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Lack of data validation - Input Length

Need

Implementation of input length validation to prevent excessive data submission

Context

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

Description

Non compliant code

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

shelf.Response handleRequest(shelf.Request request) {
var content = request.readAsString();

// process content
// ...

return shelf.Response.ok('Request processed.');
}

The above Dart code represents a simple request handler for a backend application using the Shelf framework. The vulnerability lies in the handleRequest function, which reads the request content as a string without validating its length.

In this function, request.readAsString() is used to read the entire request body as a string. However, there is no check on the length of this content. This means that an attacker can send a request with a massive amount of characters in the body, potentially causing a Denial-of-Service (DoS) attack by consuming server resources, or exploiting other potential vulnerabilities related to large inputs.

The lack of input length validation can lead to various issues, including:

  • Buffer overflow: If the server stores the input in a buffer without checking its size, it can overflow, potentially leading to arbitrary code execution or other unexpected behavior.
  • Memory exhaustion: Storing the large input can consume a lot of memory, potentially causing the server to run out of memory and crash.
  • Performance degradation: Even if the server can handle the large input, processing it can take a lot of time and computational resources, degrading the performance of the server.

To mitigate this vulnerability, it's recommended to validate the length of the input before processing it. This can be done by checking the length of the string, or by using a middleware that limits the size of the request body.

Steps

  • Add a middleware function to validate the length of the request body.
  • In the middleware function, check the length of the request body before it is processed.
  • If the length of the request body exceeds the maximum allowed length, return an error response.
  • The maximum allowed length should be defined based on the requirements of the application.
  • Ensure that this validation is applied to all relevant endpoints.

Compliant code

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

const int MAX_CONTENT_LENGTH = 1000; // Define your maximum content length

shelf.Middleware validateContentLength() {
return (shelf.Handler innerHandler) {
return (shelf.Request request) async {
var content = await request.readAsString();
if (content.length > MAX_CONTENT_LENGTH) {
return shelf.Response(413, body: 'Request body too large');
}
return innerHandler(request.change(body: content));
};
};
}

shelf.Response handleRequest(shelf.Request request) {
var content = request.readAsString();

// process content
// ...

return shelf.Response.ok('Request processed.');
}

void main() {
var handler = const shelf.Pipeline()
.addMiddleware(validateContentLength())
.addHandler(handleRequest);

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

The above code fixes the vulnerability by adding a middleware function validateContentLength() that validates the length of the request body before it is processed by the handleRequest() function.

The middleware function reads the request body as a string and checks its length. If the length of the request body exceeds the maximum allowed length (MAX_CONTENT_LENGTH), the middleware function returns an HTTP 413 (Payload Too Large) response. Otherwise, it forwards the request to the next handler in the pipeline.

The validateContentLength() middleware is added to the pipeline using the addMiddleware() method. This ensures that the validation is applied to all requests handled by the pipeline.

The MAX_CONTENT_LENGTH constant is used to define the maximum allowed length of the request body. This value should be set based on the requirements of your application.

The main() function sets up a server that listens for requests on localhost port 8080. The server uses the handleRequest() function to handle requests, with the validateContentLength() middleware applied to all requests.

References