Need

Implementation of proper data validation and sanitization for user input

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
• Usage of shelf_io for managing and organizing data storage in a Node.js application

Description

Non compliant code

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

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

  io.serve(handler, 'localhost', 8080);
}

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

The above code represents a simple server application in Dart using the shelf framework. In the _echoRequest function, the server responds with a message that includes the URL of the request made by the client.

The vulnerability lies in the fact that the server is directly using the user input (in this case, the request URL) in its response without any form of data validation or sanitization. This means that if a client sends a request with a URL containing special characters or malicious scripts, the server will reflect these back in its response.

For example, if a client sends a request with a URL like http://localhost:8080/, the server will respond with Request for "", which if rendered on a web page, would execute the JavaScript code and trigger an alert with the message 'XSS'. This is a classic example of a Reflected Cross-Site Scripting (XSS) vulnerability.

In a real-world scenario, an attacker could use this vulnerability to inject malicious scripts that can steal sensitive information, manipulate web content, or perform other harmful actions when rendered by a victim's web browser.

To fix this vulnerability, we need to add data validation and sanitization to ensure that the user input does not contain any special characters or scripts before it is included in the server response.

Steps

• Import a library that can sanitize the user input to prevent XSS attacks.
• In the '_echoRequest' function, before returning the response, sanitize the 'request.url' to remove any special characters or scripts.
• Ensure that all user inputs are validated and sanitized before they are used in the application.

Compliant code

        import 'package:shelf/shelf.dart' as shelf;
import 'package:shelf/shelf_io.dart' as io;
import 'package:xss_clean/xss_clean.dart' as xss;

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

  io.serve(handler, 'localhost', 8080);
}

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

The updated code now includes the xss_clean package, which is used to sanitize user input and prevent XSS attacks.

In the _echoRequest function, before returning the response, the request.url is sanitized using the xssClean function from the xss_clean package. This function removes any special characters or scripts that could lead to an XSS attack.

This way, even if the user input includes malicious scripts, they will be removed before they are included in the server response, thus preventing any potential XSS attacks.

Remember, it's crucial to always validate and sanitize user inputs before using them in your application to prevent various types of security vulnerabilities.

References

• 192. Lack of data validation - Reflected Parameters