Need

Implementation of secure service configuration - Request Validation

Context

• Usage of Dart for building cross-platform mobile, web, and desktop applications
• Usage of Shelf for managing and organizing data in a Node.js application

Description

Non compliant code

        import 'package:shelf/shelf.dart' as shelf;

void main() {
  var handler = const shelf.Pipeline().addMiddleware(shelf.logRequests())
      .addHandler(_echoRequest);

  io.serve(handler, 'localhost', 8080).then((server) {
    print('Serving at http://${server.address.host}:${server.port}');
  });
}

shelf.Response _echoRequest(shelf.Request request) {
  return shelf.Response.ok('Request for "${request.url}"');
}
        
        

The above code represents a simple server setup using the Shelf framework in Dart. This server listens on localhost at port 8080 and responds with a simple message for every request it receives.

However, this code is vulnerable to Cross-Site Scripting (XSS) attacks because it lacks request validation. Specifically, the _echoRequest function returns a response that includes the request URL directly without sanitizing or validating it. This means that if an attacker sends a request with a malicious script in the URL, the server will return that script as part of its response, potentially leading to an XSS attack if that response is then rendered in a client-side context.

For example, an attacker could send a request to http://localhost:8080/, and the server would return that script as part of its response. If this response is then rendered in a browser, the malicious script could be executed, leading to an XSS attack.

To fix this vulnerability, the server should validate and sanitize all inputs, including the request URL, before using them. This can be done using various input validation and sanitization libraries available in Dart.

Steps

• Identify the points in your code where user input is processed or included in responses.
• Implement input validation to ensure only expected types of data are accepted. This can be done using regular expressions or built-in validation functions.
• Encode the output to ensure that it is displayed as data to the user and not executed as code. This can be done using functions that convert special characters to their HTML encoded equivalents.
• Use a security-focused library or framework that automatically handles these types of vulnerabilities.
• Regularly update and patch your libraries and frameworks to ensure you have the latest security fixes.
• Consider using a web application firewall (WAF) to help protect against XSS attacks.

Compliant code

        import 'package:shelf/shelf.dart' as shelf;
import 'package:html_escape/html_escape.dart';

void main() {
  var handler = const shelf.Pipeline().addMiddleware(shelf.logRequests())
      .addHandler(_echoRequest);

  io.serve(handler, 'localhost', 8080).then((server) {
    print('Serving at http://${server.address.host}:${server.port}');
  });
}

shelf.Response _echoRequest(shelf.Request request) {
  var safeUrl = escapeHtml(request.url.toString());
  return shelf.Response.ok('Request for "$safeUrl"');
}
        
        

The original code was vulnerable to Cross-Site Scripting (XSS) attacks because it did not validate or sanitize the user input before including it in the response. This could allow an attacker to inject malicious scripts into the response, which would then be executed by the user's browser.

The fixed code mitigates this vulnerability by using the html_escape package to sanitize the user input. The escapeHtml function is used to replace any special characters in the user input with their HTML encoded equivalents. This ensures that any input is treated as data by the user's browser, rather than being executed as code.

In addition to this, it's recommended to use security-focused libraries or frameworks that automatically handle these types of vulnerabilities, regularly update and patch your libraries and frameworks to ensure you have the latest security fixes, and consider using a web application firewall (WAF) to help protect against XSS attacks.

References

• 339. Insecure service configuration - Request Validation