Need

Implementation of robust data validation for user input

Context

• Usage of Python 3 for writing and executing Python code
• Usage of Django for building web applications in Python

Description

Non compliant code

        from django.http import HttpResponse
from django.views import View

class MyView(View):
    def get(self, request, *args, **kwargs):
        user_input = request.GET.get('user_input')
        return HttpResponse('User input is: ' + user_input)
        
        

In the above code, the get method of MyView class is vulnerable to Cross-Site Scripting (XSS) attack. The user input is directly taken from the GET request parameters and is included in the HTTP response without any validation or escaping.

The user_input variable contains the user input from the GET request parameters. This value is then concatenated with the string 'User input is: ' and sent back in the HTTP response. If an attacker sends a GET request with malicious script tags as part of the 'user_input' parameter, the script will be executed in the browser of any user who views the response.

This is an example of a reflected XSS vulnerability because the malicious script is included directly in the response. The lack of data validation means that any script can be executed, making this a serious security vulnerability.

To avoid this vulnerability, you should always validate and sanitize user inputs, especially those included in HTTP responses. This can be done using Django's built-in escaping functions or by using a library like Django's Bleach.

Steps

• Use Django's built-in escaping functions to sanitize the user input before including it in the HttpResponse.
• Consider using Django's forms for handling user input, which includes built-in validation and sanitization.
• Avoid directly concatenating user input into server responses.
• Implement a Content Security Policy (CSP) to mitigate the risk of XSS attacks.

Compliant code

        from django.http import HttpResponse
from django.views import View
from django.utils.html import escape

class MyView(View):
    def get(self, request, *args, **kwargs):
        user_input = request.GET.get('user_input')
        sanitized_input = escape(user_input)
        return HttpResponse('User input is: ' + sanitized_input)
        
        

The original code was vulnerable to Cross-Site Scripting (XSS) attacks because it directly included user input in the server response without any validation or sanitization. This could allow an attacker to inject malicious scripts into the server response, which would then be executed in the user's browser.

The fixed code mitigates this vulnerability by using Django's built-in escape function to sanitize the user input before including it in the HttpResponse. This function replaces any special characters in the user input with their HTML-encoded equivalents, preventing them from being interpreted as code by the user's browser.

In addition to this, it's recommended to use Django's forms for handling user input, which includes built-in validation and sanitization. This can provide an additional layer of security by ensuring that the user input meets certain criteria before it's processed by the server.

Furthermore, it's advisable to implement a Content Security Policy (CSP) to mitigate the risk of XSS attacks. A CSP is a security measure that helps detect and mitigate certain types of attacks, including XSS and data injection attacks. It's a declarative policy that lets the server inform the browser about the sources from which the application expects to load resources.

References

• 192. Lack of data validation - Reflected Parameters