Spring Security supports method security using Reactor’s Context which is setup using ReactiveSecurityContextHolder. For example, this demonstrates how to retrieve the currently logged in user’s message.

Authentication authentication = new TestingAuthenticationToken("user", "password", "ROLE_USER");

Mono<String> messageByUsername = ReactiveSecurityContextHolder.getContext()
    .map(SecurityContext::getAuthentication)
    .map(Authentication::getName)
    .flatMap(this::findMessageByUsername)
    // In a WebFlux application the `subscriberContext` is automatically setup using `ReactorContextWebFilter`
    .subscriberContext(ReactiveSecurityContextHolder.withAuthentication(authentication));

StepVerifier.create(messageByUsername)
    .expectNext("Hi user")
    .verifyComplete();

with this::findMessageByUsername defined as:

Mono<String> findMessageByUsername(String username) {
    return Mono.just("Hi " + username);
}

Below is a minimal method security configuration when using method security in reactive applications.

@EnableReactiveMethodSecurity
public class SecurityConfig {
    @Bean
    public MapReactiveUserDetailsService userDetailsService() {
        User.UserBuilder userBuilder = User.withDefaultPasswordEncoder();
        UserDetails rob = userBuilder.username("rob")
            .password("rob")
            .roles("USER")
            .build();
        UserDetails admin = userBuilder.username("admin")
            .password("admin")
            .roles("USER","ADMIN")
            .build();
        return new MapReactiveUserDetailsService(rob, admin);
    }
}

Consider the following class:

@Component
public class HelloWorldMessageService {
    @PreAuthorize("hasRole('ADMIN')")
    public Mono<String> findMessage() {
        return Mono.just("Hello World!");
    }
}

Combined with our configuration above, @PreAuthorize("hasRole('ADMIN')") will ensure that findByMessage is only invoked by a user with the role ADMIN. It is important to note that any of the expressions in standard method security work for @EnableReactiveMethodSecurity. However, at this time we only support return type of Boolean or boolean of the expression. This means that the expression must not block.

When integrating with Chapter 21, WebFlux Security, the Reactor Context is automatically established by Spring Security according to the authenticated user.

@EnableWebFluxSecurity
@EnableReactiveMethodSecurity
public class SecurityConfig {

    @Bean
    SecurityWebFilterChain springWebFilterChain(ServerHttpSecurity http) throws Exception {
        return http
            // Demonstrate that method security works
            // Best practice to use both for defense in depth
            .authorizeExchange(exchanges ->
                exchanges
                    .anyExchange().permitAll()
            )
            .httpBasic(withDefaults())
            .build();
    }

    @Bean
    MapReactiveUserDetailsService userDetailsService() {
        User.UserBuilder userBuilder = User.withDefaultPasswordEncoder();
        UserDetails rob = userBuilder.username("rob")
            .password("rob")
            .roles("USER")
            .build();
        UserDetails admin = userBuilder.username("admin")
            .password("admin")
            .roles("USER","ADMIN")
            .build();
        return new MapReactiveUserDetailsService(rob, admin);
    }
}

You can find a complete sample in hellowebflux-method