Zuul Proxy CORS header contains multiple values, headers repeated twice - Java Spring Boot CORS filter config
<p>Why would I be getting every CORS header doubled? I am using the Zuul Proxy to have the request to a service proxied through an API gateway.</p>

<p>I must have something misconfigured with my spring security filtering order.</p>

<p>When I access a route that requires authentication I am getting an error like: </p>

<h2>Request to service through API Gateway error</h2>

<pre><code>XMLHttpRequest cannot load https://myservice.mydomain.com:8095/service/v1/account/txHistory?acctId=0.  
The 'Access-Control-Allow-Origin' header contains multiple values '*, *', but only one is allowed. 
Origin 'http://localhost:9000' is therefore not allowed access.
</code></pre>

<h2>Chrome network log</h2>

<p>I checked the response in Chrome devtools and sure enough the CORS headers are repeated twice: <a href="https://i.stack.imgur.com/sTLVo.png" rel="noreferrer"><img src="https://i.stack.imgur.com/sTLVo.png" alt="CORS Headers repeated"></a></p>

<p>So this looks like somehow my CORS filter is being called twice for each reply. I don't know why that would be happening at this point. It could be that my filter is added before the ChannelProcessingFilter.</p>

<h2>Code for API Gateway CORS filter:</h2>

<pre><code>public class SimpleCORSFilter implements Filter {

    @Override
    public void init(FilterConfig filterConfig) throws ServletException {}

    @Override
    public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException {
        HttpServletResponse res = (HttpServletResponse) response;
        res.setHeader("Access-Control-Allow-Origin", "*");
        res.setHeader("Access-Control-Allow-Methods", "POST, GET, OPTIONS, DELETE, PUT");
        res.setHeader("Access-Control-Max-Age", "3600");
        res.setHeader("Access-Control-Allow-Headers", "Authorization, Content-Type, Accept, x-requested-with, Cache-Control");
        chain.doFilter(request, res);
    }

    @Override
    public void destroy() {}
}
</code></pre>

<h2>My API Gateway security configuration:</h2>

<pre><code>@Configuration
@EnableWebSecurity
@EnableGlobalMethodSecurity(prePostEnabled = true)
public class SecurityConfig extends WebSecurityConfigurerAdapter {

    @Bean
    @Override
    public AuthenticationManager authenticationManagerBean() throws Exception {
        return super.authenticationManagerBean();
    }

    @Inject
    public void setUserDetailsService(UserDetailsService userDetailsService) {
        this.userDetailsService = userDetailsService;
    }

    private UserDetailsService userDetailsService;

    @Override
    protected void configure(AuthenticationManagerBuilder authenticationManagerBuilder) throws Exception {
        authenticationManagerBuilder.userDetailsService(userDetailsService)
                .passwordEncoder(new BCryptPasswordEncoder());
    }

    @Override
    protected void configure(HttpSecurity httpSecurity) throws Exception {
        httpSecurity
            .sessionManagement()
                .sessionCreationPolicy(SessionCreationPolicy.STATELESS)
                .and()
           .authorizeRequests()
                .antMatchers(HttpMethod.OPTIONS, "/**").permitAll()
                .antMatchers("/health","/metrics", "/v1/users/register").permitAll()
                .antMatchers("/mappings", "/v1/**", "/service/**").authenticated()
                .and()
            .httpBasic()
                .realmName("apiRealm")
                .and()
            .csrf()
                .disable()
            .headers()
                .frameOptions().disable()
            .and().addFilterBefore(new SimpleCORSFilter(), ChannelProcessingFilter.class);
    }

}
</code></pre>

<p>I could solve this by checking if the header is null and then setting it only if it is empty or null, though that does not seem like the best solution. I would like to understand what I have done to cause the headers to be preset twice.</p>