.errorResponseHandler(errorResponseHandler) <6>
		);

	return http.build();
}
----
<1> `authenticationConverter()`: Adds an `AuthenticationConverter` (_pre-processor_) used when attempting to extract client credentials from `HttpServletRequest` to an instance of `OAuth2ClientAuthenticationToken`.
<2> `authenticationConverters()`: Sets the `Consumer` providing access to the `List` of default and (optionally) added ``AuthenticationConverter``'s allowing the ability to add, remove, or customize a specific `AuthenticationConverter`.
<3> `authenticationProvider()`: Adds an `AuthenticationProvider` (_main processor_) used for authenticating the `OAuth2ClientAuthenticationToken`.
<4> `authenticationProviders()`: Sets the `Consumer` providing access to the `List` of default and (optionally) added ``AuthenticationProvider``'s allowing the ability to add, remove, or customize a specific `AuthenticationProvider`.
<5> `authenticationSuccessHandler()`: The `AuthenticationSuccessHandler` (_post-processor_) used for handling a successful client authentication and associating the `OAuth2ClientAuthenticationToken` to the `SecurityContext`.
<6> `errorResponseHandler()`: The `AuthenticationFailureHandler` (_post-processor_) used for handling a failed client authentication and returning the https://datatracker.ietf.org/doc/html/rfc6749#section-5.2[`OAuth2Error` response].

`OAuth2ClientAuthenticationConfigurer` configures the `OAuth2ClientAuthenticationFilter` and registers it with the OAuth2 authorization server `SecurityFilterChain` `@Bean`.
`OAuth2ClientAuthenticationFilter` is the `Filter` that processes client authentication requests.

By default, client authentication is required for the xref:protocol-endpoints.adoc#oauth2-token-endpoint[OAuth2 Token endpoint], the xref:protocol-endpoints.adoc#oauth2-token-introspection-endpoint[OAuth2 Token Introspection endpoint], and the xref:protocol-endpoints.adoc#oauth2-token-revocation-endpoint[OAuth2 Token Revocation endpoint].
The supported client authentication methods are `client_secret_basic`, `client_secret_post`, `private_key_jwt`, `client_secret_jwt`, `tls_client_auth`, `self_signed_tls_client_auth`, and `none` (public clients).

`OAuth2ClientAuthenticationFilter` is configured with the following defaults:

* `*AuthenticationConverter*` -- A `DelegatingAuthenticationConverter` composed of  `JwtClientAssertionAuthenticationConverter`, `X509ClientCertificateAuthenticationConverter`, `ClientSecretBasicAuthenticationConverter`, `ClientSecretPostAuthenticationConverter`, and `PublicClientAuthenticationConverter`.
* `*AuthenticationManager*` -- An `AuthenticationManager` composed of `JwtClientAssertionAuthenticationProvider`, `X509ClientCertificateAuthenticationProvider`, `ClientSecretAuthenticationProvider`, and `PublicClientAuthenticationProvider`.
* `*AuthenticationSuccessHandler*` -- An internal implementation that associates the "`authenticated`" `OAuth2ClientAuthenticationToken` (current `Authentication`) to the `SecurityContext`.
* `*AuthenticationFailureHandler*` -- An internal implementation that uses the `OAuth2Error` associated with the `OAuth2AuthenticationException` to return the OAuth2 error response.

[[configuring-client-authentication-customizing-jwt-client-assertion-validation]]
=== Customizing Jwt Client Assertion Validation

`JwtClientAssertionDecoderFactory.DEFAULT_JWT_VALIDATOR_FACTORY` is the default factory that provides an `OAuth2TokenValidator<Jwt>` for the specified `RegisteredClient` and is used for validating the `iss`, `sub`, `aud`, `exp` and `nbf` claims of the `Jwt` client assertion.

`JwtClientAssertionDecoderFactory` provides the ability to override the default `Jwt` client assertion validation by supplying a custom factory of type `Function<RegisteredClient, OAuth2TokenValidator<Jwt>>` to `setJwtValidatorFactory()`.

[NOTE]
`JwtClientAssertionDecoderFactory` is the default `JwtDecoderFactory` used by `JwtClientAssertionAuthenticationProvider` that provides a `JwtDecoder` for the specified `RegisteredClient` and is used for authenticating a `Jwt` Bearer Token during OAuth2 client authentication.

A common use case for customizing `JwtClientAssertionDecoderFactory` is to validate additional claims in the `Jwt` client assertion.

The following example shows how to configure `JwtClientAssertionAuthenticationProvider` with a customized `JwtClientAssertionDecoderFactory` that validates an additional claim in the `Jwt` client assertion:

[source,java]
----
@Bean
public SecurityFilterChain authorizationServerSecurityFilterChain(HttpSecurity http) throws Exception {
	OAuth2AuthorizationServerConfigurer authorizationServerConfigurer =
		new OAuth2AuthorizationServerConfigurer();
	http.apply(authorizationServerConfigurer);

	authorizationServerConfigurer
		.clientAuthentication(clientAuthentication ->
			clientAuthentication
				.authenticationProviders(configureJwtClientAssertionValidator())
		);

	return http.build();
}

private Consumer<List<AuthenticationProvider>> configureJwtClientAssertionValidator() {
	return (authenticationProviders) ->
		authenticationProviders.forEach((authenticationProvider) -> {
			if (authenticationProvider instanceof JwtClientAssertionAuthenticationProvider) {
				// Customize JwtClientAssertionDecoderFactory
				JwtClientAssertionDecoderFactory jwtDecoderFactory = new JwtClientAssertionDecoderFactory();
				Function<RegisteredClient, OAuth2TokenValidator<Jwt>> jwtValidatorFactory = (registeredClient) ->
					new DelegatingOAuth2TokenValidator<>(
						// Use default validators
						JwtClientAssertionDecoderFactory.DEFAULT_JWT_VALIDATOR_FACTORY.apply(registeredClient),
						// Add custom validator
						new JwtClaimValidator<>("claim", "value"::equals));
				jwtDecoderFactory.setJwtValidatorFactory(jwtValidatorFactory);

				((JwtClientAssertionAuthenticationProvider) authenticationProvider)
					.setJwtDecoderFactory(jwtDecoderFactory);
			}
		});
}
----

[[configuring-client-authentication-customizing-mutual-tls-client-authentication]]
=== Customizing Mutual-TLS Client Authentication

`X509ClientCertificateAuthenticationProvider` is used for authenticating the client `X509Certificate` chain received when `ClientAuthenticationMethod.TLS_CLIENT_AUTH` or `ClientAuthenticationMethod.SELF_SIGNED_TLS_CLIENT_AUTH` method is used during OAuth2 client authentication.
It is also composed with a _"Certificate Verifier"_, which is used to verify the contents of the client `X509Certificate` after the TLS handshake has successfully completed.

[[customizing-mutual-tls-client-authentication-pki-mutual-tls-method]]
==== PKI Mutual-TLS Method

For the PKI Mutual-TLS (`ClientAuthenticationMethod.TLS_CLIENT_AUTH`) method, the default implementation of the certificate verifier verifies the subject distinguished name of the client `X509Certificate` against the setting `RegisteredClient.getClientSettings.getX509CertificateSubjectDN()`.

If you need to verify another attribute of the client `X509Certificate`, for example, a Subject Alternative Name (SAN) entry, the following example shows how to configure `X509ClientCertificateAuthenticationProvider` with a custom implementation of a certificate verifier:

[source,java]
----
@Bean
public SecurityFilterChain authorizationServerSecurityFilterChain(HttpSecurity http) throws Exception {
	OAuth2AuthorizationServerConfigurer authorizationServerConfigurer =
			new OAuth2AuthorizationServerConfigurer();
	http.apply(authorizationServerConfigurer);

	authorizationServerConfigurer
			.clientAuthentication(clientAuthentication ->
					clientAuthentication
							.authenticationProviders(configureX509ClientCertificateVerifier())
			);

	return http.build();
}

private Consumer<List<AuthenticationProvider>> configureX509ClientCertificateVerifier() {
	return (authenticationProviders) ->
			authenticationProviders.forEach((authenticationProvider) -> {
				if (authenticationProvider instanceof X509ClientCertificateAuthenticationProvider) {
					Consumer<OAuth2ClientAuthenticationContext> certificateVerifier = (clientAuthenticationContext) -> {
						OAuth2ClientAuthenticationToken clientAuthentication = clientAuthenticationContext.getAuthentication();
						RegisteredClient registeredClient = clientAuthenticationContext.getRegisteredClient();
						X509Certificate[] clientCertificateChain = (X509Certificate[]) clientAuthentication.getCredentials();
						X509Certificate clientCertificate = clientCertificateChain[0];

						// TODO Verify Subject Alternative Name (SAN) entry

					};

					((X509ClientCertificateAuthenticationProvider) authenticationProvider)
							.setCertificateVerifier(certificateVerifier);
				}
			});
}
----

[[customizing-mutual-tls-client-authentication-self-signed-certificate-mutual-tls-method]]
==== Self-Signed Certificate Mutual-TLS Method

For the Self-Signed Certificate Mutual-TLS (`ClientAuthenticationMethod.SELF_SIGNED_TLS_CLIENT_AUTH`) method, the default implementation of the certificate verifier will retrieve the client's JSON Web Key Set using the setting `RegisteredClient.getClientSettings.getJwkSetUrl()` and expect to find a match against the client `X509Certificate` received during the TLS handshake.

[NOTE]
The `RegisteredClient.getClientSettings.getJwkSetUrl()` setting is used to retrieve the client's certificates via a JSON Web Key (JWK) Set.
A certificate is represented with the `x5c` parameter of an individual JWK within the set.

[[customizing-mutual-tls-client-authentication-client-certificate-bound-access-tokens]]
==== Client Certificate-Bound Access Tokens

When Mutual-TLS client authentication is used at the token endpoint, the authorization server is able to bind the issued access token to the client's `X509Certificate`.
The binding is accomplished by computing the SHA-256 thumbprint of the client's `X509Certificate` and associating the thumbprint with the access token.
For example, a JWT access token would include a `x5t#S256` claim, containing the `X509Certificate` thumbprint, within the top-level `cnf` (confirmation method) claim.

Binding the access token to the client's `X509Certificate` provides the ability to implement a proof-of-possession mechanism during protected resource access.
For example, the protected resource would obtain the client's `X509Certificate` used during Mutual-TLS authentication and then verify that the certificate thumbprint matches the `x5t#S256` claim associated with the access token.

The following example shows how to enable certificate-bound access tokens for a client:

[source,java]
----
RegisteredClient mtlsClient = RegisteredClient.withId(UUID.randomUUID().toString())
		.clientId("mtls-client")
		.clientAuthenticationMethod(ClientAuthenticationMethod.TLS_CLIENT_AUTH)
		.authorizationGrantType(AuthorizationGrantType.CLIENT_CREDENTIALS)
		.scope("scope-a")
		.clientSettings(
				ClientSettings.builder()
						.x509CertificateSubjectDN("CN=mtls-client,OU=Spring Samples,O=Spring,C=US")
						.build()
		)
		.tokenSettings(
				TokenSettings.builder()
						.x509CertificateBoundAccessTokens(true)
						.build()
		)
		.build();
----
