I'm trying to learn Spring Boot by writing a simple REST application that would log users in (POST /login) and display info about current user (GET /). I'm using Redis for sessions. 
POST /login works as expected: it returns the principal and sets the session cookie both in browser and Redis.
Consequent GET / requests, however, returns anonymousUser. What am I missing?
pom.xml:
<dependencies>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-data-redis</artifactId>
    </dependency>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-web</artifactId>
    </dependency>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-security</artifactId>
    </dependency>
    <dependency>
        <groupId>org.springframework.session</groupId>
        <artifactId>spring-session-data-redis</artifactId>
    </dependency>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-test</artifactId>
        <scope>test</scope>
    </dependency>
</dependencies>
application.properties:
spring.session.store-type=redis
server.servlet.session.timeout=3600s
spring.session.redis.flush-mode=on-save
spring.session.redis.namespace=spring:session
spring.redis.host=localhost
spring.redis.port=6379
IndexController.java:
@Controller
public class IndexController {
    private AuthenticationManager authenticationManager;
    IndexController(AuthenticationManager authenticationManager) {
        this.authenticationManager = authenticationManager;
    }
    @GetMapping
    ResponseEntity index(HttpServletRequest request, HttpSession session) {
        Authentication authentication = SecurityContextHolder.getContext().getAuthentication();
        return new ResponseEntity<>(authentication.getPrincipal(), HttpStatus.OK);
    }
    @PostMapping("/login")
    ResponseEntity login(@RequestBody LoginRequest loginRequest) {
        String username = loginRequest.getUsername();
        String password = loginRequest.getPassword();
        UsernamePasswordAuthenticationToken token = new UsernamePasswordAuthenticationToken(username, password);
        Authentication authentication = this.authenticationManager.authenticate(token);
        return new ResponseEntity<>(authentication.getPrincipal(), HttpStatus.OK);
    }
}
Config.java:
@EnableGlobalMethodSecurity(prePostEnabled = true)
@EnableWebSecurity
@EnableRedisHttpSession
@Configuration
public class Config extends WebSecurityConfigurerAdapter {
    @Autowired
    private CustomUserDetailsService userDetailsService;
    @Bean
    public PasswordEncoder passwordEncoder() {
        return new BCryptPasswordEncoder();
    }
    @Bean
    @Override
    public AuthenticationManager authenticationManagerBean() throws Exception {
        return super.authenticationManagerBean();
    }
    @Override
    protected void configure(AuthenticationManagerBuilder auth) throws Exception {
        auth.userDetailsService(userDetailsService);
    }
    @Override
    protected void configure(HttpSecurity http) throws Exception {
        http
            .sessionManagement()
            .sessionCreationPolicy(SessionCreationPolicy.ALWAYS)
            .and()
            .csrf()
            .disable()
            .authorizeRequests()
            .anyRequest()
            .permitAll();
    }
}
