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XPath injection

Need​

Prevention of XPath injection attacks

Context​

  • Usage of Dart for building cross-platform mobile, web, and desktop applications
  • Usage of Shelf for managing data models and database interactions

Description​

Non compliant code​

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

shelf.Response handleRequest(shelf.Request request) {
  var id = request.url.queryParameters['id'];
  var doc = XmlDocument.parse(request.readAsString());
  var node = doc.evaluateNodeSet('//*[id="$id"]');
  return shelf.Response.ok(node.toString());
}

In the above code, the application takes a user-provided string id from the URL query parameters and directly interpolates it into an XPath expression. This is a classic example of an XPath Injection vulnerability.

The XPath Injection vulnerability occurs when the application uses input data directly in the construction of an XPath query for XML data access. By sending irregular input, an attacker could modify the XPath query to access data that they should not be able to access, or even to disrupt the service.

In the given code, the id is directly used in the XPath expression without any form of sanitization or validation. An attacker can provide a malicious id that changes the XPath expression to access other nodes in the XML document. For example, if an attacker provides an id of '1' or '1'='1', the XPath expression becomes '//*[id="1" or "1"="1"]', which selects all nodes, potentially revealing sensitive information.

Steps​

  • Validate the 'id' parameter to ensure it only contains expected values. This could be done using a whitelist of acceptable characters or a regular expression.
  • Use parameterized XPath queries instead of string concatenation to prevent injection attacks. This ensures that user input is always treated as literal text and not part of the XPath query.
  • Consider using a less powerful language than XPath if you don't need its full functionality. This can reduce the attack surface.
  • Implement error handling to catch any exceptions that may occur during the execution of the XPath query. This can prevent the application from crashing and can also provide more information about the error, which can be useful for debugging.

Compliant code​

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

shelf.Response handleRequest(shelf.Request request) {
  var id = request.url.queryParameters['id'];
  
  // Validate the 'id' parameter
  if (!RegExp(r'^[a-zA-Z0-9]+$').hasMatch(id)) {
    return shelf.Response.forbidden('Invalid ID');
  }

  var doc = XmlDocument.parse(request.readAsString());
  
  // Use parameterized XPath queries
  var node = doc.findAllElements('id').where((element) => element.text == id);
  
  return shelf.Response.ok(node.toString());
}

The updated code includes a validation step for the 'id' parameter. This is done using a regular expression that only allows alphanumeric characters. If the 'id' parameter contains any other characters, the function will return a 403 Forbidden response.

The XPath query has been replaced with a parameterized query. This is done using the findAllElements method, which returns all elements with the specified tag name. The where method is then used to filter these elements based on their text content. This ensures that the 'id' parameter is always treated as literal text and not part of the XPath query.

This approach reduces the risk of XPath injection attacks by ensuring that user input is properly validated and sanitized. It also makes the code more robust by handling potential errors that could occur if the 'id' parameter is not valid.

References​

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