Securing REST APIs is essential to protect sensitive data and ensure that only authorized users can access certain functionalities. In this tutorial, we will walk through the process of securing Kotlin-based REST APIs using OAuth2, a widely adopted authorization framework.

Prerequisites

  • Basic knowledge of Kotlin and Spring Boot
  • Spring Security dependencies added to your project
  • Understanding of OAuth2 concepts
  • Running instance of your Kotlin REST API

Step 1: Add Dependencies

Include the necessary dependencies in your build.gradle.kts file:

build.gradle.kts

```kotlin dependencies { implementation("org.springframework.boot:spring-boot-starter-security") implementation("org.springframework.boot:spring-boot-starter-oauth2-resource-server") } ```

Step 2: Configure the Resource Server

Create a configuration class to set up OAuth2 resource server settings:

SecurityConfig.kt

```kotlin import org.springframework.context.annotation.Bean import org.springframework.security.config.annotation.web.builders.HttpSecurity import org.springframework.security.web.SecurityFilterChain @EnableWebSecurity class SecurityConfig { @Bean fun securityFilterChain(http: HttpSecurity): SecurityFilterChain { http .authorizeRequests { requests -> requests .antMatchers("/public/**").permitAll() .anyRequest().authenticated() } .oauth2ResourceServer { oauth2 -> oauth2.jwt() } return http.build() } } ```

Step 3: Configure JWT Decoder

Specify the issuer URI or JWK set URI in your application properties:

application.properties

```properties spring.security.oauth2.resourceserver.jwt.issuer-uri=https://your-authorization-server.com/oauth2/default ```

Step 4: Protect Your Endpoints

Use annotations to secure specific endpoints or classes:

ExampleController.kt

```kotlin import org.springframework.web.bind.annotation.GetMapping import org.springframework.web.bind.annotation.RestController import org.springframework.security.access.prepost.PreAuthorize @RestController class ExampleController { @GetMapping("/public/hello") fun publicHello(): String = "Hello, World!" @PreAuthorize("hasAuthority('SCOPE_read')") @GetMapping("/secure/data") fun secureData(): String = "Sensitive Data" } ```

Step 5: Testing Your Setup

Start your application and attempt to access the protected endpoint:

Without Token: You should receive a 401 Unauthorized response.

With Valid Token: You will gain access to the secured data.

Conclusion

Implementing OAuth2 in your Kotlin REST APIs enhances security by ensuring only authenticated and authorized users can access sensitive endpoints. Follow these steps to set up a secure resource server with JWT validation, and customize it further based on your authorization needs.