Spring Webflux + Spring Security – PreAuthorize not working? Worry not, we’ve got you covered!

Are you tired of banging your head against the wall, trying to figure out why Spring Security’s `@PreAuthorize` annotation is not working with Spring Webflux? Well, you’re in luck! In this article, we’ll delve into the world of reactive security and explore the reasons behind this annoying issue. By the end of this journey, you’ll be equipped with the knowledge to get `@PreAuthorize` working seamlessly with your Spring Webflux application using Kotlin.

The Setup

Before we dive into the meat of the matter, let’s set the stage. You have a Spring Webflux application written in Kotlin, and you’re trying to use Spring Security to secure your REST endpoints. You’ve added the necessary dependencies to your build.gradle file:

dependencies {
    implementation("org.springframework.boot:spring-boot-starter-security")
    implementation("org.springframework.boot:spring-boot-starter-webflux")
}

You’ve also configured your SecurityConfig class, adding the necessary annotations and configurations:

@Configuration
@EnableWebFluxSecurity
class SecurityConfig {
    @Bean
    fun securityWebFilterChain(http: ServerHttpSecurity): SecurityWebFilterChain {
        return http
            .authorizeExchange()
            .anyExchange().permitAll()
            .and()
            .oauth2ResourceServer()
            .jwt()
            .and()
            .build()
    }
}

The Problem

@RestController
@RequestMapping("/api/v1")
class MyController {
    @GetMapping("/protected-endpoint")
    @PreAuthorize("hasAuthority('ROLE_ADMIN')")
    fun protectedEndpoint():Mono<String>{
        return Mono.just("Hello, Admin!")
    }
}

However, to your surprise, the `@PreAuthorize` annotation is not working as expected. You’re able to access the `/protected-endpoint` endpoint without providing any credentials or authority. What’s going on?

Understanding the Issue

The root of the problem lies in the fact that Spring Webflux uses a different security context than traditional Spring MVC applications. In a reactive application, the security context is not thread-bound, and the `@PreAuthorize` annotation relies on the `SecurityContextHolder` to fetch the current authentication object.

In a Webflux application, the `SecurityContextHolder` is not automatically populated with the authentication object. You need to explicitly set the security context on the `ServerWebExchange` object.

Solution 1: Using `ServerWebExchange`

One way to fix this issue is by injecting the `ServerWebExchange` object into your controller method and setting the security context manually:

@RestController
@RequestMapping("/api/v1")
class MyController {
    @GetMapping("/protected-endpoint")
    fun protectedEndpoint(exchange: ServerWebExchange): Mono<String> {
        exchange.attributes[ServerWebExchangeAttributeName.AUTHENTICATION] = authentication
        return Mono.just("Hello, Admin!")
    }
}

However, this approach can become cumbersome and tedious, especially when you have multiple endpoints that require authentication. There must be a better way…

Solution 2: Using `SecurityContextRepository`

A more elegant solution involves creating a custom `SecurityContextRepository` that sets the security context on the `ServerWebExchange` object:

@Component
class CustomSecurityContextRepository : SecurityContextRepository {
    override fun load(exchange: ServerWebExchange): Mono<SecurityContext> {
        val authentication = // fetch authentication object from your authentication provider
        return Mono.just(SecurityContextImpl(authentication))
    }

    override fun save(exchange: ServerWebExchange, context: SecurityContext): Mono<Void> {
        return Mono.empty()
    }

    override fun cleanup(exchange: ServerWebExchange): Mono<Void> {
        return Mono.empty()
    }
}

Now, you need to register your custom `SecurityContextRepository` in the SecurityConfig class:

@Configuration
@EnableWebFluxSecurity
class SecurityConfig {
    @Bean
    fun securityWebFilterChain(http: ServerHttpSecurity): SecurityWebFilterChain {
        return http
            .authorizeExchange()
            .anyExchange().permitAll()
            .and()
            .oauth2ResourceServer()
            .jwt()
            .and()
            .addSecurityContextRepository(customSecurityContextRepository())
            .build()
    }

    @Bean
    fun customSecurityContextRepository(): SecurityContextRepository {
        return CustomSecurityContextRepository()
    }
}

With this custom `SecurityContextRepository` in place, your `@PreAuthorize` annotation should now work as expected:

@RestController
@RequestMapping("/api/v1")
class MyController {
    @GetMapping("/protected-endpoint")
    @PreAuthorize("hasAuthority('ROLE_ADMIN')")
    fun protectedEndpoint(): Mono<String> {
        return Mono.just("Hello, Admin!")
    }
}

Conclusion

In conclusion, securing a Spring Webflux application with Spring Security’s `@PreAuthorize` annotation requires a bit more effort than traditional Spring MVC applications. By understanding the nuances of reactive security and using a custom `SecurityContextRepository`, you can get `@PreAuthorize` working seamlessly with your Kotlin-based Spring Webflux application.

Remember, security is a critical aspect of any application, and it’s essential to get it right from the start. By following the instructions outlined in this article, you’ll be well on your way to building a secure and robust Spring Webflux application.

Keyword	Count
	5
	4
@PreAuthorize	3
Kotlin	2

• Make sure to add the necessary dependencies to your build.gradle file.
• Configure your SecurityConfig class correctly, including the custom SecurityContextRepository.
• Use the @PreAuthorize annotation on your controller methods to secure specific endpoints.
• Test your application thoroughly to ensure that the security configuration is working as expected.

1. Understand the fundamental differences between traditional Spring MVC and Spring Webflux applications.
2. Learn how to use Spring Security’s @PreAuthorize annotation effectively in a Webflux application.
3. Implement a custom SecurityContextRepository to set the security context on the ServerWebExchange object.

If you have any further questions or need additional guidance, feel free to ask in the comments section below. Happy coding!

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