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Lack of data validation - HTML code

Need​

Implementation of proper data validation for HTML code

Context​

  • Usage of Python 3.0+ for developing Python applications
  • Usage of Django for building web applications in Python

Description​

Non compliant code​

from django.http import JsonResponse

def some_view(request):
    user_input = request.GET.get('user_input')
    data = {"message": user_input}
    return JsonResponse(data)

The above code is a simple Django view that takes user input from the GET parameters and directly includes it in the JSON response. This is a very basic example of a lack of data validation.

The vulnerability here is that there's no validation or sanitization of the 'user_input' parameter before it's included in the JSON response. This means that if a user were to input HTML code as the 'user_input' parameter, this HTML code would be included directly in the response and could potentially be interpreted by the client's browser.

For example, if a user were to input <script>alert('Hello, world!');</script>, the resulting JSON would be {"message": "<script>alert('Hello, world!');</script>"}. If this JSON is then used in a context where it's inserted into the DOM of a web page, the script could be executed, leading to Cross-Site Scripting (XSS) attacks.

The lack of data validation can lead to various security vulnerabilities including but not limited to XSS. It's always a good practice to validate and sanitize user inputs to prevent such vulnerabilities.

Steps​

  • Import the 'escape' function from django.utils.html.
  • Apply the 'escape' function to the 'user_input' variable before assigning it to the 'message' key in the 'data' dictionary. This will convert any HTML characters into their escaped equivalents, preventing them from being interpreted as actual HTML code.
  • Ensure that any other user inputs are also properly escaped before they are included in the JSON response.

Compliant code​

from django.http import JsonResponse
from django.utils.html import escape

def some_view(request):
    user_input = request.GET.get('user_input')
    escaped_input = escape(user_input)
    data = {"message": escaped_input}
    return JsonResponse(data)

The original code was vulnerable to HTML injection because it directly included user input in the JSON response without any form of validation or sanitization. This means that if a user entered valid HTML code as their input, it would be interpreted as actual HTML by any site that received the JSON response.

The fixed code mitigates this vulnerability by importing the escape function from django.utils.html and applying it to the user_input variable before it is included in the data dictionary. The escape function converts any HTML characters in the input into their escaped equivalents, which prevents them from being interpreted as actual HTML code.

For example, the &lt; character would be converted to &lt;, the > character would be converted to &gt;, and so on. This means that even if a user enters valid HTML code as their input, it will not be interpreted as HTML by the site that receives the JSON response.

It's important to note that this fix only applies to the user_input variable. If there are any other user inputs that are included in the JSON response, they should also be properly escaped to prevent HTML injection.

References​

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