[[webflux-oauth2-resource-server]]
= OAuth 2.0 Resource Server

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.

[NOTE]
====
A complete working example for {gh-samples-url}/reactive/webflux/java/oauth2/resource-server[*JWTs*]  is available in the {gh-samples-url}[Spring Security repository].
====

[[webflux-oauth2resourceserver-jwt-minimaldependencies]]
== Minimal Dependencies for JWT

Most Resource Server support is collected into `spring-security-oauth2-resource-server`.
However, the support for decoding and verifying JWTs is in `spring-security-oauth2-jose`, meaning that both are necessary in order to have a working resource server that supports JWT-encoded Bearer Tokens.

[[webflux-oauth2resourceserver-jwt-minimalconfiguration]]
== Minimal Configuration for JWTs

When using https://spring.io/projects/spring-boot[Spring Boot], configuring an application as a resource server consists of two basic steps.
First, include the needed dependencies and second, indicate the location of the authorization server.

=== Specifying the Authorization Server

In a Spring Boot application, to specify which authorization server to use, simply do:

[source,yml]
----
spring:
  security:
    oauth2:
      resourceserver:
        jwt:
          issuer-uri: https://idp.example.com/issuer
----

Where `https://idp.example.com/issuer` is the value contained in the `iss` claim for JWT tokens that the authorization server will issue.
Resource Server will use this property to further self-configure, discover the authorization server's public keys, and subsequently validate incoming JWTs.

[NOTE]
To use the `issuer-uri` property, it must also be true that one of `https://idp.example.com/issuer/.well-known/openid-configuration`, `https://idp.example.com/.well-known/openid-configuration/issuer`, or `https://idp.example.com/.well-known/oauth-authorization-server/issuer` is a supported endpoint for the authorization server.
This endpoint is referred to as a https://openid.net/specs/openid-connect-discovery-1_0.html#ProviderConfig[Provider Configuration] endpoint or a https://tools.ietf.org/html/rfc8414#section-3[Authorization Server Metadata] endpoint.

And that's it!

=== Startup Expectations

When this property and these dependencies are used, Resource Server will automatically configure itself to validate JWT-encoded Bearer Tokens.

It achieves this through a deterministic startup process:

1. Hit the Provider Configuration or Authorization Server Metadata endpoint, processing the response for the `jwks_url` property
2. Configure the validation strategy to query `jwks_url` for valid public keys
3. Configure the validation strategy to validate each JWTs `iss` claim against `https://idp.example.com`.

A consequence of this process is that the authorization server must be up and receiving requests in order for Resource Server to successfully start up.

[NOTE]
If the authorization server is down when Resource Server queries it (given appropriate timeouts), then startup will fail.

=== Runtime Expectations

Once the application is started up, Resource Server will attempt to process any request containing an `Authorization: Bearer` header:

[source,html]
----
GET / HTTP/1.1
Authorization: Bearer some-token-value # Resource Server will process this
----

So long as this scheme is indicated, Resource Server will attempt to process the request according to the Bearer Token specification.

Given a well-formed JWT, Resource Server will:

1. Validate its signature against a public key obtained from the `jwks_url` endpoint during startup and matched against the JWTs header
2. Validate the JWTs `exp` and `nbf` timestamps and the JWTs `iss` claim, and
3. Map each scope to an authority with the prefix `SCOPE_`.

[NOTE]
As the authorization server makes available new keys, Spring Security will automatically rotate the keys used to validate the JWT tokens.

The resulting `Authentication#getPrincipal`, by default, is a Spring Security `Jwt` object, and `Authentication#getName` maps to the JWT's `sub` property, if one is present.

From here, consider jumping to:

<<webflux-oauth2resourceserver-jwt-jwkseturi,How to Configure without Tying Resource Server startup to an authorization server's availability>>

<<webflux-oauth2resourceserver-jwt-sansboot,How to Configure without Spring Boot>>

[[webflux-oauth2resourceserver-jwt-jwkseturi]]
=== Specifying the Authorization Server JWK Set Uri Directly

If the authorization server doesn't support any configuration endpoints, or if Resource Server must be able to start up independently from the authorization server, then the `jwk-set-uri` can be supplied as well:

[source,yaml]
----
spring:
  security:
    oauth2:
      resourceserver:
        jwt:
          issuer-uri: https://idp.example.com
          jwk-set-uri: https://idp.example.com/.well-known/jwks.json
----

[NOTE]
The JWK Set uri is not standardized, but can typically be found in the authorization server's documentation

Consequently, Resource Server will not ping the authorization server at startup.
We still specify the `issuer-uri` so that Resource Server still validates the `iss` claim on incoming JWTs.

[NOTE]
This property can also be supplied directly on the <<webflux-oauth2resourceserver-jwt-jwkseturi-dsl,DSL>>.

[[webflux-oauth2resourceserver-jwt-sansboot]]
=== Overriding or Replacing Boot Auto Configuration

There are two ``@Bean``s that Spring Boot generates on Resource Server's behalf.

The first is a `SecurityWebFilterChain` that configures the app as a resource server. When including `spring-security-oauth2-jose`, this `SecurityWebFilterChain` looks like:

.Resource Server SecurityWebFilterChain
====
.Java
[source,java,role="primary"]
----
@Bean
SecurityWebFilterChain springSecurityFilterChain(ServerHttpSecurity http) {
	http
		.authorizeExchange(exchanges -> exchanges
			.anyExchange().authenticated()
		)
		.oauth2ResourceServer(OAuth2ResourceServerSpec::jwt)
	return http.build();
}
----

.Kotlin
[source,kotlin,role="secondary"]
----
@Bean
fun springSecurityFilterChain(http: ServerHttpSecurity): SecurityWebFilterChain {
    return http {
        authorizeExchange {
            authorize(anyExchange, authenticated)
        }
        oauth2ResourceServer {
            jwt { }
        }
    }
}
----
====

If the application doesn't expose a `SecurityWebFilterChain` bean, then Spring Boot will expose the above default one.

Replacing this is as simple as exposing the bean within the application:

.Replacing SecurityWebFilterChain
====
.Java
[source,java,role="primary"]
----
@Bean
SecurityWebFilterChain springSecurityFilterChain(ServerHttpSecurity http) {
	http
		.authorizeExchange(exchanges -> exchanges
			.pathMatchers("/message/**").hasAuthority("SCOPE_message:read")
			.anyExchange().authenticated()
		)
		.oauth2ResourceServer(oauth2 -> oauth2
			.jwt(withDefaults())
		);
	return http.build();
}
----

.Kotlin
[source,kotlin,role="secondary"]
----
@Bean
fun springSecurityFilterChain(http: ServerHttpSecurity): SecurityWebFilterChain {
    return http {
        authorizeExchange {
            authorize("/message/**", hasAuthority("SCOPE_message:read"))
            authorize(anyExchange, authenticated)
        }
        oauth2ResourceServer {
            jwt { }
        }
    }
}
----
====

The above requires the scope of `message:read` for any URL that starts with `/messages/`.

Methods on the `oauth2ResourceServer` DSL will also override or replace auto configuration.

For example, the second `@Bean` Spring Boot creates is a `ReactiveJwtDecoder`, which decodes `String` tokens into validated instances of `Jwt`:

.ReactiveJwtDecoder
====
.Java
[source,java,role="primary"]
----
@Bean
public ReactiveJwtDecoder jwtDecoder() {
    return ReactiveJwtDecoders.fromIssuerLocation(issuerUri);
}
----

.Kotlin
[source,kotlin,role="secondary"]
----
@Bean
fun jwtDecoder(): ReactiveJwtDecoder {
    return ReactiveJwtDecoders.fromIssuerLocation(issuerUri)
}
----
====

[NOTE]
Calling `{security-api-url}org/springframework/security/oauth2/jwt/ReactiveJwtDecoders.html#fromIssuerLocation-java.lang.String-[ReactiveJwtDecoders#fromIssuerLocation]` is what invokes the Provider Configuration or Authorization Server Metadata endpoint in order to derive the JWK Set Uri.
If the application doesn't expose a `ReactiveJwtDecoder` bean, then Spring Boot will expose the above default one.

And its configuration can be overridden using `jwkSetUri()` or replaced using `decoder()`.

[[webflux-oauth2resourceserver-jwt-jwkseturi-dsl]]
==== Using `jwkSetUri()`

An authorization server's JWK Set Uri can be configured <<webflux-oauth2resourceserver-jwt-jwkseturi,as a configuration property>> or it can be supplied in the DSL:

====
.Java
[source,java,role="primary"]
----
@Bean
SecurityWebFilterChain springSecurityFilterChain(ServerHttpSecurity http) {
	http
		.authorizeExchange(exchanges -> exchanges
			.anyExchange().authenticated()
		)
		.oauth2ResourceServer(oauth2 -> oauth2
			.jwt(jwt -> jwt
				.jwkSetUri("https://idp.example.com/.well-known/jwks.json")
			)
		);
	return http.build();
}
----

.Kotlin
[source,kotlin,role="secondary"]
----
@Bean
fun springSecurityFilterChain(http: ServerHttpSecurity): SecurityWebFilterChain {
    return http {
        authorizeExchange {
            authorize(anyExchange, authenticated)
        }
        oauth2ResourceServer {
            jwt {
                jwkSetUri = "https://idp.example.com/.well-known/jwks.json"
            }
        }
    }
}
----
====

Using `jwkSetUri()` takes precedence over any configuration property.

[[webflux-oauth2resourceserver-jwt-decoder-dsl]]
==== Using `decoder()`

More powerful than `jwkSetUri()` is `decoder()`, which will completely replace any Boot auto configuration of `JwtDecoder`:

====
.Java
[source,java,role="primary"]
----
@Bean
SecurityWebFilterChain springSecurityFilterChain(ServerHttpSecurity http) {
	http
		.authorizeExchange(exchanges -> exchanges
			.anyExchange().authenticated()
		)
		.oauth2ResourceServer(oauth2 -> oauth2
			.jwt(jwt -> jwt
				.decoder(myCustomDecoder())
			)
		);
    return http.build();
}
----

.Kotlin
[source,kotlin,role="secondary"]
----
@Bean
fun springSecurityFilterChain(http: ServerHttpSecurity): SecurityWebFilterChain {
    return http {
        authorizeExchange {
            authorize(anyExchange, authenticated)
        }
        oauth2ResourceServer {
            jwt {
                jwtDecoder = myCustomDecoder()
            }
        }
    }
}
----
====

This is handy when deeper configuration, like <<webflux-oauth2resourceserver-jwt-validation,validation>>, is necessary.

[[webflux-oauth2resourceserver-decoder-bean]]
==== Exposing a `ReactiveJwtDecoder` `@Bean`

Or, exposing a `ReactiveJwtDecoder` `@Bean` has the same effect as `decoder()`:

====
.Java
[source,java,role="primary"]
----
@Bean
public ReactiveJwtDecoder jwtDecoder() {
    return NimbusReactiveJwtDecoder.withJwkSetUri(jwkSetUri).build();
}
----

.Kotlin
[source,kotlin,role="secondary"]
----
@Bean
fun jwtDecoder(): ReactiveJwtDecoder {
    return ReactiveJwtDecoders.fromIssuerLocation(issuerUri)
}
----
====

[[webflux-oauth2resourceserver-jwt-decoder-algorithm]]
== Configuring Trusted Algorithms

By default, `NimbusReactiveJwtDecoder`, and hence Resource Server, will only trust and verify tokens using `RS256`.

You can customize this via <<webflux-oauth2resourceserver-jwt-boot-algorithm,Spring Boot>> or <<webflux-oauth2resourceserver-jwt-decoder-builder,the NimbusJwtDecoder builder>>.

[[webflux-oauth2resourceserver-jwt-boot-algorithm]]
=== Via Spring Boot

The simplest way to set the algorithm is as a property:

[source,yaml]
----
spring:
  security:
    oauth2:
      resourceserver:
        jwt:
          jws-algorithm: RS512
          jwk-set-uri: https://idp.example.org/.well-known/jwks.json
----

[[webflux-oauth2resourceserver-jwt-decoder-builder]]
=== Using a Builder

For greater power, though, we can use a builder that ships with `NimbusReactiveJwtDecoder`:

====
.Java
[source,java,role="primary"]
----
@Bean
ReactiveJwtDecoder jwtDecoder() {
    return NimbusReactiveJwtDecoder.withJwkSetUri(this.jwkSetUri)
            .jwsAlgorithm(RS512).build();
}
----

.Kotlin
[source,kotlin,role="secondary"]
----
@Bean
fun jwtDecoder(): ReactiveJwtDecoder {
    return NimbusReactiveJwtDecoder.withJwkSetUri(this.jwkSetUri)
            .jwsAlgorithm(RS512).build()
}
----
====

Calling `jwsAlgorithm` more than once will configure `NimbusReactiveJwtDecoder` to trust more than one algorithm, like so:

====
.Java
[source,java,role="primary"]
----
@Bean
ReactiveJwtDecoder jwtDecoder() {
    return NimbusReactiveJwtDecoder.withJwkSetUri(this.jwkSetUri)
            .jwsAlgorithm(RS512).jwsAlgorithm(ES512).build();
}
----

.Kotlin
[source,kotlin,role="secondary"]
----
@Bean
fun jwtDecoder(): ReactiveJwtDecoder {
    return NimbusReactiveJwtDecoder.withJwkSetUri(this.jwkSetUri)
            .jwsAlgorithm(RS512).jwsAlgorithm(ES512).build()
}
----
====

Or, you can call `jwsAlgorithms`:

====
.Java
[source,java,role="primary"]
----
@Bean
ReactiveJwtDecoder jwtDecoder() {
    return NimbusReactiveJwtDecoder.withJwkSetUri(this.jwkSetUri)
            .jwsAlgorithms(algorithms -> {
                    algorithms.add(RS512);
                    algorithms.add(ES512);
            }).build();
}
----

.Kotlin
[source,kotlin,role="secondary"]
----
@Bean
fun jwtDecoder(): ReactiveJwtDecoder {
    return NimbusReactiveJwtDecoder.withJwkSetUri(this.jwkSetUri)
            .jwsAlgorithms {
                it.add(RS512)
                it.add(ES512)
            }
            .build()
}
----
====

[[webflux-oauth2resourceserver-jwt-decoder-public-key]]
=== Trusting a Single Asymmetric Key

Simpler than backing a Resource Server with a JWK Set endpoint is to hard-code an RSA public key.
The public key can be provided via <<webflux-oauth2resourceserver-jwt-decoder-public-key-boot,Spring Boot>> or by <<webflux-oauth2resourceserver-jwt-decoder-public-key-builder,Using a Builder>>.

[[webflux-oauth2resourceserver-jwt-decoder-public-key-boot]]
==== Via Spring Boot

Specifying a key via Spring Boot is quite simple.
The key's location can be specified like so:

[source,yaml]
----
spring:
  security:
    oauth2:
      resourceserver:
        jwt:
          public-key-location: classpath:my-key.pub
----

Or, to allow for a more sophisticated lookup, you can post-process the `RsaKeyConversionServicePostProcessor`:

.BeanFactoryPostProcessor
====
.Java
[source,java,role="primary"]
----
@Bean
BeanFactoryPostProcessor conversionServiceCustomizer() {
    return beanFactory ->
        beanFactory.getBean(RsaKeyConversionServicePostProcessor.class)
                .setResourceLoader(new CustomResourceLoader());
}
----

.Kotlin
[source,kotlin,role="secondary"]
----
@Bean
fun conversionServiceCustomizer(): BeanFactoryPostProcessor {
    return BeanFactoryPostProcessor { beanFactory: ConfigurableListableBeanFactory ->
        beanFactory.getBean<RsaKeyConversionServicePostProcessor>()
                .setResourceLoader(CustomResourceLoader())
    }
}
----
====

Specify your key's location:

[source,yaml]
----
key.location: hfds://my-key.pub
----

And then autowire the value:

====
.Java
[source,java,role="primary"]
----
@Value("${key.location}")
RSAPublicKey key;
----

.Kotlin
[source,kotlin,role="secondary"]
----
@Value("\${key.location}")
val key: RSAPublicKey? = null
----
====

[[webflux-oauth2resourceserver-jwt-decoder-public-key-builder]]
==== Using a Builder

To wire an `RSAPublicKey` directly, you can simply use the appropriate `NimbusReactiveJwtDecoder` builder, like so:

====
.Java
[source,java,role="primary"]
----
@Bean
public ReactiveJwtDecoder jwtDecoder() {
    return NimbusReactiveJwtDecoder.withPublicKey(this.key).build();
}
----

.Kotlin
[source,kotlin,role="secondary"]
----
@Bean
fun jwtDecoder(): ReactiveJwtDecoder {
    return NimbusReactiveJwtDecoder.withPublicKey(key).build()
}
----
====

[[webflux-oauth2resourceserver-jwt-decoder-secret-key]]
=== Trusting a Single Symmetric Key

Using a single symmetric key is also simple.
You can simply load in your `SecretKey` and use the appropriate `NimbusReactiveJwtDecoder` builder, like so:

====
.Java
[source,java,role="primary"]
----
@Bean
public ReactiveJwtDecoder jwtDecoder() {
    return NimbusReactiveJwtDecoder.withSecretKey(this.key).build();
}
----

.Kotlin
[source,kotlin,role="secondary"]
----
@Bean
fun jwtDecoder(): ReactiveJwtDecoder {
    return NimbusReactiveJwtDecoder.withSecretKey(this.key).build()
}
----
====

[[webflux-oauth2resourceserver-jwt-authorization]]
=== Configuring Authorization

A JWT that is issued from an OAuth 2.0 Authorization Server will typically either have a `scope` or `scp` attribute, indicating the scopes (or authorities) it's been granted, for example:

`{ ..., "scope" : "messages contacts"}`

When this is the case, Resource Server will attempt to coerce these scopes into a list of granted authorities, prefixing each scope with the string "SCOPE_".

This means that to protect an endpoint or method with a scope derived from a JWT, the corresponding expressions should include this prefix:

====
.Java
[source,java,role="primary"]
----
@Bean
SecurityWebFilterChain springSecurityFilterChain(ServerHttpSecurity http) {
	http
		.authorizeExchange(exchanges -> exchanges
			.mvcMatchers("/contacts/**").hasAuthority("SCOPE_contacts")
			.mvcMatchers("/messages/**").hasAuthority("SCOPE_messages")
			.anyExchange().authenticated()
		)
		.oauth2ResourceServer(OAuth2ResourceServerSpec::jwt);
    return http.build();
}
----

.Kotlin
[source,kotlin,role="secondary"]
----
@Bean
fun springSecurityFilterChain(http: ServerHttpSecurity): SecurityWebFilterChain {
    return http {
        authorizeExchange {
            authorize("/contacts/**", hasAuthority("SCOPE_contacts"))
            authorize("/messages/**", hasAuthority("SCOPE_messages"))
            authorize(anyExchange, authenticated)
        }
        oauth2ResourceServer {
            jwt { }
        }
    }
}
----
====

Or similarly with method security:

====
.Java
[source,java,role="primary"]
----
@PreAuthorize("hasAuthority('SCOPE_messages')")
public Flux<Message> getMessages(...) {}
----

.Kotlin
[source,kotlin,role="secondary"]
----
@PreAuthorize("hasAuthority('SCOPE_messages')")
fun getMessages(): Flux<Message> { }
----
====

[[webflux-oauth2resourceserver-jwt-authorization-extraction]]
==== Extracting Authorities Manually

However, there are a number of circumstances where this default is insufficient.
For example, some authorization servers don't use the `scope` attribute, but instead have their own custom attribute.
Or, at other times, the resource server may need to adapt the attribute or a composition of attributes into internalized authorities.

To this end, the DSL exposes `jwtAuthenticationConverter()`:

====
.Java
[source,java,role="primary"]
----
@Bean
SecurityWebFilterChain springSecurityFilterChain(ServerHttpSecurity http) {
	http
		.authorizeExchange(exchanges -> exchanges
			.anyExchange().authenticated()
		)
		.oauth2ResourceServer(oauth2 -> oauth2
			.jwt(jwt -> jwt
				.jwtAuthenticationConverter(grantedAuthoritiesExtractor())
			)
		);
	return http.build();
}

Converter<Jwt, Mono<AbstractAuthenticationToken>> grantedAuthoritiesExtractor() {
    JwtAuthenticationConverter jwtAuthenticationConverter =
            new JwtAuthenticationConverter();
    jwtAuthenticationConverter.setJwtGrantedAuthoritiesConverter
            (new GrantedAuthoritiesExtractor());
    return new ReactiveJwtAuthenticationConverterAdapter(jwtAuthenticationConverter);
}
----

.Kotlin
[source,kotlin,role="secondary"]
----
@Bean
fun springSecurityFilterChain(http: ServerHttpSecurity): SecurityWebFilterChain {
    return http {
        authorizeExchange {
            authorize(anyExchange, authenticated)
        }
        oauth2ResourceServer {
            jwt {
                jwtAuthenticationConverter = grantedAuthoritiesExtractor()
            }
        }
    }
}

fun grantedAuthoritiesExtractor(): Converter<Jwt, Mono<AbstractAuthenticationToken>> {
    val jwtAuthenticationConverter = JwtAuthenticationConverter()
    jwtAuthenticationConverter.setJwtGrantedAuthoritiesConverter(GrantedAuthoritiesExtractor())
    return ReactiveJwtAuthenticationConverterAdapter(jwtAuthenticationConverter)
}
----
====

which is responsible for converting a `Jwt` into an `Authentication`.
As part of its configuration, we can supply a subsidiary converter to go from `Jwt` to a `Collection` of granted authorities.

That final converter might be something like `GrantedAuthoritiesExtractor` below:

====
.Java
[source,java,role="primary"]
----
static class GrantedAuthoritiesExtractor
        implements Converter<Jwt, Collection<GrantedAuthority>> {

    public Collection<GrantedAuthority> convert(Jwt jwt) {
        Collection<?> authorities = (Collection<?>)
                jwt.getClaims().getOrDefault("mycustomclaim", Collections.emptyList());

        return authorities.stream()
                .map(Object::toString)
                .map(SimpleGrantedAuthority::new)
                .collect(Collectors.toList());
    }
}
----

.Kotlin
[source,kotlin,role="secondary"]
----
internal class GrantedAuthoritiesExtractor : Converter<Jwt, Collection<GrantedAuthority>> {
    override fun convert(jwt: Jwt): Collection<GrantedAuthority> {
        val authorities: List<Any> = jwt.claims
                .getOrDefault("mycustomclaim", emptyList<Any>()) as List<Any>
        return authorities
                .map { it.toString() }
                .map { SimpleGrantedAuthority(it) }
    }
}
----
====

For more flexibility, the DSL supports entirely replacing the converter with any class that implements `Converter<Jwt, Mono<AbstractAuthenticationToken>>`:

====
.Java
[source,java,role="primary"]
----
static class CustomAuthenticationConverter implements Converter<Jwt, Mono<AbstractAuthenticationToken>> {
    public AbstractAuthenticationToken convert(Jwt jwt) {
        return Mono.just(jwt).map(this::doConversion);
    }
}
----

.Kotlin
[source,kotlin,role="secondary"]
----
internal class CustomAuthenticationConverter : Converter<Jwt, Mono<AbstractAuthenticationToken>> {
    override fun convert(jwt: Jwt): Mono<AbstractAuthenticationToken> {
        return Mono.just(jwt).map(this::doConversion)
    }
}
----
====

[[webflux-oauth2resourceserver-jwt-validation]]
=== Configuring Validation

Using <<webflux-oauth2resourceserver-jwt-minimalconfiguration,minimal Spring Boot configuration>>, indicating the authorization server's issuer uri, Resource Server will default to verifying the `iss` claim as well as the `exp` and `nbf` timestamp claims.

In circumstances where validation needs to be customized, Resource Server ships with two standard validators and also accepts custom `OAuth2TokenValidator` instances.

[[webflux-oauth2resourceserver-jwt-validation-clockskew]]
==== Customizing Timestamp Validation

JWT's typically have a window of validity, with the start of the window indicated in the `nbf` claim and the end indicated in the `exp` claim.

However, every server can experience clock drift, which can cause tokens to appear expired to one server, but not to another.
This can cause some implementation heartburn as the number of collaborating servers increases in a distributed system.

Resource Server uses `JwtTimestampValidator` to verify a token's validity window, and it can be configured with a `clockSkew` to alleviate the above problem:

====
.Java
[source,java,role="primary"]
----
@Bean
ReactiveJwtDecoder jwtDecoder() {
     NimbusReactiveJwtDecoder jwtDecoder = (NimbusReactiveJwtDecoder)
             ReactiveJwtDecoders.fromIssuerLocation(issuerUri);

     OAuth2TokenValidator<Jwt> withClockSkew = new DelegatingOAuth2TokenValidator<>(
            new JwtTimestampValidator(Duration.ofSeconds(60)),
            new IssuerValidator(issuerUri));

     jwtDecoder.setJwtValidator(withClockSkew);

     return jwtDecoder;
}
----

.Kotlin
[source,kotlin,role="secondary"]
----
@Bean
fun jwtDecoder(): ReactiveJwtDecoder {
    val jwtDecoder = ReactiveJwtDecoders.fromIssuerLocation(issuerUri) as NimbusReactiveJwtDecoder
    val withClockSkew: OAuth2TokenValidator<Jwt> = DelegatingOAuth2TokenValidator(
            JwtTimestampValidator(Duration.ofSeconds(60)),
            JwtIssuerValidator(issuerUri))
    jwtDecoder.setJwtValidator(withClockSkew)
    return jwtDecoder
}
----
====

[NOTE]
By default, Resource Server configures a clock skew of 60 seconds.

[[webflux-oauth2resourceserver-validation-custom]]
==== Configuring a Custom Validator

Adding a check for the `aud` claim is simple with the `OAuth2TokenValidator` API:

====
.Java
[source,java,role="primary"]
----
public class AudienceValidator implements OAuth2TokenValidator<Jwt> {
    OAuth2Error error = new OAuth2Error("invalid_token", "The required audience is missing", null);

    public OAuth2TokenValidatorResult validate(Jwt jwt) {
        if (jwt.getAudience().contains("messaging")) {
            return OAuth2TokenValidatorResult.success();
        } else {
            return OAuth2TokenValidatorResult.failure(error);
        }
    }
}
----

.Kotlin
[source,kotlin,role="secondary"]
----
class AudienceValidator : OAuth2TokenValidator<Jwt> {
    var error: OAuth2Error = OAuth2Error("invalid_token", "The required audience is missing", null)
    override fun validate(jwt: Jwt): OAuth2TokenValidatorResult {
        return if (jwt.audience.contains("messaging")) {
            OAuth2TokenValidatorResult.success()
        } else {
            OAuth2TokenValidatorResult.failure(error)
        }
    }
}
----
====

Then, to add into a resource server, it's a matter of specifying the `ReactiveJwtDecoder` instance:

====
.Java
[source,java,role="primary"]
----
@Bean
ReactiveJwtDecoder jwtDecoder() {
    NimbusReactiveJwtDecoder jwtDecoder = (NimbusReactiveJwtDecoder)
            ReactiveJwtDecoders.fromIssuerLocation(issuerUri);

    OAuth2TokenValidator<Jwt> audienceValidator = new AudienceValidator();
    OAuth2TokenValidator<Jwt> withIssuer = JwtValidators.createDefaultWithIssuer(issuerUri);
    OAuth2TokenValidator<Jwt> withAudience = new DelegatingOAuth2TokenValidator<>(withIssuer, audienceValidator);

    jwtDecoder.setJwtValidator(withAudience);

    return jwtDecoder;
}
----

.Kotlin
[source,kotlin,role="secondary"]
----
@Bean
fun jwtDecoder(): ReactiveJwtDecoder {
    val jwtDecoder = ReactiveJwtDecoders.fromIssuerLocation(issuerUri) as NimbusReactiveJwtDecoder
    val audienceValidator: OAuth2TokenValidator<Jwt> = AudienceValidator()
    val withIssuer: OAuth2TokenValidator<Jwt> = JwtValidators.createDefaultWithIssuer(issuerUri)
    val withAudience: OAuth2TokenValidator<Jwt> = DelegatingOAuth2TokenValidator(withIssuer, audienceValidator)
    jwtDecoder.setJwtValidator(withAudience)
    return jwtDecoder
}
----
====

[[webflux-oauth2resourceserver-opaque-minimaldependencies]]
=== Minimal Dependencies for Introspection
As described in xref:servlet/oauth2/oauth2-resourceserver.adoc#oauth2resourceserver-jwt-minimaldependencies[Minimal Dependencies for JWT] most of Resource Server support is collected in `spring-security-oauth2-resource-server`.
However unless a custom <<webflux-oauth2resourceserver-opaque-introspector-bean,`ReactiveOpaqueTokenIntrospector`>> is provided, the Resource Server will fallback to ReactiveOpaqueTokenIntrospector.
Meaning that both `spring-security-oauth2-resource-server` and `oauth2-oidc-sdk` are necessary in order to have a working minimal Resource Server that supports opaque Bearer Tokens.
Please refer to `spring-security-oauth2-resource-server` in order to determin the correct version for `oauth2-oidc-sdk`.

[[webflux-oauth2resourceserver-opaque-minimalconfiguration]]
=== Minimal Configuration for Introspection

Typically, an opaque token can be verified via an https://tools.ietf.org/html/rfc7662[OAuth 2.0 Introspection Endpoint], hosted by the authorization server.
This can be handy when revocation is a requirement.

When using https://spring.io/projects/spring-boot[Spring Boot], configuring an application as a resource server that uses introspection consists of two basic steps.
First, include the needed dependencies and second, indicate the introspection endpoint details.

[[webflux-oauth2resourceserver-opaque-introspectionuri]]
==== Specifying the Authorization Server

To specify where the introspection endpoint is, simply do:

[source,yaml]
----
security:
  oauth2:
    resourceserver:
      opaque-token:
        introspection-uri: https://idp.example.com/introspect
        client-id: client
        client-secret: secret
----

Where `https://idp.example.com/introspect` is the introspection endpoint hosted by your authorization server and `client-id` and `client-secret` are the credentials needed to hit that endpoint.

Resource Server will use these properties to further self-configure and subsequently validate incoming JWTs.

[NOTE]
When using introspection, the authorization server's word is the law.
If the authorization server responses that the token is valid, then it is.

And that's it!

==== Startup Expectations

When this property and these dependencies are used, Resource Server will automatically configure itself to validate Opaque Bearer Tokens.

This startup process is quite a bit simpler than for JWTs since no endpoints need to be discovered and no additional validation rules get added.

==== Runtime Expectations

Once the application is started up, Resource Server will attempt to process any request containing an `Authorization: Bearer` header:

[source,http]
----
GET / HTTP/1.1
Authorization: Bearer some-token-value # Resource Server will process this
----

So long as this scheme is indicated, Resource Server will attempt to process the request according to the Bearer Token specification.

Given an Opaque Token, Resource Server will

1. Query the provided introspection endpoint using the provided credentials and the token
2. Inspect the response for an `{ 'active' : true }` attribute
3. Map each scope to an authority with the prefix `SCOPE_`

The resulting `Authentication#getPrincipal`, by default, is a Spring Security `{security-api-url}org/springframework/security/oauth2/core/OAuth2AuthenticatedPrincipal.html[OAuth2AuthenticatedPrincipal]` object, and `Authentication#getName` maps to the token's `sub` property, if one is present.

From here, you may want to jump to:

* <<webflux-oauth2resourceserver-opaque-attributes,Looking Up Attributes Post-Authentication>>
* <<webflux-oauth2resourceserver-opaque-authorization-extraction,Extracting Authorities Manually>>
* <<webflux-oauth2resourceserver-opaque-jwt-introspector,Using Introspection with JWTs>>

[[webflux-oauth2resourceserver-opaque-attributes]]
=== Looking Up Attributes Post-Authentication

Once a token is authenticated, an instance of `BearerTokenAuthentication` is set in the `SecurityContext`.

This means that it's available in `@Controller` methods when using `@EnableWebFlux` in your configuration:

====
.Java
[source,java,role="primary"]
----
@GetMapping("/foo")
public Mono<String> foo(BearerTokenAuthentication authentication) {
    return Mono.just(authentication.getTokenAttributes().get("sub") + " is the subject");
}
----

.Kotlin
[source,kotlin,role="secondary"]
----
@GetMapping("/foo")
fun foo(authentication: BearerTokenAuthentication): Mono<String> {
    return Mono.just(authentication.tokenAttributes["sub"].toString() + " is the subject")
}
----
====

Since `BearerTokenAuthentication` holds an `OAuth2AuthenticatedPrincipal`, that also means that it's available to controller methods, too:

====
.Java
[source,java,role="primary"]
----
@GetMapping("/foo")
public Mono<String> foo(@AuthenticationPrincipal OAuth2AuthenticatedPrincipal principal) {
    return Mono.just(principal.getAttribute("sub") + " is the subject");
}
----

.Kotlin
[source,kotlin,role="secondary"]
----
@GetMapping("/foo")
fun foo(@AuthenticationPrincipal principal: OAuth2AuthenticatedPrincipal): Mono<String> {
    return Mono.just(principal.getAttribute<Any>("sub").toString() + " is the subject")
}
----
====

==== Looking Up Attributes Via SpEL

Of course, this also means that attributes can be accessed via SpEL.

For example, if using `@EnableReactiveMethodSecurity` so that you can use `@PreAuthorize` annotations, you can do:

====
.Java
[source,java,role="primary"]
----
@PreAuthorize("principal?.attributes['sub'] == 'foo'")
public Mono<String> forFoosEyesOnly() {
    return Mono.just("foo");
}
----

.Kotlin
[source,kotlin,role="secondary"]
----
@PreAuthorize("principal.attributes['sub'] == 'foo'")
fun forFoosEyesOnly(): Mono<String> {
    return Mono.just("foo")
}
----
====

[[webflux-oauth2resourceserver-opaque-sansboot]]
=== Overriding or Replacing Boot Auto Configuration

There are two ``@Bean``s that Spring Boot generates on Resource Server's behalf.

The first is a `SecurityWebFilterChain` that configures the app as a resource server.
When use Opaque Token, this `SecurityWebFilterChain` looks like:

====
.Java
[source,java,role="primary"]
----
@Bean
SecurityWebFilterChain springSecurityFilterChain(ServerHttpSecurity http) {
	http
		.authorizeExchange(exchanges -> exchanges
			.anyExchange().authenticated()
		)
		.oauth2ResourceServer(ServerHttpSecurity.OAuth2ResourceServerSpec::opaqueToken)
	return http.build();
}
----

.Kotlin
[source,kotlin,role="secondary"]
----
@Bean
fun springSecurityFilterChain(http: ServerHttpSecurity): SecurityWebFilterChain {
    return http {
        authorizeExchange {
            authorize(anyExchange, authenticated)
        }
        oauth2ResourceServer {
            opaqueToken { }
        }
    }
}
----
====

If the application doesn't expose a `SecurityWebFilterChain` bean, then Spring Boot will expose the above default one.

Replacing this is as simple as exposing the bean within the application:

.Replacing SecurityWebFilterChain
====
.Java
[source,java,role="primary"]
----
@EnableWebFluxSecurity
public class MyCustomSecurityConfiguration {
    @Bean
    SecurityWebFilterChain springSecurityFilterChain(ServerHttpSecurity http) {
        http
            .authorizeExchange(exchanges -> exchanges
                .pathMatchers("/messages/**").hasAuthority("SCOPE_message:read")
                .anyExchange().authenticated()
            )
            .oauth2ResourceServer(oauth2 -> oauth2
                .opaqueToken(opaqueToken -> opaqueToken
                    .introspector(myIntrospector())
                )
            );
        return http.build();
    }
}
----

.Kotlin
[source,kotlin,role="secondary"]
----
@Bean
fun springSecurityFilterChain(http: ServerHttpSecurity): SecurityWebFilterChain {
    return http {
        authorizeExchange {
            authorize("/messages/**", hasAuthority("SCOPE_message:read"))
            authorize(anyExchange, authenticated)
        }
        oauth2ResourceServer {
            opaqueToken {
                introspector = myIntrospector()
            }
        }
    }
}
----
====

The above requires the scope of `message:read` for any URL that starts with `/messages/`.

Methods on the `oauth2ResourceServer` DSL will also override or replace auto configuration.

For example, the second `@Bean` Spring Boot creates is a `ReactiveOpaqueTokenIntrospector`, which decodes `String` tokens into validated instances of `OAuth2AuthenticatedPrincipal`:

====
.Java
[source,java,role="primary"]
----
@Bean
public ReactiveOpaqueTokenIntrospector introspector() {
    return new NimbusReactiveOpaqueTokenIntrospector(introspectionUri, clientId, clientSecret);
}
----

.Kotlin
[source,kotlin,role="secondary"]
----
@Bean
fun introspector(): ReactiveOpaqueTokenIntrospector {
    return NimbusReactiveOpaqueTokenIntrospector(introspectionUri, clientId, clientSecret)
}
----
====

If the application doesn't expose a `ReactiveOpaqueTokenIntrospector` bean, then Spring Boot will expose the above default one.

And its configuration can be overridden using `introspectionUri()` and `introspectionClientCredentials()` or replaced using `introspector()`.

[[webflux-oauth2resourceserver-opaque-introspectionuri-dsl]]
==== Using `introspectionUri()`

An authorization server's Introspection Uri can be configured <<webflux-oauth2resourceserver-opaque-introspectionuri,as a configuration property>> or it can be supplied in the DSL:

====
.Java
[source,java,role="primary"]
----
@EnableWebFluxSecurity
public class DirectlyConfiguredIntrospectionUri {
    @Bean
    SecurityWebFilterChain springSecurityFilterChain(ServerHttpSecurity http) {
        http
            .authorizeExchange(exchanges -> exchanges
                .anyExchange().authenticated()
            )
            .oauth2ResourceServer(oauth2 -> oauth2
                .opaqueToken(opaqueToken -> opaqueToken
                    .introspectionUri("https://idp.example.com/introspect")
                    .introspectionClientCredentials("client", "secret")
                )
            );
        return http.build();
    }
}
----

.Kotlin
[source,kotlin,role="secondary"]
----
@Bean
fun springSecurityFilterChain(http: ServerHttpSecurity): SecurityWebFilterChain {
    return http {
        authorizeExchange {
            authorize(anyExchange, authenticated)
        }
        oauth2ResourceServer {
            opaqueToken {
                introspectionUri = "https://idp.example.com/introspect"
                introspectionClientCredentials("client", "secret")
            }
        }
    }
}
----
====

Using `introspectionUri()` takes precedence over any configuration property.

[[webflux-oauth2resourceserver-opaque-introspector-dsl]]
==== Using `introspector()`

More powerful than `introspectionUri()` is `introspector()`, which will completely replace any Boot auto configuration of `ReactiveOpaqueTokenIntrospector`:

====
.Java
[source,java,role="primary"]
----
@EnableWebFluxSecurity
public class DirectlyConfiguredIntrospector {
    @Bean
    SecurityWebFilterChain springSecurityFilterChain(ServerHttpSecurity http) {
        http
            .authorizeExchange(exchanges -> exchanges
                .anyExchange().authenticated()
            )
            .oauth2ResourceServer(oauth2 -> oauth2
                .opaqueToken(opaqueToken -> opaqueToken
                    .introspector(myCustomIntrospector())
                )
            );
        return http.build();
    }
}
----

.Kotlin
[source,kotlin,role="secondary"]
----
@Bean
fun springSecurityFilterChain(http: ServerHttpSecurity): SecurityWebFilterChain {
    return http {
        authorizeExchange {
            authorize(anyExchange, authenticated)
        }
        oauth2ResourceServer {
            opaqueToken {
                introspector = myCustomIntrospector()
            }
        }
    }
}
----
====

This is handy when deeper configuration, like <<webflux-oauth2resourceserver-opaque-authorization-extraction,authority mapping>>or <<webflux-oauth2resourceserver-opaque-jwt-introspector,JWT revocation>> is necessary.

[[webflux-oauth2resourceserver-opaque-introspector-bean]]
==== Exposing a `ReactiveOpaqueTokenIntrospector` `@Bean`

Or, exposing a `ReactiveOpaqueTokenIntrospector` `@Bean` has the same effect as `introspector()`:

====
.Java
[source,java,role="primary"]
----
@Bean
public ReactiveOpaqueTokenIntrospector introspector() {
    return new NimbusReactiveOpaqueTokenIntrospector(introspectionUri, clientId, clientSecret);
}
----

.Kotlin
[source,kotlin,role="secondary"]
----
@Bean
fun introspector(): ReactiveOpaqueTokenIntrospector {
    return NimbusReactiveOpaqueTokenIntrospector(introspectionUri, clientId, clientSecret)
}
----
====

[[webflux-oauth2resourceserver-opaque-authorization]]
=== Configuring Authorization

An OAuth 2.0 Introspection endpoint will typically return a `scope` attribute, indicating the scopes (or authorities) it's been granted, for example:

`{ ..., "scope" : "messages contacts"}`

When this is the case, Resource Server will attempt to coerce these scopes into a list of granted authorities, prefixing each scope with the string "SCOPE_".

This means that to protect an endpoint or method with a scope derived from an Opaque Token, the corresponding expressions should include this prefix:

====
.Java
[source,java,role="primary"]
----
@EnableWebFluxSecurity
public class MappedAuthorities {
    @Bean
    SecurityWebFilterChain springSecurityFilterChain(ServerHttpSecurity http) {
        http
            .authorizeExchange(exchange -> exchange
                .pathMatchers("/contacts/**").hasAuthority("SCOPE_contacts")
                .pathMatchers("/messages/**").hasAuthority("SCOPE_messages")
                .anyExchange().authenticated()
            )
            .oauth2ResourceServer(ServerHttpSecurity.OAuth2ResourceServerSpec::opaqueToken);
        return http.build();
    }
}
----

.Kotlin
[source,kotlin,role="secondary"]
----
@Bean
fun springSecurityFilterChain(http: ServerHttpSecurity): SecurityWebFilterChain {
    return http {
        authorizeExchange {
            authorize("/contacts/**", hasAuthority("SCOPE_contacts"))
            authorize("/messages/**", hasAuthority("SCOPE_messages"))
            authorize(anyExchange, authenticated)
        }
        oauth2ResourceServer {
            opaqueToken { }
        }
    }
}
----
====

Or similarly with method security:

====
.Java
[source,java,role="primary"]
----
@PreAuthorize("hasAuthority('SCOPE_messages')")
public Flux<Message> getMessages(...) {}
----

.Kotlin
[source,kotlin,role="secondary"]
----
@PreAuthorize("hasAuthority('SCOPE_messages')")
fun getMessages(): Flux<Message> { }
----
====

[[webflux-oauth2resourceserver-opaque-authorization-extraction]]
==== Extracting Authorities Manually

By default, Opaque Token support will extract the scope claim from an introspection response and parse it into individual `GrantedAuthority` instances.

For example, if the introspection response were:

[source,json]
----
{
    "active" : true,
    "scope" : "message:read message:write"
}
----

Then Resource Server would generate an `Authentication` with two authorities, one for `message:read` and the other for `message:write`.

This can, of course, be customized using a custom `ReactiveOpaqueTokenIntrospector` that takes a look at the attribute set and converts in its own way:

====
.Java
[source,java,role="primary"]
----
public class CustomAuthoritiesOpaqueTokenIntrospector implements ReactiveOpaqueTokenIntrospector {
    private ReactiveOpaqueTokenIntrospector delegate =
            new NimbusReactiveOpaqueTokenIntrospector("https://idp.example.org/introspect", "client", "secret");

    public Mono<OAuth2AuthenticatedPrincipal> introspect(String token) {
        return this.delegate.introspect(token)
                .map(principal -> new DefaultOAuth2AuthenticatedPrincipal(
                        principal.getName(), principal.getAttributes(), extractAuthorities(principal)));
    }

    private Collection<GrantedAuthority> extractAuthorities(OAuth2AuthenticatedPrincipal principal) {
        List<String> scopes = principal.getAttribute(OAuth2IntrospectionClaimNames.SCOPE);
        return scopes.stream()
                .map(SimpleGrantedAuthority::new)
                .collect(Collectors.toList());
    }
}
----

.Kotlin
[source,kotlin,role="secondary"]
----
class CustomAuthoritiesOpaqueTokenIntrospector : ReactiveOpaqueTokenIntrospector {
    private val delegate: ReactiveOpaqueTokenIntrospector = NimbusReactiveOpaqueTokenIntrospector("https://idp.example.org/introspect", "client", "secret")
    override fun introspect(token: String): Mono<OAuth2AuthenticatedPrincipal> {
        return delegate.introspect(token)
                .map { principal: OAuth2AuthenticatedPrincipal ->
                    DefaultOAuth2AuthenticatedPrincipal(
                            principal.name, principal.attributes, extractAuthorities(principal))
                }
    }

    private fun extractAuthorities(principal: OAuth2AuthenticatedPrincipal): Collection<GrantedAuthority> {
        val scopes = principal.getAttribute<List<String>>(OAuth2IntrospectionClaimNames.SCOPE)
        return scopes
                .map { SimpleGrantedAuthority(it) }
    }
}
----
====

Thereafter, this custom introspector can be configured simply by exposing it as a `@Bean`:

====
.Java
[source,java,role="primary"]
----
@Bean
public ReactiveOpaqueTokenIntrospector introspector() {
    return new CustomAuthoritiesOpaqueTokenIntrospector();
}
----

.Kotlin
[source,kotlin,role="secondary"]
----
@Bean
fun introspector(): ReactiveOpaqueTokenIntrospector {
    return CustomAuthoritiesOpaqueTokenIntrospector()
}
----
====

[[webflux-oauth2resourceserver-opaque-jwt-introspector]]
=== Using Introspection with JWTs

A common question is whether or not introspection is compatible with JWTs.
Spring Security's Opaque Token support has been designed to not care about the format of the token -- it will gladly pass any token to the introspection endpoint provided.

So, let's say that you've got a requirement that requires you to check with the authorization server on each request, in case the JWT has been revoked.

Even though you are using the JWT format for the token, your validation method is introspection, meaning you'd want to do:

[source,yaml]
----
spring:
  security:
    oauth2:
      resourceserver:
        opaque-token:
          introspection-uri: https://idp.example.org/introspection
          client-id: client
          client-secret: secret
----

In this case, the resulting `Authentication` would be `BearerTokenAuthentication`.
Any attributes in the corresponding `OAuth2AuthenticatedPrincipal` would be whatever was returned by the introspection endpoint.

But, let's say that, oddly enough, the introspection endpoint only returns whether or not the token is active.
Now what?

In this case, you can create a custom `ReactiveOpaqueTokenIntrospector` that still hits the endpoint, but then updates the returned principal to have the JWTs claims as the attributes:

====
.Java
[source,java,role="primary"]
----
public class JwtOpaqueTokenIntrospector implements ReactiveOpaqueTokenIntrospector {
	private ReactiveOpaqueTokenIntrospector delegate =
			new NimbusReactiveOpaqueTokenIntrospector("https://idp.example.org/introspect", "client", "secret");
	private ReactiveJwtDecoder jwtDecoder = new NimbusReactiveJwtDecoder(new ParseOnlyJWTProcessor());

	public Mono<OAuth2AuthenticatedPrincipal> introspect(String token) {
		return this.delegate.introspect(token)
				.flatMap(principal -> this.jwtDecoder.decode(token))
				.map(jwt -> new DefaultOAuth2AuthenticatedPrincipal(jwt.getClaims(), NO_AUTHORITIES));
	}

	private static class ParseOnlyJWTProcessor implements Converter<JWT, Mono<JWTClaimsSet>> {
		public Mono<JWTClaimsSet> convert(JWT jwt) {
			try {
				return Mono.just(jwt.getJWTClaimsSet());
			} catch (Exception ex) {
				return Mono.error(ex);
			}
		}
	}
}
----

.Kotlin
[source,kotlin,role="secondary"]
----
class JwtOpaqueTokenIntrospector : ReactiveOpaqueTokenIntrospector {
    private val delegate: ReactiveOpaqueTokenIntrospector = NimbusReactiveOpaqueTokenIntrospector("https://idp.example.org/introspect", "client", "secret")
    private val jwtDecoder: ReactiveJwtDecoder = NimbusReactiveJwtDecoder(ParseOnlyJWTProcessor())
    override fun introspect(token: String): Mono<OAuth2AuthenticatedPrincipal> {
        return delegate.introspect(token)
                .flatMap { jwtDecoder.decode(token) }
                .map { jwt: Jwt -> DefaultOAuth2AuthenticatedPrincipal(jwt.claims, NO_AUTHORITIES) }
    }

    private class ParseOnlyJWTProcessor : Converter<JWT, Mono<JWTClaimsSet>> {
        override fun convert(jwt: JWT): Mono<JWTClaimsSet> {
            return try {
                Mono.just(jwt.jwtClaimsSet)
            } catch (e: Exception) {
                Mono.error(e)
            }
        }
    }
}
----
====

Thereafter, this custom introspector can be configured simply by exposing it as a `@Bean`:

====
.Java
[source,java,role="primary"]
----
@Bean
public ReactiveOpaqueTokenIntrospector introspector() {
    return new JwtOpaqueTokenIntropsector();
}
----

.Kotlin
[source,kotlin,role="secondary"]
----
@Bean
fun introspector(): ReactiveOpaqueTokenIntrospector {
    return JwtOpaqueTokenIntrospector()
}
----
====

[[webflux-oauth2resourceserver-opaque-userinfo]]
=== Calling a `/userinfo` Endpoint

Generally speaking, a Resource Server doesn't care about the underlying user, but instead about the authorities that have been granted.

That said, at times it can be valuable to tie the authorization statement back to a user.

If an application is also using `spring-security-oauth2-client`, having set up the appropriate `ClientRegistrationRepository`, then this is quite simple with a custom `OpaqueTokenIntrospector`.
This implementation below does three things:

* Delegates to the introspection endpoint, to affirm the token's validity
* Looks up the appropriate client registration associated with the `/userinfo` endpoint
* Invokes and returns the response from the `/userinfo` endpoint

====
.Java
[source,java,role="primary"]
----
public class UserInfoOpaqueTokenIntrospector implements ReactiveOpaqueTokenIntrospector {
	private final ReactiveOpaqueTokenIntrospector delegate =
			new NimbusReactiveOpaqueTokenIntrospector("https://idp.example.org/introspect", "client", "secret");
	private final ReactiveOAuth2UserService<OAuth2UserRequest, OAuth2User> oauth2UserService =
			new DefaultReactiveOAuth2UserService();

	private final ReactiveClientRegistrationRepository repository;

	// ... constructor

	@Override
	public Mono<OAuth2AuthenticatedPrincipal> introspect(String token) {
		return Mono.zip(this.delegate.introspect(token), this.repository.findByRegistrationId("registration-id"))
				.map(t -> {
					OAuth2AuthenticatedPrincipal authorized = t.getT1();
					ClientRegistration clientRegistration = t.getT2();
					Instant issuedAt = authorized.getAttribute(ISSUED_AT);
					Instant expiresAt = authorized.getAttribute(OAuth2IntrospectionClaimNames.EXPIRES_AT);
					OAuth2AccessToken accessToken = new OAuth2AccessToken(BEARER, token, issuedAt, expiresAt);
					return new OAuth2UserRequest(clientRegistration, accessToken);
				})
				.flatMap(this.oauth2UserService::loadUser);
	}
}
----

.Kotlin
[source,kotlin,role="secondary"]
----
class UserInfoOpaqueTokenIntrospector : ReactiveOpaqueTokenIntrospector {
    private val delegate: ReactiveOpaqueTokenIntrospector = NimbusReactiveOpaqueTokenIntrospector("https://idp.example.org/introspect", "client", "secret")
    private val oauth2UserService: ReactiveOAuth2UserService<OAuth2UserRequest, OAuth2User> = DefaultReactiveOAuth2UserService()
    private val repository: ReactiveClientRegistrationRepository? = null

    // ... constructor
    override fun introspect(token: String?): Mono<OAuth2AuthenticatedPrincipal> {
        return Mono.zip<OAuth2AuthenticatedPrincipal, ClientRegistration>(delegate.introspect(token), repository!!.findByRegistrationId("registration-id"))
                .map<OAuth2UserRequest> { t: Tuple2<OAuth2AuthenticatedPrincipal, ClientRegistration> ->
                    val authorized = t.t1
                    val clientRegistration = t.t2
                    val issuedAt: Instant? = authorized.getAttribute(ISSUED_AT)
                    val expiresAt: Instant? = authorized.getAttribute(OAuth2IntrospectionClaimNames.EXPIRES_AT)
                    val accessToken = OAuth2AccessToken(BEARER, token, issuedAt, expiresAt)
                    OAuth2UserRequest(clientRegistration, accessToken)
                }
                .flatMap { userRequest: OAuth2UserRequest -> oauth2UserService.loadUser(userRequest) }
    }
}
----
====

If you aren't using `spring-security-oauth2-client`, it's still quite simple.
You will simply need to invoke the `/userinfo` with your own instance of `WebClient`:

====
.Java
[source,java,role="primary"]
----
public class UserInfoOpaqueTokenIntrospector implements ReactiveOpaqueTokenIntrospector {
    private final ReactiveOpaqueTokenIntrospector delegate =
            new NimbusReactiveOpaqueTokenIntrospector("https://idp.example.org/introspect", "client", "secret");
    private final WebClient rest = WebClient.create();

    @Override
    public Mono<OAuth2AuthenticatedPrincipal> introspect(String token) {
        return this.delegate.introspect(token)
		        .map(this::makeUserInfoRequest);
    }
}
----

.Kotlin
[source,kotlin,role="secondary"]
----
class UserInfoOpaqueTokenIntrospector : ReactiveOpaqueTokenIntrospector {
    private val delegate: ReactiveOpaqueTokenIntrospector = NimbusReactiveOpaqueTokenIntrospector("https://idp.example.org/introspect", "client", "secret")
    private val rest: WebClient = WebClient.create()

    override fun introspect(token: String): Mono<OAuth2AuthenticatedPrincipal> {
        return delegate.introspect(token)
                .map(this::makeUserInfoRequest)
    }
}
----
====

Either way, having created your `ReactiveOpaqueTokenIntrospector`, you should publish it as a `@Bean` to override the defaults:

====
.Java
[source,java,role="primary"]
----
@Bean
ReactiveOpaqueTokenIntrospector introspector() {
    return new UserInfoOpaqueTokenIntrospector();
}
----

.Kotlin
[source,kotlin,role="secondary"]
----
@Bean
fun introspector(): ReactiveOpaqueTokenIntrospector {
    return UserInfoOpaqueTokenIntrospector()
}
----
====

[[webflux-oauth2resourceserver-multitenancy]]
== Multi-tenancy

A resource server is considered multi-tenant when there are multiple strategies for verifying a bearer token, keyed by some tenant identifier.

For example, your resource server may accept bearer tokens from two different authorization servers.
Or, your authorization server may represent a multiplicity of issuers.

In each case, there are two things that need to be done and trade-offs associated with how you choose to do them:

1. Resolve the tenant
2. Propagate the tenant

=== Resolving the Tenant By Claim

One way to differentiate tenants is by the issuer claim. Since the issuer claim accompanies signed JWTs, this can be done with the `JwtIssuerReactiveAuthenticationManagerResolver`, like so:

====
.Java
[source,java,role="primary"]
----
JwtIssuerReactiveAuthenticationManagerResolver authenticationManagerResolver = new JwtIssuerReactiveAuthenticationManagerResolver
    ("https://idp.example.org/issuerOne", "https://idp.example.org/issuerTwo");

http
    .authorizeExchange(exchanges -> exchanges
        .anyExchange().authenticated()
    )
    .oauth2ResourceServer(oauth2 -> oauth2
        .authenticationManagerResolver(authenticationManagerResolver)
    );
----

.Kotlin
[source,kotlin,role="secondary"]
----
val customAuthenticationManagerResolver = JwtIssuerReactiveAuthenticationManagerResolver("https://idp.example.org/issuerOne", "https://idp.example.org/issuerTwo")

return http {
    authorizeExchange {
        authorize(anyExchange, authenticated)
    }
    oauth2ResourceServer {
        authenticationManagerResolver = customAuthenticationManagerResolver
    }
}
----
====

This is nice because the issuer endpoints are loaded lazily.
In fact, the corresponding `JwtReactiveAuthenticationManager` is instantiated only when the first request with the corresponding issuer is sent.
This allows for an application startup that is independent from those authorization servers being up and available.

==== Dynamic Tenants

Of course, you may not want to restart the application each time a new tenant is added.
In this case, you can configure the `JwtIssuerReactiveAuthenticationManagerResolver` with a repository of `ReactiveAuthenticationManager` instances, which you can edit at runtime, like so:

====
.Java
[source,java,role="primary"]
----
private Mono<ReactiveAuthenticationManager> addManager(
		Map<String, ReactiveAuthenticationManager> authenticationManagers, String issuer) {

	return Mono.fromCallable(() -> ReactiveJwtDecoders.fromIssuerLocation(issuer))
            .subscribeOn(Schedulers.boundedElastic())
            .map(JwtReactiveAuthenticationManager::new)
            .doOnNext(authenticationManager -> authenticationManagers.put(issuer, authenticationManager));
}

// ...

JwtIssuerReactiveAuthenticationManagerResolver authenticationManagerResolver =
        new JwtIssuerReactiveAuthenticationManagerResolver(authenticationManagers::get);

http
    .authorizeExchange(exchanges -> exchanges
        .anyExchange().authenticated()
    )
    .oauth2ResourceServer(oauth2 -> oauth2
        .authenticationManagerResolver(authenticationManagerResolver)
    );
----

.Kotlin
[source,kotlin,role="secondary"]
----
private fun addManager(
        authenticationManagers: MutableMap<String, ReactiveAuthenticationManager>, issuer: String): Mono<JwtReactiveAuthenticationManager> {
    return Mono.fromCallable { ReactiveJwtDecoders.fromIssuerLocation(issuer) }
            .subscribeOn(Schedulers.boundedElastic())
            .map { jwtDecoder: ReactiveJwtDecoder -> JwtReactiveAuthenticationManager(jwtDecoder) }
            .doOnNext { authenticationManager: JwtReactiveAuthenticationManager -> authenticationManagers[issuer] = authenticationManager }
}

// ...

var customAuthenticationManagerResolver = JwtIssuerReactiveAuthenticationManagerResolver(authenticationManagers::get)
return http {
    authorizeExchange {
        authorize(anyExchange, authenticated)
    }
    oauth2ResourceServer {
        authenticationManagerResolver = customAuthenticationManagerResolver
    }
}
----
====

In this case, you construct `JwtIssuerReactiveAuthenticationManagerResolver` with a strategy for obtaining the `ReactiveAuthenticationManager` given the issuer.
This approach allows us to add and remove elements from the repository (shown as a `Map` in the snippet) at runtime.

NOTE: It would be unsafe to simply take any issuer and construct an `ReactiveAuthenticationManager` from it.
The issuer should be one that the code can verify from a trusted source like an allowed list of issuers.

[[webflux-oauth2resourceserver-bearertoken-resolver]]
== Bearer Token Resolution

By default, Resource Server looks for a bearer token in the `Authorization` header.
This, however, can be customized.

For example, you may have a need to read the bearer token from a custom header.
To achieve this, you can wire an instance of `ServerBearerTokenAuthenticationConverter` into the DSL, as you can see in the following example:

.Custom Bearer Token Header
====
.Java
[source,java,role="primary"]
----
ServerBearerTokenAuthenticationConverter converter = new ServerBearerTokenAuthenticationConverter();
converter.setBearerTokenHeaderName(HttpHeaders.PROXY_AUTHORIZATION);
http
    .oauth2ResourceServer(oauth2 -> oauth2
        .bearerTokenConverter(converter)
    );
----

.Kotlin
[source,kotlin,role="secondary"]
----
val converter = ServerBearerTokenAuthenticationConverter()
converter.setBearerTokenHeaderName(HttpHeaders.PROXY_AUTHORIZATION)
return http {
    oauth2ResourceServer {
        bearerTokenConverter = converter
    }
}
----
====

== Bearer Token Propagation

Now that you're in possession of a bearer token, it might be handy to pass that to downstream services.
This is quite simple with `{security-api-url}org/springframework/security/oauth2/server/resource/web/reactive/function/client/ServerBearerExchangeFilterFunction.html[ServerBearerExchangeFilterFunction]`, which you can see in the following example:

====
.Java
[source,java,role="primary"]
----
@Bean
public WebClient rest() {
    return WebClient.builder()
            .filter(new ServerBearerExchangeFilterFunction())
            .build();
}
----

.Kotlin
[source,kotlin,role="secondary"]
----
@Bean
fun rest(): WebClient {
    return WebClient.builder()
            .filter(ServerBearerExchangeFilterFunction())
            .build()
}
----
====

When the above `WebClient` is used to perform requests, Spring Security will look up the current `Authentication` and extract any `{security-api-url}org/springframework/security/oauth2/core/AbstractOAuth2Token.html[AbstractOAuth2Token]` credential.
Then, it will propagate that token in the `Authorization` header.

For example:

====
.Java
[source,java,role="primary"]
----
this.rest.get()
        .uri("https://other-service.example.com/endpoint")
        .retrieve()
        .bodyToMono(String.class)
----

.Kotlin
[source,kotlin,role="secondary"]
----
this.rest.get()
        .uri("https://other-service.example.com/endpoint")
        .retrieve()
        .bodyToMono<String>()
----
====

Will invoke the `https://other-service.example.com/endpoint`, adding the bearer token `Authorization` header for you.

In places where you need to override this behavior, it's a simple matter of supplying the header yourself, like so:

====
.Java
[source,java,role="primary"]
----
this.rest.get()
        .uri("https://other-service.example.com/endpoint")
        .headers(headers -> headers.setBearerAuth(overridingToken))
        .retrieve()
        .bodyToMono(String.class)
----

.Kotlin
[source,kotlin,role="secondary"]
----
rest.get()
        .uri("https://other-service.example.com/endpoint")
        .headers { it.setBearerAuth(overridingToken) }
        .retrieve()
        .bodyToMono<String>()
----
====

In this case, the filter will fall back and simply forward the request onto the rest of the web filter chain.

[NOTE]
Unlike the https://docs.spring.io/spring-security/site/docs/current-SNAPSHOT/api/org/springframework/security/oauth2/client/web/reactive/function/client/ServerOAuth2AuthorizedClientExchangeFilterFunction.html[OAuth 2.0 Client filter function], this filter function makes no attempt to renew the token, should it be expired.
To obtain this level of support, please use the OAuth 2.0 Client filter.
