Return 401 for admin access without permission

src/main/java/com/openisle/config/CustomAccessDeniedHandler.java

@@ -0,0 +1,25 @@
+package com.openisle.config;
+
+import jakarta.servlet.ServletException;
+import jakarta.servlet.http.HttpServletRequest;
+import jakarta.servlet.http.HttpServletResponse;
+import org.springframework.security.access.AccessDeniedException;
+import org.springframework.security.web.access.AccessDeniedHandler;
+import org.springframework.stereotype.Component;
+
+import java.io.IOException;
+
+/**
+ * Returns 401 Unauthorized when an authenticated user lacks required privileges.
+ */
+@Component
+public class CustomAccessDeniedHandler implements AccessDeniedHandler {
+    @Override
+    public void handle(HttpServletRequest request,
+                       HttpServletResponse response,
+                       AccessDeniedException accessDeniedException) throws IOException, ServletException {
+        response.setStatus(HttpServletResponse.SC_UNAUTHORIZED);
+        response.setContentType("application/json");
+        response.getWriter().write("{\"error\": \"Unauthorized\"}");
+    }
+}

src/main/java/com/openisle/config/SecurityConfig.java

@@ -17,6 +17,7 @@ import org.springframework.security.core.userdetails.UsernameNotFoundException;
 import org.springframework.security.crypto.bcrypt.BCryptPasswordEncoder;
 import org.springframework.security.crypto.password.PasswordEncoder;
 import org.springframework.security.web.SecurityFilterChain;
+import org.springframework.security.web.access.AccessDeniedHandler;
 import org.springframework.security.web.authentication.UsernamePasswordAuthenticationFilter;
 import org.springframework.web.filter.OncePerRequestFilter;
 
@@ -31,6 +32,7 @@ import java.io.IOException;
 public class SecurityConfig {
     private final JwtService jwtService;
     private final UserRepository userRepository;
+    private final AccessDeniedHandler customAccessDeniedHandler;
 
     @Bean
     public PasswordEncoder passwordEncoder() {
@@ -61,6 +63,7 @@ public class SecurityConfig {
     public SecurityFilterChain filterChain(HttpSecurity http) throws Exception {
         http.csrf(csrf -> csrf.disable())
             .sessionManagement(sm -> sm.sessionCreationPolicy(SessionCreationPolicy.STATELESS))
+            .exceptionHandling(eh -> eh.accessDeniedHandler(customAccessDeniedHandler))
             .authorizeHttpRequests(auth -> auth
                     .requestMatchers(HttpMethod.POST, "/api/auth/**").permitAll()
                     .requestMatchers(HttpMethod.GET, "/api/posts/**").permitAll()