Troubleshooting CORS Errors on Go Server and Flutter Web Client

Frontend and Backend Development
1

Despite enabling CORS in my Go server, my Flutter web client still yields CORS errors. Is the issue with the backend configuration or the Flutter request code?

func startServer() {
    conn, err := openDatabase()
    if err != nil {
        log.Fatal("Database connection failed")
    }
    initializeSchema(conn)
    preloadData(conn)

    corsOpts := cors.Options{
        AllowedOrigins: []string{"https://example.app", "*"},
        Methods:      []string{"GET", "POST", "PUT", "DELETE"},
        Headers:      []string{"Origin", "Content-Type", "Accept", "Authorization"},
        Credentials:  true,
        MaxTimeout:   8 * time.Hour,
    }
    router := setupRoutes(conn)
    router.Use(cors.Middleware(corsOpts))
    router.Run(":8080")
}

Future authenticate() async {
  final payload = {
    'username': emailInput.text,
    'password': passInput.text
  };
  try {
    final response = await http.post(
      Uri.parse('
      '
      '
      Uri.parse('$apiEndpoint/login'),
      headers: {'Content-Type': 'application/json'},
      body: jsonEncode(payload),
    );
    if (response.statusCode == 200) {
      Navigator.pushReplacement(context, MaterialPageRoute(builder: (_) => HomeScreen()));
    } else {
      print('Authentication failed');
    }
  } catch (error) {
    print('Error: $error');
  }
}
2

hey swiftcoder15, maybe try removing the ‘*’ from allowed origins as it can conflict with specified domains. i suspect flutter might be sending extra headers too. anyone else noticed similar misconfigurations? would love to hear ur checks on raw requests…

3

In my experience, a frequent source of issues in similar setups is the inclusion of both a wildcard and an explicit domain in CORS settings. My project involved a backend in which I initially faced unexpected CORS errors. By removing the wildcard and strictly specifying the exact origin, the issue was resolved. Additionally, I found that examining the network requests using browser developer tools helped identify discrepancies in headers between the client and server. Carefully reviewing these interactions allowed me to adjust settings to align with the CORS preflight expectations, thereby resolving the problem.

4

hey swiftcoder15, hav u checked if flutter sends extra headers that preflight might not like? sometimes little mismatches in header names cause issues. what do your logs reveal? curious if others have seen similar quirks!

5

hey swiftcoder15, try removin the wildcard completely. flutter might send extra headers that mess things up. check your backend log to see if any preflight is blocked and adjust your server config accordingly. hope it helps!

6

Based on my experience troubleshooting similar scenarios, the issue might indeed lie in the way CORS is handled by both the backend and the client. Specifically, I found that using a wildcard along with a specific domain in the allowed origins can lead to unpredictable behavior. This setup can cause conflicts, especially when credentials are involved. It is useful to simplify the CORS configuration by either using a single allowed origin or clearly separating development and production settings. Verifying the actual headers sent by the Flutter client using browser dev tools might also shed light on minor discrepancies that could trigger errors.